AMINO-ACID CATABOLISM 29 



carboxylase, and they can therefore be used for assaying 

 codecarboxylase . 



More recent work has shown that the tyrosine de- 

 carboxylase will also attack phenylalanine [43^2] and m- 

 tyrosine [53], and that Esch. colt possesses a specific enzyme 

 which decarboxylates a,e-diaminopimelic acid to lysine. 

 Unlike the other decarboxylases, the activity of the latter is 

 high even in cells grown in the absence of the substrate, and 

 furthermore there is evidence that the reaction is reversible: 

 like the other decarboxylases the prosthetic group appears to 

 be pyridoxal phosphate [18]. Cultures of Proteus spp. grown 

 in an amino-acid medium have been found to contain amines 

 derived from the branched chain amino-acids valine, leucine 

 and isoleucine, but their mode of formation is not yet 

 known [49]. 



Fermentations usually give rise to acidic end-products, 

 consequently as the fermentation proceeds, the pH of the 

 medium eventually reaches a value below which no further 

 growth is possible. The highly basic amines formed as the 

 result of decarboxylase activity tend to counteract this fall 

 in pH, and Gale has therefore proposed that the decar- 

 boxylases may be regarded as a type of 'neutralization 

 mechanism' [27]. Since the partial pressure of CO 2 in an 

 acidic medium is low, another possible function of the 

 decarboxylases is to make this essential metabolite available 

 inside the cell. The observation that H. parainfluenzae is 

 exacting towards putrescine, the amine formed by the 

 decarboxylation of ornithine, implies that the amines them- 

 selves may be of some significance in intermediary meta- 

 bolism [31]. 



REFERENCES 



1. Adler, E., Gunther, G. and Everett, J. E. (1938), Z. physiol. 



Chem., 255, 27 



2. Hellstrom, V., Gunther, G. and von Euler, H. (1938), 



Z. physiol. Chem., 255, 14 



3. Barker, H. A. (1937), Enzvmologia, 2, 175 



4. Volcani, B. E. and Cardon, B. P. (1948), J. biol. Chem., 



173. 803 



5. and Wiken, T. (1948), Arch. Biochem., 17, 149 



