Dynamic structure of the lower density lipoproteins. I. Incorporation of high levels of labelled lipids into very low and low density lipoproteins

Selectively labelled lipids have been incorporated into the surface monolayer of human serum low density lipoprotein (LDL) and very low density lipoprotein (VLDL). From 3 to 17 mol% of phosphatidylcholine, selectively deuterated at various positions along the sn-2-acyl chain, was transferred from unilamellar vesicles to VLDL using a partially purified phosphatidylcholine transfer protein. Selectively deuterated palmitic acids were incorporated into LDL (6-20 mol%) and into VLDL (7-10 mol%). Electron microscopy, light scattering, and 31P nuclear magnetic resonance indicated that particle size remained unchanged. Gel exclusion chromatography and chemical analysis showed no difference in hydrodynamic properties and only slight alteration to particle component ratios. Biological activity of labelled VLDL was measured from the rate of cholesterol esterification by cultured J774A.1 cells. Effect of labelling LDL was evaluated by monitoring LDL uptake and degradation by cultured human skin fibroblasts. In all cases the lipoproteins containing labels were indistinguishable from their native counterparts.