Participation of a cytochrome P450 enzyme from the 2C subfamily in progesterone 21-hydroxylation in sheep liver

Abstract

Progesterone 21-hydroxylation in hepatic microsomes from adult male sheep is a quantitatively important metabolic pathway (0.27±0.08 nmol deoxycorticosterone formed/min/mg protein; representing 13–25% of total progesterone conversion). This study was undertaken to determine whether the ovine hepatic progesterone 21-hydroxylase may be another member of the P450 2C subfamily, normally associated with progesterone 21-hydroxylation in rodent liver. An IgG preparation raised in rabbits against purified rat liver microsomal cytochrome P450 2C6 was found to recognize a single antigen (M w 52 kDa) in sheep liver microsomes. This protein was present in sheep liver (apparent concentration 16±4 ng/ μg microsomal protein) representing approx. 28% of the corresponding content of P450 2C6 in untreated rat liver. Preincubation of the anti- P450 2C6 IgG with hepatic microsomes was found to decrease the rate of progesterone 21-hydroxylation to 50–80% of uninhibited control. Taken together, from these findings it is apparent that a P450 enzyme, most likely from the 2C subfamily, catalyses deoxycorticosterone formation from progesterone in sheep liver and that this is a quantitatively important pathway of progesterone hydroxylation in these fractions.