The relationship between enkephalin degradation and opiate receptor occupancy.

Enkephalin, a brain peptide with morphine-like activities, is rapidly inactivated by a membrane-associated aminopeptidase present in brain. Evidence presented here suggests that enkephalin binding to opiate receptors is coupled to subsequent aminopeptidase degradation. This conclusion is based upon the following observations. 1) Enkephalin degradation, at concentrations 4 orders of magnitude below the Km, proceeds at essentially the same rate whether or not the peptide is bound to receptors; 2) opiates, such as morphine and naloxone as well as enkephalin, have no direct effect upon the rate of aminopeptidase action at the concentrations used in our experiments. Thus, receptor binding may serve as a concentration mechanism which ensures the efficient inactivation of enkephalin by a nearby peptidase after it has interacted with the receptor. The aminopeptidase functions optimally at. neutral pH and obeys Michaelis-Menten kinetics over at least 5 orders of magnitude of substrate concentration (K,,, for methionine enkephalin is 4.5 x 10m5 M). Many peptides, such as angiotensin, bacitracin, bradykinin, somatostatin, and substance P are inhibitors of the enzyme. However, the most effective inhibitor (Kt = 2 x 10e7 M) is the peptide analogue, puromycin. This property of puromycin may find application in studies of the physiological role of enkephalins and other brain peptides.