Action of lipoprotein lipase on mixed triacylglycerol/phosphatidylcholine monolayers. Activation by apolipoprotein C-II

Activation of lipoprotein lipase (LPL) by apolipoprotein C-II (apo-C-II) was studied using mixed monomolecular films of trioctanoylglycerol/didodecanoylphosphatidylcholine in order to mimic the interface of the physiological substrates, triacylglycerol-rich plasma lipoproteins. Lipoprotein lipase was found to be best adapted to catalyze the hydrolysis of triacylglycerols in a mixed triacylglycerol/phosphatidylcholine monolayer at an optimal molar ratio of 1:1 of the lipids. The activation of lipoprotein lipase by apo-C-II increased from 2-fold to infinity when a pure triacylglycerol monolayer was diluted progressively with phosphatidylcholine. Apo-C-II increased the specific activity of lipoprotein lipase without having any detectable effect on the amount of enzyme adsorbed to the lipid-water interface. Using synthetic peptide fragments apo-C-II (43-78), apo-C-II (55-78), apo-C-II (60-78) and apo-C-II (66-78) we could confirm that the minimal sequence region of apo-C-II required to produce the full LPL-activating effect is contained in the carboxyl-terminal part of apo-C-II, i.e. residues 55-78. The lipid binding of the activator was not necessary, further in accordance with earlier results. Modification with phenylmethane[³⁵S]sulfonyl fluoride of native apo-C-II and synthetic fragments apo-C-II (43-78) and apo-C-II (55-78) resulted in the incorporation of one ³⁵S-sulfonyl group per each corresponding peptide. Sulfonylation abolished the LPL-activating ability of the peptides, as well as their esterase activity, without qualitatively affecting their binding to the lipid monolayer. The above results are consistent with the participation of a reactive Ser residue in apo-C-II in the enhancement of the turnover number of lipoprotein lipase.