Insulin receptors in rabbit erythrocyte

The existence of insulin receptors in rabbit erythrocytes was studied by evaluating the specific binding of 125I-insulin to erythrocyte membranes. The binding of 125I-insulin was pH, time and temperature dependent. Maximal binding was achieved by incubation for 20 hr at 0 degrees C. The optimum pH was 7.4. Treatment with cations and enzymes enhanced the specific binding except for with trypsin, the treatment which greatly reduced the binding. Unlabeled insulin over a wide range of concentrations competitively inhibited the binding of 125I-insulin, while the binding was little affected by structurally unrelated hormones. Scatchard plot was represented as a concave curve. Binding sites of relatively high affinity (K1 = 0.9 X 10(9) M-1) and low capacity (8.0 X 10(13)/g protein) could be distinguished from those of lower affinity (K2 = 0.8 X 10(7) M-1) and higher capacity (1.8 X 10(15)/g protein). Hill's analysis and dissociation of 125I-insulin from membranes demonstrated the characteristics of negative cooperation between receptor sites. Both incorporation of H3(32)PO4 to erythrocyte membranes and uptake of 45Ca were significantly reduced by the addition of unlabeled insulin. Unlabeled insulin produced no effect on uptake of 45Ca into trypsin-treated erythrocytes. On the basis of these results, it was suggested that rabbit erythrocytes might possess biologically significant insulin receptors located on the cell membranes.