Reduction of hydroperoxides in the prostaglandin biosynthetic pathway by a microsomal peroxidase.

Abstract

In the cascade of reactions leading to prostaglandins, a peroxidatic enzyme reduces the hydroperoxides formed by oxygenation of unsaturated fatty acids. This manuscript describes the enzymology of a peroxidase which utilizes prostaglandin Gz, Ui-hydroperoxyprostaglandin E, 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid, 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid, or HzO, as substrate. A variety of radical-scavenging, reducing co-substrates stimulated the reaction and were oxidized in conjunction with hydroperoxide reduction. Enzyme-dependent co-substrate oxidation was studied qualitatively with phenol and quantitatively with cis-5-fluoro-2-methyl-l-[p-(methylthio)benzilidinelindene-3-acetic acid (sulindac sulfide). Molar equivalence between sulindac sulfide oxidation and 15hydroperoxyprostaglandin E, reduction was observed under certain conditions. The kinetics of the peroxidase reaction indicated substrate-dependent, irreversible self-deactivation. Furthermore, EPR measurements demonstrated the generation of radicals during enzymatic hydroperoxide reduction. These signals were quenched by each stimulatory co-substrate. Consequently, autodeactivation of peroxidase activity is interpreted as oxidative attack on the enzyme, a mechanism consistent with co-substrate oxidation by a highly reactive species. Stimulation of the peroxidase would, therefore, reflect the protective effect of radical-scavenging, reducing agents, leading to increased substrate turnovers prior to destructive attack. In duo, this system could provide oxidizing equivalents either for essential metabolic processes or a self-destruct mechanism in response to excessive hydroperoxide biosynthesis.