BIOLOGICAL FUNCTION OF PEROXIDASES 435 



second group remain colorless. The division is, however, by no means sharp, 

 and plant tissues are known which contain both polyphenol oxidase and 

 peroxidase. So far peroxidase has been studied only in plant juices and 

 breis. The Szent-Gyorgyi school assumes that hydrogen peroxide, produced, 

 for example, by the action of ascorbic acid oxidase on ascorbic acid, reacts at 

 first with polyphenols of the benzopyrane class (such as quercetin or eriodic- 

 tyol), oxidizing these with the help of peroxidase to quinones which in turn 

 oxidize ascorbic acid {lS7o). The scheme raises several unsolved problems, 

 such as competition of ascorbic acid oxidase and quinones for ascorbic acid, 

 competition of polyphenol oxidase (if present) and peroxidase for the poly- 

 phenols, and competition of peroxidase and catalase for hydrogen peroxide. 

 While it is known that catalase does not prevent peroxidative action, no 

 carefully controlled quantitative experiments have been carried out. We 

 have mentioned the fact that the interaction of pure peroxidase with ascorbic 

 acid is rapid and does not appear to need mediators. 



It is not certain that such systems are major pathways of respiration, 

 they may serve rather for the removal of hydrogen peroxide. Kursanov and 

 Kryukova (1624) have found that polar plants with high respiration contain 

 more peroxidase than plants of the warm southern districts of Russia. 



A biological role of peroxidases as dihydroxymaleic acid oxidases is still 

 more questionable. Catalase inhibits this reaction and the enzyme is 

 destroyed in it. 



The cytochrome peroxidase of yeast also can only function if it is in great 

 excess over catalase. In yeast this appears to be so, but it is still unknown 

 whether other tissues contain cytochrome peroxidase in sufficiently high 

 concentrations to compete with catalase. 



3.9.2. Animal Peroxidases. The biological significance of peroxidases in 

 animal cells is still more uncertain. It appears unlikely that milk oxidase 

 plays any biological role. Elliot (007) found nitrite, tryptophane, and tyro- 

 sine to be the only substances of biological interest in the animal body which 

 are attacked by the enzyme; the first two are not attacked by horse-radish 

 peroxidase. It is of interest to note that this peroxidase cannot be derived 

 from the peroxidase of leucocytes from which it differs chemically. 



Xo biological substrate has yet been found for myeloperoxidase. In 

 infections this enzyme is liberated from leucocytes and is found in the serum 

 (lOGG) or in empyema fluid (2G). Singer (2507) has observed a decrease in 

 the "oxidase." i.e., myeloperoxidase (cf. Agner, 26), of polymorphonuclear 

 leucocytes in infections. It is not impossible, although still unproven,that 

 the system plays a role in the detoxication of bacterial toxins.* 



A substance catalyzing the oxidation of ascorbic acid by the peroxidase 

 system more powerfully than adrenaline has been reported to be present in 

 the adrenals by Tauber (274-S). 



The peroxidase system catalyzes the formation of thyroxine from diiodo- 

 tyrosine in vitro (Harington and Rivers, 1127). In vivo it may not only form 

 the diphenyl ether linkage, but also liberate iodine from iodide and thus 



* Agner (28a) has recently shown that diphtheria and tetanus toxins are destroyed 

 by peroxidase systems. 



