SALT CONCENTRATION AND IONIC STRENGTH 815 



The principles brought out above are, however, applicable to inhibition. 

 Indeed, in most instances the investigation of the effects of solvents or 

 dielectric constant on inhibition kinetics would be more straightforward 

 and more readily interpretable than in the case of enzyme kinetics, because 

 one would not have to deal with a sequence of reactions. Changes observed 

 in Ki would reflect directly alteration in the affinity of the enzyme for 

 the inhibitor, although the same care would have to be exercised in the 

 attribution of the results to a change in the dielectric constant. 



EFFECTS OF SALT CONCENTRATION AND IONIC 

 STRENGTH 



Changes in the electrolyte concentration of media will usually alter 

 the kinetics of enzyme reactions and the susceptibility of the enzymes to 

 inhibitors. As in the variation of the dielectric constant by the addition 

 of organic substances, the effects upon enzymes of adding neutral salts may 

 be due to a variety of mechanisms, some nonspecific (such as brought about 

 by changes in the ionic strength) and some specific (such as result from 

 particular reactions of certain ions with either the enzyme or the other 

 components of the reaction). If one could be certain in any case that the 

 variation of the salt concentration is producing effects upon the kinetics 

 that are entirely attributable to changes in the ionic strength, it would 

 be easier to interpret the results quantitatively and to obtain useful infor- 

 mation relative to the molecular events that are involved in the catalysis 

 or the inhibition. Inasmuch as the primary object here is to determine the 

 effects of the ionic strength on enzyme and inhibition kinetics, it will first 

 be necessary to outline and discuss briefly the other types of effects that 

 may be observed upon changing the ionic composition of the medium. 



General Classification of Salt Effects 



The possible effects of changing the salt concentration on enzymes may 

 be most broadly designated as (1) those producing changes in the solvent 

 water, (2) those directly affecting the interaction between the enzyme and 

 the other components of the system, and (3) those indirectly affecting such 

 interactions. 



{a) Changes in the properties of water. Addition of electrolytes alters the 

 bulk dielectric constant of water. This effect is linear with the salt con- 

 centration and was expressed by Hasted et al., (1948) as: 



D = Do + 2 6c (15-67) 



where D is the dielectric constant of the salt solution, D^ is the dielectric 

 constant of pure water at 25°, c is the salt concentration, and S is the molar 



