COENZYMES DERIVED FROM B VITAMINS 165 



hydrogen from the thiamine system, and these atoms are then aerobically 

 metabolized via a hydrogen transport system. 



pyruvic acid+H 3 P04+riboflavin-containing enzyme > 



phosphoryl-acetyl intermediate +C0 2 +reduced flavoprotein 

 This type of reaction has been thoroughly studied using preparations 

 from Lactobacillus debruckii as the enzyme source. 133 Phosphoric acid is 

 an essential component of this system and undoubtedly is utilized in the 

 same manner as in the reactions previously described. If, however, the 

 phosphoryl-acetyl intermediate is not required for synthetic purposes, it 

 is degraded, its available energy dissipated as heat, and acetic acid 

 becomes the end product of the process: 



phosphoryl-acetyl intermediate > 



acetyl phosphate — >. acetic acid+H 3 P0 4 



(7) Aerobic Utilization of Pyruvic Acid. Many of the diverse "re- 

 actions" in pyruvic acid metabolism formerly postulated are now believed 

 to consist of a series of two or more enzymatic steps. All the processes 

 have a common initial reaction catalyzed by thiamine pyrophosphate. 

 This reaction is analogous to the one just described occurring in L. 

 debruckii, except that in this case a pantothenic acid enzyme picks up 

 the phosphoryl-acetyl intermediate from the thiamine system: 



pyruvic acid+H 3 P04+riboflavin-containing enzyme + pantothenic acid 

 coenzyme >■ C0 2 +reduced flavoprotein 4 phosphoryl acetyl inter- 

 mediate associated with pantothenic acid coenzyme 



The many different ways in which the phosphoryl-acetyl intermediate 

 can be utilized are taken up in the discussion of pantothenic acid function, 

 but some of the important end products which have been associated with 

 pyruvic acid and thiamine metabolism will be enumerated here. The 

 phosphoryl-acetyl intermediate produced from pyruvic acid by the 

 thiamine-catalyzed reaction is, in the presence of a suitable pantothenic 

 acid system, used for: acetylating choline; acetylating aromatic amines; 

 forming acetoacetic acid and its homologues, which are intermediates in 

 fatty acid synthesis; condensing with oxalacetic acid to form cis-aconitic 

 acid, which is a precursor of citric acid, a-ketoglutaric acid (and glutamic 

 acid), the C 4 dicarboxylic acids, etc. For this reason thiamine or its 

 coenzyme has been reported at one time or another as a necessary catalyst 

 for each of these processes. 



The condensation of the reactive intermediate with oxalacetic acid 

 initiates the tricarboxylic acid cycle by which pyruvic acid is completely 

 "oxidized" to carbon dioxide and water (p. 224). Consequently, normal 

 pyruvic acid metabolism in animal tissues can proceed only in the 



