Coated vesicles participate in the receptor-mediated endocytosis of insulin.

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Abstract
Coated vesicles were purified from rat liver differential ultracentrifugation. EM of these preparations reveal only the polyhedral structures typical of coated vesicles. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the coated vesicle preparation followed by Coomassie Blue staining of proteins reveals a protein composition also typical of coated vesicles. These rat liver coated vesicles possess a latent insulin binding capability. That is, little if any specific binding of 125I-insulin to coated vesicles is observed in the absence of detergent. Coated vesicles treated with the detergent octyl glucoside exhibit a substantial specific 125I-insulin binding capacity. The insulin binding structure of coated vesicles were visualized by cross-linking 125I-insulin to detergent-solubilized coated vesicles using the bifunctional reagent disuccinimidyl suberate followed by electrophoresis and autoradiography. The receptor structure thus identified is identical to that of the high-affinity insulin receptor present in a variety of tissues. Liver coated vesicles were isolated from rats which had received injections of 125I-insulin in the hepatic portal vein. Insulin administered in this fashion was rapidly and specifically taken up by liver coated vesicles. These data are compatible with a functional role for coated vesicles in the receptor-mediated endocytosis of insulin.

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