An acid-loading chloride transport pathway in the intraerythrocytic malaria parasite, Plasmodium falciparum

The intraerythrocytic malaria parasite exerts tight control over its ionic composition. In this study, a combination of fluorescent ion indicators and ³⁶Cl^- flux measurements was used to investigate the transport of Cl^- and the Cl^--dependent transport of "H⁺-equivalents" in mature (trophozoite stage) parasites, isolated from their host erythrocytes. Removal of extracellular Cl^-, resulting in an outward [Cl^-] gradient, gave rise to a cytosolic alkalinization (i.e. a net efflux of H⁺-equivalents). This was reversed on restoration of extracellular Cl^-. The flux of H ⁺-equivalents was inhibited by 4,4′-diisothiocyanostilbene-2, 2′-disulfonic acid and, when measured in ATP-depleted parasites, showed a pronounced dependence on the pH of the parasite cytosol; the flux was low at cytosolic pH values < 7.2 but increased steeply with cytosolic pH at values > 7.2. ³⁶Cl^- influx measurements revealed the presence of a Cl^- uptake mechanism with characteristics similar to those of the Cl^--dependent H⁺-equivalent flux. The intracellular concentration of Cl^- in the parasite was estimated to be ∼48 mM in situ. The data are consistent with the intraerythrocytic parasite having in its plasma membrane a 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid-sensitive transporter that, under physiological conditions, imports Cl ^- together with H⁺-equivalents, resulting in an intracellular Cl^- concentration well above that which would occur if Cl^- ions were distributed passively in accordance with the parasite's large, inwardly negative membrane potential.