PQQ

Pyrroloquinoline quinone (PQQ) is a quinone molecule that was first identified as an enzymatic cofactor in bacteria, acting as a prosthetic group similar to how B-vitamins work in humans.^[1] It is doubtful that PQQ is an enzymatic cofactor in humans, although it still appears to have affinity to proteins in the human body and can bind to them to confer biolgical effects. The proteins that seem to bind to PQQ are called quinoproteins,^[2] and via modifying their actions in the body PQQ can exert biological activity.

PQQ was once thought to be a novel vitamin compound, although this view has since had doubts cast upon it and is no longer seen as accurate. Despite the lack of a vitamin role in mammals, it does appear to have growth promoting properties in rodents and may be active in humans following supplementation

PQQ naturally occurs in most foods (in miniscule amounts) although the highest levels can be found in:

• Fermented Soybeans products such as Nattō (highest estimate of 61+/-31 ng/g wet weight,^[3] lower estimates in the range of 1.42 +/- 0.32ng/g^[4])
• Green Soybeans (9.26+/-3.82ng/g wet weight)^[3]
• Spinach (7.02 +/- 2.17ng/g fresh weight)^[4]
• Rape blossoms (blossoms of the brassica napus plant at 5.44 +/- 0.8ng/g fresh weight)^[4]
• Field Mustard (5.54 +/-1.50ng/g fresh weight)^[4]
• Tofu (24.4+/-12.5ng/g wet weight)^[3]
• Teas from Camellia Sinensis, aka Green Tea (around 30ng/g dry weight of leaves)^[3] with the lower range of estimates at 0.16 +/- 0.05^[4]
• Green peppers, Parsely, and Kiwi fruits (around 30ng/g wet weight or so)^[3] although some estimates are lower (2.12 +/- 0.40ng/g for green peppers)^[4]
• Human Breast milk at 140-180ng/mL (total PQQ and IPQ)^[5]

Overall content of PQQ in foods seems to range from 0.19-7.02ng/g fresh weight in one study^[4] up to 3.7-61ng/g in another,^[3] low numbers may not adequately reflect total content in foods due to excluding IPQ in the measurements whereas higher levels tend to include both PQQ and IPQ.^[5]

PQQ is present in a wide variety of foods, but currently the estimates of its contents are quite variable. This may be due to confusion as to whether solely PQQ should be counted or PQQ conjugates (it is not known if these confer dietary benefit). In general, the PQQ content of food products listed above is substantially lower than the content of supplemented PQQ (10-20mg) and food ingestion is unlikely to replicate the effects of supplementation due to the magnitude of difference

It should be noted that due to an affinity of PQQ to bind to amino acids and form imidazolopyrroloquinoline derivatives that the PQQ content of foods may not be the same as the total bioactive amounts of PQQ,^[6] probably due to rapid association with proteins forming amino acid conjugates (Imidazolopyrroloquinoline, or IPQ).^[7] Human milk, for example, contained 15% PQQ and 85% IPQ derivatives. That being said, no direct studies have been undertaken to see whether PQQ and IPQ have similar or different properties in vivo.

PQQ may form conjugates with dietary protein similar to how it is known to react with proteins in the body, but it is not known if this potential interaction with dietary protein is beneficial or negatively influences bioavailability

Pyrroloquinoline quinone is heat-stable and water soluble,^[1] and appears to be stable at ambient temperatures in the form of PQQ disodium salt either as trihydrate (12.7% water^[8]) or pentahydrate (22.9% water^[9]). It is thought to be a relatively stable REDOX factor in vivo, and is able to carry generally around 20,000 REDOX reactions before degradation,^[10][11] and when it carries out REDOX reactions by itself it gets converted into its reduced form known as pyrroloquinoline dihydroquinone (PQQH₂)^[12] and is replenished (back to the PQQ form) by glutathione.^[12]

PQQ binds to proteins via forming a schiff base, which is a spontaneous (no enzyme required) reaction to amino acids found in the protein structures such as lysine.^[13]

Pyrroloquinline quinone (PQQ) is a quinone structure with three carboxylic acid groups which are used to bind to proteins, and two ketone groups which are involved in the REDOX capacities of the molecule

In some in vitro studies, combining PQQ with reducing agents (SIN-1, sodium borohydride) can form a green precipitate^[14] and the reddish coloration of PQQ turns increasing brown when water content is removed.^[8]

PQQ (as a powder) appears to be able to change color depending on its hydration status and oxidation status

PQQ was initially thought to be synthesized via the α-amino adipic acid-Δ-semialdehyde (AASDH; also known as U26) enzyme^[15] although this seems to be incorrect^[16][17] since despite this protein having many PQQ binding sites^[18] its mRNA levels are not negatively regulated by PQQ levels^[19] which would most likely occur if the enzyme synthesized PQQ. It is known to be synthesized (in bacteria) from the amino acids L-tyrosine and glutamate^[20][21] in a process requiring a series of enzymes labelled PqqA-F where PqqA formed the peptide precursor and the other enzymes structurally modify it into active PQQ.^[22]

Although mammalian synthesis is not certain, PQQ does occur normally in the mammalian body^[23] and approxiamtely 100-400 nanograms of PQQ are thought to be made in humans each day;^[3][24] leading some authors to claim an estimated tissue concentration of approximately 0.8−5.9ng/g in humans.^[3]

Since complete deprivation from the diet of animals has been shown to hinder growth and reproductive performance,^[25][7] it was initially thought that this (paired with the initial guess of endogenous synthesis via AASDH) indicated a vitamin deficiency. However, due to the definition of vitamins being one that requires a disease state to occur during deficiency^[15] and no apparent dysfunction aside from impaired growth seen with PQQ deficeincy it was not classified as an essential vitamin;^[16] this claim of no vitamin-like property being supported by the idea that AASDH is not actually used for PQQ sythesis in humans.^[16][17]

Pyrroloquinoline quinone (PQQ) is known to occur in both the diet and in mammalian tissue, and appears to have biological activity in the body. It was initially thought to be a new vitamin, but this conclusion seems unlikely and it is more likely a bioactive non-vitamin compound.

PQQ has been investigated for being a growth factor in youth (since deprivation in rats impairs growth^[25][7]), secondary to its effects at improving mitochondrial biogenesis (making more mitochondria) at seemingly effective doses of 0.2-0.3mg/kg foodstuff (in mice),^[26] which is surprisingly close to the levels found in human breast milk.^[5] Preliminary evidence for mitochondrial efficacy has also been noted in adult humans given 0.075-0.3mg/kg daily,^[27] with the latter dose being close to the recommended 20mg serving for a 150lb adult.

