LAUSR

We hypothesized that the testis‐specific isoform of angiotensin‐converting enzyme (tACE) is released during bovine sperm capacitation, and its peptidase activity is required for capacitation. Specific objectives of this study were to (i) develop an anti‐tACE antibody; (ii) characterize expression of tACE in bovine testes and sperm; and (iii) determine the role of tACE in capacitation. A 110‐kDa protein, consistent with the mass of tACE, was detected in sperm extract by our anti‐tACE immunoserum. This immunotarget localized at the acrosomal region and principal piece, but was only expressed in testis of mature bulls. When bull sperm were incubated in Sp‐TALP (0 and 4 hr) plus 10 µg/ml heparin (capacitation group) or 10 µg/ml heparin + 10 µM captopril (an ACE inhibitor) for 4 hr, the number of acrosome‐reacted (40.1 vs. 24.0%, respectively) and hyperactivated (15.0 vs. 9.7%) sperm increased, and tyrosine phosphoprotein content were higher (p < 0.05) for sperm in heparin alone. tACE activity was also higher (0.04 U/ml; p < 0.01) in incubation medium of sperm exposed to heparin compared to 0‐ and 4‐hr incubation controls or heparin + captopril conditions (0, 0.005, and 0.009 U/ml, respectively). Furthermore, capacitation‐associated shedding of a portion of tACE into the medium decreased sperm content of the 110‐kDa tACE, but concurrently increased the abundance of a 60‐kDa tACE variant. Thus, a portion of the extracellular region of tACE (containing its catalytic site) is released from bovine sperm during capacitation, and tACE activity may be required for sperm capacitation.