Unique features of the insulin receptor in rat brain

We examined the structure of the affinity-labeled insulin receptors in rat brain, rat liver, and human IM-9 lymphocytes using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In gels run under reducing conditions, the alpha-subunit of the insulin receptor in brain had an apparent Mr of 127,000 distinctly lower than that seen in both rat liver and human lymphocytes (apparent Mr = 136,000). Exposure to neuraminidase increased the electrophoretic mobility of the liver receptor, but had no effect on the insulin receptor in brain. The carbohydrate moieties of the insulin receptors in rat brain and liver were further examined by chromatography on wheat-germ agglutinin agarose. The receptors in both tissues adsorbed to the wheat-germ agglutinin; elution with 0.3 M N-acetyl glucosamine resulted in slightly better recovery of the brain than of the liver receptor. Exposure to neuraminidase virtually abolished the interaction of the liver receptor with the lectin, whereas adsorption of the brain receptor was unaffected by neuraminidase. These results indicate that the insulin receptor in brain is distinguished from those in peripheral tissues by structural alterations, including changes in the carbohydrate moiety of the receptor. Such alterations contrast sharply with the previously observed similarities in insulin binding properties between insulin receptors in brain and other tissues. The implications of such structural alterations for the program of insulin action expressed by the receptors in brain remain to be explored.