The lipoprotein lipase-catalyzed hydrolysis of triacylglycerol was determined in a lipid monolayer containing egg phosphatidylcholine and tri[14C]oleoylglycerol. In the presence of purified bovine milk lipoprotein lipase and fatty acid-free albumin, the rate of hydrolysis of tri[14C]oleoylglycerol, as determined by the decrease in surface activity, was dependent upon enzyme concentration and was enhanced by the addition of apolipoprotein C-II, the activator protein for the enzyme. Increasing the triacylglycerol content of the phospholipid monolayer from 1 to 6 mol% (relative to phospholipid) enhanced the rate of catalysis in the presence and absence of apolipoprotein C-II. However, at low substrate concentrations (less than 4 mol% tri[14C]oleoylglycerol), the activation factor for apolipoprotein C-II was greater than at high (4–6 mol%) triacylglycerol concentrations. The addition of sphingomyelin to the phosphatidylcholine monolayer decreased lipoprotein lipase activity. Based on these monolayer studies, we conclude that lipoprotein lipase catalyzes the hydrolysis of triacylglycerol at a phospholipid interface and that the rate of catalysis is dependent on the lipid composition of the monolayer
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