714 14. EFFECTS OF pH ON ENZYME INHIBITION 



the cell contents. In the following preliminary treatments of the problem, 

 several assumptions are made: (1) the external concentration of the inhi- 

 bitor remains constant, i.e., the inhibitor in the medium is not depleted 

 significantly because of the penetration into the cells, (2) the external pH 

 remains constant at its initial value, (3) the pH and the distribution of the 

 inhibitor within the cells are homogeneous, (4) the system has reached equi- 

 librium, and (5) only the uncharged form of the inhibitor penetrates into 

 the cell. The buffering capacity of the cell is very important in determining 

 the distribution of the inhibitor and we shall first treat the two extreme 

 situations — where the cell is completely buffered so that the internal pH 

 does not change and where the cell is unbuffered. 



Case I: cells completely buffered. The equilibria involved may be repre- 

 sented by the following diagram: 



H„ + I„ — HI„ 



(14-150) 



The addition of the inhibitor will result in the entrance of HI into the cells 

 and this HI will dissociate according to the intracellular pH and the \)K^ 

 of the inhibitor. For each molecule of HI that enters the cell, one hydrogen 

 ion is formed upon dissociation and one ion of I~ is released if the inhibitor 

 is an acid. If the inhibitor is a weak base, the I form will penetrate and 

 within the cell can pick up a proton to form HI"*". The exchange of protons 

 in either case must occur with the cell buffers in order that the pH will 

 not change. The uncharged form will enter the cell until its external and 

 internal concentrations are equal. The concentration of the charged form 

 within the cell may now be calculated. 



The equilibria outside and inside the cell may be wi'itten as: 



(H)„(I)„ (H),(I), 



(HI)„ (HI), 



and, since (HI)^ = (HI)j: 



= K, (14-151) 



(I), = (I), 41^ (14-152) 



(■tl}t 



(I)o is related to (I,)^,, the total inhibitor concentration in the medium, 

 through the pH functions used earlier in the chapter, i.e., (I),, = {lt)oLfi- 

 The subscript on the left of the pH function will, in each case, indicate 

 whether the pH to be used is that outside or inside the cells. The internal 



