The thioredoxin system of the malaria parasite Plasmodium falciparum - Glutathione reduction revisited

In most living cells, redox homeostasis is based both on the glutathione an d the thioredoxin system. In the malaria parasite Plasmodium falciparum ant ioxidative proteins represent promising targets for the development of anti parasitic drugs. We cloned and expressed a thioredoxin of P. falciparum (pf trx), and we improved the stable expression of the thioredoxin reductase (P fTrxR) of the parasite by multiple silent mutagenesis. Both proteins were b iochemically characterized and compared with the human host thioredoxin sys tem. Intriguingly, the 13-kDa protein PfTrx is a better substrate for human TrxR (K-m = 2 muM, K-cat = 3300 min(-1)) than for P. falciparum TrxR (K-m = 10.4 muM, k(cat) = 3100 min(-1)). Possessing a midpoint potential of -270 mV, PfTrx was found to reduce the disease-related metabolites S-nitrosoglu tathione and GSSG;. The rate constant Fz, for the reaction between reduced P. falciparum thioredoxin and GSSG was determined to be 0.039 muM(-1) min(- 1) at 25 degreesC and pH 7.4. The k(2) for thioredoxins from man, Drosophil a melanogaster, and Escherichia coli was similar to5 times lower. Our data suggest that GSSG reduction can be supported at a high rate by the TrxR/Trx system in glutathione reductase-deficient cells; this may be relevant for certain stages of the malarial parasite but also for cells containing high [GSSG] of other organisms like dormant forms of Neurospora, glutathione red uctase-deficient yeast mutants, or CD4(+) lymphocytes of AIDS patients.