COENZYMES DERIVED FROM B VITAMINS 181 



reactions: (1) simple decarboxylations, (2) oxidative transfer of amino 

 groups, and (3) condensations establishing carbon-to-carbon bonds or 

 the reverse process in which these bonds are cleaved. However, in every 

 reaction known to be catalyzed by this coenzyme there is a common type 

 of substrate, an a-amino acid, and in every instance the reaction involves 

 a group or atom attached to the a- carbon atom. 



H COOH COOH H COOH 



R— C— C— H R— C— C— H R— C— i— H 



H NH 2 NH 2 H NH 2 



decarboxylation transamination tryptophan 



synthesis and cleavage 



I. Amino Acid Decarboxylation. Amino acid decarboxylation is un- 

 doubtedly an important means by which bacteria can metabolize some 

 amino acids. This type of reaction also provides these organisms a means 

 of producing alkaline substances with which they can alter an undesirable 

 acidic environment. 217 In addition, the decarboxylation of amino acids 

 seems to be the most likely source for certain polyamines which have been 

 shown to be bacterial metabolites. 



Decarboxylation of a-amino acids by mammalian tissues can be demon- 

 strated, although they never possess activity comparable to that observed 

 in many bacteria. This type of reaction is not believed to be an important 

 general mechanism for the catabolism of amino acids, but it may be the 

 process by which are formed histamine (from histidine), taurine (from 

 cysteic acid), ethanolamine (from serine), ^-alanine (from aspartic acid), 

 adrenalin (from 3,4-dihydroxy phenylalanine — "dopa") and putrescine 

 (from ornithine). Detailed reviews of mammalian and bacterial decar- 

 boxylases were published in 1945 and 1946. 218, 21T 



Pyridoxal phosphate has been conclusively shown to be the catalyst 

 for these six reactions: 



H H 



R— C— COOH — > C0 2 +R— C— H 



NH 2 NH 2 



1. tyrosine >■ C0 2 +tyramine. 219 



2. dopa — >• C02+3,4-dihydroxyphenylethylamine. 219 - 197 (precursor of 



adrenaline.) 



3. lysine >■ C0 2 +cadaverine. 220 



4. ornithine > C0 2 +putrescine. 221 



5. arginine >■ C0 2 +argamine. 221 - 21B 



6. glutamic acid >■ C0 2 +7-aminobutyric acid. 216 



