Enkephalin catabolism in vitro and in vivo.

Metabolism of an endogenous opiate like compound, enkephalin, was examined using high pressure liquid chromatography. The “opiate receptor” fraction of brain commonly used to measure enkephalin contains a large amount of amino-peptidase activity capable of metabolizing enkephalin but not β-endorphin. The regional distribution and kinetic parameters of this enzyme were examined. The release of tyrosine from enkephalin was at least 20 times faster than glycine or leucine. The enzyme can be inhibited by O-phenanthroline. When carrier free H enkephalin was injected intraventricularly, more than 90% was broken down within l min. The rapid metabolism of the enkephalins and related compounds should be prevented if meaningful assessments of relative potency are to be made.