OXIDASES 399 



the action is either absent or very rare. In such cases she 

 finds the direct action to be due to pyrocatechin which, on 

 exposure to air, rapidly oxidizes and then acts as an organic 

 peroxide, thus enabling the peroxidase, which is almost uni- 

 versally present, to transfer oxygen to the oxidizable substance. 



From later investigations this same author* concludes 

 that the direct oxidase reaction is due to the presence of a 

 peroxidase (which will blue guaicum only in the presence of 

 hydrogen peroxide) and an aromatic substance having a cate- 

 chol grouping. On injuring the tissues, the peroxidase acti- 

 vates the oxidation of the aromatic compound with the 

 formation of a peroxide. The system thus formed, peroxide 

 peroxidase, will then blue the guaicum. The presence of 

 tannins may inhibit or mask the reaction. 



Reed f finds that oxidases in general when purified to the" 

 extreme limit still give the catalase reaction, i.e. split hydrogen 

 peroxide into water and oxygen, and that the peroxidase re- 

 action is quite independent of the amount of hydrogen perox- 

 ide decomposed. He believes that the peroxidase combines 

 with oxygen from hydrogen peroxide or from oxygenase to 

 form an intermediate compound, which is a more potent oxi- 

 dizing agent than the original source of oxygen. It is this 

 intermediate compound which effects the final oxidation 

 changes. Since in certain plants, e.g. the pine-apple, the per- 

 oxidase reaction is independent of any ability to decompose 

 hydrogen peroxide, no catalase is present in certain stages of 

 development of the fruit, he concludes that the substances 

 which bring about the decomposition of hydrogen peroxide 

 are not necessarily concerned with the enzymes which accelerate 

 peroxide oxidations. 



According to Ewart | there is no justification for the dis- 

 tinction drawn between oxidase and peroxidase since their 

 supposed fractional precipitation by means of alcohol is merely 

 due to the attenuation of the solution. The organic oxidases 

 are proteins which may or may not be combined with metals. 

 They vary according to their strength ; a strong solution will 

 bring about direct oxidation from the oxygen held in solution, 

 whilst the weak can only transfer oxygen from compounds 



* Onslow : " Biochem. Journ.," 1919, 13, i. 



t Reed : " Bot. Gaz.," 1916, 61, 523 ; 62, 53, 233, 303, 409. 



Ewart: "British Assoc. Rep.," 1915. 



