Covalent modification of the permeability pathways induced in the human erythrocyte membrane by the malarial parasite Plasmodium falciparum

Abstract

The intraerythrocytic malarial parasite Plasmodium falciparum induces permeation pathways in the plasma membrane of its host, the red blood cell. The pathways display porelike properties with selectivity toward anions and neutral molecules. They are shown here to be susceptible to chemical modification by 4,4′-diisothiocyano-2,2′-dihydrostilbene disulfonic acid (H₂DIDS), an amino-reactive reagent which is impermeant to uninfected cells. At pH 7.4 the reagent affected transport only marginally while freely entering into infected cells and reacting with intracellular hemoglobin. On the other hand, at pH above 8.5, the compound blocked the pathways efficiently (IC₅₀ = 50 μM, at 37°C for 10 min) as judged by four criteria: (1) selective lysis of infected erythrocytes in the presence of isotonic polyols; (2) uptake of [¹⁴C] sorbitol into infected cells; (3) uptake of the fluorescent anion Nbd-taurine into infected cells under conditions in which the native anion transport system was inhibited; and (4) labeling of intracellular hemoglobin by the permeating reagent [³H]H₂DIDS. The inhibitory effect was observed only with mature forms of parasitized cells, i.e., from the trophozoite stage and onward, while the pathways of immature ring forms were refractive. However, when the probe was incorporated into the interior of hemoglobin-depleted resealed ghosts prepared from ring forms, it was found to inhibit the pore-mediated transport. On the basis of these and other studies we postulate that the H₂DIDS-sensitive sites on the pathways are endofacial, thus requiring penetration of the probe (probably through the same pathway) for their inactivation. Labeling studies with the radiolabeled modifier implicate 120-Kd, 63-Kd, and/or 51-Kd polypeptides as candidates for the pore components.