METABOLIC FUNCTIONS OF B VITAMINS 233 



glycine v "* serine (p-aminobenzoic acid, folic acid, vitamin B12) 38 

 homocysteine v "*, methionine (p-aminobenzoic acid, vitamin B 12 ) 39 

 serine >■ tryptophan (pyridoxal) 40 



Degradation of Amino Acids. The degradation of amino acids can be 

 conveniently divided into three groups: (1) those in which the amino 

 acid is first deaminated; (2) those in which the amino acid is converted 

 into another amino acid; and (3) those in which the amino acid is decar- 

 boxylated. 



The following types of reactions (discussed in detail in the preceding 

 chapter) which cause the deamination of amino acids have been shown 

 to be dependent upon the presence of coenzymes of the vitamins indicated : 



(1) Oxidative deamination of glycine and most d- and L-amino acids 

 by riboflavin-containing enzymes. 



(2) Oxidative deamination of L-glutamic acid by enzymes whose co- 

 enzymes contain nicotinic acid. 



(3) Transaminations which result in the deamination of glutamic, 

 aspartic and cysteic acids, alanine, and probably other amino acids by 

 pyridoxal-containing transaminases. 



(4) The simultaneous deamination and degradation of tryptophan by a 

 system requiring pyridoxal phosphate. 



(5) Deamination of aspartic acid, threonine, and serine by biotin- 

 reactivated systems. 



The B-vitamin-catalyzed reactions transforming one amino acid into 

 another which were previously listed above undoubtedly are important 

 to the organisms, not only from the standpoint of a means of synthesis, 

 but also as a means of catabolic utilization of certain amino acids. 



The decarboxylation of amino acids results in the formation of mono- 

 amines, diamines, y-aminobutyric acid, and /^-alanine, and it can be 

 anticipated that pyridoxal phosphate will be an essential part of most, if 

 not all, the enzymes carrying out this type of degradation. 



A check of the reactions just discussed directly implicates all of the 

 typical B vitamins, except thiamine and pantothenic acid, in the catalysis 

 of one or more reactions in which amino acids participate. 



Pyrimidines and Purines. The demonstration of a direct involvement 

 of a B vitamin in reactions utilized for the biosynthesis of purines and 

 pyrimidines has yet to be accomplished. However, by the use of inhibitors 

 and isotopically labelled substrates the general routes of synthesis have 

 been indicated, the important role of the single carbon unit established, 

 and the essentiality of p-aminobenzoic acid (or folic acid) and vitamin 

 B12 demonstrated (Chapter IIB). In purine syntheses these two vitamins 

 may in many instances be required not only for the introduction of the 

 single carbon unit into the purine nucleus, but also for the biosynthesis 



