Neutral metalloendopeptidase in human male genital tract. Comparison to angiotensin I-converting enzyme.

High concentrations of neutral metalloendopeptidase (NEP) (enkephalinase) were found in human male genital tract immunohistochemically and by enzyme activity assays, and its distribution was compared with that of angiotensin-converting enzyme (ACE) (kininase II). Whereas the two enzymes colocalize on the luminal aspect of proximal tubular epithelium and are not found elsewhere in the nephron, their distribution in the male genitalia is different. Seminal fluid is rich in NEP and ACE, but after ultracentrifugation ACE remains soluble while NEP sediments. NEP activity is low in testicular homogenate but high in the particulate fraction of epididymides and prostates. ACE, on the other hand, is active in the particulate fraction of testes and in the soluble fraction of epididymides and prostates. Prostatic NEP had a slightly higher molecular weight than the renal NEP, which was reduced by neuraminidase in electroblotting. Testicular and seminal plasma ACE also had a slightly higher molecular weight than the purified renal enzyme (150,000), probably caused by removal of an "anchor" peptide during purification. In the prostate, NEP was found by three different immunohistochemical techniques in luminal epithelial cells and in lumina. The function of NEP in the genital tract may be related to sperm maturation and proacrosin activation.