SALT COXCEXTRATIOX AXD IONIC STRENGTH 833 



in the rate in the low ionic strength range and a decrease at higher salt 

 concentrations. The possible mechanisms responsible for a positive effect 

 of salts on enzyme rates will be outlined and then we shall consider various 

 explanations for the biphasic phenomenon. An increase in rate may be 

 due to the following changes. 



(a) Increase of A'a- This implies changes in charge during the dissociation 

 of the ES complex. 



(6) Decrease of K„^. If K,^^ = Kg, there is an increased affinity of the en- 

 zyme for the substrate and this indicates either the reduction of an electro- 

 static repulsion or a large salting-out term due to non])olar forces. If 

 ^m — ^ii^v ^ difference in the charge on the different activated complexes 

 is suggested, or an increase in Ji-^ relative to ko. 



(c) Increase in the concentration of active enzyme. This could be brought 

 about in a variety of ways: shifts in acid-base equilibria, depoljTnerization 

 of the enzyme, or altered binding of an ionic cofactor. 



(d) Increase in the concentration of active substrate. In most cases this 

 would be restricted to changes in acid-base equilibria. 



Both of the first two mechanisms have been demonstrated. Other possi- 

 bilities may be visualized for more complex enzyme systems. 



The "bell-shaped" curves recorded for skin proteinase C, deoxyribo- 

 nuclease, polynucleotide phosphorylase, cytochrome oxidase, and acet^d- 

 cholinesterase can all hardly be explained on the basis of ionic strength alone. 

 Activity coefficients begin to rise at higher salt concentrations but this 

 is usually above an ionic strength of 1.0. On the other hand, the maxima in 

 the curves occur at ionic strengths much lower than this in most cases. A 

 dual effect on Kj,^ and A'g, the change in one increasing and a change in the 

 other decreasing the rate, is not feasible because marked effect of ionic 

 strength should occur with both at low salt concentrations. Indeed, the up- 

 per limbs of the curves are usually in ranges of the ionic strength where 

 little effect would be expected. It is more likely that the reduction in the 

 rate is due to an inhibition by the ions superimposed on the ionic strength 

 effect. The ions would be rather weak inhibitors and their actions would 

 be manifest only at higher concentrations. Assuming the simple Debye- 

 Hiickel formulation for the effect of ionic strength on A-, and a competitive 

 inhibition by the added ions (or usually one of the ions), we may -^Tite: 



^"^ ^^' (15-99) 



(S) + K, 



s 



for a uni-univalent salt. The complete activity coefficient expression can 

 be used and would be preferable, but complicates the following treatment 



