The addition of PPi promoted the acidification of a subcellular compartment in cell homogenates of Toxoplasma gondii tachyzoites, implying the presence of a proton-translocating pyrophosphatase. The proton gradient was collapsed by addition of the K+/H+ antiporter nigericin, and was also inhibited by addition of the PPi analogue aminomethylenediphosphonate (AMDP). Both proton transport and PPi hydrolysis were dependent upon K+, but Na+ caused partial inhibition of these activities. PPi hydrolysis was sensitive in a dose-dependent manner to AMDP, imidodiphosphate, NaF and to the thiol reagent N-ethylmaleimide. This activity was unaffected by common inhibitors of phosphohydrolases, except that NaO3V (sodium orthovanadate) stimulated the activity by 87%. Immunofluorescence microscopy, using antisera raised against conserved peptide sequences of a plant vacuolar pyrophosphatase, suggested that the pyrophosphatase in T. gondii tachyzoites was located in the plasma membrane and intracellular vacuoles of the parasite. High-field 31P-NMR spectroscopy showed that PPi was more abundant than ATP in tachyzoites. Bisphosphonates (PPi analogues), drugs that are used in the treatment of bone diseases, inhibited proton transport and PPi hydrolysis in tachyzoite homogenates, and also inhibited intracellular proliferation of tachyzoites in tissue culture cells.

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