N-Oxidation of 4-chloroaniline by prostaglandin synthase. Redox cycling of radical intermediate(s).

4-Chloroaniline undergoes N-oxidation in ram seminal-vesicle microsomal preparations supplemented with arachidonic acid to yield N-(4-chlorophenyl)-hydroxylamine and 1-chloro-4-nitrosobenzene. H2O2 also supports metabolism of the amine substrate to the same organic-solvent-extractable products, suggesting that the hydroperoxidase activity of prostaglandin synthase is responsible for the co-oxidation. Analysis of the reaction mixtures by e.s.r. spectrometry reveals the formation of a radical intermediate bearing the characteristics of a strongly immobilized nitroxide. Arylamine-stimulated O2.- release can be observed when the arachidonic acid-containing incubation media are supplemented with NADPH. Redox cycling of the nitroxide/hydroxylamine couple is presumed to represent the major source of O2.-, but additional mechanisms, such as redox changes of nitro anion radicals resulting from potential further metabolism of 1-chloro-4-nitrosobenzene, cannot be excluded. The concerted action of carrier-bound nitroxides and O2.- in initiating damage of cellular macromolecules is discussed.