The major effects of insulin on glucose, fatty acid and amino acid metabolism and on ion flux are initiated by the interaction of insulin with certain unidentified components of the cell membrane. These early events in the action of insulin were studied further with purified plasma membranes prepared from rat epididymal adipose cells.
The binding of 125I-labeled insulin to fat cell plasma membranes gives results quantitatively similar to those reported previously with unlabeled insulin. In addition, intact fat cells, plasma membranes from fat cells and a soluble extract from the plasma membranes contain a highly active insulin degrading system. Both the capacity of the plasma membranes to bind insulin and the insulin degrading activity of the membrane extract can be inhibited by the addition of unlabeled insulin to the incubation medium even though albumin is present in excess. The degraded products have not been fully identified but are soluble in 10 per cent trichloroacetic acid, nonreactive with anti-insulin guinea pig serum, nonadsorbable to a 125I adsorbing resin and, therefore, probably represent fragments of the insulin molecule. When isolated fat cells are treated with trypsin they become insulin unresponsive. Plasma membranes derived from such cells have a reduced capacity to bind insulin, and the insulin degrading system of the membranes or in the soluble extract from the membranes is inactive. This suggests that both the insulin receptor and the insulin degrading system are located on the external surface of the plasma membranes of the fat cells. It is postulated that an insulin degrading system of the plasma membrane may be one of the physiological mechanisms by which the response of fat cells to insulin is terminated.