P-31-NMR STUDY OF THE PHOSPHOLIPID MOIETY OF LIPOPHORIN SUBSPECIES

P-31-NMR spectra of four distinct subspecies of Manduca sexta hemolymph lipophorin revealed the presence of two resonances separated by 0.6 ppm. Phospholipid analysis of the lipoproteins showed that phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were present and their mass ratio correlated well to the intensity of the two resonances in each of the different subspecies. The two resonances persisted in P-31-NMR spectra of organic solvent extracts of lipophorin. These results, together with the fact that PE, but not PC, can form an intramolecular hydrogen bond between the phosphate oxygen and the amino group of ethanolamine, resulting in deshielding of the phosphorus nucleus (and a 0.6 ppm downfield shift), strongly suggest the resonances observed represent the PC and PE components of these lipoproteins. P-31-NMR line-width data obtained as a function of temperature and solvent viscosity were used to calculate the chemical shift anisotropy (DELTA-sigma), intrinsic viscosity (eta'), and lateral diffusion coefficients (D(T)) of PC and PE in different lipophorin subspecies. Eta' and D(T) for PC and PE were similar among high-density lipophorins but differed in low-density lipophorin (LDLp). These differences may be related to the large increase in diacylglycerol content in this particle and/or the association of up to 16 molecules of apolipophorin III. On the basis of the known lipid compositional differences between LDLp and high-density lipophorin subspecies, we propose that uptake of large amounts of diacylglycerol during LDLp formation results in partitioning of this lipid to the surface monolayer where it intercalates between phospholipid molecules. Diacylglycerol intercalation creates gaps between phospholipid head groups that expose the hydrophobic surface. This surface is stabilized by apolipophorin III binding. Together these modifications in the surface monolayer are reflected in the phospholipid P-31-NMR line widths, eta', and D(T) of this lipoprotein.