Arachidonic acid‐mediated cooxidation of all‐trans‐retinoic acid in microsomal fractions from human liver

Abstract

1 The quantitative importance of prostaglandin H synthase (PGHS)-mediated cooxidation of alltrans-retinoic acid (ATRA) was evaluated in human liver microsomes (n=17) in relation to CYPdependent ATRA 4-hydroxylation. 2 Observed rates of ATRA cooxidation (4.6 ± 20 pmol mg protein 71 min 71 ) and 4-hydroxylation (8.7 ± 45 pmol mg protein 71 min 71 ) were quantitatively similar and exhibited similar individual variation (4 and 5 fold, respectively).3 From kinetic studies cooxidation was an e cient process in human hepatic microsomes (V max K m 71 =0.25) compared with NADPH-and NADH-mediated 4-hydroxylation by CYP (V max K m 71 =0.14 and 0.02, respectively). 4 The capacity of lipid hydroperoxide metabolites of arachidonic acid to mediate ATRA oxidation was established directly, but downstream products (D, E, F and I-series prostaglandins) were inactive. 5 cDNA-expressed CYPs supported ATRA oxidation by lipid hydroperoxides. Whereas CYPs 2C8, 2C9 and 3A4, but not CYPs 1A2 or 2E1, were e ective catalysts of the NADPH-mediated reaction, cooxidation supported by 15(S)-hydroperoxyeicosatetraenoic acid was mediated by all ®ve CYPs. The cooxidation reaction in human hepatic microsomes was inhibited by the CYP inhibitor miconazole. 6 These ®ndings indicate that ATRA oxidation is quantitatively signi®cant in human liver. Lipid hydroperoxides generated by intracellular enzymes such as prostaglandin synthase and lipoxygenases are sources of activated oxygen for CYP-mediated deactivation of ATRA to polar products.