394 ENZYMES 



oxide, the test tube being kept at a constant temperature 

 whilst the experiment is going on. For the comparison, a 

 standard blue aqueous solution of indigo carmine is made ; 

 the time required for the blue of the guaiacum mixture to 

 match the colour of the standard blue is taken as the index of 

 the peroxidasic activity. 



It should be remarked that, according to Aso, the presence 

 of certain substances, e.g., tannin or sodium fluoride, interferes 

 with the colour reactions normally given by oxidases. 



GENERAL CONSIDERATIONS. 



Up to comparatively recent times an oxidase was con- 

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

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

 of peroxidase and peroxide. According to them, there are 

 three categories of oxidizing ferments. 



(a) Oxygenases which produce the peroxide. 



(d) Peroxidases which transfer oxygen from the peroxide 

 to the substance to be oxidized. 



(c) Catalases which destroy peroxides so that oxygen is 

 given off. 



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, unless a peroxide, such as hydrogen peroxide, be 

 added. 



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

 the potato oxygenase, peroxidase and peroxide ; the peroxi- 

 dase transfers oxygen from the peroxide to the guaiacum, and 

 the oxygenase re-oxidizes the reduced peroxide. This may 

 be termed the direct action.f On the other hand, in the 

 cucumber juice, only peroxidase is present, so that in order to 

 obtain the blue reaction with guaiacum, hydrogen peroxide, 

 or other peroxide, must be added. This is the indirect action. 



This idea has been accepted by Palladin,t who considers 



* Bach and Chodat : " Biochem. Centrbl.," 1903, i, 416 ; Bach : " Ber. deut. 

 chem. Gesells.," 1906, 39, 2126; icoy, 40, 230; 1908, 41, 216. 

 fWheldale : " Proc. Roy. Soc, Lond.," B., 191 1, 84, 121. 

 X Palladin : " Ber. deut. hot, Gesells.," igo6, 24, 97. 



