FRANK M. HUENNEKENS 



cytidine diphosphate chohne from phosphochohne and cytidine 

 triphosphate (24), and 6'-adenosyl methionine from ATP and 

 methionine (11). However, all of these reactions yield stable 

 intermediates of the coenzyme and the mobile group and have 

 been included, therefore, in Table I. Guanosine disphosphate 

 is, however, a true coenzyme, since it functions catalytically, 

 and is regenerated at the end of the reaction, in the process of 

 oxidative phosphorylation at the substrate level (see equation 



(7)). 



Pyridoxal phosphate participates in reactions where a- 

 amino acids are decarboxylated or racemized, and uridine 

 diphosphate is concerned in the structural conversion of galac- 

 tose- 1 -phosphate to glucose-1 -phosphate. However, each of 

 these coenzymes also is involved in normal reactions of equations 

 (1) and (2), and even in the exceptions cited here, the coenzymes 

 "anchor" the amino acid and sugar residues. 



There are several notable omissions in the above table. 

 Ascorbic acid has often been considered to be involved in elec- 

 tron transport, but the high level of this substance in tissues, 

 compared to other vitamins, has made it appear to be an excep- 

 tion to the "vitamin-trace substance" theory. Recently, how- 

 ever, it has been shown to participate at catalytic levels in the 

 oxidative breakdown of tyrosine. Biotin, too, has been im- 

 plicated in carboxylation reactions, although it has been difficult 

 to demonstrate it as a coenzyme in these processes. It is quite 

 possible that both of these substances, ascorbic acid and biotin, 

 actually exist as more complex coenzymes, perhaps of analogous 

 structure to those listed in Table I. This is not to argue that all 

 coenzymes must contain something in addition to the moiety 

 bearing the "active center" (to which the group being transferred 

 is attached), since lipoic acid is certainly anomalous in this 

 respect, although Reed (36) has suggested that lipoic acid may 

 be conjugated to diphosphothiamin — a suggestion challenged 

 by Gunsalus (18). 



Vitamin B12 has also been omitted from consideration, since 

 its structure is not completely established, although it does con- 



498 



