Origin of reactive oxygen species in erythrocytes infected with Plasmodium falciparum

Transport pathways in the malaria-infected erythrocyte. Their characterization and their use as potential targets for chemotherapy

The intraerythrocytic malarial parasite is involved in an extremely intensive anabolic activity while it resides in its metabolically quiescent host cell. The necessary fast uptake of nutrients and the discharge of waste products are guaranteed by parasite-induced alterations of the constitutive transporters of the host cell and the production of new parallel pathways. The membrane of the host cell thus becomes permeable to phospholipids, purine bases and nucleosides, small non-electrolytes, anions and cations. While the new pathways are quantitatively unimportant for the translocation of a particular solute, classical inhibitors of native transporters can be used to inhibit parasite growth. Several compounds were found to inhibit effectively the new pathways and, consequently, parasite growth. The pathways have also been used to introduce cytotoxic agents. The parasitophorous membrane consists of channels that are highly permeable to small solutes and display no ion selectivity. Transport of some cations and anions across the parasite membrane is rapid and insensitive to classical inhibitors, and in some cases it is mediated by specific antiporters that respond to their respective inhibitors. Macromolecules have been shown to reach the parasitophorous space through a duct contiguous with the host cell membrane, and subsequently to be endocytosed at the parasite membrane. The simultaneous presence of the parasitophorous membrane channels and the duct, however, is incompatible with experimental evidence. No specific inhibitors have been found as yet that would efficiently inhibit transport through the channels or the duct.

Redox processes in malaria and other parasitic diseases. Determination of intracellular glutathione

The role of oxidative stress resulting from production of reactive oxygen species and/or from suppression of the cellular antioxidant capacity in parasitic infections is shortly reviewed. The experimental part of the paper deals with the glutathione (GSH)--glutathione reductase (GR) system, a cornerstone of intracellular antioxidant defence mechanisms. For studying this system in parasitic diseases such as malaria new or modified methods are required. Total glutathione comprising GSH and glutathione disulphide (GSSG) in blood samples was assayed as follows. One volume of blood (> or = 10 microliters) is mixed with two volumes of 5% sulphosalicylic acid; after centrifugation (5 min, 10000 g), 10 microliters of supernatant is taken for spectrophotometric analysis using the 5,5'-dithiobis(2-nitrobenzoate) (DTNB)-glutathione recycling assay. When compared with the original method, the procedure reported here is more sensitive, less time-consuming, avoids unfavourable pH-values and leads to a sample which when frozen is stable for months. In a pilot study, the method was applied to 14 patients suffering from malaria caused by Plasmodium falciparum. The concentrations of erythrocyte glutathione were significantly decreased in the patients (1.42 +/- 0.47 mM, mean +/- SD) when compared to age- and sex-matched controls (2.11 +/- 0.45 mM, P < 0.01). The findings are contrasted with P. falciparum cultures in vitro where glutathione levels are known to be elevated. Based on the characteristics of GR a concept of determining the redox state of single cells is introduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Hexose-monophosphate shunt activity in intact Plasmodium falciparum-infected erythrocytes and in free parasites

The hexose monophosphate shunt (HMS) produces NADPH for reductive antioxidant protection and for metabolic regulation, as well as ribose-5-phosphate needed for the synthesis of nucleic acids. Since malaria-infected red blood cells (RBC) are under endogenous oxidant stress, it was interesting to determine HMS activity in intact infected cells, as well as in free parasites. HMS activity was determined by measuring the evolution of 14CO2 from D-[1-14C]glucose in normal RBC, in intact Plasmodium falciparum-infected RBC (IRBC) and in free parasites. The HMS activity of IRBC was found to be 78 times higher than that of normal RBC. This activity increased with parasite maturation from the ring stage toward the trophozoite stage, and declined at the schizont stage. The HMS activity of the parasite contributes 82% of the total observed in the intact IRBC, and that of the host cell is increased some 24-fold. The increased reducing capacity of IRBC and free parasites were also evidenced by the larger ability for reductive accumulation of methylene blue. Since the endogenous oxidative stress is produced by the parasite digestion of the host cell's hemoglobin, inhibition of this process with protease inhibitors, by alkalinization of the parasite's food vacuole, or by the application of antimalarial drugs, resulted in 20-44% inhibition of IRBC HMS activity. A similar inhibition was observed in the presence of scavengers of oxidative radicals, uric and benzoic acids. These inhibitors had only a minor effect on the HMS activity of free parasites. D-[1-14C]glucose and D-[6-14C]glucose contributed equally to newly synthesized nucleic acids, suggesting that ribose-5-phosphate needed for this synthesis is contributed by the non-oxidative activity of HMS. These results imply that a major portion of parasite HMS activity and the activated HMS of the host cell are devoted to counteract the endogenously generated oxidative stress.