VOL. 4 (1950) 



SPECIFICITY OF DECARBOXYLASES 



131 



derivative. With the bacterial preparations, the rate of CO, formation from o-hydroxy- 

 phenylalanine was practically zero. 



TABLE I 



DECARBOXYLATION OF TYROSINE AND ITS ISOMERS 



+ signifies decarboxylation 

 — signifies no decarboxylation 



Results of competition experiments suggest that the two enzymes responsible for 

 these decarboxylation reactions are the bacterial tyrosine decarboxylase and the mam- 

 malian DOPA decarboxylase. One molecule of each DL-amino-acid gives one-half of 

 a molecule of CO2 formed; we therefore assume that only one of the two steroisomers, 

 the L-form, is decarboxylated. 



These findings demonstrate the importance of the phenolic hydroxyl groups and 

 their positions on the benzene ring for the reaction between enzyme and substrate. It 

 seems safe to assume that these groups react with the protein part of the decarboxylase 

 system. 



The nature of the forces which are at work between enzyme protein and substrate 

 is not known. In the case under consideration, it seems possible that the reaction 

 between the phenolic hydroxyl groups and the enzyme involves the formation of a 

 hydrogen bond, with the hydroxyl group either as a "donor" or an "acceptor". At any 

 rate, the results obtained can be understood if it is assumed that the substrate must be 

 held by a group in the enzyme situated so that it can react with a hydroxyl group in 



OH * * 



I H H H 



HC CH HC C^ HC O^ HC CH 



HC CH 



HC CH 



HC CH 



HC 



\C^ 



\OH 



R R R R 



Bacterial enzyme Mammalian enzj'^me 



Fig. I. The asterisk marks the position of the active group in the enzyme in relation to the substrate. 

 References p. 136I1J7. 



