MECHANISMS OF OXIDATION 137 



which is desirable, although it is generally agreed that enzyme 

 action plays an important part in the process. Enzymes as- 

 sociated with the common end products of respiration are 

 subjects for first consideration and of them most attention 

 has been given to dehydrase, oxidase, catalase, zymase, and 

 carboxylase. 



In considering the possible mechanisms of oxidation it 

 must be borne in mind that they have a physical as well as a 

 chemical aspect. The former aspect has been specially em- 

 phasized by Warburg, who considers oxidation to be largely a 

 question of surface catalysis in which some form of iron plays 

 an important part. From experiments on pigeon's blood, 

 Warburg * showed that when the corpuscles are hsemolysed 

 by alternate freezing and thawing, and the blood then centri- 

 fuged, respiration was continued on the tissue residues, nuclei, 

 etc., of the lower layer, while the supernatant serum, containing 

 the haemoglobin, showed no respiration ; this difference in 

 activity he considered to indicate the significance of solid 

 surfaces ; similar conclusions were arrived at from experiments 

 on sea-urchin's eggs. 



Warburg also showed that the oxidation of amino acids 

 could be carried out at ordinary temperatures by adsorbing 

 them upon charcoal and exposing to air, whereby they yielded 

 the same oxidation products as when oxidized in the living 

 cell, namely, carbon dioxide, water and ammonia. 



The well-known depression of respiratory activity in the 

 living cell produced by narcotics Warburg attributes to the 

 fact that these substances are readily adsorbed upon the oxi- 

 dizing surface and so reduce their oxidative efficiency. This 

 view he supported experimentally on the carbon model by 

 showing that dimethyl urea, and similar narcotic substances, 

 which were readily adsorbed by the charcoal, greatly reduced 

 its oxidizing powers for amino acids. 



Warburg, moreover, showed that iron played an important 

 part in this charcoal model, since if chemically pure carbon, 

 obtained by igniting benzoic acid, be employed, practically 



* Warburg : " Biochem. Zeit.," 1921, 1 19, 134 '• " Zeit - Electrochem.," 

 1922, 28, 70. See also Vol. I., p. 411. 



