What is PQQ
PQQ is short for pyrroloquinoline quinone also called methoxatin, is a known potent antioxidant cofactor that is widely distributed in animal and plant tissues and is an indispensable nutrient in mammals, as PQQ deprivation affects mitochondria and mitochondrial function in these species 1). From a chemical perspective, assays that measure redox cycling indicate that PQQ is also 100–1000 times more efficient than other quinones and enediols, such as vitamin C (ascorbic acid) 2). PQQ can undergo thousands of reductive or oxidative cycles without degradation or polymerization 3). Nutritional studies in rodents have revealed that PQQ deficiency exhibits diverse systemic responses, including growth impairment, immune dysfunction, and abnormal reproductive performance 4). Although PQQ is not currently classified as a vitamin, PQQ has been implicated as an important nutrient in mammals. In recent years, PQQ has been receiving much attention owing to its physiological importance and pharmacological effects.
Although PQQ is not biosynthesized in mammals, trace amounts of PQQ have been found in human and rat tissues at picomolar to nanomolar levels 5) and an especially large amount has been found in human milk 6) because of its presence in daily foods, including vegetables, fruits, teas and meats 7).
Pyrroloquinoline quinone (PQQ) is a ubiquitous molecule found in plants, many simple and single cell eukaryotes (e.g., yeast), and certain bacteria. Recently, PQQ produced by rhizobacterium has been identified as an important plant growth factor 8) and is a possible source of PQQ in plant-derived food. In this regard, the ubiquitous presence of PQQ in a broad range of plants leads to a relatively constant exposure in animal diets. More importantly, levels of PQQ from dietary intake from plants are sufficient to maintain the concentration of PQQ typical of tissues 9). Pyrroloquinoline quinone (PQQ) influences a multitude of physiological and biochemical processes and has been established to be beneficial for growth and stress tolerance in both bacteria and higher organisms 10). Most importantly, nutritional studies have revealed that PQQ deficiency in mice and rats exhibits various systemic responses, including growth impairment, compromised immune responsiveness, abnormal reproductive performance, and reduced respiratory quotient 11). Moreover, in 2003, Kasahara and Kato 12) reported that PQQ could qualify as a newcomer to the B group of vitamins. These authors cloned a presumed mouse homolog (U26) of the yeast gene, 2-aminoadipic acid reductase (LYS2), and proposed that U26 could be involved in the metabolic degradation of dietary lysine, acting as a PQQ-dependent 2-aminoadipic 6-semialdehyde dehydrogenase, because U26 contained the putative PQQ-binding motif that is conserved among bacterial PQQ-dependent dehydrogenases 13). However, claims for a mammalian vitamin have been questioned because conclusive evidence for the existence of a mammalian PQQ-dependent enzyme is lacking 14). Although currently there remains controversy over whether PQQ is an essential vitamin in mammals, PQQ has been discovered to have a diverse range of physiological properties that could be beneficial to human health over the last decade.
