Binding and degradation of 125I-insulin by rat hepatocytes

The binding and the velocity of degradation of 125I-insulin in the absence or presence of varying concentrations of native procline insulin were studied using isolated rat hepatocytes. At insulin concentrations ranging from 5 X 10(-11) to 10(-6) M, insulin degradation velocity showed a first order dependence on the total concentration of insulin bound at steady state. The overall reaction had an apparent rate constant of 0.030 +/- 0.011 min-1. Furthermore, the degradation of a given amount of 125I-insulin bound to cells was more rapid and extensive than the degradation of the same amount of insulin which had been newly exposed to fresh cells. Mid pretreatment of isolated hepatocytes with trypsin or chymotrypsin at concentrations of 5 to 20 mug/ml depressed to the same degree the amount of 125-I-insulin bound at steady state and the 125I-insulin degradation velocity. Peptide or protein hormones unrelated to insulin, including the oxidized A and B chains of insulin, failed to depress the amount of insulin bound or the velocity of insulin degradation when present at concentrations of 10-5 or 10-6 M. Over a wide range of concentrations, various synthetic insulin analogues and naturally occurring insulins depressed to the same degree the amount of 125I-insulin bound at steady state and the 125I-insulin degradation velocity. These observations suggest that insulin bound to hepatocyte plasma membranes is the substrate for insulin degradation by the liver.