INDOLE FORMATION 173 



indole is tryptophan. The bacterial reaction occurs aerobically, 

 and the oxidation of one molecule of tryptophan to indole is 

 accompanied by the consumption of five atoms of oxygen. 

 We can make guesses at the nature of possible intermediate 

 substances, but if theoretically possible intermediates such 

 as indole-acetic, -propionic, -pyruvic, -acrylic, -lactic acids, 

 or the corresponding aldehydes, are added either to cultures 

 or to washed suspensions of active organism, they do 

 not give rise to indole and consequently cannot be indole 

 precursors. In an atmosphere of hydrogen Esch. coli produces 

 indole-propionic acid from tryptophan, but this must involve a 

 different metabolic path from that followed aerobically, as 

 indole-propionic acid is not decomposed further under aerobic 

 conditions. It seems that the first step in the aerobic decom- 

 position of tryptophan is not related to those steps discovered 

 for other amino-acids. 



Recent studies with the bread mould, Neurospora crassa, 

 have shown that tryptophan is synthesised by this organism 

 from indole and L-serine and that the reaction is reversible: 



+ CH2OH.CHNH0.COOH X \ CH2.CHNH2. 



I III COOH 



H H + ^2^ 



The same workers suggested that a similar reaction is involved 

 in the synthesis and breakdown of tryptophan by Esch. coli. 

 Further studies have not confirmed this suggestion, as a cell- 

 free preparation has been obtained from Esch. coli. and the 

 breakdown of tryptophan by the preparation results in the 

 liberation of indole, ammonia, and pyruvic acid. 



/\ CH2.CHNH2.COOH /\ 



I I I -fHgO^ ^1 I I+NH3+CH3.CO.COOH 



H H 



The preparation will not attack either serine or alanine so 

 these cannot be intermediates in the reaction. The oxygen 



