Neuropeptide enzyme hydrolysis in human saliva

The possible hydrolysis of neuropeptides by human saliva was studied using leucine enkephalin as a model. The data obtained indicate that in the presence of saliva this substrate is partially hydrolysed, and that its disappearance corresponds to the appearance of peptides whose composition is consistent with that of the substrate hydrolysis by-products. The formation of these peptides indicates the presence of all three classes of enzymes known to hydrolyse enkephalins in other tissues: aminopeptidases, dipeptidylaminopeptidases and dipeptidylcarboxypeptidases. The activity of these enzymes appears to be altered by the presence of low molecular-weight substances, whose inhibitory activity is apparent on all three classes of enkephalin-degrading enzymes. Substrate degradation was higher in male than female saliva; these differences appear to be caused by lower activity of the enzymes, and higher activity of the low molecular-weight inhibitors, measurable in female as compared to male saliva.