Comparison of proteases from chloroquine-sensitive and chloroquine-resistant strains of plasmodium falciparum

An aminopeptidase and four hemoglobin-degrading acid proteases have been isolated from cloned strains of chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. Aminopeptidases from both strains showed similar properties including molecular weights of 63,000 and noncompetitive inhibition by chloroquine; K i = 535 and 410 μ M for enzymes from the sensitive and resistant strains respectively. The acid proteases from the chloroquine-sensitive strain included a low molecular weight enzyme in the soluble fraction (protease S), an enzyme weakly associated with membrane (protease M 2 ), and two enzymes strongly associated with membrane (proteases M 3 and M 4 ). The acid proteases from the chloroquine-resistant strain included protease S, protease M 2, a second enzyme weakly associated with membrane (protease M 1 ), and protease M 3. All of the acid proteases were inhibited by ferriprotoporphyrin IX and by the chloroquine-ferriprotoporphyrin IX complex, I 50 = 5–25 μ M. The data were consistent with a model for chloroquine action wherein chloroquine acts to divert ferriprotoporphyrin IX from sequestration into malarial pigment, leaving ferriprotoporphyrin IX (or its chloroquine complex) to interfere with digestion of host cytosol by inhibiting hemoglobin-degrading proteases. However, the similarities among the proteases from chloroquine-sensitive and chloroquine-resistant strains of parasites suggest that chloroquine resistance does not result from changes in parasite proteases.