We evaluated the antimalarial effects of vinyl sulfone cysteine proteinase inhibitors. A number of vinyl sulfones strongly inhibited falcipain, a Plasmodium falciparum cysteine proteinase that is a critical hemoglobinase. In studies of cultured parasites, nanomolar concentrations of three vinyl sulfones inhibited parasite hemoglobin degradation, metabolic activity, and development. The antimalarial effects correlated with the inhibition of falcipain. Our results suggest that vinyl sulfones or related cysteine proteinase inhibitors may have promise as antimalarial agents.

Erythrocytic malaria parasites degrade hemoglobin as a principal source of amino acids for parasite protein synthesis. We have previously shown that a Plasmodium falciparum trophozoite cysteine proteinase, now termed falcipain, is required for hemoglobin degradation, and we have hypothesized that this proteinase is responsible for initial cleavages of hemoglobin. To further evaluate the biological role of falcipain, we expressed the enzyme in bacterial and viral expression systems. After expression in the baculovirus system, falcipain was enzymatically active and had biochemical properties very similar to those of the native proteinase. Recombinant falcipain rapidly hydrolyzed both denatured and native hemoglobin. Hemoglobin hydrolysis was blocked by cysteine proteinase inhibitors but not by inhibitors of other classes of proteinases. Our results support our hypothesis that falcipain is a critical malarial hemoglobinase that is responsible for both initial cleavages of hemoglobin and the subsequent hydrolysis of globin into small peptides.