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

Abstract

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 14CO 2 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 cells are devoted to counteract the endogenously generated oxidative stress.