Effect of sulfhydryl reagents on pancreatic islet secretion granule-plasma membrane interaction

To determine what role, if any, sulfhydryl groups may play in the fusion of islet secretion granules with the plasma membrane that takes place during exocytosis, we have studied the effect of several sulfhydryl-binding reagents, reducing agents, and oxidizing agents on the binding of 125I-labeled inside-out plasma membrane vesicles to isolated secretion granules. Three sulfhydryl-binding reagents, p-hydroxymercuribenzoate, Hg++, and N-ethyl maleimide, stimulated this binding, and the stimulation was greater in the absence of Ca++ than in its presence. In contrast, the three reducing agents used, glutathione, cysteine, and sodium bisulfite, inhibited the binding. Of the oxidizing agents, oxidized glutathione inhibited binding, whereas menadione and o-iodosobenzoate stimulated. The actions of Hg++ and glutathione were found to be on the secretion granules rather than the plasma membrane vesicles. It is concluded that the presence of a preponderance of sulfhydryl groups on the secretion granule membranes tends to limit their interaction with the plasma membrane and that these must be removed or masked for maximum fusion to occur.