Structure-activity studies of hydrophobic amino acid replacements at positions 9, 11 and 16 of glucagon

We have designed and synthesized eight compounds 2-9 which incorporate neutral, hydrophobic amino acid residues in positions 9, 11 and 16 of the glucagon molecule: (2) [desHis¹,Val⁹,Ile^11,16] glucagon amide, (3) [desHis¹,Val^9,11,16]glucagon amide, (4) [desHis¹,Val⁹,Leu^11,16]glucagon amide, (5) [desHis¹,Nle⁹,Ile^11,16]glucagon amide, (6) [desHis¹,Nle⁹,Val^11,16]glucagon amide, (7) [desHis¹,- Nle⁹,Leu^11,16]glucagon amide. (8) [desHis¹,Val⁹,Leu^11,16,Lys^17,18,Glu²¹]glucagon amide and (9) [desHis¹,Nle⁹,Leu^11,16,Lys^17,18,Glu²¹]glucagon amide. The effect of neutral, hydrophobic residues at positions 9, 11 and 16 led to good binding to the glucagon receptor. Compared to glucagon (IC₅₀ = 1.5 nM), analogues 2-9 were found to have IC₅₀ values of 6.0, 6.0, 11.0, 9.0, 2.5, 2.8, 6.5 and 7.0 nM, respectively. When these compounds were tested for their ability to block adenylate cyclase (AC) activity, they were found to be antagonists having no stimulation of adenyl cyclase, with pA₂ values of 6.15, 6.20, 6.30, 7.25, 6.10, 7.30, 6.25 and 7.25, respectively.