124 THE BIOCHEMISTRY OF B VITAMINS 



vitamin in the cells or tissues of an organism requiring the vitamin. 

 Although this method is naturally limited to organisms which cannot 

 synthesize adequate amounts of a given vitamin, its applicability has 

 been greatly extended by inducing mutations in the genes controlling 

 vitamin syntheses, thus artificially producing organisms with the desired 

 nutritional requirements. 



Reactions requiring a specific vitamin can be blocked even in organisms 

 which do not normally require the vitamin. This is accomplished by 

 subjecting the cells to some treatment which inactivates the coenzyme. 

 Coenzymes essential for some reactions can be destroyed by placing the 

 organism in an unfavorable environment. A method of general applica- 

 tion for inactivating specific enzymes within the cell involves the use of 

 inhibitors. The methods by which inhibitors can be used in investigating 

 biological reactions is discussed in more detail in the last section of this 

 monograph. 



If the metabolic processes mediated by a coenzyme derived from a 

 vitamin are blocked by the use of any of the procedures just described, 

 by-products will accumulate. Chemical identification of the substances 

 accumulating indicates the nature of the substrate of the transformation 

 which has been blocked. Finding biochemical substances (chemically un- 

 related to the vitamin) which will partially or wholly counteract the 

 changes produced in the deficient organism serves as a means of identi- 

 fication of some of the products which would normally be formed from 

 the blocked reactions. 



The chemical changes resulting from inactivation of a vitamin-contain- 

 ing system can be demonstrated by use of classical chemical procedures 

 only when the substances involved are present in measurable concentra- 

 tions. In many important instances, however, the intracellular concen- 

 trations of the reactants and products participating in a series of reactions 

 are so low that changes from the normal metabolic pattern cannot be 

 observed by ordinary methods. This obstacle was very effectively sur- 

 mounted when the isotopes of carbon, hydrogen, nitrogen, sulfur and 

 phosphorus became available for biological tracer studies. The use of 

 compounds containing labelled atoms has been responsible for a number 

 of recent contributions which have established coenzymatic functions of 

 the vitamins: (1) the unexpected discovery of two extremely important 

 biochemical intermediates (activated derivatives of formic and acetic 

 acids) ; (2) the disclosure of a number of fundamental metabolic routes; 

 and (3) the verification of the existence of postulated reactions previously 

 undemonstrable. 



The purpose of this chapter is to present a discussion of the individual 

 coenzymes and their enzymes. These discussions will not include those 



