Properties of permeation pathways induced in the human red cell membrane by malaria parasites

The intracellular development of malaria parasites in mature erythrocytes imposes on the host cell a major demand for supply of nutrients and disposal of waste products. So as to cope with these demands, the erythrocyte membrane undergoes profound alterations in its basic permeability properties. A few hours after being invaded by Plasmodium falciparum parasites, and before any structural changes are apparent on the surface, the molecular traffic across the red cell membrane changes both in intensity and in composition of permeating substances. The changes are of a gradual nature, developmentally related and dependent on de novo protein synthesis, but do not occur concurrently for all the classes of permeants. Molecules which permeate very poorly into uninfected cells, such as hexitols (e.g., sorbitol and myoinositol), amino acids (e.g., glutamine, threonine, and histidine), a variety of organic acids and metal ions show a marked increase in their permeation rates across the host cell membrane. Likewise, substances whose normal permeation pathways conform with those of facilitated diffusion (e.g., hexoses, nucleosides, choline, and some amino acids), gain access into the host cytosol either by modified or additional permeation pathways. It has been proposed that three major new pathways are induced in the membrane of infected cells: (1) one of pore-like properties, which can accomodate most of the water soluble permeants, including anionic substances; (2) a protein-lipid interface, which can accomodate compounds of relatively higher hydrophobic character; and (3) modified constitutive transporters or modified lipid surroundings with altered transport activities. The pores are blocked by permeant bioflavonoid glycosides whose sites of binding are endofacial, and amount to less than a thousand per cell. In addition to serving as specific targets for transport blockers, the new sites of permeation can also serve as routes for enhanced delivery of cytotoxic agents into parasitized cells.