Influence of the structure of the lipid-water interface on the activity of hepatic lipase

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Abstract
Factors affecting the hydrolytic activity of purified rat hepatic lipase have been examined in mixed-monolayer systems. When nonsubstrate lipids [either egg sphingomyelin or .beta.-O-hexadecyl-.gamma.-O-(1-octadec-9-enyl)-DL-phosphatidylcholine (OPPC-ether)] were used as inert matrices, hydrolytic activity for both triolein and dioleoylphosphatidylethanolamine was shown to decrease with increasing surface pressure (.pi.); negligible activity occurred at .pi. .gtoreq. 30 mN/m. Examination of the effect of introduction of cholesterol into either matrix containing 2 mol % triolein indicated that the mean molecular area decreased with increasing cholesterol and that, at .pi. = 24 mN/m, triolein was fully miscible in the sphingomyelin matrix at cholesterol concentrations .ltoreq. 32.5 mol % and in the OPPC-ether matrix at cholesterol concentrations .ltoreq. 49 mol %. Above these critical concentrations of cholesterol, the phase diagrams indicate transitions that suggest that triolein is forced out of the monolayer. Introduction of increasing amounts of cholesterol into either inert matrix increased the rate of hydrolysis of triolein by hepatic lipase, although by different degrees. There are at least two factors contributing to these effects: (1) condensation of the monolayer by cholesterol, thus increasing the total surface concentration of triolein at .pi. = 24 mN/m in the constant area surface balance, and (2)some change in triolein conformation and/or accessibility since at identical surface concentrations of triolein (8.7 .+-. 0.1 pmol/cm2) and .pi. (24 mN/m) the rate of hydrolysis of triolein by hepatic lipase is 1.5-fold higher in the OPPC-ether matrix than in the egg sphingomyelin matrix. A number of human apolipoproteins [A-I, A-II, C-II, and C-III(1,2)] were observed to inhibit the hydrolysis by hepatic lipase of 2 mol % triolein in an OPPC-ether matrix at subphase concentrations of apolipoprotein .gtoreq. 0.1 nM.

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