THE EFFECT OF PROGESTERONE ON AMINO ACID METABOLISM*

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Abstract
The concentration of plasma alpha amino acid nitrogen was significantly lowered by the intramuscular administration of 50 mg. per day of progesterone in normal volunteers. This suppression (1.3-1.7 mg. per 100 ml.) was more pronounced during the peak rise in amino acid nitrogen concentration which followed a standard high protein meal than when comparisons were made in fasting subjects of 4 hours after a low protein meal. These effects were observed as soon as 7 hours after the first injection of progesterone. During 3-day fasts, the amino acid nitrogen concentrations averaged 0.5-0.7 mg. per 100 ml. lower when daily in-jections of progesterone were given. The plasma glucose concentrations were unaffected by progesterone treatment. The fact that the concentration of plasma alpha amino acid nitrogen is promptly lowered without an increase in urinary amino acid excretion strongly supports the previous suggestion that the primary action of progesterone in enhancing the net rate of protein catabolism is exerted on the utilization of amino acids by the liver. The net increase in protein catabolism would thus be secondary to the lowered plasma concentration of amino acids. Whether tissue protein anabolism is inhibited or catabolic processes are accelerated remains to be determined.