Regioselectivity of hydroxylation of prostaglandins by liver microsomes supported by NADPH versus H2O2 in methylcholanthrene-treated and control rats: formation of novel prostaglandin metabolites.

The effects of methylcholanthrene (MC) treatment of male rats on the regioselectivity of hydroxylation of prostaglandins E1 and E2 (PGE1 and PGE2) by liver microsomes, supplemented with NADPH or H2O2, was examined. In the presence of NADPH, control microsomes catalyzed the hydroxylation at omega-1 (C19) and at omega-(C20) sites with minimal formation of novel monohydroxy metabolites of PGE1 and PGE2, referred to as compounds X1 and X2, respectively. Similarly, H2O2 supported the 19-hydroxylation and the formation of compounds X1 and X2, but yielded only minimal amounts of 20-hydroxy products. With NADPH, MC-treated microsomal incubations demonstrated only minor quantitative change in the 19- and 20-hydroxylation as compared with controls, but showed a 7- to 11-fold increase in formation of compound X1 and a 10-fold increase in formation of X2. By contrast with H2O2, MC-treatment increased by about 3-fold the 19- and 20-hydroxylation of PGE1 and by 35- to 46-fold the formation of X1; similarly, there was an approximate 2-fold increase in 19- and 20-hydroxylation of PGE2 and a 10-fold increase in formation of X2. These findings suggest that several monooxygenases are involved in catalyzing the hydroxylation at the various sites of the PGE molecule. Inhibitors of monooxygenases (SKF 525A, alpha-naphthoflavone, and imidazole derivatives) provided further evidence that the hydroxylation at the three sites of PGEs is catalyzed by different P-450 monooxygenases. It is striking that the inhibitors had a much lesser effect on the 20-hydroxylation of PGE1 as compared with other sites of hydroxylation. Structural identification of compounds X1 and X2 was elucidated as follows. Resistance of the PGB derivative of X1 to periodate oxidation and mass fragmentation analysis of the t-butyldimethylsilyl ether methyl ester, placed the hydroxylation at C17 or C18. Finally, mass fragmentation of trimethylsilyl ether methyl ester PGB derivatives of X1 and X2 provided conclusive evidence that X1 and X2 are 18-hydroxy-PGE1 and 18-hydroxy-PGE2, respectively. The above findings indicate that the high regioselectivity of hydroxylation of PGE1 and PGE2, resulting in the formation of 18-hydroxy-PGE1 and 18-hydroxy-PGE2, respectively, is catalyzed by P-450 isozyme(s) which are induced by MC, possibly by P-450c.