COENZYMES DERIVED FROM B VITAMINS 147 



tides or a reduced thiamine system which has catalyzed an oxidative 

 decarboxylation (p. 165). 



The reactions of flavoproteins could equally well be classified on the 

 basis of the substances which have been found to be hydrogen acceptors: 

 (1) the cytochromes, (2) molecular oxygen, (3) fumaric acid, and (4) 

 artificial acceptors such as methylene blue. 



An interpretation of the primary intracellular functions of flavoproteins 

 on the basis of results obtained from isolated proteins is hazardous. A 

 number of instances have been observed in which it appears that the 

 enzyme after isolation has lost its ability to react with certain substrates. 

 The rate at which some of the isolated enzymes catalyze reactions in vitro 

 is too slow to enable them to be of importance in vivo. With some flavo- 

 proteins which have been isolated it has been impossible to reconstruct 

 systems using substrates known to occur in living cells. It is still impos- 

 sible to characterize completely the most important (from a quantitative 

 standpoint) reaction which flavoproteins mediate in aerobic organisms, 

 the dehydrogenation of reduced dihydro-diphosphopyridine nucleotide. 



The changes which may occur in the enzymatic capacities of the flavo- 

 proteins during their concentration make it difficult to establish the iden- 

 tity of enzymes studied by different investigators. On several occasions 

 enzymes have been obtained from the same source material in separate 

 laboratories and not all their properties have, checked. Consequently, the 

 reactions of the flavoproteins will be classified on a general basis accord- 

 ing to their substrates and no attempt will be made to characterize the 

 individual enzymes as reported. 



Amino acid oxidases. Oxidation of amino acids is a two-step process 

 in which (1) the a-amino group is dehydrogenated to form an a-imino 

 acid, and (2) the imino group hydrolyzes spontaneously to yield ammonia 

 and the corresponding keto acid. 



R— C 



NH 2 NH O 



-2H || +H2O || 



COOH — >• R— C— COOH — > R— C— COOH + NH 3 

 H 



a-amino acid a-imino acid a-keto acid 



Three types of amino acid oxidases which are flavoproteins have been 

 recognized: (1) D-amino, (2) L-amino, and (3) glycine. The potential 

 of these systems is such that the reverse process could not be effectively 

 utilized directly for amino acid synthesis. 



D-amino acid oxidase catalyzes the oxidation of all the common 

 D-amino acids except D-lysine, although there are extreme differences in 

 the rate of reaction of the individual amino acids. 67 The enzyme occurs 

 in most animal tissues, but its purpose is not understood since its sub- 



