744 



14. EFFECTS OF pH ON ENZYME INHIBITION 



The inhibition of bioluminescence in Achromobacter by 2-methyl-l,4- 

 naphthoqiiinone (menadione) shows a characteristic variation from what 

 would be expected on the basis of a simple ionization (Fig. 14-29) (McElroy 

 and Kipnis, 1947). Although more experimental points might be desired, 

 it would appear that there is a rather sudden break in the curve at com- 

 plete inhibition. The cause for this is not known. In such cases, one possi- 



FiG. 14-28. Variation of intracellular competitive inhibition with the 



external pH (Eq. 14-191). (!;)„ = 1 mif, (S^), = 0.1 mM, iv,- == 1 mM, 



K, = 10 mM, K^ = 10-8, and K^ = 10-«. 



bility comes to mind. If the metabolism or function is active in proportion 

 to the concentration of some enzyme, or some product of an enzyme 

 reaction, but exhibits a threshold, below which the process fails, the type 

 of curve in Fig. 14-29 might be expected. The curve from zero to complete 

 inhibition may be part of a full sigmoid curve representing the inhibition 

 of an enzyme system. When a certain inhibition has been reached, the pro- 

 cess that depends on this enzyme will fail. Looked at in another way, below 

 a threshold pH^ the metabolic process cannot operate because the inhi- 

 bition on an enzyme is greater than the maximal inhibition allowable for 

 the process to occur. But as the pH^ is increased, when the required amount 



