Regulation of carboxyl ester lipase adsorption to surfaces. 2. Physical state specificity

Abstract

The physical specificity of adsorption of porcine pancreatic carboxylester lipase to mixed-lipid surface was examined by using films at the argon-buffer interface. They were comprised of 1-palmitoyl-2-oleoylphosphatidylcholine and triolein, 1,3-diolein, methyl oleate, oleonitrile, oleyl alcohol, or 13,16-docosadienoic acid. Under conditions where the surfaces are thermodynamically well-defined, each of these binary systems exhibits the formation of a lipid-lipid complex that is completely miscible with uncomplexed non-phospholipid [Smaby, J. M., and Brockman, H. L. (1985) Biophys. J. 48, 701-707]. Initial rates of adsorption of enzyme to the complexes were .gtoreq. 5% of those measured in the absence of phospholipid and comparable to its rate of adsorption to phospholipid alone. This occurred despite there being up to 46% of the surface area occupied by non-phospholipid in the complexes. Equilibrium binding measurements were made at a composition where phospholipid-fatty acid complex was the predominant species. These showed that the low rates were due to an absence of adsorption sites relative to surfaces of fatty acid alone. With diolein or fatty acid and phosphilipid, equilibrium binding was also measured at compositions intermediate between that of the complex and pure non-phospholipid. In both systems surface concentrations of enzyme varied nonideally with respect to either the mole fraction or area fraction of complex and uncomplexed diolein or fatty acid in the film. At area fractions of uncomplexed lipid of 0.35 and 0.67, dissociation constants for enzyme absorption were increased 10-20-fold relative to pure fatty acid or diolein. Thus, lipid-lipid interactions can be important regulators of lipid-protein interactions. The inhibition of adsorption by complexes is not attributable to the absolute value of the interfacial tension or to the ionization state of the fatty acid but is related to the fraction of the surface area occupied by complex. Qualitatively, such behavior can be understood by consideration of the relative sizes of the enzyme molecule and the lipid domains.