Structure-activity relationship analysis: Developing glucagon agonists and antagonists for studies of glucagon action in normal and diabetic states

Several glucagon analogues containing substitutions in the N-terminal region, in particular residues 1, 5, 6, 9 and 10 (histidine, threonine, phenylalanine, aspartic acid and tyrosine, respectively), were synthesized. In addition four β-methylphenylalanine isomers were introduced at position ten to assess the role of these topographical modifications on hormone activity, and to study the effect of constraint and biased conformational preferences of the side chain moieties on biological activity. All the analogues were synthesized by solid-phase methodology, purified to homogeneity by reverse-phase high-performance liquid chromatography, and characterized by electrospray mass spectroscopy, amino acid analysis and thin layer chromatography. Following characterization they were analyzed using rat liver plasma membranes for receptor-binding affinity as well as their ability to stimulate adenylate cyclase. Structure-activity relationship analysis provided critical information about the conformational, chemical and structural properties of amino acid residues required for receptor recognition and signal transduction in the glucagon sequence. His¹ was confirmed to operate along with Asp⁹ for the activation and binding to the glucagon receptor. These new findings should permit the design of more pure and potent glucagon receptor antagonists by focusing on the role of Phe⁶ and other residues in the N-terminal region. A newly developed assay for examining low levels of cAMP accumulation in response to glucagon antagonists, agonists and partial agonists was developed. Previously reported glucagon receptor antagonists had partial agonist activity in rat hepatocytes. This assay system, in conjunction with binding and adenylate cyclase studies in both hepatocytes and liver plasma membranes, redefines the major characteristics of pure glucagon antagonists. The most potent glucagon receptor antagonist [des-His¹, des-Phe⁶, Glu⁹]glucagon-NH₂ was studied using conformational analysis and 2D NMR techniques to analyze the secondary structure of the analogue. Proton resonance assignments using COSY, NOESY and TOCSY in d₆-DMSO were made