Insulin-receptor interactions. Presence of a positive cooperative effect

Competitive binding curves for 125I-insulin on a cultured rat hepatoma, H4-II-E-C'3, indicated that low competitive insulin concentrations increased label binding. This upward deflection of the binding curve or "hook effect" occurred at physiological concentrations of insulin. Lectins capable of mimicking insulin also stimulated insulin binding on whole cells, membranes, and solubilized receptors, apparently by increasing the affinity of the receptor for the hormone. An anti-receptor antibody and derived FAB fragments also elicited a similar increase in insulin binding affinity. At insulin levels below 10 ng/ml, the binding curves exhibited sigmoidicity consistent with positive cooperativity. This property appears to be intrinsic to the receptor and may be due to its oligomeric structure.