Enzymes and inhibitors in leu-enkephalin in metabolism in human plasma.

The enzymes degrading leucine enkephalin in human plasma and the inhibitors active on these enzymes were studied by kinetic and chromatographic techniques. Data obtained evidence the existence of complex kinetics of leu-enkephalin hydrolysis and of formation of its hydrolysis by-products. These appear to originate from the combined effect of further hydrolysis of the enkephalin's fragments after their release and of competition between the different enzymes present in plasma. Chromatographic separation of plasma proteolysis inhibitors indicates the existence of several pools of substances acting on all three enzyme groups that degrade leu-enkephalin. The partial specificity of these substances induces competition effects: consequently, the actual protection over leu-enkephalin is considerably lower that the total inhibitory activity. That notwithstanding, plasma inhibitors control enkephalin hydrolysis to a relevant extent, while they modify only slightly the ratio of hydrolysis between the different enzymes. This latter parameter--and specifically the large prevalence of aminopeptidases over dipeptidylaminopeptidases and dipeptidylcarboxypeptidases--appears controlled mainly by kinetic factors.