Insulin Degradation by Mononuclear Cells

Mononuclear cells from peripheral blood possess insulin receptors that are altered in number or binding affinity in certain metabolic diseases as obesity. The monocyte, and not the lymphocyte, is the cell with the capacity to specifically bind insulin. Furthermore, this binding appears to mirror the receptor status on such insulin target tissues as liver, muscle, and fat. Since liver, muscle, and fat also degrade insulin, mononuclear cells from the blood of normal volunteers were examined for insulin-degrading activity. Intact cells were incubated with ¹²⁵I-insulin and the amount of degraded insulin was measured by the trichloroacetic acid-precipitation technique. Insulin-degrading activity increased when the number of cells and the time of incubation were increased. Total insulin binding behaved in a similar fashion. Very little degradation was seen at 4° or 15°. The K_m for insulin-degrading activity was 7.03 × 10⁻⁸ M. Homogenized mononuclear cells degraded two to five times more insulin than did intact cells and also demonstrated cell concentration, time, and temperature dependence for degradation. The K_m of degradation for homogenized mononuclear cells was 2.2 × 10⁸ M. Subcellular fractionation revealed significant degrading activity in the 100,000 × g supernatant, but little activity in the 100,000 × g pellet. A purified lymphocyte preparation did not bind insulin and contained little insulin-degrading activity.