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.
Keywords: bull sperm; capacitation; tACE.
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