Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P47989 (xanthine oxidase)
 8,633 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Benznidazole (Bz) (N-benzyl-2-nitro-1-imidazole-acetamide) is a drug used against Chagas' disease. Rat liver microsomal and cytosolic fractions, but not mitochondria, exhibited Bz nitroreductase activity under anaerobic conditions in the presence of NADPH. Microsomal nitroreductase activity was enhanced by FAD and was inhibited totally by oxygen and partially by carbon monoxide. Liver cystosol fraction was able to reduce Bz nitrogroups in the presence of either N-methylnicotinamide or hypoxanthine as substrates. These enzyme activities were inhibited by menadione or allopurinol respectively. Under every experimental condition leading to enzymatic reduction of Bz nitrogroups and its inhibition or enhancement, reactive metabolites that bind covalently to proteins were also produced. This covalent binding was effectively prevented by reduced glutathione. Results suggest the participation of cytochrome P-450 and cytochrome c reductase in liver microsomal processes and of xanthine oxidase and aldehyde oxidase in liver cytosolic processes of Bz nitroreduction and activation to reactive metabolites that bind covalently to proteins. Possible pharmacological and toxicological implications of the described observations were discussed.
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PMID:Reductive metabolism and activation of benznidazole. 671 14

This is a report of similarities and differences among various ethnically defined populations with respect to their capacities to metabolize the prototype drugs antipyrine, caffeine, and debrisoquine. There were equal levels of the three main metabolites of antipyrine in the urine of Caucasians and Orientals; differences in antipyrine clearance between English and Indian subjects appeared to have environmental causes. Exploration of various metabolite ratios of caffeine in the urine of Caucasians and Orientals living in Canada showed three patterns: 1) no interethnic difference occurred in the ratio thought to indicate xanthine oxidase activity; 2) products of 7-demethylation and of hydroxylation of paraxanthine , both probably produced by cytochrome P-450, showed different averages in the populations; 3) the new secondary metabolite acetylformyl -methyluracil proved to be a useful indicator of the genetically controlled acetylator status, thereby confirming the well-known population difference for acetylator gene frequency. Analysis of data on debriosquine hydroxylation suggested that interpretation of the standardized metabolic ratio may be appropriate for Caucasian and Oriental groups but is misleading for published data from Saudi Arabia, Nigeria, and Ghana; even these two closely related West African populations seem to differ in debrisoquine metabolism.
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PMID:Pharmacoanthropology: drug metabolism. 671 41

The mechanism of cytochrome P-450-dependent oxidation of ethanol has been investigated using reconstituted phospholipid vesicles containing purified preparations of rabbit liver microsomal NADPH-cytochrome P-450 reductase and cytochrome P-450 LM2. Incorporation of cytochrome b5 into the vesicles resulted in a 5-fold enhancement of cytochrome P-450-catalyzed O-dealkylation of 7-ethoxycoumarin, whereas the cytochrome P-450-dependent ethanol oxidation was slightly inhibited. Superoxide dismutase, added in increasing amounts to the vesicles, inhibited the formation of superoxide anions and, in a concomitant manner, also the production of acetaldehyde from ethanol in the system. Also horseradish peroxidase inhibited ethanol oxidation catalyzed by the vesicles; acetaldehyde formation and H2O2 formation decreased in a concomitant manner as the amount of the peroxidase was increased. Externally added hydrogen peroxide markedly stimulated cytochrome P-450-dependent ethanol oxidation, but not until the concentration of H2O2 reached 0.3 mM, whereas the hydroxyl radical scavenger mannitol completely inhibited the cytochrome P-450-dependent acetaldehyde production. Oxidation of ethanol was also accomplished using vesicles containing cytochrome b5 instead of cytochrome P-450 and in other systems regenerating superoxide anions, e.g. the xanthine-xanthine oxidase system and dihydroxyfumarate. The results are consistent with an iron-catalyzed Haber-Weiss mechanism for regeneration of hydroxyl radicals which subsequently react with ethanol, thereby giving the corresponding aldehyde.
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PMID:The mechanism of cytochrome P-450-dependent oxidation of ethanol in reconstituted membrane vesicles. 678 51

