Catalysts of Organic and Inorganic Origin. 



305 



decant and filter the clear liquid. In this way a strong watery extract can 

 be obtained suitable for immediate use, and practically free from the 

 chromogen. The oxidases of all the plants examined appear to be soluble 

 in water, or, at least, to pass through a filter paper. By half saturation with 

 alcohol they can be precipitated, possibly clinging to precipitated proteids, 

 but with excess of alcohol are attenuated and finally destroyed. 



The Distribution of the Oxideise in the Cell. 



Kapidly expressed and filtered apple sap does not. decompose hydrogen 

 peroxide, and gives no blue with guaiacum. Later filtrates, when the odd 

 pulp cells on the filter paper become brownish, show a very feeble decom- 

 position of H2O2, and give a faint blue with guaiacum on long standing. 



If fragments of fresh pulp crushed between filter paper till dry are added 

 to colourless apple sap, the latter only becomes brownish-yellow after three 

 days, and the former browns distinctly, whereas in water it remains prac- 

 tically colourless. 



Apparently, therefore, the oxidase is in the protoplasm, and not in the cell ; 

 sap and the browning is more readily produced when the tannic acid is inside 

 the cell than when it is outside. 



In the case of the potato the expressed sap, however obtained, rapidly 

 discolours in air, and contains both oxidase and chromogen. If, however, 

 slices of fresh potato are immersed in colourless apple sap (filtered and 

 boiled), they slowly turn deep brown. The brown colour is on the surface 

 layers, and is mainly in the protoplasm, which darkens strongly, and the 

 cell-walls slightly, on adding ferric chloride. The tannic acid of the apple 

 sap and the oxidase of the potato meet mainly in the protoplasm of the 

 latter, and the sap outside is only slightly discoloured, and, if plenty of 

 potato is used, contains much less tannic acid. Potato oxidase will therefore 

 oxidise apple tannin, but much more slowly than apple oxidase in situ, and 

 it appears to be located in the protoplasm of the potato cells. Possibly the 

 potato oxidase may be aided by the less soluble phosphates retained by the 

 potato cells, or may work better in a less acid medium.* At least, if the 

 apple sap is nearly neutralised by the addition of dilute ammonia, the browning 

 of the potato slices in apple sap is slightly accelerated, and these become 

 very dark or black on the addition of ferric chloride. On the other hand, if an 

 excess of apple sap is used, the browned potato pulp loses its oxidase properties, 

 and. as the liquid gains none, the oxidase has evidently been destroyed. 



* According to Hunger ('Bull. d. D. Bot. Gesell.,' vol. 19, p. 374 (1901)), tannins and 

 glucose often mask the presence of an oxidase or prevent its action. This certainly does 

 not apply in the case of the apple. ' 



