Abstract
Arsenite oxidase, an oxomolybdenum enzyme, catalyses the oxidation of arsenite, (As III) to arsenate (As V) (Anderson et al. J. Biol. Chem. 267 (1992) 23674–23682). The oxidized enzyme is inactivated by diethylpyrocarbonate (DEPC). Spectroscopic evidence and the reversibility of the inhibition with hydroxylamine suggest that at least one essential histidine is modified in this reaction. In contrast, chemical or substrate-dependent reduction of arsenite oxidase protects the enzyme from inactivation by DEPC. Radiolabelled DEPC binds to approximately the same number of histidines in reduced and oxidized enzyme (3.0 ± 0.6 and 3.3 ± 0.6, respectively), although the reduced enzyme remains fully active. When oxidized arsenite oxidase is treated first with DEPC, and then reduced and treated with radiolabelled DEPC, one additional amino acid is modified. These results suggest that a conformational change occurs on reduction of oxidized arsenite oxidase which exposes one histidine while sequestering another.
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