174 THE BIOCHEMISTRY OF B VITAMINS 



occurring substances, more complex in structure than biotin, will be found 

 to be a coenzyme. 



Biotin and certain of its isotels form very stable complexes with avidin 

 and other proteins, a combination that cannot be effectively dissociated 

 by any means yet tried. The release of the biotin component can be 

 accomplished only by destroying the protein. 1ST A similar nondissociation 

 of coenzyme-apoenzyme may exist in biotin-containing enzymes. 



An insight into the chemical nature of one biotin coenzyme is given 

 by the investigations on the reactivation of amino acid deaminases 

 (p. 173). The system which these investigators have developed should 

 serve as a method which could be adapted for the quantitative determina- 

 tion of this biotin coenzyme provided the preparations were aged a suffi- 

 cient length of time so that the component parts of the cleaved (?) 

 coenzyme in the apoenzyme preparation were no longer active. 



Is biotin always an essential cell constituent? In certain "biotin-requir- 

 ing" bacteria, biotin seems to be essential for only two processes, one 

 related to the synthesis of aspartic acid and one to oleic acid production. 

 Any other metabolites produced by biotin-catalyzed reactions are appar- 

 ently dispensable or else can be obtained by alternate mechanisms. Con- 

 sequently, if these particular organisms are furnished an exogenous supply 

 of aspartic and oleic acids, they no longer require detectable amounts of 

 biotin for growth and reproduction. In this instance the synthesis of 

 demonstrable quantities of biotin by the organism does not occur. Hence, 

 this may be one instance in which a B vitamin becomes nonessential for 

 life. Possibilities which have not been completely eliminated are that 

 extremely minute quantities of the vitamin are present in the culture 

 medium as impurities, or else that small amounts are being synthesized 

 intracellularly. Such undetectable amounts could still be performing 

 certain duties essential for the survival of the organism. Since the addition 

 of avidin to the medium does not alter the situation, the presence of an 

 exogenous source seems unlikely unless the impurity is one of the avidin- 

 uncombinable isotels. 185 Intracellular synthesis, however, cannot yet be 

 conclusively ruled out, since current assay procedures may not determine 

 all forms of bound biotin. 



The intracellular concentration of both vitamin B 6 and folic acid may 

 likewise be reduced to negligible quantities under similar circumstances 

 wherein products of their functioning are supplied preformed to bacterial 

 cells (pp. 187 and 202). 



The Coenzyme Activating a-Amino Acids 



Most organisms are capable of synthesizing from other metabolic inter- 

 mediates at least part of their amino acid requirements. Processes of a 



