December 2007 - The Indian Society for Parasitology

94 Banyal and Sharmaneither Glu-Cys nor GSH can traverse the parasitemembrane. This allows the parasite to efficientlyengage in de novo glutathione synthesis without losingthe intermediate or the final products and to makeabundant use of the reducing capacity of GSH (Roth,1987).Glutathione and hemoglobin digestion: Hemoglobinis degraded by Plasmodium during intraerythrocyticstages and in the process haem is released (Banyal andFitch, 1982). Haem is a toxic molecule to malariaparasite. The parasite detoxifies haem bypolymerization and degradation. The parasiteconverts almost 30% of haem into haemozoin ormalarial pigment, while rest of the haem is degradedby reduced glutathione. Glutathione degrades haemwhether it is free in solution or it is boundnonspecifically to protein by conjugation. Glutathionemetabolism is therefore, essential for the survival ofmalaria parasite (Ginsburg et al., 1998; Padmanabanet al., 2007).Fig. 3. Trypanothione.synthesis of GSH and its subsequent conjugation tospermidine. GSH is synthesized by two enzymes,which are common with malarial parasite, while theconjugation of GSH to spermidine is catalyzed by twotrypanothione specific enzymes (Opperdoes andCoombs, 2007). Since this thiol is absent from humansand is essential for the survival of these parasites, theenzymes that make and use this molecule are targetsfor the development of new drugs to treat the diseasescaused by these parasitic protozoa (Schmidt andKrauth-Siegel, 2002). Trypanothione-dependentenzymes include reductases, peroxidases, glyoxalasesand transferases.ENZYMES OF GLUTATHIONE METABOLISMIN PARASITIC PROTOZOA: Although themetabolic pathway has not been investigated in anydetail, -glutamylcysteine synthetase (Lueder andPhillips, 1996; Luersen et al., 2000), glutathionesynthetase (Meierjohann et al., 2002a), glutathione-S-transferases (Harwaldt et al., 2002) and glutathionereductase (Farber et al., 1996) have been extensivelystudied in various protozoa, which suggests that aglutathione metabolic pathway similar to mammals ispresent in these parasites. Such studies may perhapshelp in structure based approaches to the developmentof antimalarials (Brady and Cameron, 2004).1. γ-glutamylcysteine synthetase(γ-GCS;glutamate-cysteine ligase EC 6.3.2.2): γ-GCScatalyses the first and rate limiting step in glutathionebiosynthesis. The gene for γ-GCS has been isolatedand characterized in various parasitic protozoa(Lueder and Phillips, 1996; Luersen et al., 2000). γ-GCS activity was 3.76-fold higher in P. berghei ascompared to normal mice erythrocytes and 1.4-foldhigher compared to P. berghei infected erythrocytes(our unpublished data).Glutathione and drug resistance: Most of theantimalarial drugs interact with haem and form acomplex, which is highly toxic to biologicalmembranes (Fitch et al., 1982). Glutathione competeswith antimalarial drugs for degrading haem toxin, andis thus associated with drug resistance. Growthinhibition of P. falciparum chloroquine-sensitivestrain (3D7) by buthionine (S,R)- sulfoximine (BSO, aspecific inhibitor of -GCS) and by methylene blue, aninhibitor of GR, was significantly more pronouncedthan inhibition of P. falciparum CQ-resistant strain(Dd 2) growth by antimalarial drugs (Meierjohann etal., 2002a). Also in vivo studies on P. berghei and P.vinckei-infected mice showed that drugs such asacetaminophen, indomethacin and disulphiram,which produce an indirect decrease in GSH, potentiatethe antimalarial action of the sub-curative doses of CQand amodiaquine (Deharo et al., 2003; Zuluaga et al.,2007).Glutathione metabolism and trypanosomatids: Inplace of glutathione, trypanosomatids includingcausative agents of African sleeping sickness(Trypanosoma brucei gambiense), Chagas disease (T.cruzi) and different leishmanial parasites utilizetrypanothione, a conjugate of GSH and spermidine, tomaintain the redox balance of the cells (Fairlamb et al.,1985). It is an unusual form of glutathione containingtwo molecules of glutathione joined by a spermidine(polyamine) linker (Fig. 3). Trypanothione intrypanosomatids is synthesized in four steps via theCOOHONHNHH 2 NNHOOSHNHOSHOH 2 NNHNHNHCOOHO