The role of phenylalanine at position 6 in glucagon's mechanism of biological action: Multiple replacement analogues of glucagon

Extensive evidence gathered from structure-activity relationship analysis has identified and confirmed specific positions in the glucagon sequence that are important either for binding to its receptor or for signal transduction. Fifteen glucagon analogues have been designed and synthesized by incorporating structural changes in the N-terminal region of glucagon, in particular histidine-1, phenylalanine-6, and aspartic acid-9. This investigation was conducted to study the role of phenylalanine at position 6 on the glucagon mechanism of action. These glucagon analogues have been made by either deleting or substituting hydrophobic groups, hydrophilic groups, aromatic amino acids, or a D-phenylalanine residue at this position. The structures of the new analogues are as follows: [des-His¹,des- Phe⁶,Glu⁹]glucagon-NH₂ (1); [des-His¹,Ala⁶Glu⁹]glucagon-NH₂ (2); [des- His¹,Tyr⁶,Glu⁹]glucagon-NH₂ (3); [des-His¹,Trp⁶,Glu⁹]-glucagon-NH₂ (4); [des-His¹,D-Phe⁶,Glu⁹]glucagon-NH₂ (5); [des- His¹,Nl3⁶,Glu⁹]glucagon-NH₂ (6); [des-His¹,Asp⁶,Glu⁹]glucagon-NH₂ (7); [des-His¹,des-Gly⁴,Glu⁹]glucagon-NH₂ (8); [des-Phe⁶,- Glu⁹]glucagon-NH₂ (9); [des-Phe⁶]glucagon-NH₃ (10); [des-His¹-,des- Phe⁶]glucagon-NH₂ (11); [des-His¹,des-Phe⁶,Glu⁹]glucagon (12); [des- Phe⁶,Glu⁹]glucagon (13); des-Phe⁶]glucagon (14); and [des-His¹,des- Phe⁶]glucagon (15). The receptor binding potencies IC₅₀ values are 48 (1), 126 (2), 40 (3), 19 (4), 100 (5), 48 (6), 2000 (7), 52 (8), 113 (9), 512 (10, 128 (11) 1000 (12), 2000 (13), 500 (14), and 200 nM (15). All analogues were found to be antagonists unable to activate the adenylate cyclase system even at concentrations as high as 10^-4 M except for analogues 6 and 8, which were found to be weak partial agonists/partial antagonists with maximum stimulation between 6-12%. In competitive inhibition experiments, all the analogues caused a right shift of the glucagon-stimulated adenylate cyclase dose-response curve. The pA₂ values were 8.20 (1), 6.40 (2), 6.20 (3), 6.25 (4), 6.30 (5), 6.30 (7), 6.05 (8), 6.20 (9), 6.30 (10), 6.25 (11), 6.10 (12), 6.20 (13), 6.20 (14), and 6.35 (15).