1 66 RESPIRATION 



And here opinions differ as to the starting-point of oxidation. 

 In all probability it is not alcohol, but a precursor of alcohol ; 

 and in tracing the various stages in the oxidative degradation 

 of these substances, the realms of speculation are entered. 

 Of the suggestions made, the following have the merit of at 

 least being within the range of possibility : — 



The close connection between sugar and organic acids has 

 been remarked upon on an earlier page, and also the formation 

 of aldehyde in fermentation : the oxidation of aldehyde to 

 acetic acid is a familiar activity of bacteria. 



The dehydrogenation of two molecules of acetic acid would 

 give rise to succinic acid — 



CH 3 . COOH CH 2 . COOH 



CH 3 . COOH CH 2 . COOH 



+ 2H. 

 Succinic acid 



The latter, as shown by Thunberg,* is readily converted by 

 the enzyme contained in muscle into fumaric acid — 



CH 2 . COOH CH . COOH 



-> II + 2H. 



CH 2 . COOH CH . COOH 



Fumaric acid 



It has further been found by Quastel and Whetham f that 

 resting Bacillus coli and B. pyocyantus are able to effect the 

 same conversion of succinic into fumaric acid. These authors 

 also found evidence of the conversion of fumaric into malic 

 acid by these bacteria, and Batelli and Stern % have shown 

 that muscle also contains an enzyme capable of bringing about 

 this change by the hydration of the fumaric acid— 



CH . COOH CH 2 . COOH 



|| + HOH -> | 



CH . COOH CHOH . COOH. 



Malic acid 



Malic acid, as was shown by Hahn and Haarmann,§ yields 



on dehydrogenation oxalacetic acid — 



CH 2 . COOH CH 2 . COOH 

 I -> J + 2H 



CHOH . COOH CO . COOH 



* Thunberg : " Zentrlbl. Physiol.," 1916, 31, 91. 

 f Quastel and Whetham : " Biochem. Journ.," 1924, 18, 519. 

 J Batelli and Stern : " Compt. rend. Soc. biol.," 1921, 84, 305. 

 § Hahn and Haarmann : " Zeit. Biol.," 1928, 87, 465 ; 88, 91. 



