BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 115 



phosphate during transaminations is of the same category. A phenomenon 

 entirely different from this duality in forms of a single coenzyme occurs 

 when there are synthesized from a single vitamin two specific coenzymes 

 which cannot be substituted for one another in an enzyme system. Nico- 

 tinic acid and riboflavin are the only vitamins whose coenzymes are known 

 at this time to fall definitely in this category. 



All the reactions catalyzed by thiamine, pyridoxal, and pantothenic 

 acid can be explained on the basis of a single coenzyme for each vitamin. 



Most of the known enzymatic reactions involving riboflavin require 

 the more complex coenzyme (called the dinucleotide) . A coenzyme less 

 complex in structure, riboflavin phosphate (designated as the mono- 

 nucleotide), however, activates at least three apoenzymes (p. 146). One 

 of these apoenzymes, that of the old yellow enzyme, can be reactivated 

 equally well by either the mononucleotide or the dinucleotide. The oxida- 

 tion mediated by this yellow enzyme proceeds at such a sluggish rate that 

 it could not be of use in a metabolically active cell; hence it is believed 

 to be a "derived" enzyme formed during its isolation from a more reactive 

 flavoprotein. Two other recognized mononucleotide enzymes, cytochrome c 

 reductase and L-amino acid oxidase, are capable of catalyzing the transfer 

 of hydrogen atoms at a rate rapid enough to meet adequately the demands 

 imposed upon them by natural systems. No report has been made con- 

 cerning the question of whether or not equally active systems could be 

 formed by using the dinucleotide as the coenzyme. 



Most of the reactions catalyzed by nicotinic acid, however, fall into 

 two distinct classes, each of which utilizes a specific coenzyme. The only 

 difference in chemical structure between the two nicotinic acid coenzymes 

 is one phosphate group. There are a few systems which are known to be 

 activated by either coenzyme. Upon reinvestigation of some systems for 

 which such claims had originally been made, it was shown that the co- 

 enzyme containing the extra phosphate group was degraded by traces of 

 phosphatases present as impurities in the apoenzyme preparation. 21 These 

 phosphatases convert the triphospho-coenzyme (coenzyme II), which 

 might be inactive itself, into the active diphospho-compound (coenzyme 

 I), and it would appear that either coenzyme could function. 



It may be well, before leaving the topic of vitamin-coenzyme conver- 

 sion, to speculate as to why a vitamin must be transformed into a co- 

 enzyme before it is active. In the case of no coenzyme now known does it 

 appear from the standpoint of theory of how they act that the nonvitamin 

 portion should be useful or necessary for the catalytic activity observed. 

 Thus in oxidation and reduction reactions, the addition or donation of 

 hydrogen atoms during the transfer between substrates takes place in the 



