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The traditional assessment of stallion sperm comprises evaluation of sperm motility and membrane integrity and identification of abnormal morphology of the spermatozoa. More recently, the progressive introduction of flow cytometry is increasing the number of tests available. However, compared with other sperm structures and functions, the evaluation of mitochondria has received less attention in stallion andrology. Recent research indicates that sperm mitochondria are key structures in sperm function suffering major changes during biotechnological procedures such as cryopreservation. In this paper, mitochondrial structure and function will be reviewed in the stallion, when possible specific stallion studies will be discussed, and general findings on mammalian mitochondrial function will be argued when relevant. Especial emphasis will be put on their role as source of reactive oxygen species and in their role regulating sperm lifespan, a possible target to investigate with the aim to improve the quality of frozen-thawed stallion sperm. Later on, the impact of current sperm technologies, principally cryopreservation, on mitochondrial function will be discussed pointing out novel areas of research interest with high potential to improve current sperm technologies.

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The aim of this study was to characterize angiotensin-converting enzyme (ACE) in canine testis. Detergent-extracted canine testes were sonicated in the presence of protease inhibitors and purified on an affinity column with the ACE inhibitor, lisinopril, as an affinity ligand for ACE. The fractions recovered were assessed for ACE enzyme activity via an enzyme kinetic microplate assay (at 330 nm) based on the hydrolysis of Fa-Phe-Gly-Gly (FAPGG) at pH 7.5 during an 8 min incubation. The specific activity of ACE in the starting testicular extracts was 3.53 +/- 0.99 mU mg(-1) protein with a 1588 times enrichment in ACE activity after lisinopril affinity chromatography (4239 +/- 2600 mU mg(-1) protein). The recovery efficiency of ACE after lisinopril affinity chromatography was 71.2%. The ACE activity in the detergent extracts and the purified fractions was inhibited significantly by 10 micromol captopril l(-1), a specific ACE inhibitor, and was restored to 88% of normal activity by the addition of the thiol-alkylating agent N-ethylmaleimide (0.5 mmol l(-1)) in the detergent extracts and the purified fractions incubated with captopril. The treatment of testicular extracts with 10 mmol EDTA l(-1) reduced the ACE activity significantly (5.40 +/- 1.26 versus 0.58 +/- 0.23 mU mg(-1)). The ACE activity was restored fully in the presence of zinc (5.28 +/- 0.70 mU mg(-1)). The anti-ACE antibody (raised against a 70 kDa protein from the periacrosomal plasma membrane of equine spermatozoa) recognized a 65-70 kDa protein in the detergent-extracted testes as well as in the affinity-purified fractions. This antibody also recognized a protein of similar molecular mass in ejaculated spermatozoa. ACE was localized in the periacrosomal area of the ejaculated spermatozoa and in spermatids in the seminiferous tubules. The results of this study demonstrate that ACE is present in canine testis and retains its enzyme activity after purification with lisinopril affinity chromatography. Activity of canine ACE is inhibited by captopril and EDTA and is restored in the presence of N-ethylmaleimide and zinc.