Production, assay, and occurrence of pyrroloquinoline quinone

Pyrroloquinoline-Quinone Is More Than an Antioxidant: A Vitamin-like Accessory Factor Important in Health and Disease Prevention

Pyrroloquinoline quinone (PQQ) is associated with biological processes such as mitochondriogenesis, reproduction, growth, and aging. In addition, PQQ attenuates clinically relevant dysfunctions (e.g., those associated with ischemia, inflammation and lipotoxicity). PQQ is novel among biofactors that are not currently accepted as vitamins or conditional vitamins. For example, the absence of PQQ in diets produces a response like a vitamin-related deficiency with recovery upon PQQ repletion in a dose-dependent manner. Moreover, potential health benefits, such as improved metabolic flexibility and immuno-and neuroprotection, are associated with PQQ supplementation. Here, we address PQQ's role as an enzymatic cofactor or accessory factor and highlight mechanisms underlying PQQ's actions. We review both large scale and targeted datasets demonstrating that a neonatal or perinatal PQQ deficiency reduces mitochondria content and mitochondrial-related gene expression. Data are reviewed that suggest PQQ's modulation of lactate acid and perhaps other dehydrogenases enhance NAD+-dependent sirtuin activity, along with the sirtuin targets, such as PGC-1α, NRF-1, NRF-2 and TFAM; thus, mediating mitochondrial functions. Taken together, current observations suggest vitamin-like PQQ has strong potential as a potent therapeutic nutraceutical.

Pyrroloquinoline quinone nutritional status alters lysine metabolism and modulates mitochondrial DNA content in the mouse and rat

Pyrroloquinoline quinone (PQQ) added to purified diets devoid of PQQ improves indices of perinatal development in rats and mice. Herein, PQQ nutritional status and lysine metabolism are described, prompted by a report that PQQ functions as a vitamin-like enzymatic cofactor important in lysine metabolism (Nature 422 [2003] 832). Alternatively, we propose that PQQ influences lysine metabolism, but by mechanisms that more likely involve changes in mitochondrial content. PQQ deprivation in both rats and mice resulted in a decrease in mitochondrial content. In rats, alpha-aminoadipic acid (alphaAA), which is derived from alpha-aminoadipic semialdehyde (alphaAAS) and made from lysine in mitochondria, and the plasma levels of amino acids known to be oxidized in mitochondria (e.g., Thr, Ser, and Gly) were correlated with changes in the liver mitochondrial content of PQQ-deprived rats, but not PQQ-supplemented rats. In contrast, the levels of NAD dependent alpha-aminoadipate-delta-semialdehyde dehydrogenase (AASDH), a cytosolic enzyme important to alphaAA production from alphaAAS, was not influenced by PQQ dietary status. Moreover, the levels of U26 mRNA were not significantly changed even when diets differed markedly in PQQ and dietary lysine content. U26 mRNA levels were measured, because of U26's proposed, albeit questionable role as a PQQ-dependent enzyme involved in alphaAA formation.

Pyrroloquinoline quinone modulates mitochondrial quantity and function in mice

When pyrroloquinoline quinone (PQQ) is added to an amino acid-based, but otherwise nutritionally complete basal diet, it improves growth-related variables in young mice. We examined PQQ and mitochondrial function based on observations that PQQ deficiency results in elevated plasma glucose concentrations in young mice, and PQQ addition stimulates mitochondrial complex 1 activity in vitro. PQQ-deficient weanling mice had a 20-30% reduction in the relative amount of mitochondria in liver; lower respiratory control ratios, and lower respiratory quotients than PQQ-supplemented mice (2 mg PQQ/kg diet). In mice from dams fed a conventional laboratory diet, but switched at weaning to the basal diet, plasma glucose, Ala, Gly, and Ser concentrations were elevated at 4 wk (PQQ- vs. PQQ+), but not at 8 wk. The relative mitochondrial content (ratio of mtDNA to nuclear DNA) also tended (P<0.18) to be lower (PQQ- vs. PQQ+) at 4 wk, but not at 8 wk. PQQ also counters the mitochondrial complex 1 inhibitor, diphenylene iodonium (DPI). Mice were gavaged with 0, 0.4, or 4 microg PQQ/g body weight (BW) daily for 14 d. At each PQQ level, DPI was injected (i.p.) at 0, 0.4, 0.8, or 1.6 microg DPI/g BW. The PQQ-deficient mice exposed to 0.4 or 4.0 microg DPI/g lost weight and had lower plasma glucose levels than PQQ-supplemented mice (P<0.05). In addition, fibroblasts took up (3)H-PQQ added to cell cultures, and cultured hepatocytes maintained mitochondrial PQQ concentrations similar to those observed in vivo. Collectively, these results indicate that dietary PQQ can influence mitochondrial amount and function, particularly in perinatal and weanling mice.