Figure 1. Pyrroloquinoline quinone chemical structure
PQQ chemical nature
PQQ (4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid) is a redox active o-quinone that can be reversibly reduced to pyrroloquinolinequinol or PQQH2 (4,5-dihydroxy-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid) through a semiquinone intermediate (Figure 1) 15). It has been demonstrated that PQQ stably acts as an efficient electron transfer catalyst from a number of organic substrates to molecular oxygen (O2), constructing quinoprotein model reactions. In the presence of ascorbate, NADPH, and thiol compounds such as glutathione, PQQ undergoes a two-electron reduction to form PQQH2 (pyrroloquinolinequinol) 16). Subsequently, the generated PQQH2 is oxidized back to the original quinone via the reduction of two equivalents of O2 (oxygen) to super oxide anion (O2–), which spontaneously or enzymatically dismutates to hydrogen peroxide (H2O2) 17). It is noteworthy that PQQ has the ability to catalyze continuous redox cycling so that picomole amounts of PQQ are capable of generating micromolar amounts of product 18). Meanwhile, PQQ can exert pro-oxidant actions by the formation of reactive oxygen species (ROS), such as super oxide anion (O2–) and H2O2, via its redox cycling under certain conditions and induce oxidative protein modifications, including the oxidation of cysteinyl thiols 19). PQQ also catalyzes the oxidation of primary amines, including the epsilon-amino group of lysine residues in elastin and collagen, via Schiff base formation under aerobic conditions 20). Elastin oxidation by PQQ in the presence of Cu2+ results in the formation of 2-aminoadipic semialdehyde residues and eventually its derived covalent cross-links. On the other hand, PQQ easily reacts with amino acids to form imidazole derivatives, such as imidazolopyrroloquinoline quinone, in biological samples, and these derivatives are biologically active in some cases 21). The protonated form of PQQ shown in Figure 2 dissolves only slightly in water, and the tricarboxylic acid of PQQ dissociates in neutral pH water. Therefore, PQQ disodium salt (PQQ Na2) is generally used in various examinations because of its high solubility in waster. The atomic geometry of PQQ Na2 (commercially available as BioPQQ™, a trademark of Mitsubishi Gas Chemical Co., Inc., (Tokyo, Japan)) was resolved using single-crystal X-ray structure analysis as PQQ Na2 tri-hydrate 22). PQQ disodium salt (PQQ Na2) has an advantage in the application to various experiments compared with free-form PQQ, because it can be handled easily owing to its water-soluble property 23).
Figure 2. PQQ redox reaction
Footnote: PQQH2 = pyrroloquinolinequinol (4,5-dihydroxy-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid)[Source 24) ]
PQQ health benefits
PQQ is a multifunctional bio-agent exerting anti-inflammatory 25), growth-promoting 26), anti-oxidative 27), anti-cancer and anti-aging effects 28). PQQ has been shown to affect a wide range of genes, especially those involved in mitochondria-related functions. Importantly, due to redox activity, PQQ can oxidize reduced nicotinamide adenine dinucleotide (NADH) to generate NAD+ and enhance the NAD+ dependent metabolic response in cells 29). Most of the established PQQ effects have been associated with the stimulation of peroxisome proliferator activated receptor gamma co-activator 1α (PGC-1α) through the phosphorylation of cyclic AMP response element binding protein (CREB) at serine 133 30). Transcriptional regulation of PGC-1α target genes involves the co-activation of transcription factor-mediated gene expression as well as modulation of alternative splicing of the nascent transcript 31). The regulatory effects of PGC-1α in the pathogenesis of insulin resistance and type 2 diabetes have been manifested in different animal models, and associated with the modulation of multiple genes controlling lipolysis and lipogenesis 32). Likewise, PQQ supplementation increased metabolic flexibility in obese mice offspring, simultaneously shifting liver lipid composition towards increased TAG content with a decline in the lipotoxic pools, i.e., CER and sphingomyelin 33).
PQQ is reduced easily to PQQH2 (pyrroloquinolinequinol) by reaction with reducing agents such as NADPH, sodium borohydride, glutathione, or cysteine. A couple of in vitro studies demonstrated that the reduced form of PQQ (PQQH2 or pyrroloquinolinequinol) exhibits anti-oxidative capacity 34). The aroxyl radical-scavenging activity of PQQH2 was 7.4-fold higher than that of vitamin C, which is known as the most active water-soluble anti-oxidant 35). The singlet oxygen-quenching activity of PQQH2 was found to be 6.3-fold higher than that of vitamin C 36). Interestingly, PQQH2 works as catalyst in the singlet oxygen-quenching reactions. Moreover, it has been clarified that PQQH2 may rapidly convert two molecules of α-tocopheroxyl radicals to α-tocopherol 37). These results indicate that the pro-oxidant effect of α-tocopheroxyl radicals is suppressed by the coexistence of PQQH2.
