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J. Biol. Chem., Vol. 278, Issue 9, 6809-6815, February 28, 2003

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A Second Class of Peroxidases Linked to the Trypanothione Metabolism^*

Henning Hillebrand, Armin Schmidt, and R. Luise Krauth-Siegel

From the Biochemie-Zentrum Heidelberg, Universität Heidelberg, 69120 Heidelberg, Germany

Trypanosoma brucei, the causative agent of African sleeping sickness, has three nearly identical genes encoding cysteine homologues of classical selenocysteine-containing glutathione peroxidases. The proteins are expressed in the mammalian and insect stages of the parasite. One of the genes, which contains a mitochondrial as well as a glycosomal targeting signal has been overexpressed. The recombinant T. brucei peroxidase has a high preference for the trypanothione/tryparedoxin couple as electron donor for the reduction of different hydroperoxides but accepts also T. brucei thioredoxin. The apparent rate constants k₂' for the regeneration of the reduced enzyme are 2 × 10⁵ M¹ s¹ with tryparedoxin and 5 × 10³ M¹ s¹ with thioredoxin. No saturation kinetics was observed and the rate-limiting step of the overall reaction is reduction of the hydroperoxide. With glutathione, the peroxidase has marginal activity and reduction of the enzymes becomes limiting with a k₂' value of 3 M ¹ s¹. The T. brucei peroxidase, in contrast to the related Trypanosoma cruzi enzyme, also accepts hydrogen peroxide as substrate. The catalytic efficiency of the peroxidase studied here is comparable with that of the peroxiredoxin-like tryparedoxin peroxidases, which shows that trypanosomes possess two distinct peroxidase systems both dependent on the unique dithiol trypanothione.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AJ298281.

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