Testis-specific isoform of angiotensin-converting enzyme (tACE) as a candidate marker for bull fertility.

Although a traditional bull breeding soundness evaluation is designed to identify bulls that are grossly abnormal, bulls classified as satisfactory potential breeders still vary in fertility, implying submicroscopic differences in sperm characteristics. Testis-specific isozyme of angiotensin-converting enzyme (tACE) is involved in the regulation of sperm function. Therefore, the aim of the present study was to determine tACE content, activity and localisation in bull spermatozoa and their associations with fertility. Semen from low-fertility (LF) and high-fertility (HF) Holstein bulls (n=20) with known FERTSOL rates, which represents the 56-day non-return rate, were used. There was greater tACE content (P<0.05) and tACE activity (P<0.01) in HF versus LF spermatozoa. Based on immunolocalisation, tACE was either in the acrosomal or postacrosomal region of the sperm head, with HF bulls having a higher proportion of spermatozoa with tACE in the acrosomal region than LF bulls (P<0.05). tACE content, activity, localisation to the acrosomal region and progressive motility were significantly correlated with fertility and, based on regression analysis, tACE content was predictive of fertility. tACE content and activity in semen were similar between yearling (10-13 months old) and mature (3-4 years old) bulls. Therefore, tACE has potential as a marker of field fertility in bulls at their earliest possible age.