Effect of sulfhydryl-binding reagents on release of somatostatin immunoreactivity from secretion granules

The effect of various sulfhydryl-binding reagents on the release of somatostatin immunoreactivity (SRIF-I) from secretion granules of the islets of Langerhans was studied. The granules were isolated from toadfish islet tissue by differential centrifugation in sucrose. They were then incubated in the test medium, pelleted, and the supernatant was assayed for SRIF-I using a radioimmunoassay. When the granules were incubated in isotonic saline, about 20% of the total SRIF-I in the granules was released into the medium. Three mercurials (p-hydroxymercuribenzoate, phenylmercuric acetate and Hg++) and an arsenical (diphenylchloroarsine) increased SRIF-I release 1.5 to 2-fold. In contrast, other heavy metals (Co++ and Cd++), other arsenicals (dichlorophenylarsine and AsO-2), alkylating agents (N-ethylmaleimide and iodoacetic acid), an oxidizing agent (oxidized glutathione), a reducing agent (reduced glutathione), and an amino group-binding reagent (trinitrobenzene sulfonic acid), had no effect. These results suggest that sulfhydryl groups may be present on the somatostatin secretion granules and, if so, that they differ from those previously demonstrated on the insulin- and glucagon-secretion granules.