Conformational and biological properties of di[delta-(5-nitro-2-pyrimidyl)ornithine 17,18]glucagon. Role of the arginine residues.

The reaction of nitromalondialdehyde with the arginine residues of glucagon results in the conversion of the 2 arginine residues in the peptide to delta-(5-nitro-2-pyrimidyl)ornithine to form di[delta-(5-nitro-2-pyrimidyl)ornithine 17,18]glucagon (NP-glucagon). The modified peptide does not exhibit any loss in ability to activate adenylate cyclase of rat liver plasma membranes or to stimulate glycogenolysis in cortisone-primed rabbits relative to the native hormone despite this marked alteration in structure. The CD of dilute solutions of NP-glucagon is similar to that of the native hormone. In the absence of salt, the CD of NP-glucagon is independent of peptide concentration, but structures of higher helical content are observed in concentrated peptide solutions in the presence of 0.1 M NaCl and in methanol. The extent of helix formation under these conditions is greater than that given by glucagon. Results from viscosity and proton magnetic resonance spectra confirm and extend previous studies to indicate that this fully active derivative is in a compact folded conformation.