IDENTIFICATION AND LOCALIZATION OF 2 DISTINCT MICROENVIRONMENTS FOR THE DIACYLGLYCEROL COMPONENT OF LIPOPHORIN PARTICLES BY C-13 NMR

C-13 nuclear magnetic resonance spectroscopy of lipoproteins, isolated from the insect Manduca sexta, has been employed to probe the microenvironment of diacylglycerol (DG), their major neutral lipid component, Natural abundance C-13 NMR spectra of high density lipophorin exhibited several well-separated resonances derived from its lipid moiety, including those for the carbonyl carbon atoms of phospholipid and DG fatty acyl chains in the region of 175-180 ppm. To verify the assignment of the DG acyl chain carbonyl carbon resonances, di[1-C-13]oleoylglycerol high density lipophorin was isolated after instilling a bolus of tri[1-C-13]oleoylglycerol into the midgut of larvae fed a fat-free diet. C-13 NMR spectra of the isolated lipoprotein revealed a specific and dramatic enrichment of resonances at 175.5 ppm, Expansion of this region revealed two resonances separated by 0.08 ppm. These were assigned as 1,2- and 1,3- isomers of DG, the latter presumably arising from spontaneous acyl chain migration of 1,2-DG following lipoprotein isolation. On the basis of compositional and structural analysis of this lipoprotein, it is postulated that these DG species are localized predominantly in the hydrophobic core of the particle. By contrast, natural abundance C-13 NMR spectra of the DG-rich, low density lipophorin (LDLp) subspecies revealed two additional resonances, separated by 0.2 ppm, that were tentatively assigned as 1,2- and 1,3-DG present at the surface of the particle. The verify this assignment, experiments employing phospholipase C, to convert lipophorin surface associated phospholipid into DG, were performed. The data revealed that loss of the phospholipid acyl chain carbonyl carbon resonance correlated with the appearance of two additional DG acyl chain carbonyl resonances (separated by 0.2 ppm) possessing chemical shifts similar to those observed in LDLp, Interestingly, the resonance assigned as 1,2-DG at the surface, predominanted immediately after phospholipase C hydrolysis, but its intensity decreased with time. Concomitantly, there was a corresponding increase in the resonance assigned as 1,3-DG, consistent with an interconversion of these isomers through acyl chain migration, Taken together, the results provide the first direct experimental evidence that DG molecules present at the surface monolayer and the particle core, respectively, can be distinguished. Thus it should be possible to design experiments to evaluate exchange between these two locations as well as the role of surface associated DG in binding of the exchangeable apolipoprotein, apolipophorin in.