Preparation of 2-thioltryptophan-glucagon and (tryptophan-S-glucagon)2. Differences in binding to the glucagon receptor in the hepatic adenylate cyclase system.

The synthesis of 2-thioltryptophan-glucagon is described. Oxidation of this compound gives the dimer (Trp-S-glucagon)2. Both monomer and dimer are equi-potent on a molar basis with native glucagon as activators of adenylate cyclase in hepatic plasma membranes. However, from the ability to compete with 125I-glucagon for binding at the glucagon receptor, the dimer has one-fourth the binding affinity for the receptor as does native glucagon, 2-thiol-Trp-glucagon, and Trp-(2,4-dinitrophenylsulfenyl)-glucagon which have equal affinities for the receptor. Addition of GTP, which converts the receptor from a tight-binding to a lower affinity form and which activates adenylate cyclase in the presence of glucagon, allows (Trp-S-glucagon)2 to bind equally with native glucagon and the other thiol derivatives of the hormone. This effect of GTP on the binding of the glucagon dimer and the uses of 2-thiol-Trp-glucagon in the semisynthesis of new glucagon derivatives are discussed.