306 



2. ANALOGS OF ENZYME REACTION COMPONENTS 



Table 2-9 

 Inhibition of Tyrosine : a-KETOGLUTARATE Transaminase from Dog Liver " 



" Relative binding energies were calculated on the basis of competitive inhibition 

 with K^ = 0.71 mM for L-tyrosine. No correction for ionization was made. It should 

 be noted that the different isomers of the DL-compounds may have different activities, 

 so that the binding energy of the active form should be increased. (Data from Canel- 

 lakis and Cohen, 1956 b.) 



pyruvate, essentially complete inhibition occurring with 0.4 mM in the 

 presence of 2 mM substrate (Zannoni and La Du, 1959). The inhibition is 

 negligible for the first 5 min but then increases to become complete at 

 around 20 min. This might be due to protection by the substrate and pre- 

 incubation experiments would have been informative. An equally good 

 inhibitor is m-hydroxyphenylpyruvate but phenylacetate, 2,5-dihydrox- 

 yphenylpyruvate, p-hydroxyphenyllactate, p-hydroxybenzoate, and ho- 

 mogentisate are not inhibitory. 



Other Pathways of Tyrosine Metabolism 



Little is known about the effects of analogs on tyrosine decarboxyla- 

 tion, although this might well be an important site to block if one wished 



