306 Prof. A. J. Ewart. Comparative Study of Oxidation by 



Colour and Oxidase Action. 



Owing to their striking character most attention has been directed to those 

 oxidase reactions which are accompanied by a production of colour. The 

 parsnip and carrot have fairly strong oxidases in their cortex, phloem and 

 cambium, but the former has no chromogen, the latter none oxidising further 

 on death. The most important oxidase reactions are in fact probably those 

 unaccompanied by any colour change, and in some cases coloured bodies may 

 be rendered colourless by oxidase action. 



Thus a living slice of potato stained with a watery solution of gentian 

 violet becomes slowly paler when kept moist in air. The extracted oxidase 

 of potato slowly partially decolorises gentian violet, and the action is 

 hastened by the addition of small quantities of H 2 2 . Gentian violet may 

 be heated with H2O2 without being decolorised but if a drop of a mixture of 

 CuS0 4 and sodium or potassium chloride is added to the hot liquid, the 

 latter rapidly becomes colourless. 



Copper sulphate alone is much less effective. Similar results are given by 

 eosin, indigo carmine* and methyl blue (pale purplish) but at temperatures 

 below 50° C, which is the highest to which organic oxidases can be raised 

 with safety, the reductions by the metallic oxidase are very . much slower 

 than at 95° C. to 100° 0. Further, in the case of organic oxidases, dilute 

 solutions must be used so as to avoid poisoning the oxidase. Even then only 

 partial decolorisation is shown and this is often difficult to distinguish from 

 effects due to absorption. Using a strongly oxidase diastase a faint 

 decolorisation was shown in the presence of hydrogen peroxide with dilute 

 solutions of methyl blue and eosin but none with gentian violet or indigo 

 blue with or without peroxide of hydrogen. 



Since copper sulphate and salt can decolorise indigo carmine, an oxidase 

 can also act as a reducing agent in the presence of an excess of hydrogen 

 peroxide, particularly at high temperatures. 



The Destruction of Oxidase oy Heat. 



Although oxidases are not necessarily proteins, cell oxidases seem to 

 adhere closely to proteins, and it is possible that it is the coagulation of the 

 latter by heat that renders the oxidases inactive. Their destruction by 

 absolute alcohol might arise in the same way. 



The glycerine extracts of both apple and potato develop coagulated 

 particles on boiling, at the same time that they lose their oxidase properties. 



* Prolonged boiling with excess of H 2 2 partially reduces indigo carmine to a brown 

 or greenish-brown liquid. 



