SALT COXCENTEATIOX AND IONIC STRENGTH 



829 



represented by the equation: log (A'o/A'., ) = 0.30 s. Thus the dependence 

 on the ionic strength is as expected but the numerical coefficient is less 

 than 0.509, indicating iiossibly an electrostatic effect from an adjacent 

 group (Shine and Niemann, 1955). One the other hand, later work (Martin 

 and Niemann, 1958) on the effects of NaCl, MgCl2,and CaCU on the hydro- 

 lysis of different acylated a-amino acid esters by a-chymotrypsin pointed 

 to a decrease in K, with increasing ionic strength, a slight rise in k2, and, 

 of course, an increase in the rate. These results could not be interpreted 

 easily by the usual theories. The proteinase C of rat skin with casein as the 

 substrate gave bell-shaiDed rate curves when the ionic strength was in- 

 creased by seven different salts (Martin and Axelrod, 1958). The results 

 with KC] are shown in Fig. 15-19. It may be noted that the optimal ionic 



Fig. 15-19. Variation of skin proteinase C activity, using casein as 



the substrate, with the ionic stength (altered by varying the KCl 



concentration) at pH 7.5. The activity is plotted on a relative scale. 



(From Martin and Axelrod, 1958.) 



strength differed somewhat for the different salts. Neither A', nor K,,^ in 

 the case of papain was altered by changes in the ionic stength from 0.05 

 to 0.3 (Stockell and Smith, 1957), which is surprising in view of the ionic 

 interactions assumed in the mechanism of the catalysis (Kimmel and Smith, 



