A search for the natural substrates for neutral endopeptidase (NEP; EC 3.4.24.11) in the immune system led to investigation of the enzyme9s action on thymic humoral factor gamma 2 (THF). The ectoenzyme rapidly and efficiently hydrolyses the Lys6-Phe7 bond of the octapeptide. The site of cleavage was confirmed by h.p.l.c. analysis, amino acid analysis and sequence determination of the products. Phosphoramidon (3.6 microM), a potent inhibitor of the enzyme, prevents this cleavage even during prolonged incubation. The high efficiency of hydrolysis of THF by NEP is similar to that reported for [Leu5]enkephalin, and the dipeptide Phe-Leu is the C-terminal product in the hydrolysis of both peptides. The presence of NEP, reportedly identified as the common acute lymphoblastic leukaemia antigen (CALLA), in bone-marrow cells and other cells of the immune system raises the possibility that it may play a role in modulating the activity of peptides such as THF.
