Abstract
A Mg 2+-dependent ecto-ATP diphosphohydrolase (ATPDase, EC 3.6.1.5) was present in starfish ovarian follicle cells. The enzyme which had an optimum pH between 6.0 and 7.5 could hydrolyze triphosphonucleosides (ATP, dATP, GTP, CTP, UTP and ITP) and diphosphonucleosides (ADP, dADP, GDP, CDP, UDP and IDP), but not monophosphonucleosides. The K m values for ATP and ADP were 0.17 and 0.20 mM, respectively. The ATPDase activity was insensitive to specific ATPase inhibitors, such as ouabain, vanadate and oligomycin, an adenylate kinase inhibitor, P 1, P 5-di-(adenosine-5′)pentaphosphate, and a phosphodiesterase inhibitor, 1-isobutyl-3-methylxanthine. The activity was mostly distributed in the membrane fraction of follicle cells. It is also interesting that hydrolysis of ATP and ADP occurred upon incubation of intact cells in seawater. Among various kinds of detergents, digitonin was capable of solubilizing the enzyme from the membrane fraction. Using digitonin extract, a high-performance liquid chromatography with Superose 6 column showed that the molecular weight of ecto-ATPDase in starfish follicle cells was ∼60 000.
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