162 



THE BIOCHEMISTRY OF B VITAMINS 



animal tissues, since acetaldehyde has not been shown to increase the 

 yield, i.e., both of the C 2 radicals which condense must be formed from 

 pyruvic acid. It is possible that added acetaldehyde does not unite with 

 the enzyme system to form the necessary enzyme-substrate complex, 

 whereas the "acetaldehyde," or its equivalent, produced in situ condenses 

 instantaneously. Inorganic phosphate was found to be essential for this 

 reaction, but no phosphorylated intermediates have been directly demon- 

 strated. 



The reverse reaction, in which carbon dioxide is assimilated, has been 

 reported. 140, 141 The enzyme preparation did not require a thiamine 

 coenzyme for activation, but no evidence was presented to prove that it 

 was not a bound component of the material used. 



(3) Formic Acid Production. Escherichia coli and certain other bac- 

 terial species have been shown to cleave pyruvic acid in the presence of 

 inorganic phosphates. Until recently it was assumed that formic acid and 

 acetyl phosphate were the primary products of this phosphoroclastic 

 reaction. 142 It has recently been shown, however, that the product first 

 formed from the C2 radical is not acetyl phosphate but is a related com- 

 pound whose structure is as yet unknown. 24s This compound, which in 

 this chapter is designated as the "phosphoryl-acetyl intermediate" to 

 distinguish it from acetyl phosphate, is a very reactive acetylating agent 

 as well as an efficient phosphorylating agent. It is a participant in all 

 the reactions known to be mediated by the pantothenic acid coenzyme. 

 (See p. 191 for a detailed discussion of its recognized properties.) The 

 reaction by which it is formed may be considered to be one in which (1) 

 the reactive C2 radical is combined in some fashion with phosphoric acid, 

 perhaps through a common carrier (designated in the formula as X), 

 and (2) the hydrogen atoms associate themselves with the carbon and 

 oxygen atoms which usually form carbon dioxide in the other types of 

 reactions: 



An alternate mechanism, advanced when the reaction product was pre- 

 sumed to be acetyl phosphate, was that phosphoric acid first formed an 



