Malaria parasites isolated from mouse erythrocytes are lysed by ferriprotoporphyrin IX chloride (hemin) or by a chloroquine-hemin complex in amounts that could be produced by release of less than 0.1 percent of the heme in erythrocytic hemoglobin. This effect of hemin may explain the protection against malaria provided by thalassemia and other conditions causing intracellular denaturation of hemoglobin. The toxicity of the chloroquine-hemin complex may explain the selective antimalarial action of chloroquine.

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The ferriprotoporphyrin IX (FP) molecules released by intraerythrocytic malaria parasites during hemoglobin digestion are converted to β-hematin and are stored in the parasites' food vacuoles. It has been demonstrated in cell-free medium that the incorporation of FP into β-hematin under physiological conditions requires a catalyst from parasite lysates or pre-formed β-hematin. In the present studies, lysates of Plasmodium falciparum-infected erythrocytes were suspended in 1 M NaOH and were washed with phosphate buffer, pH 7.6. When the cell extracts were incubated with hematin in 0.5 M sodium acetate buffer, pH 5, for 20 hr at 37°C, a large quantity of β-hematin was formed. To determine whether parasite components were necessary for the β-hematin formation, normal erythrocyte ghosts were similarly treated with 1 M NaOH and then incubated with hematin. In repeated experiments it was found that, on the average, 70% of the hematin was converted to β-hematin. Membranes treated with HCI or CH3COOH also promoted the formation of β-hematin, while untreated membranes were ineffective. The possibility that metabolic activities in the food vacuoles of malaria parasites may activate membrane fragments, from hemoglobin vesicles, to promote β-hematin formation is discussed in this paper.