EFFECTS OF PH, TEMPERATURE, AND ALCOHOLS ON THE REMARKABLE ACTIVATIONOF THERMOLYSIN BY SALTS

The activity of thermolysin in the hydrolysis of N-[3-(2-furyl)acryloy l] (FA)-dipeptide amides and N-carbobenzoxyl-L-aspartyl-L-phenylalane methyl ester is remarkably enhanced by high concentrations (1-5 M) of neutral salts. The activation is due to an increase in the molecular a ctivity, k(cat), while the Michaelis constant, K-m, is not affected by the addition of NaCl. In the present study, the effect of NaCl on the thermolysin-catalyzed hydrolysis of FA-glycyl-L-leucine amide (FAGLA) has been examined by changing the pH and temperature, and by adding a lcohols to the reaction mixture. The enzyme activity, expressed by k(c at)/K-m, is pH-dependent, being controlled by two functional residues with pK(a) values of 5.4 and 7.8 in the absence of NaCl. The acidic pK (a) is shifted from 5.4 to 6.7 by the addition of 4 M NaCl, while the basic one is not changed. The degree of activation at a given concentr ation of NaCl is pH dependent in a bell-shaped manner with the optimum pH around 7. Although the activity increases in both the presence and absence of NaCl with increasing temperature from 5 to 35 degrees C, t he degree of activation decreases. Alcohols inhibit thermolysin, and t he degree of activation decreases with increasing alcohol concentratio n, The degree of activation tends to increase with increasing dielectr ic constant of the medium, although it varies considerably depending o n the species of alcohol. Electrostatic interactions on the surface an d at the active site of thermolysin are suggested to play a significan t role in the remarkable activation by salts.