Progesterone is a catabolic hormone in man. In 4 subjects a progesterone induced catabolic process was accompanied by a prompt decline in the plasma concentrations of a number of free amino acids. Threonine, proline, glycine, alanine, lysine, arginine, ornithine, citrulline, cystine and serine concentrations were very significantly lowered. Valine, tyrosine, and histidine were significantly lower. The concentrations of methionine, isoleucine and leucine tended to move downward. Only the phenylalanine level was clearly uninfluenced. Previous studies had shown that total urinary amino acid N was unaffected by progesterone. Results in these experiments were corroborative in that urinary glycine, alanine, methionine and arginine were not influenced by treatment with progesterone. On the basis of these results it has been suggested that the catabolic action of progesterone is secondary to an enhancement of amino acid utilization by the liver, and that the circulating concentrations of one or several amino acids exert a negative feedback action on the rate at which amino acids are liberated from peripheral tissues. On the basis of the fact that during progesterone treatment only the concentrations of glycine, threonine, cystine and serine were lower fasting and postprandially than they were fasting on control days, it has been suggested that 1 of these 4 compounds, or a combination of several of them, may be critical in regulating peripheral protein catabolism.
