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 position of several opioid peptides have led to the interpretation of Tyr-Tic as a specific message domain for delta-opioid antagonists and to the discovery of dipeptides with substantial opioid activity. Selectivity and activity increase enormously when Tyr is substituted by 2',6'-dimethyl tyrosine (Dmt), hinting that the side chain of Dmt fits a hydrophobic cavity of the receptor very tightly and precisely. We have investigated the specificity of this fit by systematic changes of the substituents on the aromatic ring of ryr. Mono- and disubstitutions different from 2',6'- invariably lead to catastrophic decreases of activity. The only substitution compatible with retention of substantial antagonism is 2-methyl. An analysis of the conformational properties of all analogues reveals that substitutions do not affect the global shape of the molecule significantly. Accordingly, it is possible to use the shape of the different side chains to map the hydrophobic cavity of the receptor. The resulting complementary image is funnel shaped.