Biochemical markers of stallion sperm quality (review)

Seminal plasma is a multicomponent fluid that serves as a vehicle for delivering spermatozoa to the oocyte. This fluid transports male gametes and provides their protection and nutrition during further movement in the female genital tract (T.R. Talluri et al., 2017). Thereby, understanding the effect of the components of seminal plasma on reproductive cells, as well as the search for markers of cryo-resistance and sperm fertility is undoubtedly interesting for researchers. Because stallion sperm lifespan, capacitation capacity, and fertility vary widely between individuals, it is important for horse breeding to investigate the factors that influence these parameters. The purpose of our review is to analyze current publications on the study of biochemical markers that characterize sperm quality and to consider methods for determining reactive oxygen species and oxidative stress products in spermatozoa and seminal plasma. Metabolites of seminal plasma, enzymes activity in it, indicators of oxidative stress and antioxidant defense system can serve as biochemical markers of sperm quality (S. Pesch et al., 2006). To ensure motility, spermatozoa need a large amount of ATP. Monosaccharides and organic acids such as lactate, pyruvate, citrate, succinate are good energy substrates for these cells. This gives rise to interest in them as markers of fertility (C.R. Darr et al., 2016; E.B. Menezes et al., 2019; M.F. Lay et al., 2001). The concentration of nitric oxide (II) metabolites is another promising indicator for assessing the quality of stallions sperm, since it plays an important role in the regulation of sperm motility and capacitation and the fertilization process (M.B. Herrero et al., 2000; P.T. Goud et al., 2008; F. Francavilla et al., 2000). Among enzymes of seminal plasma, lactate dehydrogenase, alanine and aspartate aminosferases, γ-glutamyl transpeptidase are of interest. While semen analysis is considered the gold standard for diagnosing male fertility, it cannot detect the molecular abnormalities that are responsible for unexplained cases of male infertility. Currently, oxidative stress is considered one of the main causes of such phenomena. It damages sperm proteins, lipids and DNA, which in turn leads to poor embryo implantation and a decrease in pregnancy rates. In this review, the main producers of free radicals in sperm and the antioxidant defense system of male gametes as well as methods for the determination of reactive oxygen species and end products of oxidative stress in spermatozoa and seminal plasma are considered (A. Agarwal et al., 2003; S. Bisht et al., 2017; H. Sies, 2018). Based on the literature data, it was concluded that biochemical markers, such as seminal plasma metabolites, enzyme activity in it, and indicators of oxidative stress, have significant potential for characterizing stallion sperm quality.