Kinetic Modeling of Insulin Binding to Receptors and Degradation In Vivo in the Rabbit

In vivo metabolism of insulin, with emphasis on the distribution, binding, and degradation from receptor and nonreceptor sites, has been studied in rabbits by analyzing the kinetics of antibody-bindable, TCA-precipitable, and TCA-soluble components in plasma. Tracer amounts of ¹²⁵I-and ¹³¹I-labeled high affinity (pork) and low affinity (guinea pig) insulins were injected simultaneously into rabbits in the basal state or after a large bolus of unlabeled pork insulin. In one series of experiments, a bolus of unlabeled pork insulin was given from 5 to 30 min after the tracers. Based on the kinetic curves derived in these experiments, a model is developed for insulin metabolic pathways in vivo. A number of features are contained in the model. First, some of the kinetics are caused by distribution spaces in the body and some by specific binding sites, presumably receptors. Two equilibration phases between plasma insulin and receptors can be detected. The first and major one occurs within about one minute. The second one takes about 15 or 20 min. Partial degradation of the insulin molecule can be detected rapidly by the kinetics and is associated with the rapid (1 min) receptor phase. About 27% of the labeled insulin molecules are partially degraded via this pathway. There is, also, irreversible loss of insulin from the receptors without significant return of label to plasma during the experiment. This loss could be a result of internalization of insulin by the cells. About 48% of the insulin injected follows this pathway. The remaining degradation of insulin (∼24%) takes place at other sites in the body. The results of the kinetic analysis also suggest that there are two populations of receptors. The high affinity population has an equilibrium dissociation constant equal to about 9 × 10⁻⁹ M, a value comparable to that observed in vitro (∼5 × 10⁻⁹ M). The low affinity population has an equilibrium dissociation constant about 100 times greater than that of the high affinity population; its capacity, however, is about 30 times greater. It is, further, concluded that the TCA-soluble degradation products observed in plasma could not be accounted for by the partial and irreversible degradation pathways identified in the insulin model. They could be accounted for, however, by a 5%–15% subpopulation of injected labeled TCA-precipitable molecules that decay monoexponentially from plasma.