Lipolysis of phospholipids in model cholesteryl ester rich lipoproteins and related systems: effect of core and surface lipid phase state.

Abstract

Porcine pancreatic phospholipase A2 (PL A2) was used as a probe to study the structure of phospholipid domains of dimyristoylphosphatidylcholine (DMPC) vesicles +/- 2% cholesteryl oleate (CO), of discoidal structures formed by the interaction of apolipoprotein E (apoE) with these vesicles, and of large CO/DMPC microemulsion particles +/- apoE. Results of phospholipid hydrolysis over a range of temperatures were compared with the thermal transitions of the lipid components of the particles as measured by differential scanning calorimetry. These studies revealed that DMPC vesicles were most susceptible to digestion at or near the transition temperature. A similar result was obtained with DMPC/apoE disks; however, these particles were hydrolyzed over a broader temperature range than the vesicles. DMPC/CO vesicles were resistant to hydrolysis at every temperature tested; however, discoidal structures formed by interaction of apoE with these vesicles were hydrolyzed maximally above their thermal transition. Large microemulsion particles of CO and DMPC were poor substrates for the enzyme at every temperature; binding of apoE to these particles improved the ability of PL A2 to hydrolyze the phospholipid. These results suggest that the curvature of the surface, the presence of dissolved cholesteryl ester, and the binding of protein have profound effects on the temperature and breadth of the phospholipid phase transition and on the accessibility of the phospholipid to hydrolysis by PL A2.