COENZYMES DERIVED FROM B VITAMINS 175 



reverse type in which amino acids are degraded and metabolized are also 

 of common biological occurrence. A general method of synthesis or deg- 

 radation, reductive amination or oxidative deamination, has already 

 been pointed out in the discussion of reactions catalyzed by the coenzymes 

 of nicotinic acid and riboflavin (pp. 141 and 147). Several other types 

 of reactions, however, constitute alternate pathways by which amino 

 acids may be elaborated or utilized. At least three of these types are 

 reactions catalyzed by enzymes having a common coenzyme — one derived 

 from vitamin B 6 . In several instances, at least, reactions catalyzed by 

 this vitamin are the only methods by which an organism can adequately 

 produce particular amino acids and essential metabolites derived from 

 them. 



The Vitamin B 6 Coenzyme 



A period of almost six years separated the time when pyridoxine was 

 first synthesized (1939) and the date when the biocatalytic functions of 

 vitamin B 6 were discovered. Then, within a period of a few months, two 

 distinct metabolic processes involving entirely separate types of chemical 

 reactions were shown to be dependent upon the presence of vitamin B 6 

 derivatives. Two years later several reactions constituting a third type of 

 chemical reaction were shown to require the identical pyridoxal coenzyme 

 as did the two processes earlier recognized. 



The decarboxylation of tyrosine and several other amino acids by 

 bacterial cells had been observed to be catalyzed by an enzyme containing 

 a dissociable codecarboxylase. lss None of the coenzymes then known, 

 however, were active in reconstituting purified preparations from which 

 the coenzyme had been separated, but a concentrate having 15,000 times 

 the coenzymatic activity of the richest natural source (yeast extract) 

 had been prepared. 189 Concurrently, it was observed that the decarboxyla- 

 tion of tyrosine by resting cells of a lactic acid bacterium was influenced 

 by the medium in which the organism had been cultured. 190 For the pro- 

 duction of cells possessing optimum decarboxylase activity the medium 

 in which the organism was grown had to contain three times the amount 

 of pyridoxine needed to promote maximum growth. Since the enzymatic 

 activity of the cells varied according to the amount of pyridoxine fur- 

 nished them during growth, pyridoxine and its analogues, pyridoxamine 

 and pyridoxal (which had just become available), were tested with cell 

 suspensions of bacteria collected from cultures grown on a vitamin B 6 -free 

 medium. (The vitamin can be omitted from the medium if high levels 

 of alanine are used — see p. 187.) The addition of pyridoxal to the "vita- 

 min-deficient" cells increased the rate of decarboxylation twentyfold; 

 pyridoxamine and pyridoxine were inactive. 191 It was subsequently shown 



