Bacfenal Metabolism 77 



studied intensively with bacterial enzymes in several labora- 

 tories (7) and has been found to be unexpectedly compli- 

 cated. The reaction involves the participation of no less 

 than five coenzymes; namely, thiamine pyrophosphate, 

 lipoic acid, coenzyme A, diphosphopyridine nucleotide 

 (DPN), and adenosine diphosphate (ADP), and possibly as 

 many apoenzymes. The role of each of the coenzymes has 

 been fairly well established, although some details remain 

 to be worked out. For example, it is not yet certain whether 

 thiamine pyrophosphate and lipoic acid act entirely in- 

 dependently, or whether a compound of the two, lipoyl- 

 thiamine pyrophosphate, is an essential component of the 

 system. But it is probable that this and other controversial 

 features of the mechanism of pyruvate oxidation can be re- 

 solved in the near future. 



The mechanism of acetate oxidation in aerobic bacteria 

 has also been a subject of great interest recently. Until 1948 

 it was generally assumed on the basis of very limited data 

 that the well-known tricarboxylic acid (TCA) cycle is the 

 main pathway of pyruvate and acetate oxidation. Then 

 several observations strongly suggested that the cycle is not 

 responsible for acetate oxidation (12). For example, cell sus- 

 pensions of Azotobacter agilis were shown to oxidize 

 acetate, whereas components of the TCA cycle, like citrate 

 and a-ketoglutarate, were not oxidized under the same con- 

 ditions. In addition, numerous tracer experiments demon- 

 strated that the oxidation of C^Mabeled acetate by living 

 bacteria does not always result in the labeling of TCA- 

 cycle compounds that are added to the medium. Such ex- 

 periments stimulated an extensive reinvestigation of the 

 role of the TCA cycle by all available methods. As a result 

 it has been definitely established that Azotobacter, E. coli, 

 and several other bacteria possess the enzymatic machinery 

 necessary to operate the TCA cycle. As far as I am aware, 

 there is at present no clear-cut evidence for the existence of 

 any alternative mechanism of acetate oxidation, such as the 



