120 THE BIOCHEMISTRY OF B VITAMINS 



the conversion of a vitamin to its coenzyme. If such compounds were 

 available, they could be used to adapt the simple microbiological pro- 

 cedures used in vitamin determinations to specific assay methods for the 

 coenzymes. 



Today the only specific methods of assay for most coenzymes involve a 

 direct determination of the effect of the addition of a substance upon an 

 enzymatic reaction in which the particular coenzyme is the limiting factor. 

 This entails the preparation of the appropriate apoenzyme, either from 

 cells or tissues deficient in the coenzyme or from protein preparations in 

 which the coenzyme has been destroyed or removed. Such enzymatic assay 

 methods for coenzymes have a number of disadvantages not encountered 

 in the microbiological methods. They require equipment, chemicals, and 

 biological preparations which may not be readily available; technically, 

 more training and skill is usually needed than for a microbiological assay. 

 In addition, a general standardized procedure for all B vitamin coen- 

 zymes cannot be used, since the analysis for each coenzyme constitutes 

 a special method. 



Occurrence of Coenzymes. Since one of the reasons for grouping the 

 B vitamins together is the similarity of their natural distribution, it would 

 be of interest to know if the enzymes containing them are associated 

 together physically within cells. Some of the fundamental processes which 

 involve a number of steps may require at most a single B vitamin coen- 

 zyme, e.g., the elaborate mechanism by which glycogen is converted to 

 lactic acid in muscle requires fourteen separate enzymes, but only one 

 B vitamin coenzyme is needed (p. 219). On the other hand, there are 

 processes every step of which must be mediated by a different B vitamin. 

 The conversion of carbohydrate to fat involves a series of reactions in 

 which each pyruvic acid molecule eventually lengthens a fatty acid by 

 two carbon atoms. This conversion requires enzymes containing thiamine, 

 pantothenic acid, nicotinic acid, and probably riboflavin (p. 226). This 

 series of reactions cannot be demonstrated if the structure of the cell has 

 been destroyed. It is not known if this process is carried out by a number 

 of separate enzymes which are physically separated from one another in 

 the cell. It may be that the transformation requires an enzyme complex 

 in which the component proteins and coenzymes are actually combined, 

 and that this system would be inactive if its organization were disturbed. 



Recently a process has been described for preparing a protein complex 

 from mammalian tissue containing all the enzymes necessary for the 

 aerobic oxidation of pyruvic acid and certain fatty acids through the 

 tricarboxylic acid cycle. In its isolation, the protein complex containing 

 all the essential component enzymes separates out as a gel. 25 When the 

 gel loses its ability to carry through the series of reactions there is an 



