96 RESPIRATION 



second enzyme, an oxygenase,* whose function was to generate 

 with the help of atmospheric oxygen the requisite peroxide. 

 The oxidase system therefore was direct acting owing to 

 the fact that it contained the necessary mechanism in the 

 oxygenase for generating its own peroxide and so making 

 it independent of any added hydrogen peroxide. This view 

 was supported by the fact that by the fractional precipitation 

 with alcohol of an aqueous extract of Lactarius> the contained 

 oxidase could be separated into its two constituents : 



(a) A portion soluble in alcohol, the peroxidase constitu- 

 ent, which was without direct action on guaiacum. 



(b} A portion insoluble in alcohol, the oxygenase con- 

 stituent, which acted but feebly upon guaiacum, but showed 

 a much stronger action on the addition of the alcohol soluble 

 fraction. 



The presence of an oxidase in a plant tissue may also be 

 revealed by bruising the surface or by exposing to chloroform 

 vapour. Under such treatment the cell contents are some- 

 how disturbed, with the result that the oxidase contained 

 therein causes the formation of a coloured substance in the 

 substrate. 



The observation of Wheldale Onslow f that the oxidase 

 contained in the pea and in the potato are able to oxidize 

 pyrocatechol and allied substances,! suggest that the brown- 

 ing on injury is due to the action of an oxidizing enzyme 

 upon some such substance contained in the tissue injured. 

 In support of this view, the presence of a substance of a 

 pyrocatechol nature may be demonstrated as follows. By 

 grinding thin slices of potato under 96 per cent alcohol, the 

 oxidase system may be divided into its two constituents, an 

 alcohol soluble portion and the peroxidase which owing to 



*Bach and Chodat: " Ber. deut. chem. Gesells.," 1903, 36, 606; 1904, 

 37, 36, 1312. 



f Wheldale Onslow : "Biochem. Journ.," 1919, 13, i. 



The substances to which reference is made are protocatechuic and caffeic 

 acids. 



COOH CH = CH . COOH 



JOH I JOH \X OH 



OH OH OH 



Pyrocatechol. Protocatechuic acid. Caffeic acid. 



