Add time:07/20/2019         Source:sciencedirect.com

The metabolism of a number of primary and secondary hydroxylamines by hepatic microsomes is described. A cyanide-insensitive, reduced pyridine nucleotide-dependent hydroxylamine reductase activity that is independent of oxygen concentration catalyzes the reduction of hydroxylamine and a number of its mono- and disubstituted derivatives to the parent amines. At the pH optimum of 6.3 for the reductase, NADH is the preferred cofactor. The enzyme does not catalyze the reduction of 4-hydroxyaminoquinoline-1-oxide (HAQO) or of 1- or 2-naphthylhydroxylamine, the only known carcinogenic hydroxylamines tested. A hydroxylamine oxidase activity that requires both oxygen and reduced pyridine nucleotide oxidizes only disubstituted hydroxylamines, and the apparent initial product is the corresponding nitrone. Most nitrones undergo immediate hydrolysis in aqueous solution. At the pH optimum of 7.6 for the oxidase, NADPH is the preferred cofactor. NADPH cannot be replaced by a hydrogen peroxide-generating system, and the reaction is not affected by the addition of large amounts of exogenous catalase. Of the various organs which were assayed, the liver contained the greatest amount of both the reductase and oxidase activities; and the major portion of both activities in liver homogenates was found in the microsomal fraction. The two activities respond differently to agents such as deoxycholate, n-octylamine, and sulfhydryl inhibitors, indicating that the reduction and oxidation of the hydroxylamines are catalyzed by different enzymes or enzyme systems. Both activities are insensitive to carbon monoxide and N,N′-diphenyl-p-phenylenediamine (DPPD), an inhibitor of lipid peroxidation.

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