Catalysts of Organic and Inorganic Origin. 



285 



general revision of the oxidase ferments, and in particular a general 

 comparison with metallic oxidases. As is well known, oxidases are widely 

 distributed in plants, and are frequently responsible for the changes of colour 

 in extracted plant juices or in plant tissues after death. 



In the case of the apple and potato, the curious fact that browning took 

 place in pulp killed by immersion in poisonous metallic solutions, but not 

 when killed by heat, demanded special investigation. 



The Browning of Apples and Potatoes. 



It is generally assumed that this is due to the action of an oxidase ferment 

 upon a chromogen present in the pulp cells, such as tannic acid in the apple* 

 and tyrosin in the potato, but the oxidase is not necessarily the same in either 

 case The term "oxidase" in fact rather represents a result than a particular 

 substance, and many " oxidase " actions are not necessarily due to organised 

 ferments or enzymes at all. In a previous paper it has been shown that 

 apple pulp immersed in solutions of metallic poisons may still turn brown, 

 although ordinary ferments are destroyed by such poisons. Furthermore, 

 the reasons why the browning takes place on death, but not in the living cell, 

 and not when the cell is killed in certain special ways need investigation. Pro- 

 chromogens or zymogens may exist in the living cell which decompose into 

 interacting chromogen and enzyme on death, or the latter may be kept apart 

 in the living cell by semipermeable membranes which lose their imperme- 

 ability on death. In the latter case the localisation of the chromogen and 

 enzyme in the cell becomes a problem of special importance. 



According to Grussf " Antioxidases "t capable of arresting oxidase reactions 

 exist in various plants, and if the oxidase and the " antioxidase " balanced, a 

 chromogen and its oxidase might exist in contact in the living cell and the 

 mode of death might determine whether browning occurred or not. Behrens§ 

 considered that the browning of the apple pulp is due to a direct oxidation of 

 tannic acid to form a leathery compound with the proteids of the cell, without 

 the aid of an oxidase. 



[_ (The first point needing full investigation was the influence of poisons on 

 browning, particularly in regard to the time factor and the rapidity of 

 penetration. 



* Lindet, « Compt. Rend.,' vol. 120, p. 370 (1895). 



+ ' Biologie und Capillar analyse der Enzymen,' p. 56 (1912). 



\ To avoid possible confusion, the word "inhibitor" may be used instead of this term. 

 § 'Centralbl. f. Bakt.,' 2 Abth., vol. 4, p. 514 (1898). 



Y 2 



