834 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



and for the present piiri^ose of illustration is unnecessary. Differentiating 

 v^ with respect to the ionic strength and setting the derivative dvjds = 0, 

 the ionic strength that is optimal can be calculated from the quadratic 

 equation: 



(S) 



*'o3)< — 1.17 V Sopt -j- Kj 



' + K. 



= (15-100) 



and the value of the inhibitor constant for the ion can be determined from: 

 K, = (1.17 V So,t - Sor>t) ,^, "^ ^ (15-101) 



(b) + Krn 



For a noncompetitive type of salt inhibition the corresponding equations 

 would be: 



Sopt - 1.17 V"^ +K,=0 (15-102) 



K, = 1.17 V7^ - Sopt (15-103) 



If Kj,i also varies with the ionic strength, another term must be introduced 

 into Eq. 15-99; or the ionic strength effect may be on K^„ only. For optimal 

 ionic strengths around 0.2, values of K^ in the range 200-300 niM would 

 be expected. The use of the complete Debye-Hiickel expression would 

 lower Sgpf somewhat. 



Specific Ion Effects on Enzyme Reactions 



Because of the importance of recognizing and characterizing specific 

 ion effects in studies of the ionic strength, it may be worthwhile to cite 

 a few examples of such effects so that the various types of behavior may be 

 appreciated. Phosphoglucomutase is inhibited by several common anions, 

 which appear to compete with ribose-1 -phosphate for the enzyme site 

 (Klenow, 1955). The inhibitor constants, K,, were found to be: chloride, 

 3.6 mM; sulfate, 0.35 niM; and phosphate, 1.2 niM. These values show 

 rather tight binding of a magnitude comparable to that of the substrate, 

 for which K,,^ = 0.17 mM. Glutathione reductase is inhibited by NaCl 

 and KCl; 20 mM NaCl, for example, depresses the rate around 67% in 

 the complete assay system (Racker, 1955). It is likely that it is the Cl~ 

 ion that is the active inhibitor. Activation of many enzymes with cations 

 has been observed but activation by anions is uncommon. The best known 

 instance is the activation of cc-amylase by Cl~ (Cole, 1904; Myrback, 1926). 

 A more recent example is the strong activation of liver arylsulfatase by 

 Cl~ and some other univalent anions when ;>-nitrophenyl sulfate is the sub- 

 strate (Webb and Morrow, 1959). However, when 2-hydroxy-5-nitrophenyl 

 sulfate is the substrate, Cl~ is inhibitory. It was shown that the Cl~ has no 

 effect on the affinity of the enzyme for either substrate. Other anions — 



