160 THE BIOCHEMISTRY OF B VITAMINS 



available for recombination in any one of a number of ways to produce 

 the recognized products of pyruvic acid metabolism. 



r O -, 



H || || H || 



HC— C— C— OH — > HC— C + 2H + C0 2 

 H L I I J 



postulated 

 pyruvic acid ketenyl 



radical 



The specific products formed depend upon the manner in which the 

 enzymes utilized or disposed of the C 2 fragment (designated hereafter as 

 a ketenyl radical) and the atoms of available hydrogen. The ketenyl 

 radical can react (1) with water to form acetic acid, (2) with phosphoric 

 acid to yield a phosphorylated derivative of acetic acid, (3) with acetal- 

 dehyde (or other aldehydes) to form acetoin (or its homologs), (4) with 

 another ketenyl radical and hydrogen to form the dimer, diacetyl, or (5) 

 with the available hydrogen- atoms to form acetaldehyde. 



The available hydrogen atoms also must be utilized in some fashion if 

 they are not accepted by the hypothetical ketenyl radical (forming 

 acetaldehyde) or the dimer, diacetyl (yielding acetoin). They can be 

 accepted by another molecule of pyruvic acid, forming lactic acid; they 

 can be disposed of in the form of molecular hydrogen ; they can associate 

 themselves with the elements of carbon dioxide to produce formic acid; 

 or under aerobic conditions, the hydrogen atoms can be taken by ribo- 

 flavin-containing enzymes to be passed on and eventually accepted by 

 molecular oxygen. The three reactions of a-ketoglutaric acid known to 

 be catalyzed by thiamine pyrophosphate are analogous to three of the 

 reactions in which pyruvic acid is the substrate. 



Reactions of Pyruvic Acid Catalyzed by Thiamine Pyrophosphate. 

 Thiamine pyrophosphate and a divalent cation (Mg ++ or Mn ++ ) and the 

 appropriate apoenzyme have been shown to be the coenzymatic factors 

 for the following eight reactions: 



(1) Simple Decarboxylation. The simplest reaction which thiamine 

 pyrophosphate catalyzes is the direct decarboxylation of pyruvic acid. 

 When the keto acid molecule is cleaved, the hydrogen atoms become 

 attached to the C 2 fragment and the end products are acetaldehyde and 

 carbon dioxide: 



