Presence of sulfhydryl groups in glucagon secretion granule membranes

The effect of various sulfhydryl-binding reagents on the release of glucagon from isolated secretion granules was studied. Secretion granules isolated from toadfish islet tissue by differential centrifugation in sucrose were incubated in the test medium, the granules were centrifuged down, and the supernatant was assayed for glucagon using a RIA. When the granules were incubated in isotonic saline, 16-18% of the total glucagon in the granules was released. Whereas the addition of various dithiol-binding reagents (Cd++, Hg++, AsO2-, and dichlorophenylarsine), an oxidizing agent (oxidized glutathione), a reducing agent (reduced glutathione), and an amino group-binding reagent reagent (trinitrobenzene sulfonic acid) had no effect, the addition of Co++ to the incubation medium decreased glucagon release and the addition of the monothiol-binding reagents, p-hydroxymercuribenzoate and diphenylchloroarsine, produced a 2- to 3-fold increase in glucagon release. These results suggest that sulfhydryl groups may be present in the glucagon secretion granules and that they may be involved in glucagon release.