Enzymic and physical characterization of diacylglycerol-phosphatidylcholine interactions in bilayers and monolayers

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Abstract
The miscibility of 1,3-dioleoylglycerol (DOG) with 1-stearoyl-2-oleoylphosphatidylcholine (SOPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) dispersed in excess buffer was characterized by physical and enzymatic methods. Thermograms for all SOPC.sbd.DOG mixtures exhibit a transition at 5.3 .degree.C. Above 0.25 mole fraction of DOG, metastability is observed; after the first scan, a second peak appears at 23.4 .degree.C which corresponds to the chain melting of pure DOG. This suggests that a complex or preferred packing array is formed which has a DOG mole fraction of 0.25 (Xc). Bilayer morphology is maintained in the metastable state up to 0.8 mole fraction of DOG. Above 0.8, a novel, nonlamellar phase is formed. Fluorescence polarization of 1,6-diphenylhexatriene shows that, relative to SOPC alone, there is little change in the order of the acyl chains up to Xc followed by a large decrease above Xc. Similar results were obtained using POPC. Miscibility was also studied in lipid films at the argon.sbd.buffer interface. Isothermal phase diagrams for the mixtures at 15 and 24 .degree.C exhibited phosphatidylcholine.sbd.DOG complex formation, a region of phosphatidylcholine and complex coexistence, and a region of complex and DOG miscibility. The mole fractions of DOG in the complex (Xc) range from 0.24 to 0.27. Porcine pancreatic phospholipase A2 and pancreatic lipase plus colipase were used as probes of the surface in both the monolayer and bilayer systems. In both systems and with both enzymes, substrate hydrolysis increased abruptly with increasing DOG. Overall, the formation of a complex at the same mole fraction of DOG in bilayers as in monolayers and the similar regulation of lipolytic enzyme activity in both systems suggest that the structures of DOG.sbd.phosphatidylcholine monolayers and bilayers are governed by a similar lipid.sbd.lipid interaction.