In experiments using cultured cells, it was reported that PQQ disodium salt (PQQ Na2) prevents oxidative stress-induced neuronal death 38). It has been shown that PQQ prevented 6-hydroxydopamine (6-OHDA)-induced cell death of the dopaminergic neuroblastoma cell line SH-SY5Y and primary rat neurons and that its preventive effect was stronger than that of vitamin C and E 39). 6-OHDA is a well-known neurotoxin that compromises mitochondria complex I, resulting in the production of ROS, such as super oxide anion (O2–), hydroxyl radicals, and H2O2. Similar results were obtained in the experiment using H2O2 40). Moreover, marked decreases in ischemia damage are found in in vivo rat models, such as cardiovascular 41) or cerebral ischemia models 42). The underlying mechanisms elucidated were that PQQ acts as an anti-oxidant by scavenging super oxide anion (O2–) and protects mitochondria from oxidative stress-induced damage 43).
In humans, following a single dose of PQQ disodium salt (PQQ Na2) (0.2 mg/kg body weight), thiobarbituric acid reactive products (TBARS), which are measured by the malondialdehyde generated from lipid hydroperoxides, significantly decreased over the time course of the study 44). In addition, the change of TBARS values correlated significantly with the maximum plasma concentration (Cmax) for PQQ Na2. These results suggest that PQQ has a potential as an anti-oxidant.
Although no enzymes in animals have been identified that exploit PQQ as a cofactor, PQQ has been shown to be essential for normal growth and development in animals. When PQQ is omitted from a chemically defined diet fed to mice and rats, various systemic responses are observed including growth impairment, immune dysfunction, decreased reproductive performance, and reduced respiratory quotient 45). Oral supplementation of PQQ (above 300 ng/g diet) improves reproduction and enhances neonatal rates of growth compared with the response from diets devoid of PQQ 46). More recently, dietary supplementation of PQQ disodium salt (PQQ Na2) in broiler chicks has been shown to improve growth performance, carcass yield, immunity, and plasma status 47). Thus, this unique compound is characterized as an important growth factor or putative essential nutrient in animals, whereas the nutritional benefits of PQQ for human growth and development are still unknown. Although the detailed mechanism of PQQ action in animals still remains unclear, the ability to carry out continuous redox cycling suggests a role for PQQ as a cofactor, redox signaling molecule, or anti-oxidant.
In cultured human and mouse cells, PQQ also functions as a potential growth factor to promote cell proliferation when added to culture media 48). PQQ enhances the incorporation of [3H]-thymidine into human skin fibroblasts cultured in medium containing PQQ at concentrations as low as 3 nM. Kumazawa et al. 49) have observed that PQQ treatment stimulates activation of extracellular signal-regulated kinase 1/2 (ERK 1/2) in c-Ha-ras transformed NIH/3T3 mouse fibroblasts, resulting in increased cell proliferation. ERK, one of the mitogen-activated protein kinases, activates transcription in the ras-signaling pathway and plays a pivotal role in cell proliferation and survival 50). This signal transduction by sequential phosphorylation often is initiated by the binding of peptide growth factors to receptor tyrosine kinases (RTKs). Recently, it was shown that PQQ also significantly enhanced proliferation of human epithelial A431 cells at concentrations above 10 nM 51). Moreover, it was found that PQQ induces the activation (tyrosine autophosphorylation) of epidermal growth factor receptor (EGFR), a RTK of the ErbB family, and its downstream target ERK 1/2 in a ligand-independent manner. The activation of the ERK pathway accompanying EGFR phosphorylation via binding of EGF plays a prominent role in the proliferation of epithelial cells. On the other hand, EGFR signaling is negatively regulated by protein tyrosine phosphatase 1B (PTP1B), which catalyzes tyrosine dephosphorylation of activated EGFR, and the inhibition of PTP1B has been reported to evoke a ligand-independent activation of EGFR 52). Recent findings also indicate that PTP1B activity is modulated by post-translational modification, such as oxidation and alkylation of an extremely reactive cysteine residue at the catalytic center 53). On the basis of these facts, some scientists have elucidated that PQQ inhibits PTP1B through the oxidation of catalytic cysteinyl thiol by H2O2 produced during its redox cycling, thereby inducing the ligand-independent activation of EGFR. PTP1B has a substrate-specific ability to dephosphorylate RTKs, including the insulin receptor (IR),47) insulin-like growth factor-I receptor 54), platelet-derived growth factor receptor 55), vascular endothelial growth factor receptor 56) and nerve growth factor receptor 57), implicating the modulation of multiple growth factor-activated signaling pathways. Hence, our data suggests that inhibition of PTP1B via redox cycling by PQQ might induce a diverse range of physiological effects through potentiated RTK-mediated signaling and gene expression and exert a growth factor-like action.
