vii] OXIDIZING ENZYMES 111 



peroxidase with the formation of a brown colour (browning on injury) 

 and an oxidation product which then acts as an organic peroxide (as 

 postulated above) in the oxidase system. 



Further, the peroxidases, of plants which give the oxidase reaction 

 and brown on injury, appear to be specific in their action on the 

 catechol grouping and will not oxidize other phenolic groupings, i.e. 

 those of resorcinol, hydroquinone, pyrogallol, etc. \^^ 



In plants (e.g. Alyssum) which only give the peroxidase reaction and 

 do not brown on injury, the above-mentioned aromatic compounds are 

 not present, nor do the peroxidases of such plants act on catechol. 



It should, however, be borne in mind that in some plants the com- 

 ponents of the oxidase system may be present, but the reaction is masked 

 by the presence of an excess of tannin, sugars, etc. 



Hence it appears that the oxidase reaction of plants (as detected by 

 guaiacum) is the outcome of post-mortem changes after the death of 

 the cell. It is possible, however, that the processes giving rise to it 

 may take place to some extent, though under control, in the living cell. 



It has also been shown that in the Pear fruit and Potato tuber 

 (Wheldale Onslow, 11) it is possible by chemical methods to separate 

 the elements which go to make up the oxidase system, i.e. the peroxidase 

 and the aromatic substance. If the tissues are ground up rapidly with 

 alcohol, and filtered before oxidation can take place, and the process 

 repeated, the aromatic substance, which is soluble in alcohol, is washed 

 away, and a water extract of the residue gives only a peroxidase reaction. 

 From the alcoholic extract the aromatic substance can be precipitated 

 as a lead salt, the lead then removed as insoluble lead sulphate, and the 

 aromatic set free again in solution. If the peroxidase is then added to 

 the solution of the aromatic, the brown colour appears together with the 

 formation of the oxidation product (peroxide), and the mixture will give 

 the blue reaction of an oxidase with guaiacum. 



Expt. 118. Resolution of the components of the oxidase in the Potato tuber. (A) 

 Separation of peroxidase. Cut a few thin slices from a peeled potato and put them 

 in a mortar which contains sufficient 96 % alcohol to prevent, as far as possible, 

 exposure to the air, and pound them thoroughly. Filter quickly on a filter-pump, 

 and repeat the process several times until a colourless powder, consisting of cell- 

 residues, starch, etc. is obtained. The enzymes (including the peroxidase) of the cells 

 are precipitated by the alcohol and remain in the cell -residue. Make a water extract 

 of the white powder and filter. To a portion of the filtrate add a few drops of 

 guaiacum tincture : no blue colour is given. Add further a few drops of dilute 

 hydrogen peroxide : a blue colour appears. (B) Separation of the aromatic substance. 

 Take about 500 gms. of freshly peeled potato tuber, cut it into thin slices and drop 