Purified liver microsomal NADPH-cytochrome P-450 reductase is able to catalyze the activation of [14C]ronidazole to metabolite(s) which bind covalently to protein. Like the reaction catalyzed by microsomes, protein alkylation catalyzed by the reductase is (1) sensitive to oxygen, (2) requires reducing equivalents, (3) is inhibited by sulfhydryl-containing compounds and (4) is stimulated several fold by either flavin mononucleotide (FMN) or methytlviologen. A cytochrome P-450 dependent pathway of ronidazole activation can be demonstrated as judged by the inhibition of the reaction by carbon monoxide, metyrapone and 2,4-dichloro-6-phenylphenoxyethylamine but the involvement of specific microsomal cytochrome P-450 isozymes has not been definitively established. Milk xanthine oxidase is also capable of catalyzing ronidazole activation. Polyacrylamide sodium dodecyl sulfate (SDS)-gel electrophoresis reveals that the reactive intermediate(s) of ronidazole does not alkylate proteins selectively.
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PMID:Drug residue formation from ronidazole, a 5-nitroimidazole. II. Involvement of microsomal NADPH-cytochrome P-450 reductase in protein alkylation in vitro. 680 46

1. The subcellular distribution of nitrobenzene reduction activity in rat liver cells indicated the existence of two different enzyme systems, one localized in microsomes and the other localized in cytosol. The activity in the cytosol was mainly attributable to xanthine oxidase, judging from its substrate specificity and the inhibition by allopurinol. 2. The participation of the microsomal electron transport system in nitrobenzene reduction was examined by using antibodies against four components of the system, NADPH-cytochrome c reductase (fpT), NADH-cytochrome b5 reductase (fpD), cytochrome b5, and cytochrome P-450. Both NADH- and NADPH-dependent nitrobenzene reduction activities were strongly inhibited by anti-fpT IG and also by anti-P450 IG, but not inhibited by anti-fpD IG or anti-b5 IG. The reduction of nitrosobenzene and phenylhydroxylamine, which are supposed to be the intermediates of nitrobenzene reduction, was also examined, and it was found that NADH- and NADPH-dependent reduction of both compounds were strongly inhibited by anti-fpT IG and anti-P450 IG, but not by anti-fpD IG or anti-b5 IG. 3. Reconstruction experiments using purified NADPH-cytochrome P-450 reductase and cytochrome P-450 were also carried out and it was confirmed that the reduction of nitrobenzene, nitrosobenzene, and phenylhydroxylamine to aniline could be effected by these two components. 4. Nitrobenzene reduction by microsomes exhibited a short initial time lag and was activated by the addition of purified NADPH-cytochrome c reductase, whereas nitrosobenzene and phenylhydroxylamine reductions did not show any initial time lag and were not activated by the reductase. These observations suggest that the reduction of nitrobenzene to an intermediate, possibly nitrosobenzene or phenylhydroxylamine, limits the rate of aniline formation, and such an initial step of nitrobenzene reduction can be catalyzed by NADPH-cytochrome c reductase alone. Cytochrome P-450 is essential at least in the final step of nitrobenzene reduction to aniline. This conclusion was further confirmed by determination of these intermediates in nitrobenzene reduction.
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PMID:Participation of cytochrome P-450 in the reduction of nitro compounds by rat liver microsomes. 739 Sep 98

