The role of the Tyr10-Ser11-Lys12-Tyr13 region of glucagon in the binding interaction and activation of the glucagon receptor was investigated by means of the synthetic glucagon analogues [Phe13]glucagonamide (2), [Phe10]glucagonamide (3), [Phe10]glucagon (4), [Phe10,13]glucagon (5), [Pro11]glucagon (6), [Pro11,Gly12]glucagonamide (7), [Ala11 glucagon (8), and [Oac11-13]glucagonamide (9). These analogues were synthesized by solid-phase peptide synthesis on p-methylbenzhydrylamine or Merrifield resins with protected Nα-tert-butyloxycarbonyl amino acids. Purification by dialysis, cation-exchange chromatography, gel filtration, and preparative reverse-phase high-performance liquid chromatography (HPLC) gave products that proved homogeneous by thin-layer chromatography and HPLC and on analysis by amino acid analysis, by sequencing, and by a-chymotryptic peptide mapping with HPLC. Biological activities were examined by measurement of the stimulation of liver plasma membrane adenylate cyclase and by specific displacement of [125I]glucagon from glucagon receptors. The results of these studies indicate that while the biological “message” region of glucagon is located elsewhere, the 10–13 region has multiple roles in the glucagon-glucagon receptor interaction: (1) this region provides functional groups for direct binding interaction with the receptor, and (2) this region interacts with the receptor in such a way as to allow the “transduction message” portion of glucagon to interact and activate the receptor.
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