Control of alkaline phosphatase activity and pH stability by taurine in liquid boar semen

Alkaline phosphatase (ALP) is an enzyme present in various tissues and plays an important role in biochemical processes, by catalyzing the hydrolysis of organic phosphates and energy metabolism. Despite significant ALP activity in seminal plasma across species, its role in sperm function remains unclear. Several studies have revealed the important role of taurine in male reproductive functions, due to its antioxidant and membrane-stabilizing activity, and ability to enhance sperm quality. Therefore, this study examined the influence of taurine on ALP activity and pH stability in liquid boar semen during storage. Boar spermatozoa were exposed to different concentrations of ALP (0-5 IU/mL), and sperm motility, viability, acrosome integrity, and ALP activity were examined. In addition, liquid boar semen with varying concentrations of taurine (10-80 mM) was stored at 17°C for 7 days, and the ALP activity, pH level, and fertilization competence of spermatozoa were investigated through the storage period. Higher ALP activity was detected in fresh spermatozoa compared with capacitated spermatozoa. Motility, viability, and acrosome integrity decreased significantly in sperm incubated with 1-5 IU/mL ALP. An immunofluorescence assay revealed that ALP was localized on the acrosome, equatorial segment, and tail, and the fluorescence intensity indicated that ALP levels gradually decreased during storage. When the pH of liquid boar semen was maintained at 7.4 during storage, it showed higher ALP activity and sperm quality compared to sperm stored in a medium where pH was not maintained at a stable level. Also, when boar spermatozoa were stored in the diluent containing taurine, the pH and ALP activity were stable during the storage period. The generation of intracellular reactive oxygen species decreased in sperm stored with taurine, and higher levels of sperm motility and viability were observed. The expression of mRNA associated with fertilization competence was higher in the sperm stored with 40 mM taurine compared to that of sperm stored without taurine. Thus, it can be concluded that ALP activity and pH stability are crucial for maintaining sperm quality during liquid boar semen storage, and these factors can be regulated by the addition of taurine, suggesting its applicability in assisted reproductive techniques in mammals.