MECHANISMS OF OXIDATION IN TISSUES 1235 



in the body or is excreted combined with bases in the urine. In the 

 normal individual tyrosine, whether administered separately or in 

 combination in protein, is completely oxidised, the benzene ring being 

 broken up. In certain rare cases of disordered metabolism the 

 patient, who is otherwise apparently well, is unable to effect the total 

 oxidation of tyrosine, which is therefore excreted as homogentisic acid, 

 after undergoing only the first stage of its normal transformation in the 

 body. These various mechanisms are adjusted in each case to the 

 functional activity of the cell and are limited therefore, not by the 

 supply of oxygen or of food-stuff to be oxidised, but by the necessities 

 of the cell, i.e. the adaptations induced in it by its environmental 

 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 com- 

 bining 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 oxy haemoglobin. 

 Thus Hoppe-Seyler suggested that the activation of oxygen might 

 occur through the intermediation of reducing substances. He sup- 

 posed 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 ex- 

 posed 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 



