Binding characteristics and analgesic activity of D-ALA2-Leu5-enkephalin chloromethyl ketone

The chloromethyl ketone derivative of D-Ala2-Leu5-enkephalin (DALECK) was synthesized and its potency was tested in competing for 3H-naloxone binding sites and inducing analgesia. It was established that the compound is a potent affinity reagent at alkaline pH, blocking selectively and irreversibly the high-affinity (KD less than 1 nM) binding site. Intracisternally given DALECK showed a long-lasting, dose-dependent antinociceptive effect in the rat tail-withdrawal test. This could be completely antagonized by naloxone administration showing the reversible nature of DALECK in this in vivo assay. It is suggested that DALECK binds reversibly to the morphine receptor which mediates analgesia but irreversibly to the enkephalin receptor, the function of which remains to be elucidated.