Insulin B chain-degrading neutral peptidase activity in the rat. Tissue distribution and the effects of starvation and streptozotocin-induced diabetes

Abstract

The tissue distribution and the effects of starvation and streptozotocin-induced diabetes on insulin B chain-degrading neutral peptidase activity in the rat have been studied. The neutral peptidase activity in tissue extracts was determined by measuring the formation of trichloroacetic acid-soluble radioactivity from 125I-labeled B chain of insulin in 0.1 m Tris buffer (pH 7.2). Inhibition by several different compounds (EDTA, dithiothreitol, and potassium phosphate) which are known to inhibit the purified enzyme and the effects of pH suggest that the B chain-degrading activity measured in each of 12 tissue extracts may be similar to the neutral peptidase recently purified from rat kidney ( P. T. Varandani and L. A. Shroyer, 1977, Arch. Biochem. Biophys., 181, 82–93). Neutral peptidase activity was observed in all tissues examined and varied in the order kidney ⪢ intestine > pancreas, testis > liver > thymus > heart, skeletal muscle, diaphragm > lung, spleen > fat. Neutral peptidase activity in kidney, liver, fat, and skeletal muscle from diabetic animals was significantly depressed when compared with the levels in these tissues from normal animals. Insulin treatment of diabetic animals raised the neutral peptidase activity in kidney, liver, and fat to levels equivalent to or even exceeding normal levels; however, activity in skeletal muscle persisted at depressed levels. Heart muscle neutral peptidase activity was not significantly affected in either diabetes or starvation. In the liver, starvation reduced the level of neutral peptidase activity while subsequent refeeding raised the activity to a level exceeding the control. Opposite effects were observed in kidney: starvation increased neutral peptidase activity while refeeding brought the activity back to normal levels. Only small decreases in neutral peptidase activity were observed in fat and skeletal muscle after 24 h starvation, but were not evident after 64 h starvation. The changes in neutral peptidase activity correlated well with the changes in glutathione-insulin transhydrogenase activity previously reported in liver and kidney.