STRUCTURE-ACTIVITY STUDIES ON POSITION-14 OF HUMAN ALPHA-CALCITONIN GENE-RELATED PEPTIDE

A structure-activity study was performed to examine the role of positi on 14 of human alpha-calcitonin gene-related peptide (h-alpha-CGRP) in activating the CGRP receptor. Interestingly, position 14 of h-alpha-C GRP contains a glycyl residue and is part of an alpha-helix spanning r esidues 8-18. Analogues [Ala(14)]-h-alpha-CGRP, [Aib(14)]-h-alpha-CGRP , [Asp(14)]-h-alpha-CGRP, [Asn(14)]-h-alpha-CGRP, and [Pro(14)]-h-alph a-CGRP were synthesized by solid phase peptide methodology and purifie d by RP-HPLC. Secondary structure was measured by circular dichroism s pectroscopy. Agonist activities were determined as the analogues' abil ity to stimulate amylase secretion from guinea pig pancreatic acini an d to relax precontracted porcine coronary arteries. Analogues [Ala(1)4 ]-h-alpha-CGRP, [Aib(14)]-h-alpha-CGRP, [Asp(14)]-h-alpha-CGRP, and [A sn(14)]-h-alpha-CGRP, all containing residues with a high helical prop ensity in position 14, were potent full agonists compared to h-alpha-C GRP in both tissues. Interestingly, replacement of Gly(14) of h-alpha- CGRP with these residues did not substantially increase the helical co ntent of these analogues. [Pro(14)]-h-alpha-CGRP, predictably, has sig nificantly lower helical content and is a 20-fold less potent agonist on coronary artery, known to contain CGRP-1 receptor subtypes, and an antagonist on pancreatic acini, known to contain CGRP-2 receptor subty pes. In conclusion, the residue in position 14 plays a structural role in stabilizing the alpha-helix spanning residues 8-18. The alpha-heli x is crucial for maintaining highly potent agonist effects of h-alpha- CGRP at CGRP receptors. The wide variety of functional groups that can be tolerated in position 14 with no substantial modification of agoni st effects suggests the residue in this position is not in contact wit h the CGRP receptor. [Pro(14)]-h-alpha-CGRP may be a useful pharmacolo gical tool to distinguish between CGRP-1 and CGRP-2 receptor subtypes.