Catalysts of Organic and Inorganic Origin. 



303 



purplish-brown. Hence the chromogen after precipitation by absolute 

 alcohol can be dissolved again in water. The expressed sap from potatoes 

 turns black in a day. If boiled, treated with HC1 or absolute alcohol a 

 dirty white or grey precipitate is thrown down, and in the case of the 

 absolute alcohol the supernatant liquid is colourless and devoid of both 

 chromogen and oxidase. 



Potato Oxidase. 



Potassium chloride and bromide feebly and potassium fluoride strongly 

 retard its oxidase action on ursol tartrate and H 2 2 , on hydroquinone and 

 H 2 2 , and on pyrogallol and tyrosin. The addition of 5-per-cent. lead 

 acetate to the glycerine extract throws down a bulky white precipitate which 

 gives no blue with guaiacum, but shows an active power of decomposing H 2 2 , 

 and in the presence of the latter gives a distinct blue with guaiacum. Lead 

 acetate itself, however, gives a peroxide reaction with guaiacum. Potato 

 oxidase by itself is unable to oxidise tannic acid even in the presence of 

 H 2 2 , but if a little neutral solution of sodium phosphate is added, the liquid 

 turns slowly pale brown in the absence of H 2 2 , and more rapidly a darker 

 brown in its presence. 



Potato oxidase is peculiar in the readiness with which it oxidises tyrosin, 

 which most metallic oxidases only oxidise slowly and usually only in the 

 presence of H 2 2 . Possibly this peculiarity is due to the presence of a 

 specific sensitiser in the potato. Potassium phosphate appears to act as 

 a feeble sensitiser to the oxidase action of potassium ferricyanide on tyrosin, 

 but the nature of the sensitiser in the potato is doubtful. That such may 

 be present is indicated by the fact that potato extracts may sometimes be 

 obtained apparently capriciously, which while still reacting strongly to 

 guaiacum only react feebly to tyrosin and conversely. This is particularly 

 the case when partially sprouted tubers are used and fractional extractions 

 made. 



The Extraction of the Oxidase. 

 The oxidase of the potato is either more active or more abundant than 

 that of the apple, whereas the chromogen of the apple is more abundant 

 than that of the potato. Absolute alcohol not only does not. extract the 

 oxidase but destroys it, but if the pulp of the apple or the potato is pounded 

 with pure glycerine and filtered, the filtrate shows strong oxidase properties. 

 Glycerine also extracts some of the chromogen in each case, and hence if 

 diluted with water and exposed to air it darkens rapidly. If the glycerine 

 extract is concentrated by soaking cubes of material in glycerine, and then 

 pounding up with a little fresh glycerine, it is obtained as a clear yellowish 



