Physiological and enzymatic properties of the ram epididymal soluble form of germinal angiotensin I-converting enzyme

The 94-kDa ram epididymal fluid form of the sperm membrane-derived germinal angiotensin 1-converting enzyme (ACE) was purified by chromatography, and some of its enzymatic properties were studied. For the artificial substrate furanacryloyl-L-phenylalanylglycylglycine (FAPGG), the enzyme exhibited a Michaelis constant (K-m) of 0.18 mM and a V-max of 34 mu moles/(min(.)mg) a nd for hippuryi-L-histidyl-L-leucine a K-m of 2.65 mM and a V-max of 163 mu moles/(min(.)mg) under the defined standard conditions (300 mM NaCl and 50 mM Tris; pH 7.5 and 8.3, respectively). The FAPGG hydrolysis was decreased by 82.5% and 67.5% by EDTA and dithioerythritol, respectively, and was tot ally inhibited by specific ACE inhibitors such as captopril, P-Glu-Trp-Pro- Arg-Pro-Glu-Ile-Pro-Pro, and lisinopril. Optimum activity for FAPGG was wit h pH 6.0, 50 mM chloride, and 500 muM zinc. Under the various conditions te sted, bradykinin, angiotensin (Ang) I, Ang II, and LHRH were competitors fo r FAPGG. Bradykinin and angiotensin I were the best competitors. The enzyme cleaved Ang I into Ang II, and the optimal conditions were with pH 7.5 and 300 mM chloride. The relationship between the carboxypeptidase activity in seminal plasma and the prediction of fertility of young rams was also stud ied. These results indicated a correlation between sperm concentration and ACE activity in semen but showed no statistically significant correlation b etween such activity and fertility of the animal.. Finally, we tested the r ole of ACE in fertilization; no difference in the in vitro fertilization ra te was observed in the presence of 10(-4) M captopril.