Studies on insulin metabolism: enzymatic characteristics and biological significance of insulin-degrading activities in rat liver cell fraction (author's transl)

Abstract

Although insulin is rapidly metabolized by various tissues probably through the actions of enzymes with a high degree of specificity, the mechanism of insulin degradation in the target cell is not yet sufficiently clarified. Using the cell fractionation of rat liver, insulin degradation by plasma membrane, lysosomal, microsomal, and cytosol fractions were examined under various conditions including high and low substrate concentrations, pH 7 and pH 4, with and without glutathione, and in the presence of antiserum against pig muscle insulin-degrading enzyme (IDE). Furthermore, to clarify the biological significance of the insulin-degrading system in these cell fractions, insulin-degrading activities of liver cell fractions from streptozotocin diabetic rats were compared with those from control rats under various assay conditions.At neutral pH and low insulin concentration (0.1nM), insulin was degraded predominantly by the cytosol fraction, and its activity from diabetic rats was significantly lower than that from control rats. Insulin-degrading activity in the cytosol fraction was identical to pig IDE in most of its enzymatic properties including Km for insulin (120nM), sulfhydryl-dependency, and the effect of anti-IDE serum. At neutral pH and high insulin concentration (8.5μM), the highest activity was found in microsomes, and its activity from diabetic rats was lower than that from control rats. Insulin-degrading activity in the microsomal fraction was mainly due to glutathione-insulin transhydrogenase, judging from its Km for insulin (15μm) and the great effect of glutathione. However, the physiological importance of this enzyme was cast in doubt because of its high Km for insulin.At the acid pH, most insulin degradation occurred in the lysosomal fraction. Lysosomal activity from diabetic rats was significantly decreased at either high or low insulin concentration under acid conditions, whereas no significant differences were shown between diabetic and control rats under neutral pH. Insulin-degrading activity of plasma membranes was identical to IDE. Plasma membranes contained relatively little degrading activity under every assay condition and no changes were seen between both groups of rats.These data suggest that insulin is mainly degraded by intracellular enzymes rather than the insulin-degrading system on plasma membranes and that IDE seems to account for most of the intracellular degradation of insulin.