166 



THE BIOCHEMISTRY OF B VITAMINS 



presence of pantothenic acid and catalytic amounts of some C 4 dicar- 

 boxylic acid (precursor of oxalacetic acid) in addition to a divalent ion, 

 inorganic phosphate, thiamine pyrophosphate, a hydrogen transport 

 system, and oxygen. 



(8) Aerobic Oxidation of Pyruvic Acid in the Absence of Phosphate. 

 Cell-free preparations have been prepared from animal tissue and bacteria 

 which are capable of oxidizing pyruvic acid in the absence of inorganic 

 phosphate. 150 The reaction observed is: 



pyruvic acid + 3^ 2 — >■ acetic acid+C0 2 

 Thiamine pyrophosphate is an essential component of this system. Since 

 a requirement for, or presence of any other cofactors, could not be 

 demonstrated, the disposition of the hydrogen atoms in this system must 

 be by a mechanism which has not been previously encountered. When 

 cells metabolize pyruvic acid in this fashion they presumably cannot 

 conserve in a chemical form the energy of the oxidation. 



In the presence of thiamine pyrophosphate, the enzyme preparation 

 referred to above can carry out another reaction, not wholly unrelated — 

 the dismutation of diacetyl. 150 It may appear that this dismutation 

 represents a new type of thiamine function, since it involves neither a 

 keto acid nor a decarboxylation. However, if only one enzyme in the 

 preparation is responsible for both pyruvic acid oxidation and diacetyl 

 dismutation, the latter reaction can be considered as one in which the 

 enzyme establishes an equilibrium between two different sets of end 

 products of pyruvic acid metabolism through formation of their common 

 intermediates, namely, ketenyl radicals and available hydrogen atoms. 

 The equilibrium is such that the fragments, upon recombination, form 

 primarily acetic acid rather than diacetyl; but the mechanism for the 

 formation of acetic acid is the same as if the fragments had been formed 

 by the decomposition of pyruvic acid. The overall reaction can be repre- 

 sented in this fashion: 



O O 



H || || H 



HC— C— C— CH 



H H 



(a) + 

 2H 2 



O O 

 H || l| H 



(b) HC— C— C— CH + [2H] 



H H 



O 



H II 



HC— C 



1 ' 2H 



H OH HO H 



O 



II H 

 C— CH 



I I 



O OH 

 H || | H 

 HC— C— C— CH 

 H H H 



2HC— C + [2H] 



H in 



o o 



II II 



Net: 2 H 3 C— C— C— CH 3 + 2 H 2 

 diacetyl 



O O OH 



II II I 



2 H 3 C— C + H 3 C— C— CH— CH 3 



acetic acid acetoin 



