98 THE BIOCHEMISTRY OF B VITAMINS 



of an enzyme system. 7 Some weight was given to such interpretations, 

 since at that time it appeared that there might be almost no end to the 

 list of growth-promoting substances that would eventually be found; 

 also, none of the numerous plant and animal hormones had been impli- 

 cated in specific enzyme reactions. 



However, in the five-year period 1943 to 1948, pantothenic acid, 

 pyridoxal and inositol were found to be parts of definite enzyme systems 

 involved in fat, protein, and carbohydrate metabolism; and folic acid, 

 p-aminobenzoic acid, biotin, and a new member of the group which was 

 isolated during this period, vitamin Bi 2 , were each shown to function in 

 one or more fundamental enzymatic reactions which, at the time of this 

 writing, are in the process of being more fully characterized. During this 

 same interval it also became increasingly apparent that the number of 

 unknown B vitamins might not be as great as previously assumed, and 

 that many of the remaining uncharacterized factors of general biological 

 significance which had been reported would be found to be mixtures or 

 derivatives of compounds already known. 



Later in this chapter (p. 104) an outline will be given of the funda- 

 mental types of enzymatic reactions which have been found to constitute 

 the chemical routes over which the basic processes in carbohydrate, 

 lipide, and nitrogen metabolism must proceed. A number of these enzyme 

 systems contain as an indispensable part one of the B vitamins. Of greater 

 interest is the fact that, except for inositol, each one of the B vitamins is 

 always required in at least one group of these essential reactions, thus 

 establishing a definite association between every typical B vitamin and 

 the processes which form the foundation upon which life is built. If any 

 one of these types of reactions is lacking, a series of gaps results which 

 cannot be effectively bridged or by-passed, and the procession of re- 

 actions necessary for the maintenance of cell activity must cease.* It is 

 for this reason that these vitamins occur universally and are a pre- 

 requisite for life. 



From the standpoint of organic evolution the thesis that there exist 

 throughout the entire plant and animal kingdoms a certain number of 

 essential and fundamental types of enzymatic processes common to all 

 organisms seems not only reasonable but almost necessary. An apprecia- 

 tion of this situation has been of invaluable help to those investigators 

 interested in studying the details of metabolism, for it means that they 

 can choose as their biological tool any organism or species with which 



* See pp. 174, 187, and 202 for a discussion concerning possible exceptions — the 

 interesting cases where the reactions catalyzed by biotin, pyridoxal, and folic acid 

 may be by-passed in certain bacteria. 