Accounting for 90–95% of diabetic population, type 2 diabetes mellitus has increased rapidly in recent decades worldwide, and the morbidity and mortality associated with secondary complications of type 2 diabetes, such as retinopathy, nephropathy, and cardiovascular disease, also have increased significantly 58). Type 2 diabetes is characterized by mitochondrial disorder and chronic hyperglycemia and dyslipidemia resulting from insulin resistance of the peripheral tissues and impaired insulin secretion from the pancreas 59). Mitochondria regulate metabolic pathways through signal transduction that is essential for metabolic homeostasis and cellular function. Recent studies show that mitochondrial dysfunction of diabetic subjects is closely related to lifestyle factors, including diet, physical activity, sleep, and stress 60). Prolonged exercise and diet intervention can reverse, at least partly, the mitochondrial deficiency and improve the metabolic flexibility and insulin sensitivity in patients with type 2 diabetes 61). Recently, dietary PQQ supplementation has been revealed to enhance mitochondrial function and biogenesis and improve metabolic homeostasis in mice and rats 62). PQQ deficiency in young mice increases the plasma glucose level, reduces hepatic mitochondrial content by 20–30%, and suppresses mitochondrial respiration 63). Similarly, rats fed a diet deficient in PQQ exhibit elevated plasma lipid and ketone bodies owing to lower mitochondrial content and decreased energy expenditure 64). More importantly, PQQ supplementation reverses the mitochondrial alterations and metabolic impairment and significantly improves the lipid profile in diabetic UCD-type 2 diabetes rats 65). Mechanistically, mitochondrial biogenesis and function are stimulated by the transcriptional coactivator, peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α), through activation of the nuclear respiratory factor (NRF-1 and NRF-2) 66). The transcription factor cAMP-responsive element-binding protein (CREB) increases transcription of PGC-1α via a conserved CREB-binding site in the proximal promoter and is activated by exercise or fasting 67). Indeed, the exposure of mouse Hepa 1–6 hepatocytes to PQQ elevates PGC-1α promoter activity by enhancing CREB transcriptional activity and stimulating mitochondrial biogenesis 68). PQQ exposure also increases the levels of NRF-1 and NRF-2, resulting in the upregulation of the mitochondrial transcription factor A (Tfam) and mitochondrial gene expression. However, the molecular mechanism underlying the activation of CREB-PGC-1α signaling pathway by PQQ remains unclear.
Neuroprotection and brain function
A placebo-controlled, double-blinded study using the repeatable battery for the assessment of neuropsychological status (RBANS) was conducted with the participation of 65 Japanese subjects between 50 and 70 years old who presented with self-identified forgetfulness or forgetfulness identified by a family member, colleague, or acquaintance 69). Neuropsychological status is a neuropsychological battery developed by Randolph in the United States 70). The neuropsychological battery questions allow repeated and quick evaluation of higher brain function disorders with a variety of brain disease complications. The content of the neuropsychological status (RBANS) consists of five subtests of neurocognitive test paradigms [immediate memory, visuospatial/constructional, language, attention, and delayed memory]. Although the PQQ disodium salt (PQQ Na2) (20 mg/day) and PQQ Na2 (20 mg/day) + Coenzyme Q10 (300 mg/day) groups showed significantly better total score over time, a similar improvement over time was seen in the placebo group. Differences in immediate memory scores at week eight were significantly better in the PQQ Na2 + Coenzyme Q10 group than in the placebo group. For analysis of immediate memory, subjects were stratified into two subgroups according to baseline total scores. Although no significant difference was present between groups in the high-scoring subgroup, the PQQ Na2 + Coenzyme Q10 group in the low-scoring subgroup showed a significantly better score at week 8 and week 16 than the placebo group. This finding shows that individuals with lower neuropsychological status (RBANS) scores may achieve a better degree of improvement in response to PQQ disodium salt (PQQ Na2)-supplementation than individuals with higher scores.
