γ-Radiation-Induced Interactions between Amino Acids and Glucagon

The interaction of glucagon and phenylalanine mediated by the OH · radical causes formation of higher molecular weight products of glucagon and phenylalanine, loss of amino acid residues in glucagon, and formation of adducts of glucagon and phenylalanine. The relative yields of these products depend upon the molar ratio of phenylalanine to glucagon in solution. At low ratios, glucagon aggregation and loss of amino acid residues predominate; at high ratios, the formation of phenylalanine dimers (and possible trimers and tetramers) predominates. The formation of adducts reaches a maximum at a phenylalanine:glucagon molar ratio of 3-4, and then decreases gradually, as the molar ratio increases, but is still discernible even at high molar ratios. Mechanisms for the formation of adducts are suggested. The influence of the primary aqueous radical intermediates, OH ·, H ·, and$e{}_{{\rm aq}}^{-}$, on adduct formation has been evaluated for several different amino acids by irradiating in the presence of specific radical scavengers. For the aromatic amino acids (phenylalanine, tryptophan, and tyrosine), OH · is considerably more effective than$e{}_{{\rm aq}}^{-}$for mediating adduct formation, whereas for histidine and methionine, these primary radicals are equally effective.