COENZYMES DERIVED FROM B VITAMINS 177 



common coenzyme for these two systems. 197 Subsequently, the reconstruc- 

 tion of a transaminase system from an inactive apoenzyme and pyridoxal 

 phosphate was accomplished, thus proving conclusively the identity of 

 the vitamin B 6 coenzyme and cotransaminase. 198 



Reactions in which tryptophan either is synthesized from indole or is 

 cleaved to produce it have been demonstrated in bacteria and molds. The 

 enzymes required can be resolved into inactive components. Synthetic 

 pyridoxal phosphate can reactivate the apoenzymes 199, 200 and is pre- 

 sumably identical with naturally occurring "cotryptophanase." 



Structure of the Coenzyme. On .the basis of its chemical composition, 

 pyridoxal phosphate is probably the simplest of the B vitamin coenzymes. 

 Yet four years have elapsed since the first synthetic preparation of the 

 coenzyme was made, and its structure still cannot be stated to have been 

 proved. When pyridoxal is treated with phosphorylating agents, a phos- 

 phorylated derivative (obtained as a crude barium salt) is formed. 192 

 This "synthetic coenzyme" has the biological properties of the natural 

 codecarboxylase, cotransaminase, and cotryptophanase. 192, 198, 201 



The active compound was first prepared synthetically by American 

 scientists who have now conclusively shown it to be not the 3-phosphate 

 (the phenolic ester), 202 although in their original report properties were 

 reported for the ester which suggested it was pyridoxal-3-phosphate. 203 

 Meanwhile, Swiss chemists had prepared the acetal of pyridoxal-3- 

 phosphate by methods which leave no doubt as to the structure of their 

 product. 204 They had claimed codecarboxylase activity for their synthetic 

 product, but were unable to activate an apotransaminase system with 

 it. 205 - 206 The dispute concerning the activity of the 3»phosphate ester has 

 been resolved by simultaneously testing the products prepared in different 

 laboratories on the same biological system. 202 The activity of the 3-phos- 

 phate is so low compared to that of the active "synthetic coenzyme" of 

 the American group that the slight response elicited by the former might 

 be attribued to traces of an active isomer formed by an intramolecular 

 transesterification. 



The active synthetic derivative when cleaved yields equimolecular 

 amounts of pyridoxal and inorganic phosphate. 203 Since it is not the 

 phenolic ester, its structure is presumably that of the phosphoric ester of 

 the hydroxymethyl group (position 5 on the pyridine nucleus). Pyri- 

 doxamine can be readily produced in quantitative yields by heating 

 pyridoxal phosphate with an excess of glutamic acid in a neutral solu- 

 tion, 207 and it possesses the specific growth-promoting properties of the 

 compound prepared by a direct phosphorylation of pyridoxamine. 208 

 The esters are quite stable in alkaline solution (no destruction after 

 heating for five hours in IN NaOH at 120° C), but they are rapidly 