The dynamics and mechanisms of extracellular release of hydrogen peroxide (H2O2) from bovine pulmonary artery endothelial cells (EC) subjected to anoxia, hypoxia, and hypoxia followed by reoxygenation were examined using various inhibitors of enzymatic systems in intact cells and by direct measurement of H2O2 production from isolated EC plasma membranes. Extracellular H2O2 was measured with a fluorometric assay. EC exposed to hypoxia (3% O2) and anoxia (0% O2) released less H2O2 (29.6 +/- 1.3% and 4.2 +/- 0.7%, respectively) compared with EC exposed to normoxia (20% O2). The extracellular release of H2O2 from EC previously exposed to hypoxia for 24 h increased immediately after reoxygenation (20% O2) to 272 +/- 48%, as compared with EC exposed continuously to normoxia (100% release). Inhibition of xanthine oxidase (XO) by allopurinol did not reduce the release of H2O2 from cells exposed to normoxia or hypoxia followed by reoxygenation. Furthermore, inhibitors of cyclooxygenase (indomethacin), phospholipase A2 (quinacrine and chlorpromazine), nitric oxide synthase (L-arginine analogs), the mitochondrial electron transport chain (rotenone and cyanide), and cytochrome P-450 (methoxypsoralen) had no or minimal effect on this release. On the other hand, inhibitors of protein kinase C (calphostin and staurosporine) and NADPH oxidase (diphenyliodonium) reduced the release of H2O2 from EC in a dose-dependent manner in both exposure groups. In separate experiments, plasma membranes isolated from EC were found to produce H2O2 in the presence of NADH or NADPH as electron donors. This was inhibited by diphenyliodonium but not by allopurinol.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Release of hydrogen peroxide in response to hypoxia-reoxygenation: role of an NAD(P)H oxidase-like enzyme in endothelial cell plasma membrane. 752 30

Cellular signalling by the inflammatory cytokine tumour necrosis factor alpha (TNF alpha) has been suggested to involve generation of low levels of reactive oxygen species (ROS). Certain antioxidants and metal chelators can inhibit cytotoxicity and gene expression in response to TNF alpha in numerous cell types. However, neither the source nor function of TNF alpha-induced oxidant generation is known. Using specific inhibitors, we ruled out involvement of several oxidant-generating enzymes [cyclo-oxygenase (indomethacin), cytochrome P-450 (metyrapone), nitric oxide synthase (NG-methyl-L-arginine), NADPH oxidase (iodonium diphenyl), xanthine oxidase (allopurinol), ribonucleotide reductase (hydroxyurea)] in TNF alpha-mediated apoptosis of the murine fibrosarcoma line, L929. We also demonstrated no role for mitochondrial-derived radicals/respiratory chain in the lytic pathway using specific inhibitors/uncouplers (rotenone, KCN, carboxin, fluoroacetate, antimycin, malonate, carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and chloramphenicol-derived respiration-deficient cells. Significant ROS (H2O2, O2-.) generation was not observed in response to TNF alpha in L929 cells using four separate assays. Also, prevention of intracellular H2O2 removal by inhibition of catalase did not potentiate TNF alpha-mediated cell death. These data suggest that neither H2O2 nor O2-. plays a direct role in TNF alpha cytotoxicity. Finally, we suggest a central role for lipoxygenase in TNF alpha-mediated lysis. Three inhibitors of this radical-generating signalling pathway, including an arachidonate analogue (5,8,11,14-eicosatetraynoic acid), could protect cells against TNF alpha. The inhibitor nordihydroguaiaretic acid is also a radical scavenger, but it could not protect cells from ROS toxicity at concentrations that effectively prevented TNF alpha killing. Therefore protection by nordihydroguaiaretic acid cannot be due to scavenging of cytotoxic H2O or O2-.. The lipoxygenase product, (12S)-hydroxyeicosatetraenoic acid, was also significantly protective. As this analogue can act as a substrate for certain lipoxygenases, this effect may be due to prevention of generation of physiological products.
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PMID:Involvement of oxidants and oxidant-generating enzyme(s) in tumour-necrosis-factor-alpha-mediated apoptosis: role for lipoxygenase pathway but not mitochondrial respiratory chain. 764 35

