ORIGIN OF FATTY ACIDS 79 



(3) CH 3 CH : CH CO COOH + O = CH 3 CH : CH COOH + CO s 



Crotonic acid 

 CH 3 CH : CH COOH + 2H = CH 3 CH 2 CH 2 COOH 



Butyric acid 

 or 



(4) CH 3 CH : CH CO COOH— C0 2 = CH 3 CH : CH CHO. 



Crotonic aldehyde 

 CH 3 CH : CH CHO + CH 3 CO COOH = CH 3 CH : CH . CHOHCH, . CO . COOH 



yielding a seven-carbon compound which by a series of 

 changes similar to those undergone by the five-carbon com- 

 pound would give a six carbon acid — 



CH 3 CH 2 CH 2 CH 2 CH 2 COOH 

 Caproic acid 



While there may be some difficulty in accepting this theory 

 of the formation of fatty acids in the animal body, owing to 

 uncertainty of the production of pyruvic acid as a normal 

 product of carbohydrate metabolism, as is pointed out by 

 Leathes, this difficulty would not apply to the decomposition 

 of carbohydrate by micro-organisms. 



It has already been pointed out in Volume I. that Neuberg 

 has established the production of pyruvic acid in alcoholic 

 fermentation and has shown it to be the precursor of the 

 acetic aldehyde formed in this process; Neuberg and Arinstein,* 

 in an investigation of butyric acid and butyl alcohol fermen- 

 tation of glucose by Bac. butylicus Fitzianus, have established 

 the fact that here also pyruvic acid is the intermediate 

 substance ; this may give rise either to acetic aldehyde or to 

 butyric acid and butyl alcohol, but these products are mutually 

 exclusive, showing that butyric acid and butyl alcohol are 

 not formed by way of acetic aldehyde. This the authors 

 proved as follows : the bacillus when grown on either acetic 

 aldehyde, or its aldol CH 3 CHOH CH 2 CHO, produced no butyric 

 acid, but when grown on the aldol of pyruvic acid, it produced 

 butyric acid in quantity. The mechanism of this reaction is 

 explained as follows : — 



CH 3 CO COOH + CH 3 CO COOH = COOH 

 Pyruvic acid 



CH 3 C OH CH 2 CO COOH 

 Pyruvic acid aldol 



* Neuberg and Arinstein : " Biochem. Zeit.," 1921, H7> 26 9« 



