830 



15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



1957). The rate of the hydrolysis of carbobenzoxyglycyl-L-tryptophan by 

 pancreatic carboxypeptidase increases rapidly with ionic strength (Lumry 

 et al., 1951) up to around 0.25 and then levels off. The salt added makes 

 little difference so that the effect is presumably one of ionic strength. It 

 was claimed that the magnitude of the rate increase is greater than expected 

 from the simple Debye-Hiickel theory but unfortunately no data are given. 

 Since the substrate is negatively charged at the experimental pH, the in- 

 creased rate with higher ionic strengths would indicate that a negative 

 group, or groups, occurs on the enzyme, a decreased K,,, resulting from a 

 rise in k^, since K^,, = A'2/^'i- The following explanation for the large effects 

 was suggested: a number of negatively charged groups on the enzyme are 

 involved and although individually giving small effects, their total effect 

 may be quite large. A reduction in K,^, from 20 niM to 14 mM was found 

 for carboxypeptidase by Yanari and Mitz (1957) when the ionic strength 

 was increased from 0.2 to 0.5, the substrate being carbobenzoxyglycyl-L- 

 phenylalanine. This is not an unreasonably large effect. 



It might be expected that ionic strength effects on ribonucleases and 

 deoxyribonucleases would be very marked because of the polyionic nature 

 of both substrates and enzymes. The activity of pancreatic ribonuclease has 



-3 

 LOG (No*) 



-2 



Fig. 15-20. Eflfects of Na+ concentration on the activity of spleen deoxjrribonu- 

 clease at different pH values. (From Shack, 1959.) 



