Cooxidation of 13-cis-retinoic acid by prostaglandin H synthase

Cooxidative metabolism of 13-cis-retinoic acid (13-CIS) via prostaglandin H synthase was investigated employing ram seminal vesicle microsomes. Oxidation of 13-CIS utilizing H2O2, 13-hydroperoxy-9-cis-11-trans-octadecadienoic acid (13-OOH-18:2), or 1-hydroperoxy-5-phenyl-4-pentene was detected by measurement of O2 incorporation. UV spectroscopy and HPLC of extracted incubation mixtures demonstrated that 13-CIS was metabolized to oxidized derivatives. Similar spectral changes and HPLC profiles were obtained with H2O2, 13-OOH-18:2, or arachidonic acid as substrates. 4-Hydroxy-13-cis-retinoic acid and all trans-retinoic acid were products of cooxidation as well as other polar metabolites. Oxidation was inhibited by the antioxidant butylated hydroxyanisole and the spin trap, nitrosobenzene. These results indicate that 13-cis-retinoic acid is cooxidized by prostaglandin H synthase and suggest a free radical mechanism resembling that of lipid peroxidation.