Enzymatic synthesis/hydrolysis of cholesteryl oleate in surface films. Inhibition by lecithin and its reversal by bile salts.

Abstract

The synthesis/hydrolysis of cholesteryl oleate as catalyzed by porcine pancreatic cholesterol esterase has been studied in lipid films at the air-buffer interface. With only reactants and products initially present at the interface, equilibrium is rapidly attained at subphase enzyme concentrations of 4 x 10(-8) M or less. The equilibrium constant for the reaction, 1.4 x 10(-8) mol/cm2, is independent of pH, initial composition, and surface pressure. Lecithin, if present in molar excess relative to the sum of free and esterified cholesterol, is inhibitory. Inhibition is associated with division of the substrate into reactive and unreactive pools which are not exchangeable. Bile salts and other surfactants reverse the inhibition at concentrations one-tenth their critical micelle concentrations. Presumably this occurs through formation of a surfactant surface excess at the lipid-water interface which disrupts the unreactive lecithin-substrate complex. The adsorption of cholesterol esterase to oleic acid monolayers is first order with respect to enzyme and is saturable. At saturation, the enzyme forms a close packed monolayer at the lipid-water interface with a molecular area of 4510 A2. Adsorption of cholesterol esterase to lecithin monolayers is less than one-tenth that to oleic acid monolayers and is proportional to subphase enzyme concentration. With either lipid monolayer, enzyme denaturation at the interface was negligible. In the presence of substrate, differences in enzyme absorption can only partially account for the observed inhibition of catalysis by lecithin, indicating that the reactivity or availability of substrate to the adsorbed enzyme is also affected.