COENZYMES DERIVED FROM B VITAMINS 171 



aspartic acid; (3) the deamination of certain amino acids; and (4) the 

 biosynthesis of oleic acid. In each instance, however, more than one 

 mechanism for biotin activity can be justifiably postulated, and in no 

 case has a specific catalytic function been proved. The questions posed 

 by the many seemingly unrelated phenomena in biotin metabolism have 

 stimulated considerable interest, and they are under intensive investi- 

 gation. The answer to the basic question — does biotin have more than 

 one type of function? — should be forthcoming soon. 



The Role of Biotin in ^-Decarboxylations. The specific enzymatic 

 systems to which biotin was first tentatively assigned were ^-decar- 

 boxylases, the enzymes catalyzing the reactions: 



oxalacetic acid =^^ pyruvic acid-r-C0 2 

 oxalsuccinic acid ^ *" a-ketoglutaric -f C0 2 



The existence of these two reactions had been previously established, 

 and their importance in metabolism (tricarboxylic acid cycle, p. 224; 

 carbon dioxide fixation, p. 221) clearly recognized. Both enzymes had 

 been concentrated and shown to be specific for their respective sub- 

 strates. They did have comparable equilibrium constants and a require- 

 ment for the same cofactor, Mn ++ , 159 - 16 ° which is indicative of a common 

 mechanism. Although no essential organic cofactor can be directly demon- 

 strated, biotin has been postulated as a component of such systems 

 because of nutritional relationships between biotin and the metabolites 

 which participate in those two reactions: (1) oxalacetic acid (or its 

 amino acid analogue, aspartic acid) effectively replaced the biotin 

 requirement of microorganisms under certain conditions 161, 162 ; (2) 

 oxalacetic acid prevented the inhibition of growth effected by a biotin 

 analogue 161 ; (3) pyruvic acid and biotin in the absence of bicarbonate 

 were ineffective in meeting the aspartic acid requirements of an organism, 

 but when the cultures were grown in a bicarbonate-containing medium, 

 this vitamin could replace the amino acid 161 ; (4) the uptake of carbon 

 dioxide by a lactobacillus, followed by use of isotopically labelled bicar- 

 bonate, was not observed until sufficient biotin was added to substitute 

 for the aspartic acid requirement 164 ; the capacity of biotin-deficient 

 bacteria to decarboxylate oxalacetate is much less than that of normal 

 cells, 103 and a-ketoglutaric acid prevented the inhibition of a biotin 

 analogue. 161 The most logical explanation for these observations is that 

 the oxalacetic acid (and aspartic acid) and a-ketoglutaric acid require- 

 ments of the organisms either can be furnished in the form of the metab- 

 olites themselves, or can be supplied by the synthetic reactions under 

 discussion, provided the catalyst — a biotin enzyme — is supplied and is 



