816 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



depression of the dielectric constant. The values of d for KCl and NaCl are 

 — 5 and — 5.5 liters/mole, respectively, and thus for most enzymic work 

 Eq. 15-67 may be written simply as D = Dq — 10c. Since salt concentra- 

 tions higher than 0.5 M have seldom been used in studies of enzymes, a 

 maximal decrease in the dielectric constant of 5 would be expected. It 

 would appear likely that this factor is a relatively minor one in this range 

 of salt concentrations. Furthermore, the effect of the salt concentration 

 on the microscopic dielectric constant is actually the more important con- 

 sideration and about this one can say very little. Another effect that might 

 be brought about by the addition of salts is a reduction in the concentra- 

 tion of free water due to the hydration of the ions. However, in the range 

 of concentrations used in enzyme work this is probably a negligible factor; 

 it can be much more important in mixtures of water with organic solvents. 



(b) Changes in the activity coefficients of the enzyme reaction com/ponents . 

 This is the factor that is responsible for directly affecting the interactions 

 between charged groups. One may look at this effect in a variety of ways. 

 As the electrolyte concentration rises, the ionic atmosphere surrounding 

 each charged molecule and group is increased; i.e., the density of ions of 

 opposite charge to the central ion will increase with the number of ions 

 present. This may be said to reduce the electrical potential of each ion and 

 hence the attraction between ions of opposite sign will decrease. One may 

 also speak of a competition between the nonspecific ions and the substrate 

 (or inhibitor) for the enzyme active site. The deviation from the kinetics 

 of ideal or very dilute solutions is mainly due to this ionic atmosphere and 

 this is the basis for the Debye-Hiickel treatment of activity coefficients in 

 real solutions. All of these viewpoints are basically the same. This is the 

 only direct effect dependent on the ionic strength and it will be treated 

 quantitatively and in more detail in a later section. 



(c) Cfianges in the "properties of tJie enzyme or its reaction components. The 

 ionization constants of protein groups vary with the ionic strength and thus 

 a change in the salt concentration could alter the p-Sl^/s of enzyme groups 

 near the active center, secondarily altering the over-all charge distribution 

 and the affinity between the enzyme and whatever reacts with it. Similar 

 changes on the substrate or inhibitor molecules could also occur. The ef- 

 fects of changes in the ionic strength on the acid dissociation constants of 

 peptides and proteins are discussed thoroughly by Colm and Edsall (1943). 

 The fundamental reason for a change in the -pK,, is the changes in the ac- 

 tivity coefficients of the ionic species involved in the acid-base dissociation 

 as discussed many years ago by Bronsted (1928). The extended Debye- 

 Hiickel equation for the activity coefficient of an ion is: 



0.509z^V7" 



logy =- —^ + KsoS (15-68) 



1 + 0.327J-,, V s 



