Characteristics of Adenosinetriphosphatase of Ram Ejaculated Spermatozoa and Isolated Sperm Tails

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Abstract
Characteristics of adenosinetriphosphatase of ejaculated spermatozoa from three rams have been studied. Ram spermatozoa were found to hydrolyse ATP in preference to ADP as evidenced by the insignificant amounts of AMP recovered from the incubation medium containing both ATP and ADP as substrates. However, ram spermatozoa could dephosphorylate ADP alone at a 20% lower rate than ATP. With ATP as substrate the Km was 3.6 × 10−4 and inhibition was observed when the substrate concentration exceeded that of magnesium; this was attributed to possible binding of free ATP without splitting at the active sites. The magnesium dependent ATPase activity of sperm tails, isolated by ultrasonic disintegration and fractional centrifugation, was stimulated by sodium and potassium more at 25–36 C than at 5–25 C presumably as a result of alterations in the configuration of the enzyme. However, above 36 C or after preincubation at 45 C monovalent cations inhibited the magnesium dependent ATPase activity which was ascribed to alterations of the liquid-crystalline structure of lipids associated with the enzyme. The ATPase activity of whole sperm increased with total cation concentrations of up to 30 mM but was depressed by 100 mM. Magnesium was a less efficient activator than calcium. The pH optimum of divalent cation activated ATPase of isolated sperm tails in the presence of 0.1 M KCl was about 8.0. When the KCl concentration was increased to 0.5 M and 1.0 M the ATPase activity was markedly reduced and the pH optimum shifted to 7.2. Under these conditions, activation of the enzyme appeared to be dependent on the total divalent cation concentration rather than the nature of the cation. In view of these and other observations it is suggested that divalent cations modify the environment of the active centres by occupying fixed binding sites which may then influence orientation of the substrate on the enzyme.

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