THIOREDOXIN-DEPENDENT PEROXIDE REDUCTASE FROM YEAST

A 25-kDa antioxidant enzyme that provides protection against oxidation systems capable of generating reactive oxygen and sulfur species has previously been identified. The nature of the oxidant eliminated by an d the physiological source of reducing equivalents for, this enzyme, h owever, were not known. The 25-kDa enzyme is now shown to be a peroxid ase that reduces H2O2 and alkyl hydroperoxides with the use of hydroge ns provided by thioredoxin, thioredoxin reductase, and NADPH. This pro tein is the first peroxidase to be identified that uses thioredoxin as the immediate hydrogen donor and is thus named thioredoxin peroxidase (TPx). TPx exists as a dimer of identical 25-kDa subunits that contai n 2 cysteine residues, Cys(47) and Cys(170). Cys(47)-SH appears to be the site of oxidation by peroxides, and the oxidized Cys(47) probably reacts with Cys(170)-SH of the other subunit to form an intermolecular disulfide. Mutant TPx proteins lacking either Cys(47) or Cys(170), th erefore, do not exhibit thioredoxin-coupled peroxidase activity. The T Px disulfide is specifically reduced by thioredoxin, but can also be r educed (less effectively) by a small molecular size thiol. The Sacchar omyces cerevisiae thioredoxin reductase gene was also cloned and seque nced, and the deduced amino sequence was shown to be 51% identical wit h that of the Escherichia coli enzyme.