172 THE BIOCHEMISTRY OF B VITAMINS 



not inhibited by biotin analogues. The inability of biotin-deficient tissues 

 to metabolize pyruvic acid 165, 16C or of biotin-deficient yeast cells to 

 utilize glucose aerobically 167 can be attributed to the deficit of oxalacetic 

 acid needed to catalyze the tricarboxylic acid cycle by which these sub- 

 strates are "oxidized." 



However, the following observations which are not in line with this 

 hypothesis suggest that the function of biotin in /^-decarboxylations may 

 not be a direct one: (1) aspartic acid, but neither oxalacetic acid nor any 

 other dicarboxylic acids which can be converted to oxalacetic acid, alters 

 the biotin requirement of yeast 168 ; (2) the biotin content of oxalacetic 

 acid decarboxylase preparations from a bacterium decreased during puri- 

 fication, 169 and no biotin at all was found in a purified preparation of 

 animal origin. However, the biotin might have been in a form which was 

 inactive in the microbiological assays. 170 



Biotin Function in Aspartic Acid Synthesis. The sparing effect of 

 aspartic acid on the biotin requirements of yeast 171, 172 and bac- 

 teria, 161, 1G2 - 173> 174 can best be interpreted on the basis of biotin function- 

 ing either directly or indirectly in the synthesis of this amino acid. Since 

 aspartic acid is effective when no other C 4 dicarboxylic acids are, 168 it is 

 possible that the reaction in aspartic acid synthesis which is catalyzed 

 by biotin is not one in which oxalacetic acid is directly formed from 

 pyruvic acid by carboxylation. This would also explain why aspartic 

 acid is always more effective than oxalacetic acid in substituting for 

 biotin, and why other C 4 dicarboxylic acids, which should be easily con- 

 verted to oxalacetic acid, are inactive both as substitutes for biotin and 

 as agents for reversing biotin inhibitors. 



The diminished rate of respiration of biotin-deficient yeast 167, 175 is 

 increased by the addition of either aspartic acid or biotin plus ammonium 

 salts. Biotin alone is ineffective. This observation suggests that the 

 reaction catalyzed by biotin in the synthesis of aspartic acid is one in- 

 volving an amination. 



Biotin as a Catalyst for Deaminations. The type of reaction in which 

 biotin has been most directly implicated is the deamination of certain 

 amino acids. When bacterial cells are suspended in acid buffers they 

 rapidly lose their ability to decarboxylate aspartic, malic, and oxalacetic 

 acids 176 and to deaminate aspartic acid, threonine, and serine. 177 Extracts 

 of dried cells can be reactivated by the addition of yeast extract, or by 

 biotin plus adenylic acid (muscle) , but not by biotin alone. 178 Although 

 the biotin-adenylic acid mixture is as effective as yeast extract initially, 

 the combination becomes ineffective after the cell preparations have been 

 stored; the yeast extract, on the other hand, maintains its ability to 

 reactivate the stored preparations of the deaminase systems. These 



