All trans-retinoic acid (RA) and 13-cis-retinoic acid (13-CIS) are used in the treatment of acne, photoaging, and represent major metabolites of vitamin A. We have demonstrated peroxyl radical-dependent oxidation of 13- CIS in mouse skin. In addition, we have shown that RA and 13-CIS function as antioxidants and inhibit peroxidation of liposomes and microsomes. In contrast, RA exerts prooxidant effects in detergent micelles where it stimulates free radical propagation reactions. We propose to elucidate the mechanisms involved in the antioxidant and prooxidant properties of these retinoids by identifying the major RA and 13-CIS oxidation products generated, in phospholipid bilayers and detergent micelles, by reaction with peroxyl radicals generated chemically or during hydrocarbon autoxidation. In addition, the effects of dioxygen tension, retinoid concentration, epoxidation across the retinoid 5,6-double bonds, and the relative contributions of solvent cage effects, imposed by phospholipid bilayers versus micelles, on the antioxidant and prooxidant effects will be investigated. We have also shown that RA and 13-CIS undergo hydroperoxide- and arachidonic acid-dependent oxidation by prostagalndin H (PGH) synthase by mechanisms involving a hybrid of peroxidase and peroxyl radical chemistry. Reaction of RA and 13-CIS with PGH synthase peroxidase results in hydroxylation at the C4 position (which is also a major cytochrome P-450 metabolite) and the formation of other oxidation products. PGH synthase- catalyzed oxidation of RA and 13-CIS may represent a significant metabolic pathway in various extrahepatic tissues containing high cyclooxygenase activity and low cytochrome P-450 activity. We propose to identify the major RA and 13-CIS oxidation products generated during PGH synthase peroxidase-catalyzed oxidation in vitro, determine the regio- and stereochemistry of H-atom abstraction and the stereochemistry of hydroxylation, measure deuterium kinetic isotope effects of the H-atom abstraction reaction, determine the source of oxygen in oxidized products, and confirm the intermediacy of retinoid-derived carbon-centered, peroxyl, and alkoxyl radicals. We will also investigate the cytochrome P-450- catalyzed hydroxylation reaction with respect to deuterium kinetic isotope effects and stereochemistry of H-atom abstraction and hydroxylation. Finally we will investigate oxidation of RA and 13-CIS in cultured human keratinocytes and in the canine kidney-derived MDCK cell line in order to determine the relative contributions of peroxyl radical-, POH synthase-, and cytochrome P-450-dependent mechanisms of oxidation in these biological systems.
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