The PQQ story

About twenty years ago, the cofactor pyrroloquinoline quinone, PQQ, was discovered. Here the author gives his personal view on the reasons why this cofactor was so lately discovered and how the steps in its identification were made. The discovery not only led to subsequent studies on the physiological significance of PQQ but also initiated investigations on other enzymes where the presence of PQQ was expected, resulting in the discovery of three other quinone cofactors, TPQ, TTQ, and LTQ, which differ from PQQ as they are part of the protein chain of the enzyme to which they belong. Enzymes using quinone cofactors, the so-called quinoproteins, copper-quinoproteins, and quinohemoproteins, are mainly involved in the direct oxidation of alcohols, sugars, and amines. Some of the PQQ-containing ones participate in incomplete bacterial oxidation processes like the conversion of ethanol into vinegar and of D-glucose into (5-keto)gluconic acid. Soluble glucose dehydrogenase is the sensor in diagnostic test strips used for glucose determination in blood samples of diabetic patients. Quinohemoprotein alcohol dehydrogenases have an enantiospecificity suited for the kinetic resolution of racemic alcohols to their enantiomerically pure form, certain enantiomers being interesting candidates as building block for synthesis of high-value-added chemicals. Making up for balance after twenty years of quinoprotein research, the following conclusions can be drawn: since quinoproteins do not catalyze unique reactions, we know now that there are more enzymes which catalyze one and the same reaction than we did before, but do not understand the reason for this (compare e.g. NAD/NADP-dependent glucose dehydrogenases, flavoprotein glucose oxidase/dehydrogenase, and soluble/membrane-bound, PQQ-containing glucose dehydrogenases, enzymes all catalyzing the oxidation of beta-D-glucose to delta-gluconolactone but being quite different from each other); however, taking a pragmatic point of view, the foregoing can also be regarded as a positive development since as illustrated by the examples given above, the enlargement of the catalytic arsenal with quinoprotein enzymes provides in more possibilities for enzyme applications; the hopes that PQQ could be a new vitamin have diminished strongly after it has become clear that its occurrence is restricted to bacteria; the impact factor is broader than just the development of the field of quinoproteins, since together with that of enzymes containing a one-electron oxidized amino acid residue as cofactor, it has emphasized that cofactors not only derive from nucleotides (e.g. FAD, NAD) but also from amino acids. Finally, strong indications exist to assume that this is not the end of the story since other quinone cofactors seem awaiting their discovery.

Enhanced rat sciatic nerve regeneration through silicon tubes filled with pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ) is an antioxidant that also stimulates nerve growth factor (NGF) synthesis and secretion. In an earlier pilot study in our laboratory, Schwann cell growth was accelerated, and NGF mRNA expression and NGF secretion were promoted. The present study was designed to explore the possible nerve-inducing effect of PQQ on a nerve tube model over a 1-cm segmental deficit. An 8-mm sciatic nerve deficit was created in a rat model and bridged by a 1-cm silicone tube. Then,10 mul of 0.03 mmol/l PQQ were perfused into the silicone chamber in the PQQ group. The same volume of normal saline was delivered in the control group. Each animal underwent functional observation (SFI) at 2-week intervals and electrophysiological studies at 4-week intervals for 12 weeks. Histological and morphometrical analyses were performed at the end of the experiment, 12 weeks after tube implantation. Using a digital image-analysis system, thickness of the myelin sheath was measured, and total numbers of regenerated axons were counted. There was a significant difference in SFI, electrophysiological index (motor-nerve conduct velocity and amplitude of activity potential), and morphometrical results (regenerated axon number and thickness of myelin sheath) in nerve regeneration between the PQQ group and controls (P < 0.05). More mature, high-density, newly regenerated nerve was observed in the PQQ group. We conclude that PQQ is a potent enhancer for the regeneration of peripheral nerves.

Cofactor diversity in biological oxidations: implications and applications

Until recently, it was generally believed that enzymatic oxidation and reduction requires the participation of either a nicotinamide (NAD(P)+) or a flavin (FAD, FMN), in agreement with the existence of NAD(P)/H-dependent dehydrogenases/reductases and flavoprotein dehydrogenases/reductases/oxidases. However, during the past 20 years, the unraveling of the enzymology of the oxidation and reduction of C1-compounds by bacteria has led to the discovery of many new redox cofactors, some of them discussed here as they have a wider physiological significance than just enabling enzymatic C1-conversions to occur. A good example is the quinone cofactors, encompassing PQQ (2,7,9-tricarboxy-1H-pyrrolo[2,3-f]-quinoline-4,5-dione), TTQ (tryptophyl tryptophanquinone), TPQ (topaquinone), LTQ (lysyl topaquinone), and several others whose structures have still to be elucidated. Another example is mycothiol (1-O-(2'-[N-acetyl-L-cysteinyl]amido-2'-deoxy-alpha-D-glucopyranosyl)-D-myo-inosoitol), the counterpart of glutathione, once thought to be a universal coenzyme. Because these novel cofactors assist in reactions that can also be catalyzed by already known enzyme "classic cofactor" combinations, and first indications suggest that the chemistry of the reactions is not unique, one may wonder about the evolutionary background for this cofactor diversity. However, as will be illustrated by examples, from a practical point of view the diversity is beneficial, as it has increased the arsenal of enzymes suitable for application.

Involvement of a quinoprotein (PQQ-containing) alcohol dehydrogenase in the degradation of polypropylene glycols by the bacterium Stenotrophomonas maltophilia

Previous work has shown that when the bacterium Stenotrophomonas maltophilia is grown on polypropylene glycol, different dye-linked polypropylene glycol dehydrogenase (PPG-DH) activities are induced during growth. Here the purification and characterization of the dehydrogenase activity induced in the stationary phase, and present in the periplasmic space, is described. The homogeneous enzyme preparation obtained consists of a homodimeric protein with a molecular mass of about 123 kDa and an isoelectric point of 5.9. The cofactor of the enzyme appeared to be pyrroloquinoline quinone (PQQ), no heme c was present, and holo-enzyme contained two PQQ molecules per enzyme molecule. In these respects, PPG-DH described here is similar to already known quinoprotein alcohol dehydrogenases, but in other respects, it is different. Therefore, it is suggested that PPG-DH could be a new type of quinoprotein alcohol dehydrogenase. Based on its strong preference for polyols, PPG-DH seems well fitted to carry out the first step in the degradation of PPGs, synthetic polymers containing a variety of hydroxyl groups.