Glucagon amino groups. Evaluation of modifications leading to antagonism and agonism

Using native glucagon and [12-homoarginine]glucagon (analogue A), prepared in high yield and purity by new procedures, we have synthesized the following glucagon analogues by semisynthetic methods: [1-deshistidine][12-homoarginine]glucagon (analogue B); Nα-carbamoylglucagon (analogue C); Nα,Nε-dicarbamoylglucagon (analogue D); [1-Nα-carbamoylhistidine, 12-Nε-trinitrophenyllysine]glucagon (analogue II); [1-deshistidine] [2-Nα-trinitrophenylserine, 12-homoarginine]glucagon (analogue III); and [1-Nα-trinitrophenylhistidine, 12-homoarginine]glucagon (analogue IV). The introduction of hydrophylic groups at the α- and ε-amino positions of glucagon results in a reduction in potency. The α-position is also involved in biological activity. Carbamylation of the α-position results in a partial agonist (analogues C and D). The introduction of hydrophobic groups and the neutralization of the positive charge at the α- and ε-amino positions result in glucagon antagonists (analogues II, III, and IV). [1-Nα-Trinitrophenylhistidine, 12-homoarginine]glucagon (analogue IV) is the most potent inhibitor tested. Based on its competitive inhibitory action, this analogue appears to have about one-third the affinity of glucagon for the receptor site. These modifications at the ε-amino position cause an increase in the secondary structure of the peptide (as shown by circular dichroism studies) which may be related to their biological activities.