210 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



The carboxylases have for their coenzyme pyridoxal phosphate which 

 acts according to the mechanism described on p. 174. A whole series of 

 decarboxylases exists, each being specific for the L-form of a given amino 

 acid. Certain of them have been isolated from animal tissues such as liver 

 and kidney, but the majority have been isolated from micro-organisms in 

 which the enzymes appear if their specific substrate is present in the culture 

 medium. In micro-organisms therefore these decarboxylases are adaptive 

 enzymes. The amines produced by the decarboxylation of amino acids 

 (Table XIII) often possess pharmacological activity; this is the case for 

 histamine, the product of the decarboxylation of histidine. 



Table XIII 



Amines resulting from the decarboxylation of various amino acids. 



(b) Deaminations 

 1. Oxidative deamination 



Many cells, and in particular those of mammalian tissues can deaminate 

 amino acids to form the corresponding ketonic acids in the presence of the 

 specific enzyme and oxygen, according to the general reaction : 

 R R 



111 

 NH2— CH— COOH + — 02-^0=C— COOH-f-NHa 



2 



The enzymes catalysing this reaction are the L-amino acid oxidases and the 

 D-amino acid oxidases. The role of the latter in metabolism has not yet been 

 elucidated, for the naturally occurring amino acids are generally of the L-series. 



The D-amino acid oxidase of sheep kidney has been purified; it is a 

 flavoprotein containing FAD. It is of low specificity and catalyses the oxida- 

 tive deamination of all the amino acids of the D-series with the exception of 

 glutamic acid. It does not act on amino acids of the L-series, or on glycine. 



The L-amino acid oxidases isolated from various animal cells and micro- 

 organisms are also flavoproteins, containing FMN, but they likewise 

 are not very specific; they act on a number of amino acids, but not on all. 

 Glycine is deaminated neither by D-amino acid oxidases nor by L-amino 

 acid oxidases. Its oxidative deamination is accomplished in the presence 

 of a specific enzyme, glycine-oxidase, in the following manner : 



NH2CH2COOH + iOa -> NH3 + CHO— COOH 



glyoxylic acid 



