140 RESPIRATION 



Palladium black will serve as the catalyst ; the known 



affinity of this element for hydrogen enables it to eliminate 



hydrogen according to the equation 



/OH /OH 



CH,Cf-OH + Pd = PdH 2 + CH 3 G< 



\ H v> 



but unless methylene blue, or some other hydrogen acceptor, 

 is present to remove the hydrogen from the palladium, the 

 reaction soon comes to an end. The part played by water is 

 convincingly shown by the fact that whereas chloral CC1 3 CH0 

 is unacted upon by palladium and methylene blue, chloral 

 hydrate CC1 3 CH(0H) 2 is at once oxidised. 



Even the oxidation of carbon monoxide to the dioxide 



CO + o = co 2 



has been shown by Wieland * to involve the intermediate 



formation of formic acid 



/OH 

 CO 4- H,0 = H . C< 



^O 



which is then further oxidized to carbon dioxide and water, 



/OH 

 H . C< + O - H 2 + C0 2 . 



The principles outlined above have been applied by Wieland 

 to explain enzymic oxidations ; in his opinion oxidizing en- 

 zymes are in reality activators of hydrogen, and he terms them 

 dehydrases. Two examples will suffice to illustrate this, the 

 Schardinger reaction of milk f and the nitrate-reducing en- 

 zyme of the potato. 



The Schardinger reaction was originally designed for dis- 

 tinguishing boiled from unboiled milk ; it depends upon the 

 fact that the latter, when warmed with methylene blue and 

 a drop of acetic aldehyde, decolorizes the dye, whereas 

 boiled milk produces no such change. According to Wieland 

 this reaction is due to a dehydrase which activates the hydro- 

 gen of the aldehyde hydrate, thus enabling the methylene blue 

 to exert its action as hydrogen acceptor. 



* Wieland : " Ber. dent, cliem. Ges.," 1912, 45, 679, 2613. 

 f Ibid., 1914, 47, 2085. 



