Degradation and internalization of bound insulin in human erythrocytes

Internalization and degradation of insulin by human erythrocytes were studied. Erythrocytes were incubated with 125I-insulin at 4 degrees C, 15 degrees C, and 37 degrees C for varying time intervals. These erythrocytes were then subjected to a low pH wash to release bound insulin followed by TCA precipitation. After 4, 22, and 24 hours of insulin binding at 4 degrees C, 92 to 95% of the bound 125I-insulin was dissociable and 92 to 98% of the extractable insulin was undegraded. After 3.5 hours of incubation at 15 degrees, 82% of the bound insulin was dissociable and 60% of this was intact. However, after 60, 90, 120, and 180 minutes of incubation at 37 degrees C, only 42, 34, 24, and 37%, respectively, of the bound insulin was dissociable. The undissociated insulin in the 37 degrees C studies was considered to be intracellular. With increasing time of incubation at 37 degrees C, the extractability of cell bound insulin and the proportion of undegraded dissociable insulin were decreased. When 125I-insulin binding was 95% blocked by preincubating the erythrocytes with anti-insulin receptor antibody, 95% of the degradation of 125I-insulin was also blocked. These studies indicate that mature human erythrocytes degrade internalized insulin and this process is time, temperature, and insulin receptor dependent.