PQQ is thought to be a non-vitamin growth factor, in part due to its naturally high levels in breast milk and reduced growth in rats without dietary PQQ. It may do so via beneficially influencing mitochondrial function

It is seen as a novel REDOX catalyzing agent due to its stability, which prevents most self-oxidation (seen in catechins) and polymerization (tannins).^[10] A case has been made that PQQs effects are constant between species and bacteria, which aims to validate extrapolation from one species to humans.^[10] The potency of PQQ and its quinoproteins in REDOX cycling appears to be approximately 100-fold greater than vitamin C or other polyphenolic compounds, when in alkaline conditions.^[24][28][29]

PQQ, after associating with proteins (not in the role of a cofactor) appears to be capable of REDOX cycling suggesting that it can have conditional prooxidant and antioxidant roles. The association with proteins suggests that it can modify their structures either directly or via modifying the levels of oxidation at the level of the protein (similar to how carotenoids such as astaxanthin are located at the cellular membrane which localizes their effects)

Pyrroloquinoline quinone (PQQ) was discovered in 1979 as an enymatic cofactor in bacteria;^[30] preliminary evidence in pig kidneys and adrenal glands suggested a similar role in mammals.^{[31][32][33][34]} Doubts were later cast upon the role of PQQ as a mammalian enzymatic cofactor,^[35][36][37] and currently the consensus is that PQQ is unlikely to be an enzymatic cofactor in humans as it is in bacteria and plants.

Pyrroloquinoline quinone (PQQ) was first discovered as a bacterial enzymatic cofactor (being required by bacterial enzymes to function properly) and preliminary evidence suggested it could play the same role in mammals, which would make PQQ a vitamin. But further study found no quality evidence supporting this role in mammals; it is currently believed that PQQ does not act as an enzymatic cofactor in humans

REDOX (REDuction OXidation) signalling refers to stimulation or inhibition of cellular signalling systems by molecules that can switch from an oxidized state to a reduced state, such as the well-known REDOX-acting supplements Vitamin C and Alpha-lipoic acid.^[38] Pyrroloquinoline quinone (PQQ) may have this property as well, although its primary mode of action seems to be acting on known REDOX proteins in the cell; this is in line with its high binding affinity for some proteins, despite not acting as their coenzyme.^[39][20] For example, PQQ may function as a mammalian growth factor via signal transduction modification by both oxidation and redox cycling^[40], and has been shown to improve insulin signalling in mice by redox cycling.^[41]

PQQ may have an indirect influence on REDOX signalling in a cell by modifying the actions of proteins, which may underlie some antioxidative (and prooxidative) changes in a cell similar to any other REDOX agent

PQQ has been noted to partially inhibit thioredoxin reductase 1 (TrxR1), which is an enzyme in the cytosol that reduces thioredoxin.^[42] PQQ has low potency yet high affinity in binding to TrxR1 and seems to outcompete thioredoxin binding.^[43] When PQQ binds to TrxR1, the enzyme's activity is modified so it acts more on an alternate substrate known as juglone.^[44] Overall, NADPH oxidase activity of TrxR1 (a measure of the activity of this enzyme) is increased in the presence of 10-50µM PQQ due to increased activity of the TrxR1-Juglone interaction.^[44]

Pyrroloquinoline quinone (PQQ) binds to an antioxidant enzyme (TrxR1) and alters its function, reducing its affinity towards its normal substrate and increasing its affinity towards an alternate substrate. Overall activity of this enzyme appears to be enhanced at high concentrations of PQQ, but the effect of more physiologically realistic (nanomolar) concentrations are not known

Inhibition of TrxR1 activity is known to cause an increase in the activity of the Nrf2 protein, which acts on the nucleus (via the antioxidant response element or ARE) to increase antioxidant gene expression.^[45][46][47] Since oral supplementation of PQQ appears to influence a large amount of genes under control of TrxR1-related transcripts^[48] it is thought that TrxR1 inhibition by PQQ occurs in vivo.^[48]

It is thought that PQQ inhibits thioredoxin reductase (TrxR1) when ingested orally, since genes that would normally be activated when TrxR1 is inhibited do seem to be activated with PQQ in rats

PQQ has also been shown to inhibit glutathione reductase, but despite a decreased K_M towards juglone (which would increase NAPDH oxidation and enzyme activity) the K_cat was also reduced and enzyme activity remains similar with or without PQQ.^[44] However, GSSG reduction with 5µM PQQ was reduced approximately 2-fold relative to control.^[44]

An inhibitory effect has been noted in regards to glutathione reductase as well, although the practical significance of this particular enzyme interaction is not known

In rats, PQQ depletion is known to influence genetic expression (238 out of 10,000 tested genes) and dietary repletion is known to influence 847 transcripts;^[48] of these, the major pathways affected include Thioredoxin and MAPK signalling but also PGC-1α, a positive regulator of mitochondrial biogenesis^[49]).^[48] PQQ activates PGC-1α via CREB phosphorylation^[50] and appears to positively regulate mitochondrial biogenesis in vivo. It also has other possible roles in blood pressure regulation, cellular cholesterol homeostasis, energy production, and protection of mitochondrial activity, all of which are beneficially associated with increased PGC-1α activity^[10][49]).

When studies are undertaken in rats comparing a PQQ deficient diet, in which the rats must rely solely on de novo biogenesis of PQQ) against PQQ sufficient diets, the PQQ supplemented diets tend to promote up to 20-30% more mitochondria in the liver (on a mass basis, as assessed by mtDNA) over the rats' lifetime.^{[26][25][10][51][7][48][50]} Decreased permeability of the mitochondrial membrane has also been noted without alterations in functional capacity or mitochondrial size,^[25] along with the mitochondrial count per cell increasing 60% from 56.8+/-7.8 to 91+/-6.6 with 2mg/kg PQQ fed by gavage starting from 2 weeks of age in rats on a PQQ deficient diet.^[25]

At least one study on humans has found a considerably higher increase in PGC-1α protein levels compared with placebo for those taking 20 mg/day during a 6-week endurance exercise training program.^[52]

Pyrroloquinoline quinone (PQQ) appears to be capable to increasing the activity of PGC-1α, which then promotes mitochondrial proliferation and membrane stabilization. This occurs in rats using oral doses similar to those in humans, and occurs secondary to CREB phosphoylation; this may suggest bioenergetic benefits of supplementation. At least one human study confirmed that 20 mg, in conjunction with endurance exercise, increased PGC-1α protein levels.

