MECHANISMS OF OXIDATION IN TISSUES 1107 



mental changes. In discussing the mechanism of intracellular oxidation 

 we have therefore to consider in the first place how the dysoxidisable food- 

 stuffs are made to combine with the molecular oxygen diffusing into the 

 cells from the blood in the capillaries, and in the second place the means 

 by which these oxidative changes are strictly limited in accordance with 

 the necessities of the cell, and finally the nature of the specific oxidative 

 mechanisms for each kind of food-stuff and for the various stages in the 

 oxidation of each food-stuff. 



We are very far as yet from being able to give a definite answer to any 

 one of these questions. Even in the first problem, namely, the oxidation 

 of dysoxidisable substances, we have to confine ourselves almost exclusively 

 to speculation on possibilities. Although these substances will not unite 

 with the oxygen of the air, in which the combining activities of the oxygen 

 are satisfied by the combination of two atoms to form one molecule, many 

 of them readily undergo oxidation if subjected to the action of ' atomic * 

 oxygen or ' active ' oxygen, and it has been suggested that the problem of 

 the oxidation of the body is really bound up with the question as to the 

 mode of activation of the molecular oxygen derived from the oxyhsemoglobin. 

 Thus Hoppe-Seyler suggested that the activation of oxygen might occur 

 through the intermediation of reducing substances. He supposed that 

 reducing substances might be formed under the influence of ferments by 

 hydrolytic splitting of the food- stuffs. A reducing substance is one that 

 has sufficient affinity for oxygen at the ordinary temperature to tear asunder 

 the bonds which unite two atoms of oxygen to form one molecule, and to 

 combine with one or both of the atoms so set free. If the combination is 

 with only one atom, the other atom of the oxygen molecule is set free in an 

 active form, and is therefore able to oxidise dysoxidisable substances which 

 may be present. Thus, when a mixture of ammonia and pyrogallol is 

 exposed to the atmosphere, the oxygen is rapidly absorbed, forming a dark 

 brown solution, pyrogallol being therefore a reducing agent. But at the 

 same time a certain amount of the ammonia (a dysoxidisable substance) 

 undergoes oxidation with the formation of nitrite. In the slow spontaneous 

 oxidation of phosphorus which occurs on exposing this substance to the 

 atmosphere, ozone, 2 0, is always formed. As a type of the formation of 

 reducing substances in hydrolytic fermentations may be adduced the butyric 

 acid fermentation, in which sugar is converted into butyric acid, carbonic 

 acid, and hydrogen : 



The hydrogen produced in this process would act as a reducing agent. 

 There is no doubt that reducing substances are formed under normal circum- 

 stances in the tissues, as is shown by the methylene-blue experiment, and 

 it is possible that such reducing substances may aid in activating oxygen 

 and in the induction of certain oxidative processes. 'The activation of 

 oxygen would, however, not explain the specific character of the various 

 oxidations, and the accurate gradation of these oxidations to the necessities 

 of the cell. In many cases reducing substances may themselves act as 



