The article describes the identification and characterization of a 25-kDa antioxidant enzyme from yeast, named thioredoxin peroxidase (TPx). TPx is a peroxidase that reduces H₂O₂ and alkyl hydroperoxides using hydrogen donors from thioredoxin, thioredoxin reductase (TR), and NADPH. The protein consists of identical 25-kDa subunits containing two cysteine residues, Cys⁴⁷ and Cys¹⁷⁰, which form an intermolecular disulfide bond. The oxidized Cys⁴⁷ likely reacts with Cys¹⁷⁰ to form the disulfide bond. Mutant TPx proteins lacking either cysteine do not exhibit peroxidase activity. The disulfide bond of TPx is specifically reduced by thioredoxin but can also be reduced by small molecular weight thiol compounds. The authors also cloned and sequenced the Saccharomyces cerevisiae TR gene, finding 51% identity with the Escherichia coli enzyme. The study reveals that TPx functions as an antioxidant against both the ascorbate and thiol oxidation systems, with its specificity for the thiol system attributed to the ability of thiols to reduce the TPx disulfide bond more effectively than ascorbate. The purification and characterization of TPx and its components provide insights into the mechanisms of peroxide reduction and the role of TPx in cellular antioxidant defense.The article describes the identification and characterization of a 25-kDa antioxidant enzyme from yeast, named thioredoxin peroxidase (TPx). TPx is a peroxidase that reduces H₂O₂ and alkyl hydroperoxides using hydrogen donors from thioredoxin, thioredoxin reductase (TR), and NADPH. The protein consists of identical 25-kDa subunits containing two cysteine residues, Cys⁴⁷ and Cys¹⁷⁰, which form an intermolecular disulfide bond. The oxidized Cys⁴⁷ likely reacts with Cys¹⁷⁰ to form the disulfide bond. Mutant TPx proteins lacking either cysteine do not exhibit peroxidase activity. The disulfide bond of TPx is specifically reduced by thioredoxin but can also be reduced by small molecular weight thiol compounds. The authors also cloned and sequenced the Saccharomyces cerevisiae TR gene, finding 51% identity with the Escherichia coli enzyme. The study reveals that TPx functions as an antioxidant against both the ascorbate and thiol oxidation systems, with its specificity for the thiol system attributed to the ability of thiols to reduce the TPx disulfide bond more effectively than ascorbate. The purification and characterization of TPx and its components provide insights into the mechanisms of peroxide reduction and the role of TPx in cellular antioxidant defense.