Acid extrusion from the intraerythrocytic malaria parasite is not via a Na⁺/H⁺ exchanger

The intraerythrocytic malaria parasite, Plasmodium falciparum maintains an intracellular pH (pH_i) of around 7.3. If subjected to an experimentally imposed acidification the parasite extrudes H⁺, thereby undergoing a pH_i recovery. In a recent study, Bennett et al. [Bennett TN, Patel J, Ferdig MT, Roepe PD. P. falciparum Na⁺/H⁺ exchanger activity and quinine resistance. Mol Biochem Parasitol 2007;153:48-58] used the H⁺ ionophore nigericin, in conjunction with an acidic medium, to acidify the parasite cytosol, and then used bovine serum albumin (BSA) to scavenge the nigericin from the parasite membrane. The ensuing Na⁺-dependent pH_i recovery, seen following an increase in the extracellular pH, was attributed to a plasma membrane Na⁺/H⁺ exchanger. This is at odds with previous reports that the primary H⁺ extrusion mechanism in the parasite is a plasma membrane V-type H⁺-ATPase. Here we present evidence that the Na⁺-dependent efflux of H⁺ from parasites acidified using nigericin/BSA is attributable to Na⁺/H⁺ exchange via residual nigericin remaining in the parasite plasma membrane, rather than to endogenous transporter activity.