Comparison of the action of lipoprotein lipase on triacylglycerols and phospholipids when presented in mixed liposomes or in emulsion droplets

We have compared the action of lipoprotein lipase on liposomes of egg yolk phosphatidylcholine containing less than saturating amounts of trioleoylglycerol (less than 3%) and emulsion droplets of the same lipids. The amounts of the two types of lipid particles (expressed in terms of phosphatidylcholine) needed to reach substrate saturation of the enzyme were similar, indicating similar binding of the lipase to these two lipid/water interfaces. With liposomes, as opposed to emulsion droplets, albumin was not necessary for continued hydrolysis of triacylglycerols, presumably because product fatty acids could be accommodated in the phospholipid bilayer. The maximal rate of trioleoylglycerol hydrolysis was more than 10-fold higher, and the ratio of trioleoylglycerol/phosphatidylcholine hydrolysis was more than 50-fold higher with the emulsion droplets. Qualitatively similar results were obtained with hepatic lipase, and a lipase from Pseudomonas fluorescence. The data suggest that the lipases remained at the interface for several catalytic cycles, and that a continued supply of substrate molecules to the active site favored triacylglycerol entry from the core of the lipid particle, rather than sliding in from the side through lateral diffusion in the surface layer.