Origin of reactive oxygen species in erythrocytes infected with Plasmodium falciparum.

Oxidative radicals are demonstrably produced in malaria-infected erythrocytes. In order to verify the biochemical origin of these radicals, erythrocyte lysate was brought to acid pH to mimic the environment of the parasite food vacuole into which host cell cytosol is transferred during parasite feeding. Oxyhemoglobin, but not deoxyhemoglobin, is rapidly converted to methemoglobin at rates which decline with increasing pH. The rate of conversion is further increased in the presence of the catalase inhibitor 3-amino-1,2,4-triazole (3-AT) and the extent of inhibition of the lysate catalase increases upon acidification, implying that H2O2 is thus produced by the spontaneous dismutation of superoxide radicals generated during methemoglobin formation. Intact Plasmodium falciparum trophozoite-infected human red blood cells (TRBC) were shown to produce H2O2 and OH radicals about twice as much as normal erythrocytes, as evidenced by the inhibition of endogenous catalase activity in the presence of 3-AT and the degradation of deoxyribose, respectively. Increased H2O2 levels and catalase activity were found in both host cell and parasite compartments. No increase in H2O2 production over that observed in uninfected erythrocytes could be detected at the ring stage when host cell digestion is absent. H2O2 and OH radicals production in TRBC was considerably reduced when digestion of host cell cytosol was inhibited either by antiproteases (which reduce the proteolysis of imported catalase) or by its alkalinization with NH4Cl (which reduce methemoglobin formation). These results suggest that reactive oxygen species are produced in the parasite's food vacuole during the digestion of host cell cytosol, and are able to egress from the parasite to the host cell compartment.