AMINO-ACID CATABOLISM 27 



dioxide is not produced during the fermentation of glucose 

 by Strep, faecalis, and since growth is not possible in its 

 absence Gale has suggested that arginine dihydrolase is a 

 mechanism whereby this metabolite is made available [26]. 

 He found that strains with high arginine dihydrolase activity 

 grew better if the amount of arginine initially present in 

 the medium was such that it was not all decomposed by 

 the time the pH became unfavourable for further growth. 

 Alternatively, it is feasible that arginine serves as a source 

 of energy and that arginine dihydrolase activity is connected 

 with the organism's energy metabolism, since Knivett [38] 

 has shown that the conversion of citrulline to ornithine is 

 accompanied by the phosphorylation of ADP to ATP. 



Amino-acid decarboxylases 



Although for several years bacteria have been known to 

 form amines from amino-acids, no study of the enzymes 

 concerned was made until the work of Gale [27]. The amines 

 arise by decarboxylation of a-amino-acids: 



RCH(NH2)COOH=RCH2NH2+C02 



and the initial experiments demonstrated the existence of 

 six amino-acid decarboxylases, specific respectively for the 

 L-isomers of tyrosine, lysine, ornithine, arginine, histidine 

 and glutamic acid. They have been found in the genera 

 Escherichia, Streptococcus, Clostridium, Proteus and Lacto- 

 bacillus. Whilst an organism may possess more than one 

 decarboxylase, the distribution of the enzymes amongst the 

 strains of any one species is variable. These enzymes are 

 active in the pH range 2-6 and have a sharp optimum. In 

 general the substrates possess an a-amino and an a-carboxyl 

 group together with another polar group at the opposite end 

 of the molecule. Substitution in any of these groups yields 

 substances which are not attacked and the introduction of 

 a hydroxyl group in another part of the molecule is the only 

 known structural modification which does not affect suscep- 

 tibility to decarboxylation, e.g. ^-hydroxyglutamic acid, 

 /5-hydroxylysine and 3:4-dihydroxyphenylalanine are decar- 

 boxylated by the glutamic, lysine and tyrosine decarboxylases 



