AMINO-ACID CATABOLISM 15 



catabolism by Bacillus subtilis, another aerobe, appear to be 

 similar to those found in the Pseudomonas spp. [40]. 



Tryptophanase 



The appearance of indole in the culture medium is a 

 valuable diagnostic test in bacteriology and it is readily 

 detected by the formation of a pink compound in the 

 presence of ^-dimethylaminobenzaldehyde and acid. Hop- 

 kins and Cole were the first to isolate tryptophan and imme- 

 diately suspected and proved that it was the natural 

 precursor of this indole. The enzyme system concerned has 

 been termed tryptophanase, and is found in some species of 

 Escherichia, Proteus and Vibrio, though there are strain 

 differences in any one species. Oxygen is probably not 

 directly involved in the initial step of the tryptophanase 

 reaction, but indole tends to accumulate only in aerobic 

 conditions; e.g. in the absence of Og , Esch. coli formed 

 indolepropionic acid and little, if any, indole. When washed 

 cells of Esch. coli were incubated with tryptophan and the 

 system adequately aerated, the uptake of oxygen corres- 

 ponded to that required for the complete oxidation of the 

 alanine side chain [64]. Most of the experiments concerned 

 with the mode of action of tryptophanase have been per- 

 formed with preparations of Esch. coli, and prior to attempt- 

 ing to prepare the enzyme system in a cell-free state atten- 

 tion was directed to the factors affecting its activity in whole 

 cells [29]. Tryptophanase was found to be adaptive, and 

 extremely active cells were obtained from vigorously aerated 

 media containing tryptophan. Whether Esch. coli grown in 

 the absence of the substrate exhibits detectable trypto- 

 phanase activity appears to depend on the strain of the 

 organism concerned. The inclusion of glucose in the 

 medium may result in the suppression of indole formation, 

 and if it does, the cells from such cultures do not exhibit 

 tryptophanase activity. This effect is not due to growth in 

 an acidic medium since other sugars are metabolized with 

 the formation of acidic end-products and yet they do not 

 suppress the adaptive formation of tryptophanase. How- 

 ever, several workers have reported activity in cells derived 



