Hormonal and metabolic influences on intracellular peptidase activity

¹ Section of Endocrinology and Metabolism, Presbyterian-St. Luke's Hospital; and Department of Medicine, University of Illinois College of Medicine, Chicago, Illinois

A procedure which permits the estimation of intracellular peptidase activity of surviving rat diaphragm has been evaluated following changes in the hormonal and metabolic environment. It was found that leucine aminopeptidase (LAP) activity fluctuated with the seasons, rising during the summer and decreasing to low levels during the winter. LAP activity was significantly increased after acute and chronic fasting, cortisone administration and triiodothyronine administration. Conversely, LAP activity was reduced following adrenalectomy or thyroidectomy. The ‘stress’ of saline injection did not provoke significant changes in aminopeptidase activity and attempts to induce enzyme adaptation by injection of peptide substrate were unsuccessful. These data are consistent with the view that intracellular peptidases perform an important regulatory function in the control of protein metabolism. The possibility has been raised that the changes noted may be the result of varying degrees of intracellular exposure of substrate to enzyme.