When humans supplement PQQ (0.075-0.3mg/kg for one week at a time for each dose), urinary lactate decreased by 15% along with a reduction in urinary pyruvic acid.^[27] A minor reduction of fumarate was noted, but other Kreb's cycle intermediates (Isoaconitate, Citric acid, 2-oxoglutarate, and succinate) were not altered in the urine.^[27] It was hypothesized, on the assumption that urinary metabolites reflect cellular energy status, that this indicated an increase in mitochondrial efficiency.^[53][54]

A nonsignificant decreasing trend in urinary 4-hydroxyphenylacetate was noted with PQQ;^[27] decreases in this and other urinary metabolites tend to suggest increased β-oxidation rates.^[55]

The currently lone human study using doses of PQQ commonly found in supplements suggest that supplementation may increase mitochondrial efficiency

Pyrroloquinoline quinone (PQQ) is known to enhance signalling of some MAPK proteins, most notably ERK1/2, to significant extents, rivalling its effects on thioredoxin and PGC-1α.^[48][56] This may be secondary to oxidative changes on the PTP1B protein; the changes occur when PQQ facilitates the production of hydrogen peroxide by associating with other proteins^[57]) within a cell via direct REDOX cycling.^[40] Hydrogen peroxide then modifies PTP1B on Cys-215.^[58] The change of Cys-215 from a sulfenic acid moiety (-SOH) into a more oxidized sulfinic acid (–SO2H) or sulfonic acid (–SO3H) causes reversible inhibition of PTP1B.^[59][60]

PTP1B is a negative regulator of the insulin receptor,^[61] and is also a negative regulator of the epidermal growth factor receptor (EGFR).^[58] By alleviating a negative inhibition, PQQ (via H₂O₂) can enhance signalling through the EGFR resulting in more ERK1/2 activation.

By acting as a direct REDOX couple, PQQ can inhibit PTP1B activity via hydrogen peroxide production within a cell. This inhibition of PTP1B enhances growth factor signalling (via EGFR signalling) and can enhance insulin sensitivity in a cell (by enhancing insulin receptor signalling)

Sirtuins are a class of histone deacetylase (HDAC) enzymes that have been implicated in aging and the regulation of energy homeostasis. One of the most studied sirtuins is SIRT1, which in mammalian cells is homologous to the yeast protein Sir2 that promotes increased longevity in lower organisms such as fruit flies and worms.^[62] SIRT1 also plays a role in whole-body energy metabolism,^[63] where it functions as a metabolic sensor, helping to fine-tune metabolism during stress responses.^[64] While SIRT1 was not shown to increase maximal lifespan in mice fed normal diets, transgenic mice overexpressing this protein exhibited healthier aging, with less metabolic disorders and cancer.^[65][66] SIRT1 transgenic mice also are protected from metabolic damage induced by high fat diets,^[67][66] which tend to reduce SIRT3 expression.^[68]

Since both SIRT1 and SIRT3 are involved in mitochondrial biogenesis^[69] and PQQ has been shown to influence this process, researchers in a recent study hypothesized that PQQ may have a positive effect on mitochondria by increasing the expression or activity of SIRT1 and SIRT3. To test this idea, HepG2 cells growing in culture were treated with 10-30µM PQQ, which significantly increased expression levels and enzyme activity of both SIRT1 and SIRT3 proteins. Although more research is needed to determine whether PQQ is able to increase sirtuin activity in vivo, (let alone in healthy humans) this work raises the possibility that PQQ supplementation could help to promote optimal metabolic function and healthy aging through increased SIRT1 and SIRT3 activity.

In vitro studies have shown that PQQ activates SIRT1 and SIRT3, which belong to the sirtuin family of gene regulatory proteins implicated in metabolic control and healthy aging. More research is needed to determine whether PQQ affects sirtuin function in humans.

PQQ is absorbed well in the intestines, but its absorption is highly variable; 62% of PQQ is absorbed on average in rats in a fed state, with a range from 19-89%.^[70]

A single dose (0.2mg/kg) of PQQ ingested by humans in a fruit-flavored drink has a t_max of about two hours and a C_max of approximately 9nM.^[27] Doubling PQQ dose from 0.075mg/kg PQQ daily for one week to 0.15mg/kg and then 0.3mg/kg in healthy subjects increased plasma PQQ levels in a linear manner. Fasting blood levels of PQQ ranged from 2 to 14nM when measurements were taken on day four of supplementation.^[27] These levels may be similar to the steady state values as they were measured after the fourth day of dosing on the morning after PQQ was ingested.^[27]

Daily supplementation of pyrroloquinoline quinone (PQQ) appears to increase plasma PQQ concentrations to a steady state level of around 10nM in humans

PQQ appears to be eliminated from mice 24 hours after ingestion except in the skin and kidneys, which retain detectable levels of PQQ following oral ingestion.^[70] In the skin, it was noted that 0.3% of the ingested dose was detectable six hours following a dose and 1.3% of the oral dose was detected after 24 hours. Greater than 95% of the PQQ in the blood seems to be associated with the blood cell fraction, with less than 5% remaining in the plasma fraction.^[70]

86% of an ingested dose of PQQ in mice appears to be eliminated via the kidneys within 24 hours of oral ingestion^[70] and is excreted in a manner directly correlated with serum levels in humans;^[27] in humans, less than 0.1% of the ingested dose is detected as unmodified PQQ, suggesting that PQQ is highly metabolized prior to elimination.^[27]

Pyrroloquinoline quinone has been noted to bind directly to metals such as uranium. This explains the toxicity of uranium to bacteria, which depend on PQQ as a cofactor for enzymes;^[71] uranium displaces a calcium ion which is required to bind PQQ to certain enzymes in bacteria.^[72][73]

Pyrroloquinoline quinone has a known affinity for some minerals, but the role of PQQ in the human body in regards to minerals is not known. It is unlikely to play a role in heavy mineral elimination due to the very low serum concentrations of PQQ.

The NMDA receptor possesses a sulfhydryl REDOX modulatory site that is susceptible to oxidation^[74] where oxidation suppresses NMDA signalling and reduction enhances NMDA signalling.^[75][76] PQQ (50µM) does not affect basal currents through the receptor, but it can block reducing agents from enhancing signalling^[77][78] in the 5-200µM range. The reduction of signalling is thought to be due to acting on the REDOX site, since PQQ can reduce excitotoxicity but fails to protect from H₂O₂ (which causes toxicity independent of the NMDA receptor).^[79]

This mechanism is thought to underlie protective benefits of PQQ supplementation^[78] seen at low concentrations of 5µM (other mechanisms require PQQ concentrations of up to 50µM in order to become appreciable).^[79]

PQQ appears to have a regulatory effect on the glutamate receptor known as NMDA, by causing some oxidation of the REDOX site and preventing excess reduction from occurring it can suppress abnormal spikes in NMDA signalling; since an excess of NMDA signalling can be toxic, the result is a neuroprotective effect. This is thought to be applicable to oral supplementation due to a low concentration being required.

