254 VITAMINS 



Like nicotinic acid it had been known to organic chemists and had been 

 obtained synthetically long before its usefulness as a vitamin was dis- 

 covered. It is very soluble in water and is quite stable to boiling in 

 dilute aqueous solution. Hot alkalies, however, decompose it with the 

 formation of trimcthylamine. 



Bound choline in the form of lecithin is present in every living cell, 

 and free choline is likewise very widely distributed in biological materials. 

 At present, no information is available regarding the human requirement 

 for this dietary factor. 



OTHER DIETARY FACTORS 



There are a number of other factors that have been reported as essential 

 in the diet of experimental animals, but to discuss them in any detail 

 would be beyond the scope of this book. However, two definite chemical 

 substances in addition to those already considered have been shown quite 

 conclusively to belong to the vitamin B complex. These are para-amino- 

 benzoic acid and inositol: 



COOH 



H 

 O 



/§\ 

 HOCH ^ HCOH 



HOCH „ HCOH 



c 

 o 



H 



Inositol 



The former is probably used for the biosynthesis of pteroylglutamic acid 

 and owes its vitamin-like activity in certain species to this circumstance. 

 Inositol is required by mice and rats for normal growth and the avoidance 

 of dermatitis and loss of hair. It is not known to be required by human 

 beings. 



Another vitamin-like substance needed by certain microorganisms is 

 lipoic acid, which has recently been obtained in pure form and found to 

 have the following structure: 



CH2CH2CH(CHj)4— COOH 



s s 



a-Lipoic acid or thioctic acid 



According to Reed and De Busk it is combined in the living cell with 

 thiamine and phosphoric acid to form lipothiamide pyrophosphate, which 

 appears to be a necessary coenzyme for the oxidative decarboxylation of 

 a-keto acids, such as pyruvic acid, during metabolism. 



