Under the physiological conditions, Ca<^>2+ ions regulate sperm motility, hyperactivation, chemotaxis, acrosomal response. [Ca<^>2+] - level in cytoplasm is maintained by Ca<^>2+, Mg<^>2+-АТPase which is one of the targets of reactive oxygen species. Hyperproduction of reactive oxygen species can damage sperm cells and is considered to be one of the mechanisms of male infertility. This paper is aimed at evaluation of the H2O2 effect on the main kinetic parameters of ATP hydrolysis by Ca<^>2+, Mg<^>2+-АТPase of spermatozoa of fertile (normozoospermia) and infertility men (asthenozoospermia). Ca<^>2+, Mg<^>2+-АТPase activity has been determined spectrophotometrically by production of Pi. Concentration dependencies have been linearized in Lineweaver - Burk plot. Effective inhibitory effect of H2O2 on thapsigargin-sensitive and thapsigargin-insensitive Ca<^>2+, Mg<^>2+-АТPase activity of sperm cells in the fertile and infertile men has been demonstrated. The effects of H2O2 on the main kinetic parameters of the ATP hydrolysis with the involvement of Ca<^>2+, Mg<^>2+-АТPase has been studied. In the whole range of studied concentrations of ATP the thapsigargin-sensitive and thapsigargin-insensitive Ca<^>2+, Mg<^>2+-АТPase activity of spermatozoa in the fertile and infertile men has been reduced because of presence of H2O2 in the incubation medium. However, the optimal activity of the Ca<^>2+, Mg<^>2+-АТPase activity of sperm cells in both normozoospermic and asthenozoospermic men has been observed taking into account 5 mM of ATP in the incubation medium. By linearization of concentration curves in Lineweaver - Burk plot the main kinetic parameters of Ca<^>2+-activated, Mg<^>2+-dependent ATP hydrolysis in the sperm cells of fertile and infertile men have been determined. Under the influence of H2O2, the initial maximum rate of ATP hydrolysis by thapsigargin-sensitive Ca<^>2+, Mg<^>2+-АТPase significantly decreases only in spermatozoa of men with normozoospermia. At the same time, the affinity constant of thapsigargin-sensitive Ca<^>2+, Mg<^>2+-АТPase to ATP is significantly increased in spermatozoa of men with asthenozoospermia. A significant increase in the affinity constant of thapsigargin-insensitive Ca<^>2+, Mg<^>2+-АТPase to ATP has been found in both normozoospermic and asthenozoospermic men. Moreover, in the pathospermic men the increase in the affinity constant to ATP is more expressed than in normozoospermic men. At the same time, there is a tendency of decreasing in the maximum rate of ATP hydrolysis by thapsigargin-insensitive Ca<^>2+, Mg<^>2+-АТPase of spermatozoa of fertile men, but these changes are not significant. Under conditions of H2O2-induced oxidative stress, inhibition of Ca<^>2+, Mg<^>2+-АТPase activity in sperm cells of asthenozoospermic men occurs due to decreasing in the enzyme affinity to the substrate.

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