352 RIBOFLAVIN 



ment on the rates at which different amino acids are attacked,^** since tech- 

 niques and preparations differ, but it is generally acknowledged that methi- 

 onine shows the highest rate of oxidation and that glycine, lysine, and 

 glutamic acid are not oxidized at all. Amino acids with the amino group in 

 the |8 position, e.g., /3-alanine and /3-aminobutyric acid, and dipeptides such 

 as alanyl glycine and leucyl glycine are inactive as substrates. 



D-Amino acid oxidase is present in most animal organs, kidney and liver 

 being the best sources. There is an active D-amino oxidase in Neurospora}^ 

 Kisch^^ has claimed that there are at least three different D-amino acid 

 oxidases, depending on the species of animal from which the kidney extracts 

 are prepared. 



The physiological role of this enzyme is not understood, since the amino 

 acids which occur in cells are all of levo configuration. It has been postulated 

 that it is present to destroy D-amino acids formed by the racemization of 

 the natural amino acids. 



a. Preparatio7i of D-Amino Acid Oxidase 



Since it is not inhibited by drying or by treatment with many organic 

 solvents, it is simple to prepare D-amino oxidase by extracting ground tissue 

 (usually kidney) with acetone and drying in vacuo P The tissue powder 

 can be extracted with water. 



h. Mechanism of Action 



The oxidation of D-amino acids based upon the determination of oxygen 

 uptake, ammonia, and keto acids was represented as*^ 



NH2 NH O 



I -2H 11 4-H2O II 



R— C— COOH > R— C— COOH - > R— C— COOH + NH3 



H 



However, hydrogen peroxide is formed during the oxidation ; so the reaction 

 in highly purified preparations devoid of catalase may be written as fol- 

 lows :^^' ^^ 



NH2 O 



(1) I I' 



^ ' CH3— C— COOH -I- flavin + H2O -> CH3C— COOH + H2-flavin 



H 



(2) H2-flavin -f O2 -^ H2O2 + flavin 

 O 



(3) CH3CCOOH + H2O2 -^ CH3COOH + CO2 + H2O 

 The turnover number of D-amino acid oxidase is about 2000. 



86 J. R. Klein and P. Handler, /. Biol. Chcm. 139, 103 (1941). 



87 B. Kisch, Enzymologia 12, 97 (1936). 



