Partial Disruption of Naturally Occurring Groups of Insulin Receptors on Adipocyte Plasma Membranes by Dithiothreitol and N-ethylmaleimide: the Role of Disulfide Bonds

In this ultrastructural study, monomeric ferritin-insulin was used to further elucidate the role of disulfide bonds in maintaining the natural groups of insulin receptors on adipocyte plasma membranes. Dithiothreitol (1 mM) caused partial disruption of the occupied receptor groups with an increase in single receptors to greater than 50% of total occupied receptors. N-Ethylmaleimide (1 mM) disrupted the groups to the same extent as dithiothreitol and the effect was partly additive with the dithiothreitol effect. The magnitude of the disruption caused by dithiothreitol or N-ethylmaleimide was similar to that caused by cytochalasin B. Dithiothreitol, a reducing agent, caused a marked increase in binding of insulin to the plasma membranes while N-ethylmaleimide and cytochalasin B, both thiol reagents, had little if any effect on insulin binding. These data suggest that two different sets of disulfide bonds are involved. One set was susceptible to both reducing and thiol reagents and responsible for holding the receptor groups together, and the other set was susceptible to reducing agents only and related to the increased insulin binding caused by dithiothreitol. A proposed model is discussed.