Interleukin-2 (15 micrograms/mouse, i.p. twice daily for 4 days and once on the 5th day) significantly lowered cytochrome P-450 and heme content and increased heme oxygenase mRNA accumulation; the activities of 7-ethoxycoumarin O-deethylase, ethoxy- and pentoxyphenoxazone O-dealkylases were decreased. The activity of the type O form of hepatic xanthine oxidase increased, but there was no increase in lipid peroxide, expressed in terms of microsomal malondialdehyde. In vivo inactivation of xanthine oxidase activity by feeding mice with tungstate did not substantially change the degree of interleukin-2-induced cytochrome P-450 depression, suggesting that the two processes are not causally linked. Induction of tolerance to endotoxin by a 4-day pretreatment with lipopolysaccharide resulted in 50% protection against this depression despite inhibition of the interleukin-2 induced formation of tumor necrosis factor. This suggests that the release of tumor necrosis factor per se does not fully account for the depression of cytochrome P-450. Dexamethasone, already used in patients to reduce the toxicity of interleukin-2 therapy, provided full protection against the cytochrome P-450 depression.
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PMID:Mechanisms of interleukin-2-induced depression of hepatic cytochrome P-450 in mice. 779 64

Since a single nephron is a functional unit of the kidney, individual microdissected segments from the nephron would be ideal tissue samples for investigations on renal pharmacology. Several micromethods have enabled researchers to analyze the biochemical and pharmacological characteristics of these nephron segments. Miniaturized cuvettes containing microliter volumes of samples can be applied for general procedures of photometry like Lowry's protein determination. Fluorometry becomes a more sensitive method when enzymatic cycling systems of NAD/NADH or NADP/NADPH are combined, which have been used for assays of enzyme activities or substrate contents in minute biological samples having tissue proteins less than 1 microgram. The microchemiluminescence procedure has been successfully utilized for cellular ATP content or oxygen radical generation. Radioimmunoassay can be used to determine endogenous components such as cyclic nucleotides, eicosanoids, etc. Continuous gradient polyacrylamide microgels prepared in 5- to 10-microliters capillaries have made it possible to quantify the intranephron distribution of cytochrome P-450, xanthine oxidase and superoxide dismutase. As an example of the modern techniques, microscopic fluorometry using Fura-2AM has been established to identify agonist-induced cytosolic free calcium transients.
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PMID:[Micromethods for the determination of metabolic characteristics in individual nephron segments]. 790 40

BRL 55792, BRL 55791, and BRL 55039 are prodrugs of an active antiviral agent 9-(3-hydroxypropoxy) guanine (BRL 44385). The prodrugs were 6-deoxygenated analogs of BRL 44385, with ether groups substituted at the 9-position: BRL 55792 with an (isopropoxymethyloxy)propoxy group, BRL 55791 with a (methoxymethyloxy)propoxy group, and BRL 55039 with an ethoxypropoxy group. Conversion of the prodrugs to BRL 44385 had been demonstrated in vivo in the rat and involved 6-oxidation followed by dealkylation. Metabolism was studied in rat liver in vitro systems to find a model to evaluate BRL 44385 production. Rat hepatocytes performed both reaction steps and were used to assess which of the three prodrugs demonstrated greatest production of the active drug. BRL 55792 demonstrated greatest conversion in vitro, and this was in agreement with in vivo data. The production of BRL 44385 from BRL 55792 was also demonstrated in human hepatocyte incubations, providing evidence that these reactions can occur in humans, thereby increasing confidence that BRL 55792 would be suitable prodrug for human therapy. Further experiments were performed to investigate the enzymes involved in these conversions. The 6-oxidation step occurred in the cytosol. Use of allopurinol and menadione (xanthine and aldehyde oxidase inhibitors) indicated that these conversions were catalyzed exclusively by xanthine oxidase in the rat but mainly by aldehyde oxidase in humans. The dealkylation reaction was detected in hepatocytes but not in homogenates or subcellular fractions. Inhibition of this reaction by aminobenzotriazole and ketoconazole (P-450 inhibitors) indicated that it was mediated by cytochrome P-450.
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PMID:Use of rat and human in vitro systems to assess the effectiveness and enzymology of deoxyguanine analogs as prodrugs of an antiviral agent. 801 73


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