232 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



blue by oxidation. Various other aromatic substances, 

 such as benzidine, a-naphthol and para-phenylene-diamine 

 can be used as acceptors, but in such cases it is found that 

 hydrogen peroxide must usually be added, even to an 

 extract which behaves as a complete oxidase system to- 

 wards guiacum. Some very interesting results have 

 recently been obtained by Mrs. Muriel Wheldale Onslow, 

 relating the establishment of the oxidase system in certain 

 plants to the browning that takes place on injury and to 

 the preliminary oxidation of some aromatic cell constituent. 

 They will shortly be published in the Biochemical Journal. 



The tyrosinases are oxidising enzymes which act on 

 tyrosine. They also act on substances of allied constitu- 

 tion, such as ^-cresol. The relationships of these plant 

 enzymes to the mechanism of the oxidations occurring in 

 the animal body is not at present certain. 



Preparation of guiacum tincture. Take the inner portions of a lump of 

 resin and make a i -5 per cent, solution in 95 per cent, alcohol, by heating in a 

 flask in a boiling water bath. Add some adsorbent charcoal, boil for about 5 

 minutes and filter. The solution should be freshly prepared. 



A more reliable preparation of guiaconic acid is described by Lyle and 

 Curtman (Journ. Biol. Chem., xxxiii., p. i). 



274. Potato oxidase. Thoroughly wash and scrub a potato. 

 Peel it and pound the peel in a mortar with a little cold water. 

 Filter. Treat a small amount of the nitrate with a few drops of the 

 tincture of guiacum. A blue colour is obtained after a short time. 

 Boil a few cc. of the aqueous extract, cool and add guiacum. No 

 blue colour is obtained, showing that the enzyme is destroyed by 

 boiling. 



275. Conversion of Potato oxidase to a per oxidase. Heat 

 about 5 cc. of the potato extract to 65 C. for 10 minutes in a water 

 bath. Divide the solution into two portions, A and B. To A add a 

 few drops of guiacum tincture and a few drops of hydrogen peroxide. 

 To B add a few drops of guiacum tincture. A generally goes blue, 

 whilst B generally gives a negative or very feeble reaction. 



NOTE. The organic peroxide is more unstable to heat than the peroxi- 

 dase. After heating, the solution requires the addition of hydrogen 

 peroxide. 



