182 THE BIOCHEMISTRY OF B VITAMINS 



In every case the apoenzyme used for reconstructing the system was of 

 bacterial origin, with the exception of dopa decarboxylase. 197 No direct 

 answer has been obtained to the question of whether pyridoxal phosphate 

 functions in the decarboxylation of other amino acids by mammalian 

 tissue. Until it has, the lack of a positive demonstration should be at- 

 tributed to the weakness of such systems in mammalian tissues and the 

 difficulty of resolving the holoenzyme. 



II. Transamination. Pyridoxal phosphate is known to be the coen- 

 zyme for two transaminase reactions, the so-called glutamic-aspartic 

 system and the glutamic-alanine system. 



O NH 2 O 



HO— C— CH 2 — CH 2 — C— C— OH + HO— C— CH 2 — C— OH =^= 



i 



glutamic acid oxalacetic 



acid 



O O NH 2 O 



HO— C— CH 2 — CH 2 — C— C— OH + HO— C— CH 2 — C C— OH 



H 



a-ketoglutaric acid aspartic acid 



O NH 2 O 



2 — C ( 



HO— C— CH 2 — CH 2 — C C— OH + H 3 C— C— C— OH =^= 



H 



glutamic acid pyruvic 



acid 



O NH 2 O 

 HO— C— CH 2 — CH 2 — C— C— OH + H 3 C— C C— OH 



k 



a-ketoglutaric alanine 



acid 



A combination of these two systems gives what amounts to an aspartic- 

 alanine system: 



aspartic acid + pyruvic acid ^± oxalacetic acid+alanine. 



This reaction was originally believed to be catalyzed by a single distinct 

 enzyme, 222 having another "coenzyme" in addition to pyridoxal phos- 

 phate. It is now recognized, however, that this reaction is brought about 

 by a combination of these two enzyme systems and catalytic amounts of 

 glutamic acid (or a-ketoglutaric acid). 223 These latter substances are the 

 •"coenzyme" of the combined system; they shuttle back and forth between 



