246 VITAMINS 



presence of sulfur in the other ring (B) . Biotin and thiamine are the only 

 vitamins that contain sulfur. 







II 



c 



NH "^NH 



A 



CH CH 



I B I 

 CH2 CH-(CH,)4-C00H 



S 



Biotin, C10H16O3N2S 



Although it is readily destroyed by such oxidizing agents as hydrogen 

 peroxide, biotin is, in general, a very stable substance. It is not affected 

 by light, strong acids such as normal HCl or H2SO4, nor by exposure to a 

 degree of heat greater than that encountered during ordinary cooking 

 operations. However, it is destroyed by strong alkali. ^ In many tissues 

 it appears not to exist in a free state, but in combination with some cell 

 constituent, presumably protein. This view is supported by the recent 

 isolation from autolyzing yeast of biocytin, a peptide-hke combination 

 of biotin and the amino acid, lysine. Note that the linkage is through 

 the evsilon amino group of lysine. 







II 



I I 



HC CH 



I I 



H2C CH(CH2)4CONH— (CH2)4— CH-COOH 



S I 



NHi 



Biocytin 



Nothing is known as yet regarding the amount of biotin needed by 

 human beings. However, the quantities required by various lower organ- 

 isms are so extremely minute that it must be regarded as one of the 

 most highly active substances known. Its effect on yeast growth, for 

 example, can still be detected at dilutions of 1:300,000,000,000. 



PTEROYLGLUTAMIC ACID 



This vitamin was first observed in connection with studies on the nutri- 

 tional requirements of lactic acid bacteria. An impure preparation from 

 liver, designated as the "norite eluate factor," was shown to be necessary, 



