Metabolism of progesterone by the rabbit liver.

Previous work indicates that progesterone is metabolized extrahepatically; the kidney has been implicated as a site of its biological inactivation. This study was designed to determine the formation and retention of progesterone metabolites by kidney tissue. 1. It was found that 40–45 min after administration of progesterone-4-14C to rabbits, as much as 30% of the extractable radioactivity was free steroids. No significant synthesis of conjugated metabolites occurred in vitro. 2. The presence of unmetabolized progesterone in the rabbit kidney 40 min after administration of the labeled compound has been established. The presence of enzymes capable of 20α- and 20β-reduction, 3β-reduction, 5α-reduction and 6-hydroxylation has been established. Evidence for 5β-reduction and 3α-reduction is also presented. 3. The type and concentration of free steroids isolated from the rabbit kidney could be predicted by the relative enzyme activities in vitro. 4. Based upon the size of the organs, concentration of metabolite...