The result of another human clinical study was reported very recently 71). A randomized, placebo-controlled, double-blinded study to examine the effect of PQQ disodium salt (PQQ Na2) on cognitive functions was conducted with 41 elderly healthy subjects. Subjects were administered orally 20 mg of PQQ Na2/day or placebo for 12 weeks. For cognitive functions, selective attention by the Stroop and reverse Stroop test90) and visual-spatial cognitive function by the laptop tablet Touch M91) were evaluated. In the Stroop test, the change of Stroop interference ratios for the PQQ Na2 group was significantly smaller than for the placebo group. In the Touch M test, the stratification analyses dividing each group into two groups showed that the score significantly increased only in the lower group of the PQQ Na2 group (initial score < 70).
Relating to cognitive functions, PQQ disodium salt (PQQ Na2) shows effects on stress, fatigue, and sleep. Seventeen adult and female subjects participated in a clinical trial using an open-label trial to evaluate the effectiveness of PQQ Na2 on stress, fatigue, quality of life, and sleep 72). The participants ingested 20 mg of PQQ Na2 daily for eight weeks. The results in the Profile of Mood States–Short Form showed that all six measures of vigor, fatigue, tension-anxiety, depression, anger-hostility, and confusion significantly improved following PQQ Na2 supplementation compared with scores for those measures before supplementation of PQQ Na2. The results of the Oguri–Shirakawa–Azumi Sleep Inventory (Middle-aged and Aged version) showed significant improvement in drowsiness at awaking, sleep onset and maintenance, and sleep duration. For validation, the Pittsburgh Sleep Quality Index Japanese version also showed significant improvement in sleep-related behavior. Furthermore, the changes in these global scores were correlated with changes in the cortisol awakening response, i.e., the effects of PQQ Na2 on improvement of sleep quality are supported by a biomarker.
Recently, two papers were published regarding the effect of PQQ Na2 on health benefit in humans 73). PQQ Na2 is helpful for the improvement of skin conditions and lipid metabolism. PQQ Na2 may be useful not only for the improvement of brain functions but also for various health benefits. The underlying mechanisms of the effects of PQQ Na2 should be elucidated further.
It is well known that PQQ is distributed ubiquitously in nature and found in numerous dietary sources, including fermented soy beans (natto), tea, green peppers, parsley, kiwi fruit, and human milk 74). Various methods for instrumental analyses and bioassays for PQQ have been developed, but the PQQ content in foods varies in different reports because PQQ is chemically reactive and prone to form derivatives or condensation products with other nutrients 75). Kumazawa et al. 76) have developed a method based on gas chromatography/mass spectrometry (GC/MS) with isotopic dilution for free PQQ after derivatization with phenyltrimethylammonium hydroxide. Using this analytical method, the levels of free PQQ in various foods, including vegetables, fruits, and teas, were determined to be in the range of 3.7–61 ng/g wet weight or ng/mL in liquid foods (see Table 1 below). Recent analyses of PQQ using a reliable liquid chromatography/electrospray-ionization tandem mass spectrometry (LC/MS/MS) method elucidated that free PQQ was present in various food samples in the range of 0.19–7.02 ng/g fresh weight or ng/mL in liquid foods 77). Based on available food composition data, it is estimated that humans consume 0.1–1.0 mg PQQ and its derivatives per day 78).
