The enkephalins and opiates: structure-activity relations.

The X-ray structures of 9 "opiate" drugs which exhibit a range of pharmacological activity have been examined in detail leading to the theory that one of the reasons why the enkephalins and related peptides possess morphine-like activity is because they have a tyrosine, and hence a "tyramine", residue at the amino terminal position. This residue or a conformationally similar moiety, can be shown to be present in many opiates and analogues.