Delayed processing of the insulin proreceptor by hepatocytes from diabetic rats

The effect of diabetes on insulin receptor processing was assessed in rat hepatocytes, 2-4 weeks after the induction of diabetes with streptozotocin. Isolated hepatocytes from control and diabetic rats were labelled for 30 min with [35S]methionine in methionine-free medium and chased with complete medium for 1-3 hrs. Solubilized cell extracts were immunoprecipitated with a site-specific anti-insulin receptor antibody, proteins were separated by electrophoresis and labelling quantified following autoradiography. After 1 hr chase, only the insulin proreceptor was labelled in both groups. After 3 hrs, the ratio of labelled proreceptors to labelled insulin receptors was 0.57 +/- 0.063 in controls and 1.33 +/- 0.24 in hepatocytes from diabetic rats. Insulin added in vitro did not affect transit time. Delayed processing of the insulin proreceptor may reflect altered N-glycosylation and may also involve other glycoproteins.