Studies on the Metabolism of Radioprogesterone in Mice and Rats*

THE synthesis of progesterone-21-C14 (Riegel and Prout 1948) has made possible the investigation of certain specific problems involving the metabolism of the hormone. The presence of C14 at the 21 position serves the dual purpose of labeling the hormone as well as providing a substance suitable for study of the metabolism of the side chain. An important objective of this investigation was to determine whether animals have the ability to oxidatively metabolize the side chain of progesterone. Radioactive carbon dioxide was detected in the expired air of rats and mice after the injection of progesterone-21-C14. This metabolic pathway for the oxidation and elimination of the C-21 carbon atom of progesterone is, therefore, established. A second important objective of this investigation was to determine the distribution of progesterone or its metabolites within the tissues of the animal organism. Various tissues of rats and mice have been shown to possess different capacities for the absorption and retention