8 THE BIOCHEMISTRY OF B VITAMINS 



appear to be fundamentally more important biocatalytically than 

 pyridoxine. The three members of the group possess for animals what 

 has been called "vitamin B 6 " activity. The chemistry of these compounds 



CHO CH 2 NH 2 CH 2 OH 



A A A 



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



H 3 C— C CH H 3 C— C CH H 3 C— C CH 



\/ \/ V 



N N N 



Pyridoxal Pyridoxamine Pyridoxine 



is discussed in a series of articles by Heyl et at. and in earlier contributions 

 from the same laboratory. 7 - 8 - 9 - 10 



Biotin. This substance is the only naturally occurring one (except its 

 conjugates and possibly oxybiotin discussed below) known to be capable 

 of counteracting the deficiency induced by feeding raw egg white or 

 avidin, its active constituent, to animals. It is an example of a B vitamin 

 which appears to be of little or no practical importance nutritionally 



O 



II 

 C 



HN NH 



HC CH 



H 2 C CH— CH 2 — CH 2 — CH 2 — CH 2 — COOH 



V 



Biotin 



(because it is so readily produced by intestinal organisms) but is none- 

 theless most interesting as a universal cellular constituent which prob- 

 ably acts catalytically. It is effective in unusually low concentrations. 



Oxybiotin, the compound corresponding to biotin with oxygen replac- 

 ing the sulfur, is especially interesting as a substance which almost falls 

 within our definition of a B vitamin. Certain microorganisms, at least, 

 can use it in place of biotin and do not convert it into biotin. 11 Oxybiotin 

 in these cells can act as a part of the metabolic machinery, but since it 

 appears ordinarily not to be a constituent of cells, and certainly there 

 is no evidence that it is present in all cells, its inclusion in the group of 

 B vitamins is not justified. 



The organic chemistry of biotin and its derivatives has been reviewed 

 by Hofmann 12 and by Melville, 13 and further discussion of its chemistry 

 will be found in Section D of this volume. 



