pH Regulation in the Intracellular Malaria Parasite, Plasmodium falciparum

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 blockerN,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.