Neuropeptides and their peptidases: Functional considerations

The properties of the various brain membrane peptidases capable of hydrolysing released neuropeptides are reviewed, with particular emphasis on endopeptidase-24.11 and angiotensin converting enzyme. The substrate specificities of both enzymes are defined and their relative contribution to the degradation of tachykinins in vitro are considered. One approach to assessing the physiological roles of identified peptidases involves examining the protective effect of selective peptidase inhibitors on the degradation of peptides released from brain slices. This procedure has been applied to study the release of substance P-like immunoreactivity from slices of rat substantia nigra. Inhibition of endopeptidase-24.11, but not of angiotensin converting enzyme, produces a significant increase in recovery of substance P. The specificity and distribution of endopeptidase-24.11 would therefore not be inconsistent with a role in the physiological inactivation of tachykinins, as well as enkephalins. At peripheral sites, LHRH and atrial natriuretic peptide may be important substrates of the enzyme. The endogenous neuropeptide substrate(s) for striatal angiotensin converting enzyme remain unclear.