Abstract
Hydroxylamine metabolites, implicated in dose-dependent and idiosyncratic toxicity from arylamine drugs, and amidoximes, used as pro-drugs, are metabolized by an as yet incompletely characterized NADH-dependent microsomal reductase system. We hypothesized that NADH cytochrome b5 reductase and cytochrome b5 were responsible for this enzymatic activity in humans. Purified human soluble NADH cytochrome b5 reductase and cytochrome b5, expressed in Escherichia coli, efficiently catalyzed the reduction of sulfamethoxazole hydroxylamine, dapsone hydroxylamine, and benzamidoxime, with apparent Km values similar to those found in human liver microsomes and specific activities (Vmax) 74 to 235 times higher than in microsomes. Minimal activity was seen with either protein alone, and microsomal protein did not enhance activity other than additively. All three reduction activities were significantly correlated with immunoreactivity for cytochrome b5 in individual human liver microsomes. In addition, polyclonal antibodies to both NADH cytochrome b5 reductase and cytochrome b5 significantly inhibited reduction activity for sulfamethoxazole hydroxylamine. Finally, fibroblasts from a patient with type II hereditary methemoglobinemia (deficient in NADH cytochrome b5 reductase) showed virtually no activity for hydroxylamine reduction, compared with normal fibroblasts. These results indicate a novel direct role for NADH cytochrome b5 reductase and cytochrome b5 in xenobiotic metabolism and suggest that pharmacogenetic variability in either of these proteins may effect drug reduction capacity.
Footnotes
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This work was supported by National Institutes of Health Grant GM 61753.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.104.072389.
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ABBREVIATIONS: NDHR, NADH-dependent hydroxylamine reductase; P450, cytochrome P450; cyt b5, cytochrome b5; b5R, NADH cytochrome b5 reductase; PBS, phosphate-buffered saline; SMX-HA, sulfamethoxazole hydroxylamine; HPLC, high performance liquid chromatography; HSA, human serum albumin; HLM, human liver microsome.
- Received June 15, 2004.
- Accepted August 6, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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