144 RESPIRATION 



readily in ordinary circumstances. Dufraisse and Moureu 

 have shown that not only hydrocyanic acid but the ferrocyan- 

 ides, which according to Warburg should be without action 

 of any kind, act as depressants of oxidation. The same 

 authors * draw attention to the fact that among the most active 

 antoxygens are the phenols and tannins and they suggest that 

 the wide distribution of these substances in the plant world 

 may be the cause of the relatively slow rates of oxidation 

 in the plant as compared with the animal ; they also suggest 

 that the toxic action of phenols is due to their antoxygenic 

 property. 



Support for Wieland's theory of dehydrogenation is fur- 

 nished by the work of Thunberg, who has shown that freshly 

 minced and washed muscle contains an enzyme which in the 

 presence of methylene blue, acting as a hydrogen acceptor, 

 is able to convert succinic acid into fumaric acid : — 



CH 2 . COOH CH . COOH 



| + MB = || + MBH 2 . 



CH 2 COOH CH • COOH 



Succinic acid Fumaric acid 



Thunberg f regards hydrogen as the essential fuel of the 

 living cell and considers only those substances as possible 

 intermediate metabolites which when left in contact with 

 methylene blue in the absence of oxygen decolorize this sub- 

 stance and therefore act as hydrogen donators. His tech- 

 nique consists in placing the material under examination in 

 a test tube with freshly washed frog's muscle, to supply the 

 dehydrase, and methylene blue, filling the tube with boiled 

 water, and leaving the whole in a thermostat and examining 

 at intervals. If the methylene blue is decolorized, the sub- 

 stance in question is a hydrogen donator and consequently a 

 possible intermediate metabolite. 



It will be noted that the theory of Wieland envisages only 

 the activation of hydrogen atoms, thus making the substance 

 activated for the time being a hydrogen donator. Quastel,* 



* Moureu and Dufraisse: " Comp. rend.," 1922, 174, 258. 

 f Thunberg : "Skand. Archiv. Physiol.," 1920, 40, 1 ; " Natunviss.," 

 1922, IO, 417. 



tQuastel : " J. Hygiene," 1928,28, 139, and the literature there quoted. 



