164 THE BIOCHEMISTRY OF B VITAMINS 



product of the decomposition of acetyl phosphate. 144 On the basis of the 

 recent finding discussed above, it is anticipated that the phosphoryl- 

 acetyl intermediate will be found to be the initial product of the reaction 

 rather than acetyl phosphate itself, and that the equation representing 

 the reaction should be: 



pyruvic acid +H3PO4 — >- phosphoryl acetyl intermediate +C0 2 +H 2 



The reaction as indicated might be the result of the summation of two 

 individual enzymatic reactions — a phosphoroclastic cleavage producing 

 formic acid, immediately followed by the decomposition by a hydro- 

 genlyase of the formic acid into carbon dioxide and molecular hydrogen. 

 This mechanism is excluded, however, since the enzyme preparations do 

 not decompose formic acid. 



(5) The Acetic Acid-Lactic Acid Dismutation of Pyruvic Acid. One 

 additional means of hydrogen disposal under aerobic conditions has been 

 observed in bacterial cultures — the case in which the available hydrogen 

 atoms are accepted by a second molecule of the pyruvic acid substrate. 

 The products are acetic acid (phosphorylated derivatives?), carbon 

 dioxide, and lactic acid. 



(a) pyruvic acid +H 2 >■ (2H)+acetic acid+C0 2 



(or H3PO4?) (phosphorylated?) 



(b) pyruvic acid + (2H) >■ lactic acid 



Net: 2 pyruvic acid +H 2 — >■ lactic acid +acetic acid +C0 2 



This dismutative anaerobic utilization of pyruvic acid has been observed 

 in a number of animal tissues, 145, 14G > 147 and in several species of bac- 

 teria. 88 - 147 > 148 



There is a question which has not yet been conclusively answered: 

 Is the reaction in which the second molecule of pyruvic acid is reduced 

 an independent reaction requiring a separate enzyme? If so, is a hydrogen 

 carrier necessary to transfer the available hydrogen atoms from the 

 thiamine enzyme to the pyruvic acid reductase? In one instance in which 

 this reaction was studied in a cell-free system, the evidence favored the 

 concept of coupled reactions and indicated that hydrogen carriers are 

 needed to link the two distinct reactions of this dismutation. 88 



The use of radioactive isotopes has made it possible demonstrate the 

 assimilation of carbon dioxide by the reverse process. 149 



(6) Aerobic Production of Acetic Acid. If a hydrogen acceptor (other 

 than the intermediates or substrate) is available, pyruvic acid can be 

 metabolized in the fashion indicated below. Under natural conditions, a 

 riboflavin-containing protein is believed to accept initially the available 



