MECHANISMS OF OXIDATION IN TISSUES 1237 



a ferment known as tyrosinase, from the fact that, when added to 

 solutions of tyrosine in the presence of air, the tyrosine is oxidised with 

 the formation of a brown pigment. * The same ferment is able to effect 

 the oxidation of other aromatic substances. The browning of a 

 freshly cut potato or apple on exposure to the air is similarly ascr bed 

 to the oxidation of a chromogen by the oxygen of the air, through the 

 intermediation of an oxidase present in the cells. If benzyl alcohol or 

 salicyl aldehyde be added to a suspension of liver-cells in blood, and 

 air be allowed to bubble through the mixture for some time, the alcohol 

 or aldehyde is found to have been oxidised to the corresponding acid. 

 In the same way xanthine (C 5 H 4 N 4 2 ) added to a mixture of spleen 

 pulp and defibrinated blood is converted into uric acid (C 5 H 4 N 4 3 ). 



Bach and Chodat have shown that in many cases the oxidase is not 

 a single substance, but a mixture of an organic peroxide with a ferment, 

 peroxidase, which has the property of splitting off atomic, i.e. active, 

 oxygen from the peroxide. These peroxidases have the same effect 

 on hydrogen peroxide. They must be distinguished from the ferment 

 catalase, which is present in almost all animal and vegetable tissues, 

 and which effects a rapid decomposition of hydrogen peroxide with the 

 formation of molecular oxygen : 



2H 2 2 = 2H 2 + 2 . 



In the case of a peroxidase the equation would be represented : 



H 2 2 = H 2 + 0'. 



Many reactions are known in chemistry in which the part of a 

 peroxidase is played by an inorganic catalyst. Thus hydrogen 

 peroxide effects a slow oxidation of many organic substances, but the 

 oxidation is enormously hastened if to the mixture be added a trace of 

 a ferrous salt (Fenton's reaction). The same part may be played by 

 salts of manganese, and it is interesting to note that manganese forms 

 an essential constituent of the peroxidase laccase, which is present in 

 many plants and is responsible for the formation of the Japanese 

 lacquer. It effects a specific oxidation of hydroquinone and pyrogallol. 

 The oxidations carried out by the use of hydrogen peroxide, with or 

 without a catalyst or peroxidase, present a close resemblance to the 

 oxidations occurring in the animal body. Thus Dakin has shown 

 that saturated fatty acids, even the higher members of the series, are 

 gradually oxidised if warmed gently with hydrogen peroxide in the 

 presence of ammonia, and the course of the reaction resembles in 

 many respects that which, on other grounds, we have assumed to take 

 place in the normal metabolism of the body. 



We have no evidence that hydrogen peroxide is formed at any 



