Insulin Degradation by Liver Cell Membranes*

Abstract

Liver cell membranes actively degrade insulin, but the mechanism for this process has not been established. Exposure of [¹²⁵I]iodoinsulin for various times to a liver cell membrane preparation enriched in blood sinusoidal surfaces resulted in the generation of labeled products of varying sizes. Separation of these products on a molecular sieve column demonstrated a progressive decrease in ¹²⁵I-labeled material eluting in the insulin peak and an increase in both larger and smaller molecular weight materials. The latter could be separated into at least three distinct peaks. In addition, the material eluting in the insulin peak had a decreased immunoprecipitability, suggesting that this peak was also heterogeneous. The ¹²⁵I-labeled material extracted from the membranes eluted from the molecular sieve column predominantly in the void volume and in the insulin peak. There was an increase with time in the larger molecular weight fraction. The liver membrane insulin-degrading activity was characterized by polyacrylamide gel electrophoresis after solubilization by Triton X-100. Only one area of insulin-degrading activity was found. This activity had a mobility identical with the mobility of purified insulin protease and distinctly different from that of purified glutathione insulin transhydrogenase. Furthermore, no glutathione insulin transhydrogenase activity could be demonstrated using an assay specific for that enzyme. Thus, the insulindegrading activity of liver cell membranes appears to be due to the proteolytic enzyme insulin protease.