The Relationship between the Insulin-binding Capacity of Fat Cells and the Cellular Response to Insulin

Abstract The effects of trypsin on the insulin-binding capacity of fat cells were studied with 125Iiodoinsulin, which was shown to be a valid tracer of native insulin. The binding of insulin to isolated fat cells was approximately 5 microunits/100 mg when the concentration of the hormone in the incubation medium was 100 microunits per ml. The initial step of the insulin receptor interaction followed the law of mass action. When the cells were exposed to trypsin (1 mg per ml) for 15 sec and for 15 min, the binding capacity was reduced by more than 80 and 98%, respectively. Upon subsequent incubation of trypsin-treated cells for 2 hours after inactivation of the enzyme, the binding capacity was partly restored. However, the maximum binding capacity of cells was only 9 and 4 microunits/100 mg (depending upon the length of the initial trypsin treatment) while that of untreated cells was 62 microunits/100 mg. The apparent dissociation constant of the insulin receptor system (approximately 1 milliunit per ml or 7 nm) was not significantly altered by the above treatment. Glucose metabolism in recovered cells was less sensitive to insulin as compared with that of untreated cells; however, the metabolism in all cell preparations responded normally to higher concentrations of insulin when more than 1.5 microunits of insulin were bound to 100 mg of fat cells. This indicates that glucose metabolism in intact fat cells is stimulated maximally when only approximately 2.4% of the total insulin receptors on the cells are occupied by insulin. Since the binding follows the law of mass action, it appears that the presence of excess receptors on fat cells renders the cells highly sensitive to insulin.