818 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



tion this can be another way in which salt effects arise. Proteins such as 

 ovalbumin, y-globulin, and serum albumin appear to be electrophoretically 

 inhomogeneous and composed of a variety of units differing in total charge 

 (Cann and Phelps, 1957). Equilibrium between these is affected by salt 

 concentration and it was concluded that some structural changes are pro- 

 duced in the proteins. There is no reason why certain enzymes may not 

 behave similarly. 



(d) Specific ion binding. When an electrolyte is added to an enzyme prep- 

 aration, one must always consider the possibility that one or both of the 

 ions reacts specifically with either the enzyme or the other components of 

 the system. By a specific interaction is meant an association of an ion with 

 a group that is significantly more pronounced than would be exjDected on 

 the basis of simple electrostatic attraction such as occurs to produce the 

 ionic atmosphere. Ions such as calcium are particularly prone to associate 

 with groups on enzymes, substrates or inhibitors. The binding of Ca++ to 

 serum albumin involves an interaction energy of 4.3 kcal/mole (Katz and 

 Klotz, 1953) indicating a specific affinity for some groups on the proteins. 

 Many have assumed, however, that the common ions, Na+, K+, and Cl~, 

 are not specifically bound. Yet Lewis and Saroff (1957) have demonstrated 

 differential binding of Na+ and K+ to myosin and Saroff (1957) has reported 

 different dissociation constants for the comijlexes of various amphoteric 

 substances with Na+, K+, and Li+. It was concluded that a certain struc- 

 tural specificity is required and that a form of chelation may be involved. 

 Thus one cannot be confident of a lack of si)ecific salt effects even when 

 NaCl and KCl are used to alter the ionic strength. In this connection, stud- 

 ies designed to determine the effects of ionic strength on enzyme reactions 

 would generally be improved if more than one salt were used. In this way 

 specific effects might be detected and corrections would be possible. It is 

 actually very difficult to draw a distinct line between specific and nonspe- 

 cific effects in certain instances. It need scarcely be said that in enzyme 

 reactions involving an ionic cofactor, such as Mg++, special attention should 

 be given to the possibility of a direct competition by the added ions. 



Effects of Changes in the Ionic Strength on Dissociation Constants 



The dissociation constants of the ES or EI complexes, or of any other 

 complexes that may occur in the enzyme reaction or inhil)ition, would be 

 expected to vary with the ionic strength. The type of variation will depend 

 on the nature of the forces Ijetween the interacting species. We shall first 

 consider ionic interactions because these are treated more readily in a 

 quantitative manner. The approach is based on the variation of the activity 

 coefficients of the interacting molecules with the ionic strength. It has been 

 customary to write the expression for the dissociation constant in terms of 



