Characteristics of human erythrocyte insulin binding sites

Insulin and human erythrocyte cell membrane interactions were studied with respect to binding and dissociation. The per cent of specific binding of 125I-labeled insulin to erythrocytes was directly proportional to the cell concentration. The optimum pH for binding was 8.1. The initial binding rate was directly proportional to, and the steady state insulin binding was reversely proportional to, the incubation temperature. The per cent of specific binding of 125I-labeled insulin was 12.10 +/- 1.13 per cent (mean +/- SD)/4 X 10(9) cells (n = 10) at 0.8 ng/ml insulin. Native insulin competed with 125I-labeled insulin for binding and showed almost complete inhibition at 10(4) ng/ml. The Scatchard plots were upward concave. Maximum binding capacity was 230 binding sites per cell. The average affinity constant decreased as the per cent of fractional occupancy increased. Affinity constants for the empty and filled sites were 1.49 and 0.16 X 10(8) M-1 respectively. Bound insulin was displaced by native insulin. The dissociation rate by "dilution + native insulin" was higher than that by "dilution only". The dissociation rate was accelerated even by the physiological concentration of insulin and maximum at 100 ng/ml. It is concluded that human erythrocytes have insulin binding sites which are indistinguishable from insulin receptors on the target tissues for insulin.