Synthesis and biological evaluation of a metazocine-containing enkephalinamide. Evidence for nonidentical roles of the tyramine moiety in opiates and opioid peptides

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Abstract
To test the hypothesis that the tyramine moiety present in opiates and in opioid peptides plays an identical functional role at opioid receptors, a hybrid enkephalinamide that contains (-)-metazocine in place of Tyr1 was synthesized. The hybrid and its congeners are inactive or feebly active in the electrically stimulated guinea pig ileum and mouse vas deferens preparations. The tyramine moiety in opiates and related structures evidently does not play the same functional role as that in the opioid peptides. The different functional roles of the tyramine moiety in opiates and opioid peptides may be a consequence of different modes of interaction with common receptors.