106 THE BIOCHEMISTRY OF B VITAMINS 



In the preparation of this outline several types of reactions, which so 

 far have been demonstrated in only certain forms of life, have been 

 omitted since their inclusion would necessitate cumbersome qualifying 

 phrases in the description of the general types of reactions. For example, 

 the detoxification of amines by amide formation in the liver and the 

 formation of an acetate ester, acetylcholine, in nerve tissue represent 

 cases where the reactive acetyl derivative, which usually reacts to form 

 carbon-to-carbon bonds, has been utilized for specialized functions. From 

 the standpoint of function, the syntheses of such acetyl amides and 

 esters (requiring pantothenic acid, p. 195) can be considered entirely 

 distinct from the amide and ester syntheses necessary to produce fats 

 and proteins (no vitamin requirement). 



It may be that another general type of reaction in which carbon-to- 

 carbon bonds are formed will eventually be added to this group. Some 

 microorganisms can effect the condensation of the amino acid serine with 

 indole to form tryptophan (p. 183). Here it is the hydroxymethyl group 

 adjacent to the a-carbon atom (rather than the carboxy or amino group) 

 which reacts after the amino acid is activated by a pyridoxal-containing 

 enzyme. To date, this reaction is the only instance known in which pyri- 

 doxal catalyzes the formation of a carbon-to-carbon bond. 



Inositol has recently been implicated as a necessary component of 

 a-amylase, a hydrolytic enzyme. Attempts to associate it with other 

 hydrolytic enzymes have not yet been reported. The vitamin analyses of 

 several other purified enzymes catalyzing hydrolysis indicate that 

 inositol does not function generally in hydrolytic reactions. When poly- 

 saccharides are synthesized and broken down intracellularly, it is by a 

 phosphorolytic process rather than a hydrolytic one, i.e., the elements of 

 phosphoric acid instead of water take part in the formation and cleavage 

 of glycosidic linkages. It would be interesting to know if inositol is needed 

 for this type of reaction. If so, then inositol could take its place with the 

 other members of the B group as a compound which is essential for a 

 reaction required in the metabolism of practically all cells. The earlier 

 discovery that inositol is a component of cell lipides (which could 

 account for its nutritional importance) is probably partly responsible 

 for so little work being directed toward establishing other possible 

 functions. The association of inositol with enzymatic activity warrants 

 a thorough investigation of its catalytic role in metabolism. 



In view of the large number of separate reactions involved in the 

 metabolism of carbohydrates, fats, and proteins and the variety of 

 chemical compounds produced during these processes, it seems quite 

 significant that the types of enzymatic reactions are limited; also, that 

 each vitamin is specific for a given type of reaction, and that all the 



