THE MECHANISM OF OXIDATION IN THE TISSUES 1159 



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



H 2 2 = H 2 + (X. 



In chemistry many reactions are known 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 time in 

 the body, though there is some reason to assume its formation in the process 

 of carbon assimilation in the green leaf. If we adopt the views of Bach and 

 Chodat, we must assume that every animal cell contains organic peroxides 

 as well as peroxidases, or else that it can under physiological conditions 

 form these substances. Since there is also evidence of the presence of 

 reducing substances in the cells, we may conveniently assume, with Ehrlich, 

 that distinct side-chains of the protoplasmic molecule have specific affinities 

 for oxygen. When all these affinities are saturated, these side-chains will 

 act as peroxides, parting with their oxygen with extreme ease, whereas 

 when the greater number are unsaturated, the resultant effect will be that 

 of a reducing agent. The same protoplasmic molecule may therefore, 

 according to its state of saturation with oxygen, act either as an oxidising 

 or reducing agent, and can effect, probably through the intermediation of 

 specifically adapted oxidases, the oxidation of the various foodstuffs stored 

 up as the paraplasm of the cell. Since the oxidative processes are deter- 

 mined, not by the presence of oxygen but by the functional activities of 

 the tissue, we must assume that the peroxidases are not preformed in the 

 cell, but exist as precursors, zymogens, from which they can be set free in 

 accordance with the necessities of the cell. 



It is probable that many of the foodstuffs or other proximate con- 

 stituents are not directly accessible to oxidation, and that the first step in 

 their utilisation is a process of cleavage or hydrolysis, which itself involves 

 the presence of specific ferments. Thus, so far as we can tell, the ammo-acids 

 undergo deamination before oxidation. They can thus be stored up in the 

 cell either free or in the form of protein, and present no point of attack to 

 oxygen until the process of hydrolysis and deamination has taken place. This 

 course of events is certainly true for some of the members of the purine group. 



