Probing the shape of a hydrophobic pocket in the active site of delta-opioid antagonists

The change of selectivity and the induction of antagonism by the insertion of 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) in the second pos ition of several opioid peptides have led to the interpretation of Tyr-Tic as a specific message domain for delta -opioid antagonists and to the disco very of dipeptides with substantial opioid activity. Selectivity and activi ty increase enormously when Tyr is substituted by 2'6'-dimethyl tyrosine (D mt), hinting that the side chain of Dmt fits a hydrophobic cavity of the re ceptor very tightly and precisely. We have investigated the specificity of this fit by systematic changes of the substituents on the aromatic ring of Tyr. Mono- and disubstitutions different from 2',6'- invariably lead to cat astrophic decreases of activity. The only substitution compatible with rete ntion of substantial antagonism is T-methyl. An analysis of the conformatio nal properties of all analogues reveals that substitutions do not affect th e global shape of the molecule significantly. Accordingly, it is possible t o use the shape of the different side chains to map the hydrophobic cavity of the receptor. The resulting complementary image is funnel shaped. Copyri ght (C) 2001 European Peptide Society and John Wiley & Sons, Ltd.