Ultrastructural localization of angiotensin-converting enzyme in ejaculated human spermatozoa

Angiotensin-converting enzyme (ACE) is known to be released from human spermatozoa during capacitation. However, it has not yet been localized ultrastructurally in ejaculated sperm cells. Therefore, the purpose of the present study was to demonstrate the location of ACE by means of immunoelectron microscopy and direct immunofluorescence. In addition, ACE activity of spermatozoa was correlated with standard semen parameters. The activity of angiotensin-converting enzyme was measured in spermatozoa from 115 donors and patients attending the andrological outpatient department. Progressive motility was negatively correlated with sperm ACE activity (Spearman rank correlation r=-0.364, P < 0.0001), whereas no statistically significant correlations with sperm concentration, total motility and morphology were observed. Immunoelectron microscopy demonstrated that ACE is mainly located at the plasma membrane of the acrosomal region, equatorial segment, postacrosomal region and midpiece. In contrast, only weak ACE-like immunoreactivity was found at the flagellum. In cases of cells with missing plasma membranes ACE seems also to be located at the surface of the outer acrosomal membrane. By means of immunohistochemical methods, different patterns of ACE-like immunofluorescence were observed: (i) fluorescence of the acrosome or the entire sperm head, midpiece and flagellum; (ii) fluorescence of the postacrosomal region, midpiece and flagellum; (iii) bright fluorescence of the equatorial segment with less intensive labelling of the postacrosomal region and flagellum. Induction of the acrosome reaction by calcium ionophore A23187 resulted in an increase of spermatozoa with weak acrosomal fluorescence, indicating loss of the plasma membrane.