FOOD DIGESTION AND RESPIRATION 91 



can conveniently be prepared free from admixtures which obscure 

 their typical action. 



A preparation can be obtained from the root of the horse- 

 radish and elsewhere which has the same action on hydrogen 

 peroxide as ferrous salts have (E., p. 203). That is, it enables the 

 peroxide to oxidise lactic acid, etc. It has been called peroxidase, 

 having the general characters of an enzyme. Its action on the 

 peroxide is quite different from that of catalase, another enzyme 

 of very wide occurrence. While the latter causes the evolution 

 of gaseous oxygen, and does not increase the oxidation potential 

 of the peroxide, peroxidase causes no evolution of oxygen, but has 

 a marked effect in raising the oxidative power. Although the 

 composition of peroxidases has not yet been definitely established, 

 there is evidence that they consist essentially of the colloidal 

 hydroxide of a metal, such as iron, copper, or manganese, which is 

 capable of existence in two forms, one produced from the other 

 by an oxidation. In the actual enzyme these hydroxides are 

 associated with some stable organic colloid gum, protein, etc. 

 (P., p. 585). The function of this colloid appears to be to protect 

 the hydroxide from aggregation and loss of active surface by the 

 effect of electrolytes. 



. Thus, the concurrence of four factors is required (i) oxygen ; 

 (2) an autoxidisable substance ; (3) a peroxide, produced by the 

 action of the first on the second ; and (4) a peroxidase. In 

 many cases we can separate from cells complexes containing 

 peroxides and peroxidases ; these are often called oxydases. 



A convenient reagent in the investigation of such systems 

 is an acid contained in the gum-resin, guaiacum. This is 

 oxidised by active oxygen, not by ordinary oxygen, or only 

 very slowly. When oxidised, a blue pigment is formed. No 

 effect is produced by a peroxidase alone, nor by an organic 

 peroxide alone ; only when combined. If placed on the cut 

 surface of a potato, a blue colour is produced. Hence there 

 must be both peroxide and peroxidase present. In some 

 cases a substance is naturally present which changes in colour 

 when oxidised. This is the origin of the brown tint seen 

 to form on the cut surface of a living apple. When the cells 

 are cut across, and exposed to oxygen, an autoxidisable sub- 

 stance gives rise to a peroxide, which is then acted upon by 

 a peroxidase. The active oxygen, thus available, oxidises a 

 colourless compound, also present in the cells, forming a brown 

 pigment (E., p. 203). 



There are many subsidiary details concerning these oxidation 

 mechanisms for which space is not available here. The account 

 given is a brief summary of the state of knowledge at present, which 