100µM PQQ has been noted to protect cells from glutamate-induced cytotoxicity^[80][81] associated with an increase in antioxidant enzyme activity, as assessed by Nrf2 and HO-1.^[80] This is thought to be downstream of Akt/PI3K and GSK-3β activation,^[82] of which the former is known to occur with PQQ in the 50-100µM range in vitro.^[82]

PQQ also appears to prevent an increase in JNK signalling seen with NMDA-mediated toxicity, but it is not related to the protective effects on cellular survival^[82] and PI3K activation cannot fully predict the protective effects of PQQ.^[80]

PQQ appears to be related to an activation of PI3K/Akt signalling, which is known to cause an induction in antioxidant enzymes via Nrf2. This is thought to underlie some of the protective benefits of PQQ on cellular structure seen in vitro, but its significance to oral supplementation is not known

Protective effects against glutamate have been noted when PQQ is directly injected into the brain in a manner that is associated with the aforementioned antioxidant effects (PI3K activation and Nrf2/HO-1 induction).^[81]

Injections of PQQ into the brain are known to be neuroprotective, but it is not known if this applies to oral ingestion as well

Rotenone is a commercial pesticide that inhibits electron transport in mitochondria, resulting in high levels of reactive oxygen species production, oxidative damage and cell death through apoptosis. Based on these properties, rotenone is often used in rodent models for Parkinson’s disease, where chronic infusion causes oxidative damage that resembles many of the features of the human disease.^[83][84]

A recent study examined the ability of PQQ to confer neuroprotection in rotenone-induced models of Parkinson’s disease. Pre-treatment with 100μM PQQ protected SH-SY5Y neuroblastoma cells from rotenone-induced apoptosis by altering the balance of pro-vs. anti-apoptotic proteins, promoting cell survival in-part by reducing oxidative damage.^[85]

PQQ pre-treatment further protected mitochondria, preserving mitochondrial mass to near control levels in spite of a 36% reduction in mitochondrial mass with rotenone alone. In rotenone-induced models of Parkinson’s disease, rotenone is typically injected directly into the brain, causing neuronal damage that approximates the pathology of the human disease. Importantly, the in vitro effects of PQQ were validated in vivo, where injection of PQQ along with rotenone directly into the brain decreased neuronal damage and improved symptoms.^[85]

PQQ has been shown to have a protective effect in rodent models of Parkinson’s disease. In vitro studies further suggest that this protective effect occurs via reduction of oxidative stress and protection of mitochondria, suppressing cell death. More research is needed to determine to examine the effects of PQQ on Parkinson’s disease in humans.

In fibroblastic cells (L-M), incubation of PQQ disodium salt (approximately 100µg/mL) for 24 hours has resulted in a peak 40-fold increase in Nerve Growth Factor (NGF) synthesis, with minor (around 5 to 10-fold) increases at 10-20µg/mL^[86][87] in a manner dependent on COX2 induction^[88] and PI3K/Akt.^[89] Prostaglandins D2 and E2 (from arachidonic acid) have been reported in vitro,^[88] and while they were not tested as a mandatory intermediate the former (and its metabolite prostaglandin J2) are known to promote NGF synthesis in the 6.3-25µg/mL range^[88] via CHRT2^[90] extending to a variety of cell lines.^[91][92][93]

This increase in NGF synthesis has also been noted in isolated mouse astrocytes exceeding alpha-lipoic acid (ALA) in potency, but less than ALA in c/3T3 (embyotic fibroblast) cells.^[94]

When tested in vitro, PQQ appears to concentration-dependently increase NGF synthesis up to a peak efficacy at 100µg/mL. The increase noted in isolated cells appears to be quite large. Eicosanoid signalling appears to be involved in this phenomena, suggesting that PQQ works via manipulating the actions of eicosanoids

When fat-soluble derivatives were tested (PQQ trimethyl esters) at injections of 0.1-1mg/kg every other day, it was noted that peripheral sciatic nerves had enhanced regeneration;^[86] injections into the periphery failed to cause an increase in NGF in the neocortex, thought to be due to poor diffusion of PQQ across the blood brain barrier due to complexation with proteins in serum.^[86] A pharmaceutical modification of the PQQ enzyme (oxapyrroloquinoline; OPQ) was able to enhance brain NGF concentrations,^[86] and since OPQ is known to be metabolized into PQQ in bacteria (hypothesized to occur in rodents) and is fat soluble it was thought to act as a prodrug.

When tested later, PQQ added to silicon tubes confirmed an increase in the rate of physical recovery in a mouse model of physical nerve injury with benefits seen after four weeks extending to twelve weeks.^[95] This improvement was associated with an increase in well-myelinated neurons.^[95]

In a spinal cord injury model, 5mg/kg PQQ injected into the spine daily for a week after injury was able to suppress the expression of iNOS after one day (a biomarker for inflammation^[96][97]) and improved both locomotor performance and neuronal health (axonal density) in the area relative to control.^[98] Benefits to peripheral nerve function (in a rat model of sciatic nerve injury) have been noted orally; a low dose (20mg/kg) prevented hyperalgesia from the nerve injury while only the higher dose (40mg/kg) prevented muscular atrophy and lipid peroxidation.^[99]

The enhancement of neurogenesis has been noted in the periphery (tissue excluding the brain) with injections of low doses of PQQ, but an increase in neurogenesis in the brain has failed to be noted which is thought to be due to transportation issues to the brain. While there are no oral studies in rodents yet, PQQ has been noted to enhance peripheral neurogenesis following nerve injuries

As mentioned in the glutaminergic section, the oxidative effects of PQQ on the NMDA modulatory site^[77][78] can ultimately cause a reduction in NMDA-induced superoxide formation in the neuron^[79] at concentrations (5uM) that do not affect oxidation per se (no effect against hydrogen peroxide which circumvents the receptor).^[79]

The anti-glutaminergic effects that occur at lower concentrations may also ultimately cause anti-oxidative effects by suppressing NMDA signalling, despite this mechanism being reliant on the pro-oxidant effects of PQQ

PQQ does not appear to influence the toxicity of peroxynitrate (a combination of nitric oxide and the superoxide radical), despite inhibiting its formation.^[100] When using SIN-1 as a way to produce peroxynitrate and induce cell death in vitro, PQQ at 100uM abolished cell death prior to peroxynitrate formation with an EC₅₀ of 15+/-8.4uM, yet actually potentiated pre-existing peroxynitrate toxicity (also seen with superoxide dismutase, an anti-oxidant enzyme, when catalase was not present).^[14] The mechanism appears to be through sequestering the superoxide radical without significantly influencing nitric oxide, as PQQ does not appear to modify many parameters of nitric oxide or peroxynitrate per se yet potentiated a SIN-1 induction of cGMP and production of nitrate, theoretically caused by a backlog of nitric oxide that could not convert to peroxynitrate due to less free superoxide radicals.^[14] Interactions with PQQ and superoxide radicals has been noted previously.^[101][102]

