CHARACTERIZATION AND LOCALIZATION OF AN ATP-DIPHOSPHOHYDROLASE ON THE EXTERNAL SURFACE OF THE TEGUMENT OF SCHISTOSOMA-MANSONI

An ATP-diphosphohydrolase (EC 3.6.1.5) was identified in the tegumental fraction isolated from Schistosoma mansoni worms. Both ATP and ADP were hydrolyzed to AMP at similar rates by the enzyme. Other nucleotides were also degraded by the tegument enzyme, revealing a broad substrate specificity. Electrophoretic separation of tegumental proteins under non-denaturing conditions followed by addition of ATP or ADP as substrate revealed a single band of activity with similar mobility. In addition, similar heat-inactivation profiles were obtained for ATPase or ADPase activities, indicating that a single enzyme is responsible for degrading both nucleotides. The enzyme was not inhibited by vanadate, levamisole, tetramisole, ouabain or sodium azide. The ADPase activity was not affected by adenosine (5')-pentaphospho-(5')-adenosine (Ap5A) or by an excess of glucose and hexokinase used as an ATP-trapping system, thus excluding the presence of any significant adenylate kinase activity. The ATP-diphosphohydrolase displayed micromolar affinities for both Mg2+ and Ca2+ and the calcium-activated enzyme was inhibited by millimolar Mg2+. In intact live worms a calcium phosphate precipitate was formed on the outer tegumental surface upon incubation of the worms with either ATP or ADP, indicating the ecto-localization of this enzyme. In addition, ultrastructural hystochemical localization of the enzyme was obtained. A distinct deposition of lead phosphate granules on the outer surface of the tegument was observed by electron microscopy, in the presence of either ATP or ADP as substrate. It is suggested that the ATP-diphosphohydrolase could regulate the concentration of purine nucleotides around the parasites and hence enable them to escape the host hemostasis by preventing ADP-induced platelet activation.