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

The quantitative importance of prostaglandin H synthase (PGHS)‐mediated cooxidation of all‐ trans ‐retinoic acid (ATRA) was evaluated in human liver microsomes ( n =17) in relation to CYP‐dependent ATRA 4‐hydroxylation. Observed rates of ATRA cooxidation (4.6–20 pmol mg protein −1 min −1 ) and 4‐hydroxylation (8.7–45 pmol mg protein −1 min −1 ) were quantitatively similar and exhibited similar individual variation (4 and 5 fold, respectively). From kinetic studies cooxidation was an efficient process in human hepatic microsomes (V max K m −1 =0.25) compared with NADPH‐ and NADH‐mediated 4‐hydroxylation by CYP (V max K m −1 =0.14 and 0.02, respectively). 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. cDNA‐expressed CYPs supported ATRA oxidation by lipid hydroperoxides. Whereas CYPs 2C8, 2C9 and 3A4, but not CYPs 1A2 or 2E1, were effective catalysts of the NADPH‐mediated reaction, cooxidation supported by 15(S)‐hydroperoxyeicosatetraenoic acid was mediated by all five CYPs. The cooxidation reaction in human hepatic microsomes was inhibited by the CYP inhibitor miconazole. These findings indicate that ATRA oxidation is quantitatively significant 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.British Journal of Pharmacology (2000) 131 , 851–857; doi: 10.1038/sj.bjp.0703579