Can prevent superoxide radical induced cell death, but does not significantly influence nitric oxide cell death per se

NMDA receptors are involved in the pathology of seizures (as seizures are involved with excessive NMDA signalling^[103][104]) and the REDOX modulatory site that PQQ is known to interact with (suppressing high levels of activity) is further implicated^[105] since seizures are associated with a high level of reducing agents in the brain^[106][107] which can act upon that site to promote increased NDMA signalling;^[105] it is thought that PQQ could have a therapeutic role (seen with pharmaceutical NMDA antagonist^[108][109]) since by its oxidative role it hinders this particular site on NMDA receptors^[77][78] and PQQ is thought to not associated with side-effects from excess suppression due to only suppressing high levels of NMDA signalling but not basal levels.

When seizures occur, they are potentiated by excessive signalling through the NMDA receptors and due to this NMDA receptor antagonists (or anything that can suppress excess signalling) are thought to be therapeutic. Since PQQ has been implicated in suppressing excess NMDA signalling, it is being investigated for anti-epileptic effects

Application of 200µM PQQ to isolated neurons undergoing epileptic activity can fully abolish such activity if induced by reducing agents (no effect on epileptic activity induced by other means),^[105] supporting the role PQQ plays in epilepsy via NMDA antagonism which may occurs to limited levels at concentrations as low as 5µM.^[79]

In vitro evidence support a role for PQQ, but due to quite high concentrations being used (relative to what is seen in the blood) and a hypothesized low transportation to the brain it is not sure if this will occur in a living organism following oral ingestion

Pyrroloquinoline quinone (PQQ) appears to have protective effects against ischemia (assessed by infart size) when 10mg/kg is injected either 30 minutes prior to ischemia (reducing the infarct size from a 95+/-3.6% increase to 68.8+/-10.4%)^[110] and is slightly less effective when injected immediately after rather than preloaded (37.6% reduction seen previously reduced to 18.5%).^[110] This has been replicated elsewhere with 3-10mg/kg (70-81% protection) but not 1mg/kg was given an hour after MCAO injury.^[111]

Injections of PQQ have been noted to have protective effects in rats subject to stroke, but due to high injection doses being used and the low dose being ineffective preliminary evidence does not appear to look promising for oral supplementation of PQQ in this role; oral testing, however, has not yet been conducted

Injections (intraperitoneal) of PQQ in the range of 5-10mg/kg to rats for three days prior to tramautic brain injury was able to dose-dependently protect the brain from injury with the highest dose appearing to confer absolute protection (assessed by histology and cognitive behaviour post-injury).^[112]

When injected into rats at 10mg/kg bodyweight, PQQ does not appear to cause overt behavioural changes in regards to sedation, activty, or heart rate^[110] with no alterations in EEG readings being observed.^[110]

Several morphological changes are associated with PQQ that may confer pro-cognitive effects, such as proliferation of Schwann cells secondary to PI3K/Akt activation,^[89] PQQ is also able to induce production of Nerve Growth Factor (NGF)^[87] secondary to COX induction;^[88] increases in NGF have been observed in vivo when using trimethylesters (for permeability into the brain) with a maximal increase of 1.7-fold over baseline associated with a PQQ metabolite named oxazopyrroloquinoline.^[86]

PQQ supplementation has also been associated with preventing stress-associated (oxidative stress mediated) declines in memory^[113] reducing damage done by methylmercury toxicity,^[114][115] and reducing memory impairment induced by a lack of oxygen;^[116] at 20mg/kg bodyweight PQQ has a potency nonsignificantly different than 200mg/kg Vitamin E (as R-R-R-Alpha tocopherol) in reversing age-related memory decline in rats.^[116] which, together with its neuroprotective status, assure it a position as a rehabilitative Nootropic.

A human study found PQQ to have a modest, but significant effect on cognitive function in aged individuals with visual spatial impairment.^[117] Since cognitive function inevitably declines with aging, the goal of the study was to examine whether PQQ could help maintain or improve cognitive function in aged individuals. To examine whether PQQ could augment congnitive function, 42 individuals with an average age of 58 years were divided into placebo or PQQ supplementation groups in a randomized, placebo-controlled and double-blinded trial. Subjects took either 20mg of PQQ disodium salt (BioPQQ, manufactured by Mitsubishi Gas Chemical Co., Tokyo, Japan) or placebo after breakfast every day for 12 weeks. The Stroop test, Reverse Stroop test and Touch M test (a laptop tablet test to evaluate visual-spatial cognitive function) were administered at week 0 to establish baseline/pre-supplementation cognitive function and at week 12, to evaluate the effects of supplementation. No significant differences were noted between the PQQ group or placebo for either the Stroop or Reverse Stroop tests. In contrast, some differences were noted with the Touch M test, where a higher score indicates higher function and a score of less than 70 points is reflective of declined brain function.^[117] Although patients with a mean baseline Touch M score greater than 70 points did not show significant differences after 12 weeks PQQ supplementation, individuals with initial Touch M scores of less than 70 significantly increased from 58.1 at baseline to 71.5 after 12 weeks supplementation. While more research is needed to validate this result in additional subjects, taken at face-value this work suggests that PQQ might help recover some lost cognitive abilities in aged individuals with impaired cognitive function. This work is quite preliminary, however, and warrants further investigation until any definite conclusions can be drawn.

PQQ appears to have general nootropic benefit for those with impaired cognitive function (due to age, neural damage, etc.,) but does not have ample evidence to be claimed a cognition promoting nootropic in otherwise healthy individuals.

