FUNCTION 



2 aspartic acid 



1 («^ 



(6) (c) (d) 



2NH3 + 2 fumaric acid — ► malic — ► oxaloacetic — -> COg 



acid 



2H 



acid , 



pyruvic 

 acid 



succinic acid 



Since biotin restored the ability of E. coli to produce carbon dioxide 

 from aspartic acid, malic acid and oxaloacetic acid after the bacterial 

 cells had lost this ability on standing, they concluded that biotin 

 was concerned with the coenzyme of oxaloacetic acid decarboxylase. 



This suggestion was supported by Lardy et al.,^ who demonstrated 

 that biotin-deficient L. arabinosus could convert oxaloacetic acid to 

 aspartic acid by transamination and therefore suggested that the in- 

 ability of biotin-deficient organisms to synthesise aspartic acid lay in 

 their failure to condense pyruvic acid with carbon dioxide to produce 

 oxaloacetic acid, i.e., the reverse of reaction (d). In a medium devoid 

 of aspartic acid the addition of bicarbonate stimulated growth in the 

 presence but not in the absence of biotin. 



The hypothesis was in part supported by the work of W. Shive and 

 L. L. Rogers ^ on biotin antagonists. They found that in E. coli 

 a-ketoglutaric acid effected a 3-fold increase in the antibacterial 

 index for the competitive inhibition of biotin by desthiobiotin (page 

 452) and that L-glutamic acid had a similar effect. Aspartic acid, 

 succinic acid, fumaric acid, malic acid and czs-aconitic acid, on the 

 other hand, were inactive. This indicates that inhibition of the 

 growth effect of biotin interferes with the biosynthesis of a-keto- 

 glutaric acid which in E. coli is apparently involved in the biosynthesis 

 of aspartic acid. Thus in this organism, biotin appears to function 

 in the conversion of oxalosuccinic acid into a-ketoglutaric acid. In 

 L. arabinosus, the antibacterial index for the competitive inhibition 

 of biotin by y-(3 : 4-ureylene-cyclohexyl) -butyric acid (see page 453) 

 was increased lo-fold by the addition of aspartic or oxaloacetic acid, 

 so that in this instance biotin is apparently concerned in the synthesis 

 of oxaloacetic acid. Similar results^ with biotin antagonists were re- 

 ported by Axelrod et al. '<», who found that a number of biotin analogues 

 would neutralise the stimulatory effect of biotin on biotin-deficient 

 yeast, but that the stimulatory effect of aspartic acid was not affected 

 thereby. The analogues, therefore, presumably exert their effect by 

 preventing the conversion of biotin into a form necessary for the 

 synthesis of aspartic acid. 



Some doubt was thrown on the association of biotin with the 



443 



