584 PRINCIPLES OF GENERAL PHYSIOLOGY 



which are not, by themselves, accessible to molecular oxygen. In the presence of 

 water we generally find that the peroxides form hydrogen peroxide, which has not 

 a high oxidation potential. But it was known already to Schonbein (1860) that 

 ferrous salts, in extremely small amount, strongly accelerate the action of hydrogen 

 peroxide on oxidisable substances. Ferrous salts, in fact, as also those of copper 

 and of manganese, accelerate the oxidising power of oil of turpentine, benzaldehyde, 

 etc., no doubt by action on the peroxides produced. It is supposed by some that 

 these metallic salts combine with the peroxides to form unstable "complexes," 

 which split off the peroxide oxygen more readily than the original peroxides, like 

 the permolybdic acids of Brode's experiments. In any case, the metal reappears 

 in its original form and is thus a true catalyst. 



PEROXIDASES 



When we come to apply the above phenomena to the process of oxidation in 

 living cells, we find that the problem is by no means easy. The oxidation systems 

 met with are often of great complexity and the enzymes unstable. The result has 

 been that, although we are in possession of a large number of facts, their relation 

 to a general theory is not a simple matter to make out. 



The reader will find an excellent account of the subject in the monograph by Kastle (1910) ; 

 we must confine ourselves here to those facts which seem to give most guidance in the forma- 

 tion of a general theory. When we refer to certain preparations as coming from this or that 

 plant or animal organ, it is not to be supposed that similar substances are not of general 

 occurrence. It happens that, for various reasons, particular enzymes and so on are more 

 readily isolated from their admixture with other substances in some cases than in others. 



The gum-resin, guaiacum, happens to be a convenient test for the presence of 

 active oxygen, since one of its constituents, guaiaconic acid, is oxidised to a blue 

 substance by active oxygen, but not by ordinary oxygen, nor even by such 

 peroxides as that of hydrogen. It is best used in the form of a solution of 

 guaiaconic acid in dilute alcohol. 



Suppose that we take a scraping from the surface of a potato, some fresh blood 

 fibrin, or various other products of living cells, and apply a drop of guaiaconic acid 

 solution. A blue colour is produced, showing the presence of active oxygen. But 

 this simple experiment does not lead us far in the analysis of the mechanism. 



Now, Bach and Chodat (1903) showed that from the root of the horse-radish 

 a solution could be prepared which did not give the blue reaction spoken of, 

 neither did it give off oxygen gas when hydrogen peroxide was added. But if 

 to this solution we add hydrogen peroxide, guaiacum is oxidised with the production 

 of the blue colour. The solution must therefore contain something which activates 

 hydrogen peroxide. This constituent is destroyed by heat, precipitated by alcohol, 

 and shows the general properties of an enzyme, and was therefore called 

 " peroxida.se." 



We are next naturally led to look for evidence of the presence of hydrogen 

 peroxide, or similar peroxide, together with peroxidase, in those cases in which 

 guaiacum is blued without the necessity of adding hydrogen peroxide. An 

 experiment by Bach (1914, p. 225) is of interest here. Fresh potato juice oxidises 

 tyrosine rapidly. If acted on by alcohol, a precipitate is formed which has 

 scarcely any action on tyrosine, unless hydrogen peroxide is added. Hence 

 hydrogen peroxide can take the place of a similar substance naturally present. 



Now, when we take a solution of peroxidase and add guaiacum and hydrogen 

 peroxide, we naturally obtain the blue colour whether free oxygen is present or 

 not, since the hydrogen peroxide supplies what is required. But, suppose we take 

 potato scrapings, put them into a tube through which we lead hydrogen or coal 

 gas, until the oxygen is displaced, and then, by means of a tap funnel, previously 

 fitted, we drop guaiaconic acid on to the potato, we see that no oxidation takes 

 place until air is allowed to enter the tube. If we consider this result for a 

 moment, we shall see that its meaning must be this : Peroxidase is present as 

 usual, but the absence of active oxygen, unless air is present, shows that no 

 peroxide is available for the peroxidase to act upon. The peroxide is therefore 



