Enkephalin dipeptidyl carboxypeptidase (enkephalinase) activity: selective radioassay, properties, and regional distribution in human brain

Abstract

The compound [3H]Tyr1,D-Ala2,Leu-OH5]enkephalin has been synthesised as a potentially selective substrate for enkephalin dipeptidyl carboxypeptidase (enkephalinase) activity in brain. Incubations in the presence of homogenates and particulate fractions from rodent and human brain result in the formation of [3H]Tyr-D-Ala-Gly, which can be conveniently isolated by polystyrene bead column chromatography. The enzyme activity responsible for the hydrolysis of the Gly3-Phe4 amide bond of this substrate displays close resemblance to that hydrolysing the natural enkephalins at the same level. In addition, enkephalinase activity characterised in postmortem human brain is closely similar to that in rodent brain, with regard to optimal pH and apparent affinities of various substrates and inhibitors, including the potent compound thiorphan. Enkephalinase activity is distributed in a highly heterogeneous fashion among regions of human brain, the highest levels being found in globus pallidus and pars reticulata of the substantia nigra. This distribution is poorly correlated with that of opiate receptor binding sites but displays some resemblance to that of reported Met5-enkephalin levels.