168 THE BIOCHEMISTRY OF B VITAMINS 



Schiff's base by the elimination of the elements of water from the amino 

 group of thiamine and the carbonyl group of the substrate has been 

 postulated. 137 The essentiality of the amino group on the pyrimidine ring 

 of the vitamin suggests the formation of such an intermediate, but its 

 existence has not been demonstrated. 



Whenever the reaction catalyzed is of an oxidative type the enzyme sys- 

 tem is momentarily in possession of the equivalent of two available hydro- 

 gen atoms. The enzyme system must, therefore, exist in both an oxidized 

 and reduced state. It has been postulated that the coenzyme would be the 

 most likely component of the enzyme system to undergo a reversible 

 oxidation and reduction, since such is the case in other enzymes trans- 

 porting hydrogen atoms. The possibility that some group of the apoenzyme 

 component (rather than the coenzyme) may be the actual hydrogen 

 carrier, possibly by constituting a thioldisulfide system, should not be 

 overlooked. Attempts were first made to demonstrate a reversible reduc- 

 tion and reoxidation of the thiazole nucleus in a manner analogous to 

 the pyridine-dihydropyridine interconversions of the nicotinic acid co- 

 enzymes. This possibility is no longer seriously considered. However, it 

 has been recently pointed out that the dihydrothiamine pyrophosphate 

 has never actually been prepared, since all the attempts to reduce the 

 thiazole nucleus chemically resulted in a cleavage of the molecule at the 

 methylene bridge connecting the two aromatic nuclei of the vitamin. 152 



A second mechanism, wherein an oxidized and reduced state of thiamine 

 would also exist, has been postulated on the basis of the observed thia- 

 mine activity of "thiamine disulfide," a dimer in which the thiazole 

 nucleus opens. 153, 154 If the suggested equilibrium occurs, the structure 

 always ascribed to the vitamin represents the reduced, rather than the 

 oxidized form. Although the disulfide analogues of either thiamine or its 

 coenzyme are active when tested with intact cells or organisms, these 

 compounds do not reactivate cell free preparations of apocarboxylase. 155 

 This would indicate that the disulfides are not active oxidized forms of 

 the vitamin or coenzyme but are instead compounds which, although 

 inactive per se, can be reduced by cells to form the vitamin or coenzyme 

 having an intact thiazole nucleus. Since the decarboxylation in which 

 thiamine disulfide was tested is a nonoxidative one, it can justifiably be 

 argued that the reaction should not be used for testing the validity of 

 any hypothesis concerned with oxidized and reduced states of the 

 coenzyme. 



The Function of Thiamine. In making a statement concerning a 

 general mode of action for the thiamine coenzyme in the decarboxylation 

 of a-keto acids, one should consider three questions: 



