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.

Ferriprotoporphyrin IX (FP) and a chloroquine-FP complex lysed isolated Plasmodium falciparum parasites as judged by decreases in the turbidity of parasite suspensions and by ultrastructural changes. Exposure of parasite suspensions to 50 ,uM FP or to a complex formed from 50 ,M FP and 20 ,uM chloroquine reduced the number of identifiable parasites and caused swelling and loss of internal detail in those that were identifiable. The amount of lysis was dosedependent over the range of 10 to 50 ,uM FP. Formation of a chloroquine-FP complex reduced, but did not eliminate, the toxicity of FP. Since there is evidence indicating that a chloroquine-FP complex forms when chloroquine-susceptible parasites are exposed to chloroquine, we suggest that accumulation of this complex may account for the chemotherapeutic effect of chloroquine against P. falciparum.

No abstract

The effect of protease inhibitors obtained from the culture filtrates of actinomycetes (pepstatin, chymostatin, leupeptin, phosphoramidon and elastatinal) on the in vitro invasion of erythrocytes from rhesus and assamese monkeys by Plasmodium knowlesi merozoites was studied. The presence of these inhibitors had no effect on release of merozoites from schizonts but inhibited entry of the parasite into the red cell. Thus, at 50 micrograms/ml, chymostatin and leupeptin completely blocked the invasion whereas pepstatin and elastatinal showed 50% inhibition. Phosphoramidon at this concentration gave only 30% inhibition. Pretreatment of the erythrocytes with these inhibitors did not block invasion. Also, the intracellular development of the parasite was totally unaffected in the presence of these agents. These results suggested that proteases of merozoites might play some crucial role in the invasion process.

Three mouse hybridomas were isolated that produced IgM monoclonal antibodies (Mab) which reacted with erythrocytic stages of Plasmodium falciparum and inhibited the invasion of erythrocytes in vitro. Those Mab, initially identified by an ELISA screening of hybridoma culture medium, exhibited a strong binding to trophozoite and schizont but not to ring or merozoite stage parasites or to erythrocytes in an indirect immunofluorescence assay. All inhibited the parasite's ability to infect erythrocytes in an in vitro invasion inhibition assay. Western blot analysis of the binding of the Mab to SDS-PAGE-separated parasite antigens isolated from the ring, trophozoite, schizont or spontaneously released merozoite stages, indicated that two of the Mab bound to a Mr 105,000 antigen in trophozoites and schizonts while the third Mab did not. All three Mab also bound to Mr 30,000-40,000 antigens in all stages, however, in all instances binding to these antigens was enhanced in merozoites. It was further observed that the two Mabs that bound to a Mr 105,000 antigen: exhibited a markedly reduced binding to the Mr 105,000 antigen in merozoite preparations; exhibited different relative intensities of binding to the trophozoite and schizont antigens; both bound to the same Mr 105,000 antigen as demonstrated through Western blot analysis of antigens separated by two-dimensional gel electrophoresis. The findings suggest that the inhibitory Mab bound to different epitopes of the same antigen and that the antigen may either be processed or degraded at about the time of merozoite release and erythrocyte invasion.