Degradation, receptor binding affinity and biological potency of monoiodoinsulin in isolated rat fat cells

The degradation, binding affinity and biological potency of monoiodoinsulin was studied in isolated rat fat cells. The rate of inactivation by a concentrated cell suspension was indistinguishable from that of native insulin, whereas the relative biological potency (increase in lipid synthesis from glucose) and binding affinity (inhibition of receptor binding of 125 I-labelled insulin) was 60-80%. Assuming that the insulin receptor binding is a simple, reversible, bimolecular reaction, the following are the consequences for the interpretation of experiments at equilibrium in which an unlabelled species of insulin competes with (125I) monoiodoinsulin present in a concentration much below the dissociation constant for the insulin: 1. The dissociation constant is estimated without bias. 2. The total number of receptors is slightly underestimated.