On the Possible Role of Thiol Groups in the Insulin-Releasing Action of Mercurials, Organic Disulfides, Alkylating Agents, and Sulfonylureas

Abstract

The thiol activity of pancreatic islets was spectrophotometrically assayed as the formation of 6-mercaptonicotinic acid from the organic disulfide, 6,6'-dithiodinicotinic acid. Islets containing more than 90% beta-cells were microdissected from non-inbred ob/ob-mice. Comparisons of intact with homogenized islets indicated that the organic disulfide penetrates relatively slowly into the beta-cells. When tested at concentrations know to enhance insulin release, p-chloromercuribenzene-sulfonic acid almost completely blocked the thiol activity of intact islets, whereas no significant effect was observed with iodoacetamide, D-glucose, or glibenclamide. Although glibenclamide had no demonstrable effect on the thiol activity of free L-cysteine, the binding of glibenclamide to serum albumin was decreased by blocking the albumin thiols with azobenzene-2-sulfenyl bromide. The uptake of glibenclamide by pancreatic islets was inhibited by cysteine or reduced glutathione. Cysteine, as well as 6,6'-dithiodinicotinic acid, also seemed to interact negatively with glibenat organic mercurials and disulfides stimulate insulin release by blocking thiol groups in the beta-cell plasma membranes. The thiol groups involved in iodoacetamide-induced secretion may escape detection by the assay employed, or target groups other than thiols may be involved. The data on glibenclamide are compatible with, but do not unequivocally support, the notion that thiol groups may play a role in sulfonylurea-induced insulin release.