OXIDASES 499 



ordinary practice. The former are characterized by giving the 

 so-called direct action with an alcohoHc tincture of guaiacum, 

 blueing it at once ; * the peroxidases, on the other hand, only 

 give an indirect action, i.e. they only produce a blue with 

 guaiacum after the addition of a little hydrogen peroxide. 

 A difficulty in the way of recognising a fundamental difference 

 between oxidases and peroxidases based upon their behaviour 

 towards guaiacum, is the fact that no distinction between 

 these two groups is noticed when they are allowed to act upon 

 the following reagents : benzidene, a-naphthol, and p- 

 phenylene diamine, each dissolved in i per cent strength in 

 50 per cent alcohol ; in each case no colour results until 

 after hydrogen peroxide has been added, a fact which suggests 

 that neither oxidases nor peroxidases are sufficiently strong 

 to oxidise these substances without the assistance of hydrogen 

 peroxide (but see p. 500). 



GENERAL CONSIDERATIONS. 



Up to comparatively recent times an oxidase was con- 

 sidered to be a single enzyme, but according to Bach and 

 Chodat,t what used to be termed oxidase is really a mixture 

 of peroxidase and peroxide. According to them, there are 

 three categories of oxidizing enzymes : — 



[a] Oxygenases which produce the peroxide. 



{h) Peroxidases which transfer oxygen from peroxides to 

 the substance to be oxidized. 



[c) Catalases which act specifically upon hydrogen peroxide 

 with evolution of oxygen. 



In the colour reactions mentioned above two actions are 

 possible. Either the plant juice, e.g. of the potato, gives the 

 blue coloration with the guaiacum tincture alone, or, the blue 

 colour will not occur, as, for example, in the sap of the 

 cucumber or of the horse radish, unless a peroxide, such as 

 hydrogen peroxide, be added. 



On Bach and Chodat's hypothesis, there are present in 



* It should be noted that the presence of a large quantity of tannin 

 or sugar may interfere with the reaction and inhibit the production of the 

 blue colour. 



t Bach and Chodat : " Biochem. Centrbl.," 1903, i, 416. Bach: 

 " Ber. deut. chem. Gesells.," 1906, 39, 2126 ; 1907, 40, 230 ; 1908, 41, 216. 



32 * 



