A comparative study on the effect of chloroquine and ammonium chloride on feeding process of Plasmodium falciparum in vitro

Relationship of chloroquine-induced redistribution of a neutral aminopeptidase to hemoglobin accumulation in malaria parasites

To study the relationship between neutral aminopeptidase activity and hemoglobin accumulation in malaria parasites, we treated mice infected with Plasmodium berghei NYU-2 with chloroquine intraperitoneally in doses ranging from 0.3 to 3 micromol per 25 g mouse. Preparations of infected erythrocytes (normalized to represent 1000 parasites per 1000 erythrocytes) hydrolyzed 1200 nmol of leucine-p-nitroanilide per minute per milliliter of packed erythrocytes, which was 10x more than that of uninfected preparations. The activity in infected preparations was distinguished by resistance to ferriprotoporphyrin IX and puromycin and susceptibility to inhibition by ethanol and Tris. Chloroquine treatment caused the activity in unwashed membrane ghosts of infected preparations to decrease by 50% despite an increase in total activity. Concomitantly, hemoglobin in washed membrane ghosts increased. Electron microscopy revealed that the hemoglobin was retained in endocytic vesicles. Chloroquine-induced redistribution of a neutral aminopeptidase may be the cause of hemoglobin accumulation in endocytic vesicles of malaria parasites.

Intraerythrocytic Plasmodium falciparum utilizes only a fraction of the amino acids derived from the digestion of host cell cytosol for the biosynthesis of its proteins

It is generally accepted that intraerythrocytic malaria parasites digest hemoglobin to supply the amino acids needed for the synthesis of their own proteins. This view has never been quantitatively tested. In this investigation we have measured the degradation of hemoglobin and the increase in parasite protein content as a function of parasite maturation in cultures of Plasmodium falciparum. Defined parasite stages were obtained either from tightly synchronized cultures or from asynchronous cultures after density-fractionation. We showed that both hemoglobin digestion and total parasite protein content increased with parasite maturation, from the early trophozoite stage onwards, although the total protein content of the parasite remained significantly lower than that of other eukaryotes. The parasite digested up to 65% of the host cell's hemoglobin but utilized only up to about 16% of the amino acids derived from hemoglobin digestion. This large discrepancy is profoundly puzzling particularly in view of the need to detoxify the cell from the large quantities of ferriprotoporphyrin IX and iron released during hemoglobin digestion.

Origin of reactive oxygen species in erythrocytes infected with Plasmodium falciparum

Oxidative radicals are demonstrably produced in malaria-infected erythrocytes. In order to verify the biochemical origin of these radicals, erythrocyte lysate was brought to acid pH to mimic the environment of the parasite food vacuole into which host cell cytosol is transferred during parasite feeding. Oxyhemoglobin, but not deoxyhemoglobin, is rapidly converted to methemoglobin at rates which decline with increasing pH. The rate of conversion is further increased in the presence of the catalase inhibitor 3-amino-1,2,4-triazole (3-AT) and the extent of inhibition of the lysate catalase increases upon acidification, implying that H2O2 is thus produced by the spontaneous dismutation of superoxide radicals generated during methemoglobin formation. Intact Plasmodium falciparum trophozoite-infected human red blood cells (TRBC) were shown to produce H2O2 and OH radicals about twice as much as normal erythrocytes, as evidenced by the inhibition of endogenous catalase activity in the presence of 3-AT and the degradation of deoxyribose, respectively. Increased H2O2 levels and catalase activity were found in both host cell and parasite compartments. No increase in H2O2 production over that observed in uninfected erythrocytes could be detected at the ring stage when host cell digestion is absent. H2O2 and OH radicals production in TRBC was considerably reduced when digestion of host cell cytosol was inhibited either by antiproteases (which reduce the proteolysis of imported catalase) or by its alkalinization with NH4Cl (which reduce methemoglobin formation). These results suggest that reactive oxygen species are produced in the parasite's food vacuole during the digestion of host cell cytosol, and are able to egress from the parasite to the host cell compartment.

Chloroquine: mechanism of drug action and resistance in Plasmodium falciparum

Quinoline-containing drugs such as chloroquine and quinine have had a long and successful history in antimalarial chemotherapy. Although these drugs are known to accumulate by a weak base mechanism in the acidic food vacuoles of intraerythrocytic trophozoites and thereby prevent hemoglobin degradation from occurring in that organelle, the mechanism by which their selective toxicity for lysosomes of malaria trophozoites is achieved has been subject to much discussion and argument. In this review the recent discovery that chloroquine and related quinolines inhibit the novel heme polymerase enzyme that is also present in the trophozoite food vacuole is introduced. The proposal that this inhibition of heme polymerase can explain the specific toxicity of these drugs for the intraerythrocytic malaria parasite is then developed by showing that it is consistent with much of the disparate information currently available. The clinical usefulness of chloroquine, and in some recent cases of quinine as well, has been much reduced by the evolution and spread of chloroquine resistant malaria parasites. The mechanism of resistance involves a reduced accumulation of the drug, although again the mechanism involved is controversial. Possible explanations include an energy-dependent efflux of preaccumulated drug via an unidentified transmembrane protein pump, or an increase in vacuolar pH such that the proton gradient responsible for drug concentration is reduced. New data are also presented which show that heme polymerase isolated from chloroquine resistant trophozoites retains full sensitivity to drug inhibition, consistent with the observation that resistance involves a reduced accumulation of the drug at the (still vulnerable) target site. The significance of this result is discussed in relation to developing new strategies to overcome the problem presented by chloroquine resistant malaria parasites.

Inhibition of hemoglobin degradation in Plasmodium falciparum by chloroquine and ammonium chloride

The effect of chloroquine (CQ) and NH4Cl on hemoglobin degradation within Plasmodium falciparum was studied by SDS gel electrophoresis. CQ inhibited hemoglobin digestion such that accumulation of hemoglobin in the parasites occurred. Quantitative analysis indicated that the content of hemoglobin in isolated CQ-treated parasites was increased from 2.39 +/- 0.47 micrograms hemoglobin (Hb)/mg protein to 8.34 +/- 0.77 micrograms Hb/mg protein (P less than 0.001) within 45 min and further to 18.7 +/- 1.23 micrograms Hb/mg protein for 2 hr compared with the untreated parasites. These results suggest that the inhibition of hemoglobin degradation in malarial parasites might be the primary target of CQ antimalarial action. The CQ-like effect on the hemoglobin digestion in P. falciparum was observed with another lysosomotropic weak base, NH4Cl, suggesting that the CQ effect on hemoglobin degradation and its antimalarial action, as well as the effect of NH4Cl, are related to their properties of lysosomotropic weak base.