Digestion of the host erythrocyte by malaria parasites is the primary target for quinolinecontaining antimalarials

Intraerythrocytic malaria parasites feed on their host cell cytosol. We show that human red blood cells infected with the malaria parasite Plasmodium falciparum, produce free amino adds the composition of which resembles that of globin, the most abundant red blood cell protein. The rate of amino acid production is almost equal to the rate of efflux of these acids from the infected cell. Production of amino acids increases with parasite age: the rates of production at the young ring and the mature trophozoite stages were 3.3 and 13.5 nmol/10⁸ infected cells per min at 37°, respectively, compared with 0.04 nmol/10⁸ cells per min in uninfected cells. The quinoline-containing antimalarial drugs, chloroquine, quinine and mefloquine, inhibit amino acid production at the same concentrations at which they inhibit parasite growth, but have no effect on the endogenous parasite protein degradation. We suggest that parasite feeding on host cell cytosol is the primary target for the antimalarial action of these drugs. Chloroquine accumulation, the rate of amino add production by infected cells and the inhibitory effect of the drug, were determined simultaneously at the different stages of parasite development. At all stages the rate of amino add production and chloroquine accumulation were directly related and both were inversely related to the inhibitory effidency of the drug. The lysosomotropic agents methylamine and NH₄Cl at millimolar concentrations also inhibit amino add production, suggesting that the process is pH dependent and localized in the vacuole. Host cytosol degradation and drug accumulation both ake place in the parasite food vacuole. Our observations imply that the metabolically dependent acidification of this parasite organelle is involved in both processes.