i 4 2 RESPIRATION 



1. CH 3 Cf + HOH = CH 3 C^OH 



,OH /OH 



2. CH 3 C^ OH -> CH 3 C^ + 2H 



\H ^O 



3. NaN0 3 + 2H = NaN0 2 + H 2 0. 



Even the biological conversion of alcohol into acetic acid by 

 the vinegar plant, Bacterium aceti, was shown by Wieland to 

 be a dehydrase action which could be effected in an atmosphere 

 of nitrogen by leaving freshly washed cultures of the plant 

 in contact with alcohol and methylene blue in a flask from 

 which air had been displaced by nitrogen. In a comparatively 

 short time the methylene blue was decolorized and after some 

 days a measurable quantity of acetic acid had been produced.* 



It is evident that the theories of Warburg and Wieland are 

 fundamentally different, the former insisting upon the acti- 

 vation of oxygen while the latter postulates the activation 

 of hydrogen. 



Warburg's criticism of Wieland's hypothesis is that acti- 

 vated hydrogen when finally oxidized should produce hydrogen 

 peroxide and not water, and that hydrogen peroxide is not 

 found in living cells. To this Wieland replies that all aerobic 



tissues. Thus milk in anaerobic conditions can reduce nitrate, the nitrate 

 acting as the hydrogen acceptor ; if, however, the reaction is allowed to 

 take place in the presence of air a peroxide is formed (Haas and Hill, 

 " Biochem. J.," 1923, 17, 671, and Haas and Lee," Biochem. J.," 1924. 

 18, 614) which was shown by Thurlow (id., 1925- I9> 175) to be hydrogen 

 peroxide resulting from atmospheric oxygen acting as hydrogen acceptor 

 in place of the nitrate; in such circumstances no nitrate is reduced. 



It has since been shown by Thurlow (loc. cit.), Dixon and Thurlow 

 (" Biochem. J.," 1924, 18, 989), and Bernheim (" Biochem. J.," 1928, 22. 

 344) that, in the nitrate-reducing system, either xanthine or hypoxanthine 

 or adenine may replace the aldehyde as hydrogen donator and methylene 

 blue may replace nitrate as hydrogen acceptor. 



Subsequently Bernheim, comparing the Schardinger reaction of milk 

 with the nitrate-reducing action of potatoin thepresenceof aldehyde, arrived 

 at the conclusion that the potato enzyme is able to reduce methylene blue 

 in the presence of aldehyde as hydrogen donator in the same way as the 

 Schardinger enzyme of milk. Bach (*' Biochem. Zeit.," 1913, 52, 412) 

 and Michlin {id,. 1927, 185, 216), who had tried unsuccessfully to do this, 

 failed, presumably owing to their working at an unsuitable pH range, 

 Bernheim having shown that the methylene blue reducing range of the 

 enzyme of the potato to be 5-6 to 78, while the nitrate reducing range is 

 from 3 to 8*6, with an optimum at 5*5. 



* Wieland : " Ber. deut. chem. Ges.," 1913, 46, 3336. 



