Insulin--receptor Interactions in Adipose Tissue Cells: Direct Measurement and Properties

An assay system is described for measurement of specific binding of [(125)I]insulin to intact fat cells and membrane fractions from such cells. The binding is time- and temperature-dependent and saturable with respect to insulin; the bound insulin is displaced by native insulin but not by oxidized or reduced insulin or by a number of other peptide hormones. A maximum of about 11,000 molecules of insulin can bind per cell. The insulin-receptor association is a bimolecular reaction with a rate constant of 1.5 x 10(7) M(-1) sec(-1), while the dissociation is a strictly first-order process with a rate constant of 7.4 x 10(-4) sec(-1). A dissociation constant of 5.0 x 10(-11) M can be calculated from these rate constants, whereas a value of 6.1 x 10(-11) M is obtained on the basis of enhancement of glucose oxidation. Complex formation does not result in chemical change or inactivation of insulin or receptor. The total binding capacity of fat cells is quantitatively recovered in the particulate fraction after homogenization. The insulin-cell receptor interaction is a simple dissociable process involving a homogeneous species probably present exclusively in the cell membrane.