Studies on the Mode of Action of Insulin: Properties and Biological Activity of an Insulin-Dextran Complex

Abstract

The present study was on the mode of action of insulin using a synthetic insulindextran complex which is water-soluble but does not readily pass through the cell membrane. Crystalline bovine insulin was covalently coupled at pH 9 to dextran (av mol wt, 40,000 or 70,000) which had been activated with cyanogen bromide. After coupling, the insulin-dextran formed was precipitated with acetone and purified by gel filtration on a column of Sephadex G-7S. Bound insulin in the insulin-dextran complex was determined by a micromethod using polyacrylamide gel electrophoresis. The shape of the radioimmunoassay curve of insulin-dextran indicated that the immunochemical effectiveness of the complex is less than that of unmodified insulin. When insulindextran was injected iv into alloxan-diabetic rats, at a dose of as little as 0.4 U/kg bw the blood glucose level rapidly decreased. The hypoglycemic action of the complex was greater and more prolonged than that of unmodified insulin. Moreover, the insulin-dextran complex caused marked induction of glucokinase, pyruvate kinase and ATP citrate lyase in the liver on injection into alloxandiabetic rats at much lower dose than that of native insulin. It was also shown that insulindextran scarcely passed through the cell membrane and was not degraded appreciably on incubation with pieces of fat pads or slices of soleus muscle. (Endocrinology90: 1220, 1972)