NADPH quinone oxidoreductase

Subsequently, proteolytic fragments of the rabbit renal 25-kDa amiloride-binding protein were micro-sequenced and found to have high sequence homology with rat and human NAD(P)H quinone oxidoreductase. Indeed, enzymatic assays revealed that renal brush border membrane vesicles contain significant NADPH quinone oxidoreductase activity. Presumably NAD(P)H quinone oxidoreductase coincidentally binds amiloride analogs with the same rank order as the Na /H exchanger [39]. 

Superoxide dismutases are enzymes that catalyze the dismutation of superoxide radicals to hydrogen peroxide and molecular oxygen. Superoxide dismutases either have Cu and Zn (cytosolic isoform) or Mn " " (mitochondrial isoform). Reduction of hydrogen peroxides in cells is accomplished via catalases to water and oxygen. Cellular organelles called peroxisomes contain large quantities of catalase. In addition, NADPH quinone oxidoreductases (DT-diaphorase) also act as antioxidant enzymes by catalyzing two-electron reduction of quinones. 

Valerio LG, Kepa JK, Pickwell GV, Quattrochi LC. Induction of human NADPH quinone oxidoreductase (NQOl) gene expression by the flavonol quercetin. Toxicol Lett 2001 119 49-57. 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

Basu S, Brown JE, Flannigan GM, et al. The immunohistochemical analysis of NAD(P)H quinone oxidoreductase and NADPH cytochrome P450 reductase in human superficial bladder tumors relationship between tumor enzymology and clinical outcome following intravesical mitomycin C therapy. Int J Cancer 2004 109 703-709. 

Studies on metabolic stability using hepatocyte suspensions are not feasible for automation/HTS, but these studies do provide rather complete profiles of hepatic biotransformation without the supplements of cofactors and cosubstrates. The use of S9 in metabolic stability studies can be evaluated in a manner similar to that used for the microsomal assays, but with the possible addition of a broader panel of cofactors or cosubstrates. These include NADPH for CYP/FMO-mediated reactions, NADH for xanthine oxidoreductase and quinone oxidoreductase 2, NADPH-dependent reductions by carbonyl reductases, and NADPH/NADH-dependent reductions catalyzed by aldo-keto reductases, uridine 5 -diphosphate... 

The tocopherols are converted by oxidation to tocopheryl quinones, which upon reduction become tocopheryl hydroquinones. The reduction of a-tocopheryl quinone to a-tocopheryl hydroquinone occurs either via NADPH-cytochrome P45Q reductase, NAD(P)H quinone oxidoreductase 1, or ascorbate for the other tocopherols these pathways have not been tested. The tocophrayl hydroquinones can regenerate the toeopheroxyl radical and thus preserve a-tocophraol with different effieieneies (a > p > y-toeopheryl hydroquinone). ... 

The reductive biotransformation of drugs has been one of the least studied reactions, and many of the enzymes that are involved have not been well characterized. Some of the enzymes that catalyze reductive reactions of drugs are the cytochrome P450s, molybdenum reductases, alcohol dehydrogenases, carbonyl reductases, NADPH cytochrome P450 reductase, NAD(P)H— quinone oxidoreductases, and enzymes of the intestinal microflora (Matsunaga et al., 2006 Rosemond and Walsh, 2004). 

Both NADPH cytochrome P450 reductase (P450 reductase) and NAD(P)H-quinone oxidoreductase (NQO) are flavin adenine nucleotide-containing enzymes that catalyze the reduction of quinones and quinone-like structures. However, P450 reductase is a microsomal enzyme that catalyzes a one-electron reduction to yield semiquinone radieals that can redox cycle to produce superoxide anion radicals, whereas NQO is a cytosolic enzyme that catalyzes a two-electron reduction to yield hydroquinones (Matsunaga et al., 2006). P450 reductase also can catalyze the one-electron reduction of nitroaromatics to the nitro anion radical, which can redox cycle ... 

While isolated rat hepatocytes do not hberate appreciable amounts of 02 , simple quinones, such as 2,5-dimethyl-p-benzoquinone stimulate the formation of O2 up to 15 nmoles per min and 10 cells (Powis et al. 1981). Hepatocyte 02 formation stimulated by a variety of simple quinones and more complex antitumor quinones is maximal at a qui-none one-electron reduction potential (E, ) of -70 mV and quahtatively similar to the pattern of 02 formation seen with mitochondrial NADH ubi-quinone oxidoreductase and microsomal NADH-cytochrome bs reductase. 02 production, by microsomal NADPH-cytochrome P-450 reductase is maximal at a quinone E of -200 mV. Phenobarbital induction, which increases NADPH-cytochrome P-450 reductase, had no effect on O2 formation by hepatocytes. 

Forrest et al. (183) isolated and sequenced a gene from liver encoding an NADPH-dependent carbonyl (i.e., n-oxo-anthracycline) reductase. The enzyme belonged to a class of secondary alcohol oxidoreductases and was related to the Streptomyces sp. strain C5 ketoreductase encoded by arfl (280 renamed in this chapter to doHlJ). Interestingly, Forrest et al. (183) indicated that the liver enzyme also catalyzed reduction of the anthra eye line quinone as well. 

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