The metabolism of progesterone by animal tissue in vitro. 2. Factors influencing the metabolism of progesterone by rabbit liver and the investigation of the products of metabolism

In the first paper of this series (Taylor, 1954a), it was shown that progesterone metabolism by rat- liver homogenate is increased by addition of either diphosphopyridine nucleotide (DPN) or nicotin- amide (or both) or of citrate to the incubation medium; metabolism is not inhibited under anaero- bic conditions. In a large-scale experiment, 28 % of the progesterone incubated with rat-liver homo- genate (prepared in nicotinamide solution) was accounted for by isolation of 5a-pregnane-3:20dione, 3a-hydroxy-5oc-pregnan-20-one, unchanged progesterone, and an unidentified substance which was probably not a 'pregnanediol'. The failure to isolate any steroids of the 5,B-pregnane series was rather disconcerting in view of the fact that in humans and many other animals the reduction products of progesterone appearing in the urine are mainly of this series (see Dorfman & Ungar, 1953, for references). The metabolites of progesterone produced by the rat in vivo have not been fully characterized, but Riegel, Hartop & Kittinger (1950) demonstrated that after injection of [21-14C]progesterone into rats, most of the radioactivity was found in the faeces in a ketonic, non-alcoholic, neutral steroid which was not progesterone. It has not been established whether or not the rat excretes 'pregnanediols' normally or after progesterone administration.