SYNTHESIS, BIOLOGICAL-ACTIVITY, AND CONFORMATIONAL-ANALYSIS OF [PGLU(6),N-MEPHE(8),AIB(9)] SUBSTANCE-P (6-11) - A SELECTIVE AGONIST FOR THENK-3 RECEPTOR

A highly potent and selective agonist to the tachykinin NK-3 receptor, [pGlu6,N-Me-Phe8,Aib9] substance P (6-11) (I), was synthesized via th e solid phase method. The ED50 of I was 4 nM in the guinea pig ileum i n the absence of atropine (NK-1+ NK-3 receptors) and this agonist was 5000-fold less potent in the presence of atropine (NK-1 receptor). The analogue was virtually inactive in the rat vas deferens (NK-2 recepto r). A detailed analysis of the solution conformation of this analogue in DMSO-d6 and in a DMSO-d6/H2O cryomixture was carried out by a combi nation of H-1-nmr 2D techniques (DQF-COSY, TOCSY, NOESY and ROESY) and model building based on empirical energy calculations. Peptide I exis ts as a mixture of isomers containing cis and trans Phe-N-MePhe peptid e bonds. The main isomer, containing a cis Phe-N-MePhe peptide bond, s hows a preferred folded conformation characterized by a type VI beta-t urn with Phe and N-MePhe in the i + 1 and i + 2 positions. The turn is followed by a helical segment extending to the C-terminal. This confo rmation is compared to previously reported conformations of other sele ctive tachykinin agonists and may be a promising lead for the design o f novel NK-3 agonists with additional conformational constraints.