Hydroperoxide-dependent cooxidation of 13-cis-retinoic acid by prostaglandin H synthase

Abstract

Reverse phase high pressure liquid chromatography was employed to separate the major products resulting from the hydroperoxide-dependent cooxidation of 13-cis-retinoic acid by microsomal and purified prostaglandin H (PGH) synthase. Several major oxygenated metabolites including 4-hydroxy-, 5,6-epoxy-, and 5,8-oxy-13-cis-retinoic acid were unambiguously identified on the basis of cochromatography with authentic standards, uv spectra, and mass spectral analysis. Identical product profiles were generated regardless of the type of oxidizing substrate employed, and heat-denatured microsomes or enzyme did not support oxidation. In addition, several geometric isomers including all trans-retinoic acid were identified. Isomerization to all trans-retinoic acid in microsomes occurred in the absence of exogenous hydroperoxide, was insensitive to inhibition by antioxidant, and was eliminated when heat-denatured preparations were substituted for intact microsomes. Conversely, isomerization to at least one other isomer required the addition of hydroperoxide and was sensitive to antioxidant inhibition. Addition of antioxidant to microsomal incubation mixtures inhibited the hydroperoxide-dependent generation of 5,6-epoxy- and 5,8-oxy-13-cis-retinoic acid and other oxygenated metabolites but stimulated the formation of 4-hydroxy-13-cis-retinoic acid. Under standard conditions, 77% of the original retinoid was metabolized resulting in products containing 1.25 oxygen atoms/oxygenated metabolite, and two dioxygen molecules were consumed per hydroperoxide reduced. Purified PGH synthase also supported O2 uptake during cooxidation of 13-cis-retinoic acid by H2O2 or 5-phenyl-4-pentenyl-1-hydroperoxide, and the initial velocities of O2 uptake were directly proportional to enzyme concentration. 13-cis-Retinoic acid effectively inhibited peroxidase-dependent cooxidation of guaiacol indicating a direct interaction of retinoid with peroxidase iron-oxo intermediates, and EPR spin trapping studies demonstrated the formation of retinoid-derived free radical intermediates. Incubating H2O2 with microsomal PGH synthase resulted in the initiation of lipid peroxidation, detected via measurement of malondialdehyde generation, that was inhibited by retinoid and suggests some limited involvement of lipid peroxidation in retinoid oxidation. Incubation of 13-cis-retinoic acid with hematin and 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid in the presence of detergent, a system that generates high yields of peroxyl radicals, resulted in high yields of 5,6-epoxide; 4-hydroxy-13-cis-retinoic acid was not detected.(ABSTRACT TRUNCATED AT 400 WORDS)