One open-label human study conducted with 20mg PQQ for 8 weeks in 17 persons with fatigue or sleep impairing disorder noted that PQQ was able to significantly improve sleep quality, with improvements in sleep duration and quality appearing at the first testing period 4 weeks after usage while a decrease in sleep latency required 8 weeks to reach significance.^[118] This study also noted improved appetite, obsession, and pain ratings that may have been secondary to improved sleep; contentness with life trended toward significance over 8 weeks but did not reach.^[118]

Protective effects have been noted in cardiac myocytes subject to ischemia, secondary to scavenging of peroxynitrate radicals, at injectible doses of 15mg/kg bodyweight 30 minutes prior to ischemia.^[119][120] PQQ was studied alongside metprolol as a combiantion anti-oxidant/beta-blocker therapy, and 3mg/kg PQQ and 1mg/kg metprolol were both insignificantly different in reducing mortality (40% of control passed, 8% of PQQ and 14% of metprolol) while no deaths were recorded in combination therapy.^[121] Combination was also more effective in reducing infarct size relative to either therapy in isolation, and both groups using PQQ had a reduction of creatine kinase release that was insignificantly different between groups.^[121]

The combination therapy study noted increased cardiac mitochondrial respiration with PQQ but neither metprolol nor PQQ+metprolol, and respiration was further increased even in the contrl groups with no ischemia/reperfusion done.^[121]

Secondary to the pro-mitochondrial effects and anti-oxidative effects during ischemia/reperfusion, PQQ appears to be cardioprotective under certain contexts

In otherwise healthy humans supplementing PQQ at 0.075-0.3mg/kg for three weeks (increasing the dose each week), supplementation was associated with a decrease in C-reactive protein concentrations in serum (45%).^[27] This study also noted that urinary trimethylamine-N-oxide (TMAO) was reduced^[27] and since both C-reactive protein (CRP)^[122] and TMAO^[123] are thought to be biomarkers for atherosclerosis PQQ is thought to have a role.

In rats fed a PQQ deficient diet relative to the same diet fed with 2mg/kg PQQ, plasma diglycerides and triglycerides (DAG and TAG) were elevated 20-50% (higher value related to triglycerides) in the PQQ deficient diet relative to 2mg/kg with no significant difference in free fatty acids,^[26] which is similar to levels previously seen with this experimental protocol.^[25] The elevation of triglycerides in the deficient mice does not influence the n3/n6 omega fatty acid ratios.^[26]

The increase seen in triglycerides may be due to this study being conducted for a long period of time, where previous research has demonstrated that PQQ deficient diets reduce mitochondrial density by 20-30%^[25] and levels of mRNA for PPAR, Fatty Acid binding protein, and Acyl CoA oxidase being significantly reduced with PQQ deficiency.^[26] Additionally, higher levels of beta-hydroxybutryic acid (indicative of less beta-oxidation) were seen in PQQ deficient rats. Inducing PQQ deficiency from a sufficient state can also elevate triglyceride levels to almost two-fold the previous levels, with the trend being reversed upon acute administration of PQQ in pharmacological amounts (2mg/kg bodyweight).^[48]

Appears to reduce triglycerides very potently (to a greater extent than fish oil, empirically) in research animals relative to a PQQ deficient diet, and this is thought to be due to increased mitochondrial β-oxidation of fatty acids

The one human study to use supplemental PQQ (0.075-0.3mg/kg for three weeks in escalating doses) failed to find any significant influence on triglyceride concentrations in serum of otherwise healthy adults consuming a standard (but uncontrolled) diet.^[27] This study also noted alterations in urinary metabolites (4-Hydroxyphenylacetate and 4-Hydroxyphenylactate) suggestive of an increase in mitochondrial β-oxidation despite no apparent changes in triglycerides.^[27]

First study to assess the effects of PQQ on triglycerides has failed to find an influence in otherwise healthy humans

PQQ (500nM) has been noted to inhibit protein tyrosine phosphatase 1B (PTP1B) secondary to producing H₂O₂^[40] (H₂O₂ is known to inactivate PTP1B in a reversible manner^[58]), and aside from PTP1B being a negative regulator of a growth factor receptor (EGFR^[58]) it also negatively influences insulin receptor signalling;^[61] inhibition of PTP1B, seen also with berberine and ursolic acid (albeit by different mechanisms), tends to increase the activity of the insulin receptor.

Sequestering the hydrogen peroxide made from PQQ appears to block its inhibition on PTP1B.^[40]

Via prooxidative changes within a cell, PQQ can produce hydrogen peroxide which then impairs PTP1B function. Since PTP1B normally suppresses signalling via the insulin receptor, the result is a compensatory increase in insulin signalling

In young rats (before sexual maturation), PQQ either at 3mg/kg in the diet or having a PQQ deficient diet does not seem to significantly affect blood glucose or insulin levels.^[26] An increased glucose AUC was seen when PQQ deficient mice were subject to an oral glucose tolerance test, but no single time point was significnatly different.^[26] Injections of PQQ at 4.5mg/kg bodyweight also did not significantly influence blood sugar or insulin levels in healthy rats, but was able to significantly reduce glucose AUC (by 7%) and glucose disposition in diabetic rats fed glucose and injected with PQQ, with no effect of PQQ on fasting glucose levels in rats.^[26]

It has potential for alleviating fat-induced insulin resistance (characterized by a dysregulation in beta-oxidation of the TCA cycle) by increasing mitochondrial biogenesis in muscle cells, similar to exercise.^[124]

At this moment in time, nothing remarkable about PQQ and glucose metabolism

When comparing a rat diet deemed sufficient in dietary pyrroloquinoline quinone (PQQ; 2mg/kg) to a diet deficient in one, the deficient diet appeared to have a decreased metabolic rate (reaching only 90% of the control rats)^[26] with the difference being more prominent during the fed rather than fasted state;^[26] it appears that this decreased metabolic rate did not influence the rats of lipolysis nor glycolysis as assessed by the respiratory quotient.^[26]

Depleting the rat diet of PQQ appears to reduce their metabolic rates relative to a diet with adequate levels of PQQ, but no studies have investigated whether an increase in metabolic rate occurs with extra supplemental PQQ

Pyrroloquinoline quinone (PQQ) has been noted to inhibit RANKL-induced osteclast formation in RAW 264.7 macrophage-like cells at a concentration of 10µM, which occurred at all stages of cell maturation.^[125]

RANKL normally signals through the transcription factor NFATc1^[126][127] via a particular AP-1 signalling protein that contains c-Fos and c-Jun.^[128][129] PQQ inhibited c-Fos induction from RANKL,^[125] but other RANKL-induced proteins (NF-kB and MAPKs) were unaffected suggesting that RANKL signalling overall was unaffected.^[125]

There is a negative regulatory pathway from RANKL, where RANKL increases IFN-β production which signals via its receptor (IFNAR^[130]) to activate STAT1 and JAK1 to suppress the actions of RANKL.^[131][132] IFN-β was not affected by PQQ, but the receptor expression (and its targets) appeared to be increased which were thought to underlie the observed inhibitory effects seen with PQQ.^[125]

PQQ appears to enhance the negative feedback mechanism controlling osteoclastogenesis (production of osteoclasts, which are negative regulators of bone mass) and via this enhancement overall osteoclast activity is hindered somewhat and this is thought to promote bone mass over time. Due to a higher than normal concentration being used, it is not sure if this occurs following oral supplementation

One study using 0.075-0.3mg/kg PQQ supplementation daily for three weeks (increasing with dose each week) in otherwise healthy adults has noted a decrease in overall urinary amino acid levels by approximately 15%,^[27] with the decrease in some (serine, asparangine, aspartic acid) being biomarkers for skeletal muscle consumption of nitrogen (via being converted into glutamine and alanine^[27][133]).

