COENZYMES DERIVED FROM B VITAMINS 183 



the two enzymes and thus make possible the coupling of the two reactions. 

 The individual systems have been found to occur in animal tissue, in 

 plant material, 224 and in bacterial cells, but as yet only those of animal 

 and bacterial origin have been resolved and tested for their coenzyme 

 requirements. The existence of a third distinct transaminase is known. 

 It catalyzes a glutamic-cysteic acid system, but it has not been studied 

 from the standpoint of its component parts. 225 Although the occurrence of 

 additional transaminases in which other amino acids form part of the 

 system has been indicated, absolute proof of their existence is still lacking. 



III. Reactions in which the Methylene Groups Attached to the a-Car- 

 bon Atom React. Two instances are known in which pyridoxal phosphate 

 catalyzes a process in which it is the methylene group bonded to the 

 a-carbon atom (rather than the amino or carboxyl group) that reacts. 

 In one tryptophan is the product; in the other, it is the substrate. 



A. Synthesis of Tryptophan from Indole and Serine. A mutant of 

 Neurospora crassa can utilize indole in place of tryptophan. 226 An analysis 

 of the mechanism, using extracts from such cells, demonstrated that the 

 biosynthesis of tryptophan can be achieved by this reaction. 199 



indole serine tryptophan 



The equilibrium for this reaction must decidedly favor the synthetic 

 process, rather than its reverse, since the degradation of tryptophan into 

 serine and indole cannot be demonstrated with the enzymatic preparation. 

 B. Non-Oxidative Degradation of Tryptophan. An extract prepared 

 from Escherichia coli has been found to catalyze the breakdown of trypto- 

 phan in the following manner 201 : 



H NH 2 O 



-c— oh — > rn n + h,c— c— c— oh + nh 3 



-U- 



N N 



H 



tryptophan indole pyruvic acid ammonia 



After inactivation of the enzyme system by aging and dialysis the 

 protein can be completely reactivated by pyridoxal phosphate. The pos- 

 sibility of a two-step reaction in which one enzyme would hydrolytically 



