Pathways for the reduction of oxidized glutathione in the Plasmodium falciparum-infected erythrocyte: can parasite enzymes replace host red cell glucose-6-phosphate dehydrogenase?

Abstract

Plasmodium falciparum-infected human red cells possess at least two pathways for the generation of reduced nicotinamide adenine dinucleotide phosphate (NADPH): (1) the glucose-6-phosphate dehydrogenase (G6PD) pathway and (2) the glutamate dehydrogenase (GD) pathway using glutamate as a substrate. Uninfected erythrocytes lack the GD pathway. The NADPH generated can be used to reduce oxidized glutathione (GSSG), which accumulates in the presence of an oxidative stress. In red cell G6PD deficiency, this pathway is reduced or absent, and the host cells as well as the parasites within them are vulnerable to oxidant stress. In view of the presence of the GD pathway in parasitized red cells and the recent description of a parasite-derived G6PD enzyme, we have asked whether the pathways for the reduction of GSSG provided by the parasite can substitute for the host G6PD in red cells deficient in G6PD activity. We have devised a functional assay in which the reduction rate of GSSG is monitored in the presence of buffered infected or control red cell lysates and substrates. Infected G6PD-deficient erythrocytes were obtained from in vitro cultures after a single prior growth cycle of the parasites in G6PD deficient cells to eliminate contaminating normal red cells. The results show that only parasitized red cells can reduce GSSG via the GD pathway. In parasitized G6PD Mediterranean red cells (completely G6PD-deficient), there is a detectable GSSG reduction via the G6PD pathway, not found in uninfected lysates from the same individual. In G6PD A- (African type, featuring partial deficiency), a small increment in the G6PD-dependent reduction of GSSG can also be detected. However, when compared to G6PD normal red cells, the activities from the parasite-derived pathways are small and could not be considered substitutes for normal host enzyme activity. It is concluded that while the plasmodium provides additional pathways for the generation of NADPH that may serve its own metabolic needs, the host red cells and hence the parasite itself remain vulnerable to oxidant stress.