Preliminary evidence suggests that oral PQQ supplementation can influence skeletal muscle metabolism in otherwise healthy humans with standard supplemental doses, but the practical significance of this is not yet known

PQQ appears to have some interactions with the immune system, as deprivation of PPQ from the diet (relative to a PQQ sufficient diet) appears to cause abnormal immune function in mice, with altered immune response after stressors.^[51][7]

A study on parental (intravenous) nutrition found that the addition of 3mcg PQQ to the parental nutrition in mice was able to increase the count of CD8+ cells and lymphocytes in intestinal Peyer's Patches, although not to the level of oral control.^[134]

Application of PQQ to macrophages in vitro was able to prevent osteoclast differentiation at doses as low as 0.1uM (but more potency at 10uM) secondary to increasing IFN-β secretion; IFNβ is a negative regulator of osteoclast differentiation normally released after inflammation, and PQQ increases its release (and subsequent suppression), which is also demonstrated by increased levels of proteins induced by IFN-β (iNOS, STAT1, JAK1).^[125] PQQ was found to phosphorylate NF-kB, p38, and IKKβ in these cells which is a pro-inflammatory response in macrophages.^[125]

Practical relevance unknown

The reduced form of pyrroloquinoline quinone (PQQ), known as pyrroloquinoline equinol or dihydroquinone pyrroloquinoline (PQQH₂) appears to be able to sequester singlet oxygen (¹O₂) with a potency 6.4-fold less than β-carotene as reference yet higher than that of Vitamin E (2.2-fold) and Vitamin C (6.3-fold).^[12]

PQQH₂ appears to be produced (via reduction) from PQQ when in a buffer in the presence of glutathione^[12] and this process is known to use the semiquinone (PQQH) as an intermediate;^[57] exposure to oxygen either by ambient atmosphere or by singlet oxygen readily oxidizes PQQH₂ back into PQQ.^[12] This suggests that glutathione is capable of recycling PQQ as an antioxidant.

PQQ and its reduced form PQQH₂ appear to form a cyclical relationship where PQQH₂ sequesters oxygen radicals, and glutathione reduces it back into PQQ so it may sequester more radicals; the potency of this reaction, on a molecular level, seems intermediate to β-carotene (PQQ is lesser) and Vitamin C/E (greater)

One study assessing whether PQQ could directly sequester peroxynitrate (ONOO^-) failed to find such a property of PQQ, as despite protecting cells form the toxic effects of SIN-1 (produces nitric oxide and superoxide radicals,^[135] of which PQQ scavenged the superoxide radicals^[14]) the toxicity of peroxynitrate directly was not protected against (in fact, it appeared to be augmented at 100-300µM PQQ).^[14]

Pyrroloquinoline quinone (PQQ), even at impractically high concentrations, does not appear to direct sequester reactive nitrogen species (nitrogen based pro-oxidants) such as peroxynitrate

One human study using supplemental pyrroloquinoline quinone (PQQ) and measuring serum antioxidant capacity via TBARS and TRAP values failed to find any significant influence on TRAP values but noted a decrease in TBARS (indicative of lipid peroxidation) to the degree of 0.2% when measured at peak serum PQQ values (6-12nM) seen with up to 300µg/kg supplementation;^[27] this decrease in TBARS was noted to be significantly less than other dietary supplements such as procyanidins from Cocoa extract which (560mg) can reduce TBARS by 25-35%^[136] or sources of anthocyanins such as aronia melanocarpa or blueberry.

The decrease in serum biomarkers of lipid peroxidation that is known with PQQ supplementation is probably much too low to be indicative of anything significant

Oral ingestion of 4mg/kg PQQ to mice (more effective than both 2mg/kg and 8mg/kg, as well as the reference drug of 10mg/kg nilestriol^[137]) appears to reduce death from gamma irradiation when given an hour before and again seven days after irradiation; damage to select cells tested (white blood cells, reticulocytes, bone marrow cells) was also reduced with 4mg/kg PQQ supplementation to mice.^[137]

Oral ingestion of PQQ (estimated human equivalent of 0.32mg/kg) appears to be able to protect mice from gamma irradiation to a respectable degree

An intraperitoneal injection of pyrroloquinoline quinone (PQQ) to rats at 5mg/kg twice before CCl₄ liver toxicity appeared to exert protective effects;^[138] when tested in vitro, PQQ showed protective effects in isolated liver cells with most potency at 3µM.^[138]

Due to the involvement of pyrroloquinoline quinone (PQQ) in bacteria (from where it was discovered in 1979^[30]) and the involvement of quinoproteins in the fermentation process ^[139] (which PQQ associates with) and the above higher count of PQQ recorded in fermented foods; it is hypothesized that fermentation may increase PQQ content. Interestingly, common strains of bacteria in the human intestinal tract do not appear to synthesis much PQQ^[140][141] and in antibiotic fed mice (lacking intestinal microflora) it seems that dietary intake is the major determinent of bodily PQQ levels.^[141]

Pyrroloquinoline quinone was thought to be synthesized by intestinal bacteria due to its discovery being that of a bacterial cofactor, but preliminary evidence does not support the intestinal microflora as a major producer of PQQ in the body

Pyrroloquinoline quinone (PQQ) was once implicated in being an enzymatic cofacter for diamine oxidase (pig kidney)^[31][32] and DOPA decarboxylase (pig kidney)^[33] (as well as dopamine β-hydroxylase, albeit in the renal medulla^[34]), although it is generally accepted to not be a significant component of eukaryotic enzymes in vivo (in the role of a cofactor) like it is in bacterial and plant enzymes.^[35][36][37] Still, it is detectable in the kidney after oral ingestion in the rat^[70] and elimination of PQQ is primarily via the urine^[70] suggesting it may still play a role independent of being an enzymatic cofactor.

