HYDRATION AND LOCALIZATION OF DIACYLGLYCEROL IN THE INSECT LIPOPROTEIN LIPOPHORIN - A C-13-NMR STUDY

In order to probe the organization of diacylglycerol (DG) in lipophorin, C-13-enriched lipophorin was prepared for NMR investigations. We obtained C-13-enriched lipophorin labeled exclusively in DG by feeding insects tobacco leaves coated with [1-C-13]palmitic acid or [1-C-13]oleic acid. Lipophorins enriched up to 5% with a [C-13]fatty acid were obtained by this procedure. NMR studies of the isolated lipophorin DG showed that palmitic acid accumulates almost entirely (>90%) in the sn-1 position. Oleic acid was found equally distributed between the sn-1 and sn-2 positions, yielding a DG enriched equally at both positions. The C-13-NMR spectra of both [C-13]palmitate- and [C-13]oleate-enriched lipophorins showed that DG had one narrow carbonyl resonance indicative of rapid motion. A comparative analysis of the C-13 carbonyl chemical shift data for DG in organic solvents, aqueous solutions, and dispersions with the DG carbonyl chemical shift of native lipophorin enriched in [C-13]palmitate or [C-13]oleate shows a high degree of water exclusion from the DG carbonyls in lipophorin. This result is consistent with the existence of a lipophorin lipid core containing most of the lipophorin DG. This study represents the first attempt to elucidate the organization of DG in lipophorin. The possibility of obtaining [C-13] DG-enriched lipophorins, selectively enriched in one or both acyl chains of DG, should provide a powerful tool for further analysis of the organization and the dynamic properties of DG in native lipoproteins.