Glucagon structure and function. Preparation and characterization of nitroglucagon and aminoglucagon.

The nitration of glucagon (1%) with tetranitromethane (1 mole excess per mole of tgrosyl residue) proceeded rapidly at pH 8 and 0”, conditions wherein the reactants were almost completely insoluble. Under these conditions few polymeric and oxidative side reactions occurred, and tyrosyl residue 13 reacted much more rapidly than tyrosyl residue 10. The products of nitration were purified by gel filtration and chromatography with DEAE-celhrlose and were characterized by electrophoresis, amino acid analysis, enzymatic hydrolysis, partial structural analysis, and biological and immunological assays. Mononitroglucagon, containing 3-NO2 tyrosine at residue 13, was twice as active as glucagon in elevating blood glucose, but it stimulated rat liver adenylate cyclase no better than glucagon and was poorly (20%) recognized immunologically. Nitration of tyrosyl residue 10 in addition to residue 13 depressed biological and immunological activity, although dinitroglucagon was still more active than glucagon as measured by hyperglycemia. Reduction of mononitroglucagon with Na&O, gave monoaminoglucagon, which was about half as active as glucagon by all assays. We found diaminoglucagon to be nearly half the potency of the monoamino derivative. Among possible explanations for our results are alterations in phenolic pK, enhanced binding to receptor, protection of the hormone from proteolysis, or combinations of these factors. Whatever the explanation, our results clearly implicate the tyrosyl residues of glucagon in the biological and immunological reactivity of the hormone.