Synthesis and hormonal activity of [Tyr22] glucagon and [desHis1, Tyr22] glucagon

[Tyr22] glucagon and [desHis1, Tyr22] glucagon were synthesized by an improved solid phase procedure on a Pam-resin. The course of the synthesis was monitored by quantitative ninhydrin analysis and preview sequencing. Following cleavage by the low/high HF method the peptides were purified by ion exchange chromatography and reverse phase HPLC. The overall yield of homogeneous isolated peptide from the first amino acid was 41%. Circular dichroism measurements on dilute solutions in mixed aqueous organic solvents at pH 2, 6.9 and 9.2 showed increased beta-sheet structure relative to glucagon. [Tyr22] glucagon was a full agonist with 20-30% activity in the rabbit blood glucose assay and 10% activity in the rat liver membrane adenyl cyclase assay. [desHis1, Tyr22] glucagon had only a trace of activity in the adenyl cyclase assay (less than 0.002%) but bound to membranes in a competitive [125I] glucagon assay 1.0% as well as glucagon. The analog completely inhibited formation of cAMP by natural glucagon, with 50% inhibition at a ratio of 83:1 and pA2 = 6.7. The data are discussed in terms of models of glucagon structure in dilute solution.