The role of sulfhydryl groups in thyrotropin binding and adenylate cyclase activities of thyroid plasma membranes.

The effects of sulfhydryl (SH) reagents on the binding of TSH to thyroid membranes and on membrane adenylate cyclase activity were studied. Diamide, a SH-oxidizing agent, enhanced [125I]iodo-TSH binding to thyroid membranes in a dose-dependent manner. On the contrary, dithiothreitol (DTT), a disulfide-reducing reagent, markedly inhibited this activity in a dose-dependent manner. These effects could not be attributed to a direct interaction between the reagents and the radioactive TSH. Scatchard plot analysis of the data was limited by curvilinear plots but suggested that diamide increases while DTT decreases the affinity of the receptors for TSH in thyroid membranes, neither reagent appeared to influence the capacity of TSH binding by membranes. The effects of diamide and DTT on adenylate cyclase activity of thyroid plasma membranes were just the opposite of their effects on TSH binding. Thus, diamide inhibited adenylate cyclase activity of thyroid plasma membranes while DTT increased basal enzyme activity and slightly but not significantly increased TSH-stimulated enzyme activity. The data suggest a possible physiological role for SH and disulfide groups of thyroid membranes in modulation of thyroidal activity.