824 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



The primary salt effect is the result of the variation of the activity coeffi- 

 cients with the salt concentration. A positive salt effect (increase in the rate 

 on addition of salts) is to be expected when the ions carry charges of the 

 same sign; a negative salt effect when the ions are of opposite signs. The 

 reasoning here is the same as in the previous treatment of the effects of 

 ionic strength on dissociation constants and, indeed, the equilibrium in- 

 volved in the formation of the activated complex is considered to be the 

 critical factor in the reaction rate. A secondary salt effect that occurs in 

 some reactions was defined by Bronsted as resulting from the effects of 

 the salt concentration on acid-base equilibria whereby the actual concentra- 

 tions of the reactants or catalysts are changed. Both types of effect can 

 occur in enzyme reactions. 



We may define two rate constants, just as in the treatment of dissocia- 

 tion constants; the ordinarily determined rate constant, k, and the true 

 rate constant, fc^, as occurs in Eq. 15-82. It is obvious that: 



k = ko ^^^^ (15-83) 



The relation between these constants in terms of ionic strength may be 

 expressed either by the use of the simple Debye-Hiickel Eq. 15-73: 



log k = log h + 1.022„2i, W (15-84) 



or by the use of the complete Eq. 15-68: 



1 02z z \/ s 



log k = log h + — — j=. + Ksos (15-85) 



1 + 1.31 V s 



The latter equation is more accurate for the ranges of ionic strength usually 

 tested in enzyme reactions. The observed rate constant thus changes with 

 ionic strength to the same degree as the dissociation constant but in the 

 opposite direction. This theory has been found to predict satisfactorily 

 the rate changes of nonenzymic reactions (Benson, 1960, p. 527). 



This treatment api^lies to a bimolecular ionic reaction. Inasmuch as 

 the rate constant A'., for the Ijreakdown of the ES complex is important in 

 determining the rate of an enzyme reaction, we must now consider the ef- 

 fects of ionic strength on a dissociation reaction. The recovery from inhibi- 

 tion may also depend on the rate constant l'_i for the dissociation of the 

 EI complex. The splitting of a complex AB can be represented in the tran- 

 sition state theory as follows: 



AB — X -» A -f B (15-86) 



where the rate is proportional to tlie concentratioii of the activated complex 

 X, i.e., V = A'2(X). The activated complex is assumed to be in equilibriimi 



