The Putative Glutathione Peroxidase Gene of Plasmodium falciparum Codes for a Thioredoxin Peroxidase

Abstract

A putative glutathione peroxidase gene (Swiss-Prot accession number Z 68200) of Plasmodium falciparum, the causative agent of tropical malaria, was expressed in Escherichia coli and purified to electrophoretic homogeneity. Like phospholipid hydroperoxide glutathione peroxidase of mammals, it proved to be monomeric. It was active with H2O2 and organic hydroperoxides but, unlike phospholipid hydroperoxide glutathione peroxidase, not with phosphatidylcholine hydroperoxide. With glutathione peroxidases it shares the ping-pong mechanism with infinite Vmax and Km when analyzed with GSH as substrate. As a homologue with selenocysteine replaced by cysteine, its reactions with hydroperoxides and GSH are 3 orders of magnitude slower than those of the selenoperoxidases. Unexpectedly, the plasmodial enzyme proved to react faster with thioredoxins than with GSH and most efficiently with thioredoxin of P. falciparum (Swiss-Prot accession number 202664). It is therefore reclassified as thioredoxin peroxidase. With plasmodial thioredoxin, the enzyme also displays pingpong kinetics, yet with a limiting Km of 10 μM anda k1′ of 0.55 s -1. The apparent k1′ for oxidation with cumene, t-butyl, and hydrogen peroxides are 2.0 × 104 M-1 s-1, 3.3 × 103 M-1 s-1, and 2.5 × 103 M-1 s-1, respectively, k 2′ for reduction by autologous thioredoxin is 5.4 × 104 M-1 s-1 (21.2 M-1 s-1 for GSH). The newly discovered enzymatic function of the plasmodial gene product suggests a reconsideration of its presumed role in parasitic antioxidant defense.