科研之友
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摘要
The mitochondrion of sea urchin sperm is located at the base of the sperm head, and the flagellum extends from the mitochondrion for similar to 40 mu m. These sperm have two known flagellar, non-mitochondrial, enzymatic systems to rephosphorylate ADP. The first involves the phosphocreatine shuttle, where flagellar creatine kinase (Sp-CK) uses phosphocreatine to rephosphorylate ADP. The second system, studied in this report, is adenylate kinase (Sp-AK), which uses 2 ADP to make ATP + AMP. Cloning of Sp-AK shows that, like Sp-CK, Sp-AK has three catalytic domains. Sp-AK localizes along the entire flagellum, and most of it is tightly bound to the axoneme. Sp-AK activity and flagellar motility were studied using demembranated sperm. The specific Sp-AK inhibitor Ap5A blocks enzyme activity with an IC50 of 0.41 mu M. In 1 mM ADP, flagella reactivate motility in 5 min; 1 mu M Ap5A completely inhibits this reactivation. No inhibition of motility occurs in Ap5A when 1 mM ATP is added to the reactivation buffer. The pH optimum for Sp-AK is 7.7, an internal pH at which sperm are fully motile. The pH optimum for Sp-CK is 6.7, an internal pH at which sperm are immotile. In isolated, detergent-permeabilized flagella, assayed at pH 7.6, the K-m for Sp-AK is 0.32 mM and the V-max is 2.80 mu M ATP formed/min/mg of protein. When assayed at pH 7.6, the Sp-CK K-m is 0.25 mM and the V-max 5.25. At the measured in vivo concentrations of ADP of 114 mu M, at pH 7.6, the axonemal Sp-AK could contribute similar to 31%, and Sp-CK 69%, of the total non-mitochondrial ATP synthesis associated with the demembranated axoneme. Thus, Sp-AK could contribute substantially to ATP synthesis utilized for motility. Alternatively, Sp-AK could function in the removal of ADP, which is a potent inhibitor of dynein ATPase.
- 出版日期2007-2-2
