DIVALENT METAL-IONS AS MODULATORS OF RAT-LIVER MICROSOMAL CHOLESTEROLESTERASE

The regulatory properties of the divalent metal ions Mg2+, Ca2+ and Mn 2+ on the activity and kinetic behaviour of rat liver microsomal chole sterol esterase were studied in vitro. Me2+ and Ca2+ exhibited similar concentration and preincubation time-dependent increases in esterase activity with maximal stimulation at a concentration of 2 mM. However, Mn2+ had no effect at this concentration but displayed a potent inhib itory effect at concentrations above 20 mM. Activation of cholesterol esterase by Mg2+ and Ca2+ was selective in relation to i) the changes that cations produced in the enzyme kinetic constants, and ii) the che lating agents that reversed the metal ion-induced activation. Hence, t he maximum rate of cholesterol ester hydrolysis doubled in the presens e of Mg2+ and activation was reversed by EDTA, whereas a significant d ecrease in the apparent Km for cholesterol oleate was found when Ca2was added and this effect was blocked by ATP and EGTA. Both cations we re able to reactivate cholesterol ester hydrolase activity in metal-de pleted microsomes.