COENZYMES DERIVED FROM B VITAMINS 187 



and amine, and it would seem logical to expect either the aldehyde or 

 amine form to be active. Until recently this was thought to be the case. 236 

 Reports of recent experiments in which a highly purified preparation of 

 glutamic-aspartic transaminase was resolved indicate that in this instance, 

 at least, only pyridoxal phosphate is active in reconstituting the en- 

 zyme. 208 If this is found to be generally true when refined preparations 

 are used, the original hypothesis (which led to the discovery of cotrans- 

 aminase) may have to be modified. It may be that only the aldehyde 

 phosphate can establish the initial apoenzyme-coenzyme bond. 



The phosphorylation of the vitamin is essential for its incorporation 

 into the enzyme complexes. It has also been demonstrated that an 

 analogue of the vitamin does not associate to any degree with a decar- 

 boxylase, and hence is inactive, whereas the phosphorylated analogue is 

 an effective inhibitor. 



The Essentiality of Vitamin B G . The sparing action of alanine, 237 and 

 in particular D-alanine, 238 upon the vitamin B 6 requirements of some 

 organisms had been presumed to be due to its utilization in the synthesis 

 of pyridoxal. Although the decarboxylase and transaminase activities of 

 cells cultured upon D-alanine in the absence of a vitamin B 6 source are 

 very slight, 192 ^ 198 it has been commonly assumed that the use of D-alanine 

 results in the synthesis of only the minimum amounts of vitamin B 6 

 needed for growth. It has now been established that D-alanine is not a 

 precursor of vitamin B 6 , but is a direct product of its catalytic activity, 

 or else can indirectly function by sparing the requirement for some metab- 

 olite produced directly when vitamin B G is available. 239, 24 ° 



The phenomenon of the nonessentiality of a B vitamin was discussed 

 when biotin functions were considered (p. 174). Pyridoxal is another B 

 vitamin which may be nonessential for the growth and metabolism of 

 certain organisms. Since its primary function is the synthesis of amino 

 acids and amines, it may be possible to dispense with its reactions entirely 

 if the organism is supplied with all the essential products preformed 

 (L-amino acids, D-alanine, amines, and unidentified products in casein 

 hydrolyzates) . 238 



In higher forms of life, where amino acids and hormones like histamine 

 and adrenalin must be synthesized in situ, and where extensive degrada- 

 tion of amino acids must precede excretion, the vitamin B 6 requirement 

 could never be completely abolished. 



The Coenzymes Involved in Condensations Forming Carbon-to-Carbon 

 Bonds 



In the elaboration of more complex organic compounds from simpler 

 ones there must be condensation reactions in which carbon-to-carbon 



