204 INTERMEDIATE METABOLISM 



Now the peroxide can, in undergoing further oxidation, 

 impart an atom of oxygen to any substance which is 

 capable of oxidation. This may be either another molecule 

 of benzaldehyde — 



CgHgCO . . OH + CeHjCHO = 2C6H5COOH, 

 or any other oxidisable substance present — 



CeH.CO . . OH + Indigo = CeH^COOH + 



oxidation product of indigo. 



It is beheved, therefore, that the cell contains peroxides 

 which act hke benzaldehyde in the above reaction, taking 

 up molecular oxygen and imparting it to the food molecules 

 as atomic oxygen. 



This view is borne out by the close similarity which 

 exists between the oxidation processes which take place 

 in the body and those which occur in vitro by the action 

 of the simplest peroxide, hydrogen peroxide. To give an 

 example. Butyric acid is in the body oxidised to ace to- 

 acetic acid. The only agent capable of effecting the same 

 change outside the body is hydrogen peroxide. But it 

 cannot be hydrogen peroxide itself which is responsible 

 for oxidation in the tissues : first, because this substance 

 is toxic ; secondly, because several tissues contain a ferment 

 catalase, which decomposes it with liberation of oxygen in 

 molecular and therefore inactive form. 



The transference of atomic oxygen from the peroxide to 

 the substance undergoing oxidation is effected by means 

 of enzymes called peroxidases. The existence of such 

 enzymes has been demonstrated in certain vegetable 

 tissues. 



Hydrogen peroxide alone has a very slow oxidising 

 effect on lactic acid, but in the presence of the hving cells 

 of the horse-radish oxygen is rapidly transferred from the 

 H2O2 and oxidises the lactic acid. Now a similar accelera- 

 tion occurs in the presence of traces of ferrous or manganese 

 salts. For this reason, and also because either iron or 

 manganese is nearly always found in the ash of peroxidases, 



