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Abstract:
The first step in insulin action is interaction of the hormone with specific receptors on the plasma membrane of target cells. The receptor site is defined by its functional properties: it binds insulin with a high degree of specificity and affinity; the specific binding sites are finite in number; hormone binding is rapid and reversible; the binding sites are, or predominate, on the plasma membrane; hormone binding can be related to biological effects. The use of biologically active monoiodoinsulin permits direct study of the binding of 125I-insulin to receptor in isolated cells or in cell membranes. The binding of 125I-insulin is highly specific and strictly dependent on the biologically active structure of the insulin molecule. Both the rate and the amount of binding are time and temperature-dependent. The insulin--receptor interaction is a complex reaction, whose quantitative analysis requires consideration of, in addition to hormone binding to receptor, degradation of the hormone and of the receptor. By measuring binding at steady state, one can determine both the apparent insulin--binding capacity and the apparent dissociation constant of the insulin--receptor complex. Scatchard analyses of these data reveals heterogeneity of the binding sites with respect to equilibrium constant, a result which is also compatible with negative cooperativity between the receptor sites. Studies of insulin--receptor interactions have led to a variety of applications, two of which are of special interest from pharmacological, pathophysiological and clinical viewpoints. One is concerned with the use of insulin--receptor binding systems to study hormone structure--activity relationships and to analyze endogenous forms of insulin. These studies have shown that, in every instance, the binding affinity of an insulin or insulin analogue is in direct proportion to its biological potency. No antagonist of native insulin has been found thus far, including data obtained with proinsulin, proinsulin intermediates and Non-Suppressible Insulin-Like Activity (NSILA) or somatomedin (s). These studies have also revealed that proinsulin reacts only with insulin receptors in contrast to NSILA which, inaddition to reacting with insulin receptors, also possesses its own specific receptor sites. The other application deals with studies of receptor alteration in pathological states. This investigation has been well documented in the obese hyperglycemic (ob/ob) mouse which exhibits a receptor defect in the liver, fat, thymic lymphocyte and myocardium. The defect essentially involves a decrease in the number of binding sites and selectively affects the insulin receptor. A variety of data, both in vivo and in vitro, indicate an inverse relationship between the concentration of insulin and the number of insulin receptors, and suggest a negative feedback effect of the hormone on its own receptor...