pH regulation in the intracellular malaria parasite, Plasmodium falciparum. : H+ extrusion via a V-type H+-ATPase

The mechanism by which the intra-erythrocytic form of the human malaria parasite, Plasmodium falciparum, extrudes H⁺ ions and thereby regulates its cytosolic pH (pH_i), was investigated using saponin-permeabilized parasitized erythrocytes. The parasite was able both to maintain its resting pH_i and to recover from an imposed intracellular acidification in the absence of extracellular Na⁺, thus ruling out the involvement of a Na⁺/H⁺ exchanger in both processes. Both phenomena were ATP-dependent. Amiloride and the related compound ethylisopropylamiloride caused a substantial reduction in the resting pH(i) of the parasite, whereas EMD 96785, a potent and allegedly selective inhibitor of Na⁺/H⁺ exchange, had relatively little effect. The resting  pH_i of the parasite was also reduced by the sulfhydryl reagent N-ethylmaleimide, by the carboxyl group blocker N,N'- dicyclohexylcarbodiimide, and by bafilomycin A₁, a potent inhibitor of V- type H⁺-ATPases. Bafilomycin A₁ blocked pH(i) recovery in parasites subjected to an intracellular acidification and reduced the rate of acidification of a weakly buffered solution by parasites under resting conditions. The data are consistent with the hypothesis that the malaria parasite, like other parasitic protozoa, has in its plasma membrane a V-type H⁺-ATPase, which serves as the major route for the efflux of H⁺ ions.