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any of a group of enzymes of the oxidoreduc-tase class that catalyze the hydrogen-peroxide-mediated or organic-peroxide-mediated oxidation of various polyphenols and aliphatic and aromatic amines, as well as of fatty acids, cytochromes, and glutathione. The peroxidases whose substrates are fatty acids, cytochromes, and glutathione are called fatty-acid peroxidase, cytochrome peroxidase, and glutathione peroxidase, respectively. Depending on the source from which they are isolated, peroxidases vary in molecular weight and substrate specificity. The most widely distributed and well-studied peroxidase is of plant origin, usually obtained from horseradish; its molecular weight is 44,100. The prosthetic group of plant peroxidases resembles heme, which is the prosthetic group of hemoglobin; indeed, hemoglobin does exhibit slight peroxidase activity. Plant peroxidases are important in respiration: together with polyphenol oxidase, they catalyze the oxidative conversion of respiratory chromogens into respiratory pigments.
Small amounts of peroxidase are found in animals, and only in white blood cells and milk. Horseradish peroxidase is used in the laboratory to determine the glucose level of blood and urine and to mark lysosomes in cytochemical studies.