Functional relationships between insulin binding, action, and degradation. A reassessment.

We have evaluated the interrelationships between insulin binding, action, and degradation in primary cultures of isolated rat hepatocytes. In this cell system, over 99% of insulin degradation is cell-mediated. Whereas the Km value of insulin degradation in disrupted cells is approximately 10(-7) M, the Km value of insulin degradation in intact cells increases with increasing concentrations of insulin and parallels the Kd value of total insulin binding at all insulin concentrations. The dose-response relationship for insulin action differs from that of insulin degradation and specific insulin binding. However, when the portion of specific insulin binding which is degraded insulin is subtracted, the dose-response relationship for the remaining "specific-intact" insulin bound correlates with that of insulin action. These data indicate that the interpretation of relationships between insulin action, binding, and degradation depends on the biological system and the fraction of insulin binding utilized for analysis. In addition, our data derived in primary cultures of hepatocytes suggest that high affinity insulin binding sites may mediate insulin action, that insulin binding is rate-limiting for insulin degradation, and that all insulin binding sites, including "nonspecific" sites, may mediate insulin degradation.