822 7. MERCURIALS 



The value of x, the concentration of active enzyme regenerated, can be 

 calculated from the quadratic equation: 



x^{\ - K)-x [(R,) + (E,)] + (E,) (R,) = (7-15) 



if the total concentrations of enzyme and reactor are known. The value of 

 the final inhibition is given simply by: 



If both enzyme and reversor SH groups ionize similarly, we need not in- 

 clude the effect of (H+), and since the mercurial is assumed to be bound 

 to either the enzyme or the reversor, we need not consider (X~), the con- 

 centration of mercurial-complexing ligands in the medium. The situation 

 described here is that in which an enzyme is titrated to complete inhibition 

 and the reversor then added. If any irreversible inactivation has occurred, 

 the experimentally measured inhibition will be greater than in Eq. 7-16. 

 In most reactivation experiments, (R^) is much greater than (E,), and 

 thus the reversor concentration is not reduced significantly by the binding 

 of the mercurial. The equation for the determination of x is simplified to: 



a;=A' + a;(R,) - (E,) (R,) = (7-17) 



It is interesting to specify the conditions required to reverse the inhibition 

 to a determined value ij. Substitution of (E^) (1 — ij) for x in the solution 

 to the quadratic equation leads to 



(7-18) 



(7-19) 

 (E,) V K, 



Equation 7-18 gives the ratio of the dissociation constants of the two mer- 

 captides so that the inhibition will be reduced to ij when the enzyme and 

 reversor concentrations are given, while Eq. 7-19 shows how much reversor 

 must be present relative to the enzyme to achieve a reactivation to if when 

 the dissociation constants are known. In many experiments the reversor is 

 added at a concentration of 1-10 vaM and thus (R^)/(EJ is often 10^ and 

 10^, since enzyme concentrations commonly run from 10"^ to 10^^ raM. 

 If we assume that K^ = K^, which is a reasonable approximation in many 

 cases, the values of (R^)/(E^) shown in the following tabulation must be 

 used to reduce the inhibition to the levels indicated. Thus one would expect 

 that the concentrations of reversor often used would completely abolish 

 the inhibition, so that if the enzyme activity is experimentally not restored 