The biosynthesis of PQQ in higher organisms has not been shown and therefore, the major source of PQQ in these organisms, including plants and animals, is believed to be derived from microorganisms. The details of PQQ biosynthesis have not been resolved yet, but a putative pathway has been proposed on the basis of the functions of conserved genes in numerous bacteria 79). The majority of PQQ-producing bacteria contain six genes (pqqABCDEF) in an operon 80). These genes have been expressed in Escherichia coli, a non-PQQ producer and lead to the production of PQQ 81). Genetic knockout studies of each of these genes show that four of the six gene products (PqqA, PqqC, PqqD, and PqqE) are absolutely required for PQQ production 82). In all cases, pqqA encodes a small polypeptide, typically 20–30 amino acids in length, containing a conserved glutamate and tyrosine that serve as the backbone in PQQ biogenesis 83). The glutamate and tyrosine undergo post-translational modifications to form the intermediate 3a-(2-amino-2-carboxyethyl)-4,5-dioxo-4,5,6,7,8,9-hexahydroquinoline-7,9-dicarboxylic acid (AHQQ) 84). PqqC is the most characterized and has been shown to catalyze the eight-electron oxidation and ring cyclization of AHQQ to form PQQ 85). A large number of bacteria extracellularly excrete PQQ and is indicated as micrograms per mL of broth culture 86). On the other hand, common strains of bacteria in the human intestinal tract appear to synthesize little PQQ 87) and hence, it seems that dietary intake is the major source of PQQ in the human body.
Table 1. Concentrations of PQQ in foods
Regarding typical exposures of free PQQ, the amount for humans is estimated to vary from 100-400 μg daily 89), about the same as the daily nutritional recommendations for biotin and folic acid, respectively. However, PQQ easily forms condensation products upon interaction with amino acids 90), complicating the precision of such estimates. The primary condensation products are imidazolopyrrolo-quinoline and imidazolopyrroloquinoline derivatives with attached amino acid side chains as part of the chemical structure. For example, only about 15 percent of the PQQ is present in free form in biological fluids such as human milk, while 85 percent is present as imidazolopyrrolo-quinoline (IPQ) and derivatives 91). Thus, it is not unreasonable to assume that for humans the total exposure to PQQ derivatives may be as much as 1-2 mg per day. This amount is in the range that clearly influences optimization of growth and health in animal models 92). In the case of human milk, PQQ amounts to 1-2 μg PQQ/g of milk solid, which is also similar to the PQQ concentrations reported for cow’s milk 93). It is important to note that PQQ appears readily absorbed. Smidt et al 94) determined that the ap-parent absorption of an oral dose of 14C-PQQ ranges from 20-80 percent when administered to adult mice in the fed state. The percentages were estimated from the amount of radioactivity present in urine and tissues 24 hours after administration.
PQQ side effects
Since 2009, dietary supplements containing PQQ disodium salt (PQQ Na2) have been commercialized in the United States and no adverse effects have been reported. As for oral toxicity studies, a 14-day preliminary study and a 28-day repeated dose study, as acute studies, and a 13-week subchronic study were performed in rats 95). The median lethal dose was 1000–2000 mg PQQ Na2/kg body weight in male and 500–1000 mg PQQ Na2/kg body weight in female rats. In the 14-day study, high doses of PQQ Na2 resulted in increases in relative kidney weights with associated histopathology in female rats only, while a follow-up 28-day study in female animals resulted in increases in urinary protein and crystals. These findings were reversible and resolved during the recovery period. In the 13-week study, a number of clinical chemistry findings and histopathological changes were noted, which were deemed to be of no toxicological significance, as the levels were within the historical control range, were not dose-dependent, occurred at a similar frequency in control groups, or occurred only in the control group. Based on these findings, a no-observed-adverse effect level (NOAEL) of 100 mg PQQ Na2/kg body weight was determined in rats, the highest dose tested in the 13-week study 96). A recent study reported that the NOAEL of PQQ Na2 in rats is considered to be 400 mg PQQ Na2/kg body weight for both sex, the highest dose tested 97).
