N alpha-Malto-glucagon and N alpha-malto, S-methyl methionine27-glucagon: preparation and characterization of two partial agonists.

N alpha-Maltoglucagon was prepared by demethylation of N alpha-malto, S-methyl methionine27 glucagon, and the two derivatives were purified to greater than 99% and 99.7%, respectively. S-Methylation of glucagon lowers the reactivity of Lys-12 and provides an alternative strategy to epsilon-amino protection for directing glycosylation of glucagon to the alpha-amino group. Both derivatives are partial agonists, with their adenylate cyclase activation and binding reduced in parallel. N alpha-Maltoglucagon produces 70% and N alpha-malto, S-methyl methionine27 glucagon 40% of the maximum activity of native hormone. N alpha-Maltoglucagon binds equivalently to N alpha-biotinyl, N epsilon-acetimidoglucagon whose maximum activity is near 35%, but a pK shift of the imidazole moiety cannot account for the difference in their abilities to produce transduction. Both glycosylated derivatives bind noncooperatively and both inhibit adenylate cyclase at high concentrations. The presence of a maltose residue on the amino terminal of glucagon may be required but, alone provides insufficient structural complementarity for concanavalin A binding to occur. The glycosylated derivatives are resistant to aminopeptidase degradation, are more soluble, and the maltose residue is unlikely to cause toxicity with in vivo use. Such attributes may be advantageous in the development of other analogs.