l6 NITROGEN METABOLISM 



from cultures grown in the presence of glucose and a 

 probable explanation of their results is that tryptophanase is 

 only developed after all or most of the glucose has been 

 decomposed. There is evidence that this enzyme system is 

 not developed in cultures grown in an amino-acid rich 

 medium containing glucose because phenylalanine and 

 tyrosine exert an inhibitory effect. The specificity of trypto- 

 phanase is high and indole is only formed from compounds 

 related to L-tryptophan provided that the a-carboxyl and 

 a-amino groups, the /^-position in the side chain, and the 

 N of the indole ring are unsubstituted. 



Wood, Gunsalus and Umbreit [63] have obtained from 

 Esch. coll a cell-free preparation which attacked tryptophan 

 with the formation of equimolecular amounts of indole, 

 NH3 and pyruvic acid. Their tryptophanase preparation was 

 activated by pyridoxal phosphate and would not deaminate 

 either serine or alanine. They therefore concluded that 

 neither of these amino-acids is an intermediate in the 

 degradation of tryptophan by this route. Dawes and Hap- 

 pold have performed similar experiments and reached the 

 same conclusion. Although their system produced equi- 

 molecular amounts of indole and pyruvic acid there was an 

 excess of NH3 . Whilst no correlation with tryptophan dis- 

 appearance was attempted, these observations may indicate 

 that the initial step is one of deamination which is perhaps 

 catalysed by a type of L-amino-acid oxidase. If this were so, 

 the formation of ^-indolepyruvic acid would simultaneously 

 make hydrogen available for the reductive rupture of the 

 bond linking indole to the beta carbon of pyruvic acid: 



i/^. nCH2CH(NH2)COOH 



+ H0O, -NH3, -2H 



NH 



nCH,COCOOH 



/\ . CH3 



T^ Ml + CO 



\A/ I 



NH NH COOH 



There is little direct evidence to support this hypothesis, 



