Inhibition of the Ecto-ATPdiphosphohydrolase of Schistosoma mansoni by Thapsigargin

Abstract

ATPdiphosphohydrolases (ATPDases) are ubiquitous enzymes capable ofhydrolyzing nucleoside di- and triphosphates. Although a number ofpossible physiological roles have been proposed for ATPDases, detailedstudies on structure-function relationships have generally been hamperedby the lack of specific inhibitors of these enzymes. We have previouslycharacterized a Ca²⁺-activated ATPDase on the external surface ofthe tegument of Schistosoma mansoni, the etiologic agent of humanschistosomiasis. In the present work, we have examined the effectsof thapsigargin, a sesquiterpene lactone known as a high affinityinhibitor of sarco-endoplasmic reticulum calcium transport (SERCA)ATPase, on ATPDase activity. Whereas other lactones tested had littleor no inhibitory action, thapsigargin inhibited ATP hydrolysis by theATPDase (K _i∼20 μM). Interestingly, hydrolysis of ADP was notinhibited by thapsigargin. The lack of inhibition of ATPase activityby orthovanadate, a specific inhibitor of P-type ATPases, and theinhibition of the Mg²⁺-stimulated ATP hydrolysis by thapsigarginruled out the possibility that the observed inhibition of the ATPDaseby thapsigargin could be due to the presence of contaminating SERCAATPases in our preparation. Kinetic analysis indicated that a singleactive site in the ATPDase is responsible for hydrolysis of both ATPand ADP. Thapsigargin caused changes in both V _max and K _m for ATP,indicating a mixed type of inhibition. Inhibition by thapsigarginwas little or not affected by changes in free Ca²⁺ or Mg²⁺concentrations. These results suggest that interaction of thapsigarginwith the S. mansoni ATPDase prevents binding of ATP or its hydrolysisat the active site, while ADP can still undergo catalysis.