PQQ is not thought to play a role as a cofactor of enzymes in the kidneys like initially thought, but due to being eliminated by the kidneys and accumulating in them following oral ingestion in the rat it is still thought to play a role (perhaps as a REDOX couplet, like other mechanisms)

PQQ has been shown to be cytotoxic to U937 leukemia cells, but not NIH3T3 nor L929 cells, in a dose-dependent manner.^[142] Catalase treatment neutralized these effects, as they appear to be secondary to hydrogen peroxide production in cells which PQQ has been repeatedly shown to induce.^[143] Superoxide dismutase had no effect on PQQ cytotoxicity, while glutathione or N-AcetylCysteine increased cytotoxicity 2-5fold without affecting the cells on their own (and thus working via PQQ by increasing H₂O₂ production form PQQ 1.5-2fold).^[142] PQQ by itself decreased intracellular glutathione levels, and when glutathione was depleted (via BSO, an inhibitor of γ-glutamylcysteine synthetase) the apoptosis of cells morphed into necrosis, and this necrosis was still mediated by H₂O₂ due to being inhibited by catalase.^[142]

Induces cell death via H₂O₂, and uses glutathoine to produce even more H₂O₂ to augment its efficacy. A depletion of glutathione induces necrosis

PQQ has been implicated in reducing melanogenic (melanin producing) protein expression in cultured B16 cells, where it can inhibit tyrosinase expression and reduce gene activity^[144] and can prevent stimulation of tryosinase mRNA by alpha-melanocyte stimulating hormone.^[145]

Parkinson's disease is known to be associated with what are known as Lewy Bodies (irregular cytoplasmic inclusions^[146][147]) which are comprised of a molecule known as α-synuclein^[148] which is known to damange dopaminergic neurons and is involved in the pathology of Parkinson's disease when it aggregates.^[149][150] It is involved in normal physiological function (as a chaperone) when unaggregated,^[151] so the process of α-synuclein aggregation itself is seen as pathological.

Pyrroloquinline quinone (PQQ) is known to bind to some of these α-synuclein peptides directly via forming a schiff base with the lysine amino acids in the peptides^[13] similar to both EGCG (green tea catechins) and baicalein (skullcap)^[13] although baicalein seems relatively more potent.^[152] This direct binding also reduces formation of truncated α-synuclein^[153] (which accelerate the formation of larger aggregates^[154]) and the larger protein aggregates themselves^[13] by around 14.8-50% at 280µM.^[153] This may indirectly reduce the cytotoxicity that is seen with large aggregates,^[13] although PQQ seems to be capable of reducing cytotoxicity from pre-formed aggregates independent of the aforementioned binding.^[153]

Protein aggregates tend to occur normally in the brain, and their aggregation is accelerated and seem to be central to the development of Parkinson's Disease. PQQ appears to physically bind to these proteins in vitro to prevent the aggregation, but it occurs at a very high concentration and it does not seem likely to occur with respectable potency following oral supplementation

6-hydroxydopamine (6-OHDA), a metabolite of dopamine which is known to cause oxidative damage to dopaminergic neurons and detected at higher levels in persons with Parkinson's,^[155] may have its toxicity attenuated with coincubation of PQQ.^[156] Oxidative neurotoxicity and DNA fragmentation induced by 6-hydroxydopamine was reduced in a concentration dependent manner with concentrations of 300nM showing efficacy, yet this protective effect was not seen with vitamin C or Vitamin E, two other anti-oxidants tested at concentrations up to 100µM.^[156]

Elsewhere in isolated neurons, the protein DJ-1 (plays roles in oxidative protection^[157][158] and mutations in it underly some genetic cases of early onset Parkinson's Disease^[159]) does not have its expression altered by PQQ^[160] but 15µM PQQ appeared to preserve cell survival in the presence of oxidants by preserving the actions of DJ-1;^[160] excessive oxidation of DJ-1 at C106 ablates its antioxidant potential^[161] and PQQ appears to prevent this from occurring despite no direct binding.^[160]

There may be some protective effects at the level of dopaminergic neurons with PQQ that is not related to preventing the formation of protein aggregation, and although this happens at a much more respectable (lower) concentration it is still uncertain if this applies to oral supplementation of PQQ

Pyrroloquinline quinone appears to inhibit the formation of amyloid fibrils (Aβ_1-42; full inhibition at 70μM PQQ^[162]), and although it can also bind to α-synuclein this binding does not indirectly inhibit Aβ_1-42 aggregation.^[13]

and to reduce the cytotoxicity of these fibrils on neuronal cells.^[163]

PQQ has been shown to be cytotoxic to U937 leukemia cells, but not NIH3T3 nor L929 cells (but was observed in EL-4), in a dose-dependent manner with most significance at 20-50uM.^[142] Catalase treatment neutralized these effects, as they appear to be secondary to hydrogen peroxide production in cells which PQQ has been repeatedly shown to induce.^[143] Superoxide dismutase had no effect on PQQ cytotoxicity, while glutathione or N-AcetylCysteine increased cytotoxicity 2-5fold without affecting the cells on their own (and thus working via PQQ by increasing H₂O₂ production form PQQ 1.5-2fold).^[142] PQQ by itself decreased intracellular glutathione levels, and when glutathione was depleted (via BSO, an inhibitor of γ-glutamylcysteine synthetase) the apoptosis of cells morphed into necrosis, and this necrosis was still mediated by H₂O₂ due to being inhibited by catalase.^[142]

Glutathione can be increased by cysteine containing supplements including N-AcetylCysteine or Whey Protein

In cancer cells susceptible to PQQ's induction of H₂O₂, adding glutathione to the cell by consuming Cysteine-containing supplements can augment the efficacy of PQQ

PQQ has been associated with renal tubule inflammation at the dose of 11-12mg/kg bodyweight in rats after injections, and some symptoms of both renal and hepatic toxicity are seen with injections of 20mg/kg in rats.^[121][164] Acute death from PQQ injections between doses of 500-1000mg/kg bodyweight has been recorded in rats.^[10][164]

11-12mg/kg bodyweight, based on rudimentary body surface area conversions, is approximately 120-131mg/PQQ daily (although injections) if extrapolated to humans.

One human study using 20mg PQQ alone or in combination with 300mg CoQ10 noted that there were no toxicological signs or symptoms associated with treatment over a 12 week period,^[165] and consumption of up to 0.3mg/kg PQQ (around 20mg for a 150lb person) for one week has been noted to be safe.^[27]

Chronic toxicity to the kidneys and liver may be achieved at a relatively low dose, although acute death requires a very high and unpractical dose. Until more evidence surfaces, it would be prudent to avoid superloading

In an Ames test (TA1535, TA1537, TA98, and TA100 strains), 10-5000μg PQQ per plate (without metabolic activation) and 156-5000μg per plate (with activation) has failed to show appreciable genotoxic effects.^[166]

In lung fibroblasts derived from chinese hamsters, 12.5-400μg/mL (no metabolic activation) and 117.2-3750μg/mL (with activation; highest concentration being 10mM) and the latter concentration in isolated lymphocytes failed to exert appreciable genotoxic effects as assessed by structural abberations and polyploidy.^[166]

The aforementioned disodium salt of PQQ has failed to acutely exert genotoxic effects in mice (up to 2,000mg/kg) as assessed by a micronucleus assay and in bone marrow erythrocytes.^[166]

No genotoxiticity has been noted with the disodium salt of PQQ