THE VITAMIN B COMPLEX 



or condensation with another molecule (page 102). This adequately 

 explains the importance of aneurine in nutrition, for pyruvic acid 

 occupies one of the key positions in both fat and carbohydrate meta- 

 bolism. The conversion of pyruvic acid into acetic acid, however, 

 also involves pantothenic acid (page 391), without which biological 

 acetylations cannot take place. It is possible that the link between 

 fatty acid metabolism and the tricarboxylic acid cycle may also be 

 broken in its absence, although this has not yet been proved. 



Another member of the vitamin B complex concerned with fat 

 and carbohydrate metabolism is biotin, which appears to be a con- 

 stituent of the coenzyme of oxaloacetic acid decarboxylase (page 

 443). This is responsible for the conversion of oxaloacetic acid into 

 pyruvic acid and, presumably, for the fixation of carbon dioxide by 

 pyruvic acid. It may also be concerned with the conversion of oxalo- 

 succinic acid into a-ketoglutaric acid. 



Thus practically all the B vitamins, of which sufficient knowledge 

 is available, appear to be involved in carbohydrate and fatty acid 

 metabolism. 



In addition to their function as hydrogen carriers in association 

 with various dehydrogenases, the fiavine nucleotides are also necessary 

 for the oxidation of D-amino acids to the corresponding keto-acids, 

 and for the oxidation of diamino acids and glycine (page 195), whilst 

 cozymase is essential for the conversion of glutamic acid into a-keto- 

 glutaric acid (page 278). Thus the oxidation of amino acids also 

 depends on the presence of riboflavine and nicotinic acid. 



Amino acids undergo other changes besides oxidation, and these 

 involve another member of the vitamin B complex, pyridoxine, which 

 is apparently not concerned with carbohydrate metabolism. As 

 already noted (page 331), pyridoxal phosphate is the coenzyme re- 

 sponsible for the decarboxylation of the amino acids, L-lysine, L-tyro- 

 sine, L-histidine, L-arginine, L-ornithine and L-glutamic acid, with 

 the formation of the corresponding amines. Pyridoxal phosphate 

 also catalyses the transfer of an amino group from an amino acid to 

 a keto acid with the formation of a second amino acid, and it appears 

 to be responsible for the synthesis of tryptophan from indole and 

 serine. 



A considerable amount of research work is at present being carried 

 out on the mechanism of enzyme reactions, and the next few years 

 will doubtless provide many further examples of reactions in which 

 known members of the vitamin B complex are involved. In addition, 

 it is not unlikely that other substances having analogous properties 

 will be discovered and that these will have all the characteristics that 

 we have come to associate with the components of the vitamin B 

 complex. Thus, they should produce characteristic deficiency 



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