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

Activation of lipoprotein lipase (LPL) by apolipoprotein C-11 (apo-C-11) 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:l of the lipids.The activation of lipoprotein lipase by apo-C-I1 increased from 2-fold to infinity when a pure triacylglycerol monolayer was diluted progressively with phosphatidylcholine.Apo-C-I1 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-I1 (43-78), apo-C-I1 (55-78), apo-C-I1 (60-78) and apo-C-I1 (66-78) we could confirm that the minimal sequence region of apo-C-I1 required to produce the full LPL-activating effect is contained in the carboxyl-terminal part of apo-C-11, ie.residues 55-78.The lipid binding of the activator was not necessary, further in accordance with earlier results.Modification with phenylmethane[36S]sulfonyl fluoride of native apo-C-11 and synthetic fragments apo-C-II(43-78) and apo-C-11 (55-78) resulted in the incorporation of one 36S-sulfonyl group per each corresponding peptide.Sulfonylation abolished the LPL-activating ability of the peptides, as well as their esterase activity, * Financial support was provided by the University of Helsinki.This is Paper VI1 in a series on enzyme reactions in a membrane model.Paper VI is identified as Reference 4. These results were presented during the Scandinavian Workshop on Plasma Lipoproteins held in Punkaharju and Helsinki, Finland, June 22-29, 1982.