Divalent cation dependence and inhibition of Schistosoma mansoni ATP diphosphohydrolase by fluorosulfonylbenzoyl adenosine

Hydrolysis of ATP or ADP catalyzed by the ATP diphosphohydrolase of Schistosoma mansoni tegument was measured in the presence of different cations. ATP diphosphohydrolase was stimulated by micromolar concentrations of either Ca2+ or Mg2+, Ca2+ producing threefold higher maximal activities than Mg2+. Kinetic studies indicated that Ca2+ and Mg2+ compete for the same binding site on the enzyme. The effect of covalent labeling of ATP diphosphohydrolase with the ATP analog fluorosulfonylbenzoyl adenosine (FSO(2)BzAdo) was studied. Schistosome tegument was passed through with Sephadex G-50 filtration centrifugation columns to remove endogenous nucleotides, and this was followed by labeling with FSO(2)BzAdo. Incubation of ATP diphosphohydrolase with 1 mM FSO(2)BzAdo for 1 h inhibited ATPase or ADPase activities by 60% and 50%, respectively. Addition of ATP together with FSO(2)BzAdo provided greater than 90% protection against FSO(2)BzAdo inactivation, indicating that FSO(2)BzAdo binds to an ATP-binding site on the ATP diphosphohydrolase. Furthermore, addition of FSO(2)BzAdo to a medium containing intact worms caused 30% and 50% inhibition of ATPase and ADPase activities, respectively, indicating that the ATP-binding site of diphosphohydrolase is accessible to FSO(2)BzAdo from the external surface of S. mansoni worms.