Single-dose oral toxicity tests in rats were performed in compliance with Good Laboratory Practice. The single-dose oral toxicity tests indicated the approximate lethal dose of PQQ is less than 1,000 mg/kg body weight of rats, but higher than 500 mg/kg. Post-mortem pathological examinations of test rats suggest the kidney as the principal target organ for acute effects of PQQ. In part, this is a validation of an earlier published toxicology study in which PQQ was administered intraperitoneally to rats at a dose of 11-12 mg/kg body weight 98). Signs of renal tubular damage and inflammation were observed. When lower doses were used, however, no treatment effects or obvious pathological signs were observed 99). Likewise, in a 90-day repeated dose study in which PQQ was administered to rats by oral gavage (3, 20, or 100 mg PQQ/kg body weight) no adverse effects were observed. Moreover, at oral dosage levels from 250-2,000 mg PQQ/kg in mice, an examination for micronucleus induction in red blood cells showed no effects 100). Lastly, the results from a battery of genotoxicity tests in vitro (the Ames, micronucleus, and chromosomal aberration tests) were negative, i.e., PQQ did not cause clastogenic toxicity (chromosome breaks, rearrangements and changes in chromosomal number) 101).
Additionally, the genotoxic potential of PQQ Na2 was evaluated in a core battery of genotoxicity tests 102). The results of the bacterial mutation assay (Ames test) were negative. Weak positive results were obtained in two separate in vitro chromosomal aberration tests at the highest dosage in Chinese hamster lung fibroblasts. Upon testing in an in vitro chromosomal aberration test in human peripheral blood lymphocytes, no genotoxic activity was noted. In the in vivo micronucleus assay in mice, PQQ Na2 at doses up to 2000 mg/kg body weight demonstrated that no genotoxic effects are expressed in vivo in bone marrow erythrocytes. From these results, PQQ was concluded to have no genotoxic activity in vivo.
A placebo-controlled, double-blinded safety studies in humans have been conducted in preparation for several human use patents 103), 104). PQQ was administered at 20 or 60 mg/day for four weeks to two groups (10 each) of healthy adults given either a PQQ supplement or a placebo. These studies were double-blinded and conducted at two different commercial drug-testing facilities: the New Drug Clinical Center, Fukuhara Clinic, Eniwa, Hokkaido, Japan and Cronova Co., Ltd., Suminoeku, Osaka, Japan 105), 106). No adverse effects were observed in standard clinical tests at either dose (e.g., glucose, triglycerides, and various lipoprotein fractions). Functional tests for liver toxicity were also normal (e.g., aspartate aminotransferase and serum glutamic oxaloacetic transaminase). At 60 mg PQQ daily, the amounts of urinary N-acetyl-β-d-glucosaminidase (NAG) activity, which is a sensitive biomarker for renal tubular damage, were also within the normal range. N-acetyl-glucosaminidase is a renal hydrolytic enzyme located primarily in the lysosomal fraction of the renal tubular cell. Abnormal changes in renal tubular function or damage results in N-acetyl-β-d-glucosaminidase (NAG) elevation in urine 107).
In summary, these observations taken together suggest there is no evidence of acute side effects or overt toxicity from consuming PQQ in amounts up to 60 mg per day for humans or several hundred mg per kg of diet fed to animals 108).
There are potential benefits from PQQ supplementation related to lipidemic and glycemic control, prevention of cardiovascular and neurodegenerative diseases, and improvement of brain functions. Recent evidence suggests that PQQ can be useful for various health benefits through different mechanisms including redox activity, radical-scavenging activity, and modulation of cell signaling pathways. According to recent observations, PQQ shows no toxicity and genotoxicity in oral administration, and thus, oral supplementation of PQQ would be a promising approach to improving health status. On the other hand, the precise molecular mechanism underlying the action of PQQ is not understood fully. The mechanistic studies that aid in defining the function of PQQ could provide further benefits for human health.
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