Kinetics of insulin binding to rat white fat cells at 15°C

Abstract

The kinetics of insulin binding to isolated rat epididymal fat cells was investigated at 15°C, at which temperature the system was simplified by the absence of lysosomal insulin degradation. The data were fit by maximum likelihood criteria with differential equations describing a number of models for the interaction of insulin and cells. Among those models that yielded a fit, the selection criteria were minimization of the Akaike information criterion and compatibility of the overall equilibrium constant for the system calculated from rate constants with the previously obtained experimental value. The results of the analysis indicated that insulin, I, first reversibly bound to cell surface receptors, R, whereupon this initial insulin-receptor complex, RI, reversibly altered its state or cellular location to R'I. No evidence was found that insulin could either associate or dissociate from R'I directly. The association rate constant was a value shown to be incompatible with diffusion control. From the other rate constants, an equilibrium constant was calculated, in excellent agreement with the previously measured value (Lipkin, E.W., Teller, D.C., and de Haen, C. (1986) J. Biol. Chem. 260, 1694-1701). The kinetic analysis also yielded receptor numbers similar to those obtained by equilibrium binding studies. The nature of the R'I state is discussed in terms of an internalized state, in terms of insulin receptor complex in caveolae, in terms of receptor aggregates, and in terms of being a Michaelis complex between insulin bound to the receptor and cell surface-bound insulin protease.