110 THE BIOCHEMISTRY OF B VITAMINS 



coenzymes will not fit into this classification. The number of substances 

 known to function coenzymatically is small, and the B vitamin deriva- 

 tives constitute a major portion of the list. 



Several naturally occurring oxidizing and reducing agents, including 

 ascorbic acid and glutathione, have been classified as "enzyme activators." 

 The requirement for these substances, however, is not specific since other 

 compounds can generally be substituted. It would seem, then, that these 

 compounds should be considered as poising agents needed to adjust the 

 redox potential to a level at which the protein will be active. An excep- 

 tion to this may be the requirement for glutathione, or closely related 

 compounds, necessary for the functioning of glyoxalase. 8 Here gluta- 

 thione appears to participate chemically in the reaction. 



There are a number of important compounds which are essential for 

 certain cyclic enzymatic processes. Ornithine in the urea cycle and 

 oxalacetic acid in the tricarboxylic acid cycle (p. 224) are familiar 

 examples. These substances go through a series of enzymatic transforma- 

 tions, but are regenerated at the completion of the cycle. For this reason 

 such a substance is sometimes referred to as a "carrier" for the com- 

 pound which is produced or consumed during the process. Although it is 

 possible to show a great deal of similarity between the manner in which 

 such "carriers" function and the way in which some of the coenzymes 

 function (p. 137), these "carriers" are commonly thought of as a type of 

 catalyst distinct from the typical coenzymes. 



There are a number of questions concerning the relation of the B vita- 

 mins to their respective coenzymes and the relation of the coenzymes 

 to the proteins that will be considered in the following section of this 

 chapter. The individual coenzymes will be discussed in detail in Chapter 

 IIB. 



The Coenzymes 



Intensive research on the structures and functions of the coenzymes 

 derived from the B vitamins is currently being conducted in a number 

 of laboratories. It is probable that many important contributions directly 

 related to the topics to be discussed are about to be reported in the 

 journal literature. The information available as this book goes to press 

 will be summarized in the following paragraph. 



The coenzymes derived from thiamine, riboflavin, nicotinic acid, and 

 pyridoxine have all been isolated and their chemical structures deter- 

 mined. A crystalline preparation of a pantothenic acid-containing 

 coenzyme has not yet been announced, but the properties and hydrolytic 

 products of highly active preparations have been reported. Absolute 

 proof that biotin is a part of a specific coenzyme has not yet been pub- 



