Replacement of neutral lipids of low density lipoprotein with esters of long chain unsaturated fatty acids

A method has recently been described by which the neutral lipids of plasma low density lipoprotein (LDL) can be extracted with heptane and replaced with exogenous cholesteryl esters. In the current studies we show that, in addition to cholesteryl esters, other esters of long chain fatty acids, including triacylglycerols and methyl esters, can be used to reconstitute the core of heptane-extracted LDL. For each of these classes of esters, the common structural requirement for substantial incorporation into LDL was the presence of at least one cis-double bond in the fatty acyl chain. For example, cholesteryl oleate, triolein and methyl oleate could each be incorporated into LDL to yield a final lipid to protein mass ratio that was greater than one. In contrast, only trace amounts of esters of saturated fatty acids, such as cholesteryl stearate, tristearin or methyl stearate, could be incorporated into LDL despite the use of a variety of solvents and different temperatures of incubation. Incorporation of these saturated compounds was not enhanced by the inclusion of unsaturated cholesteryl esters or unsaturated triacylglycerols in the reconstitution reaction. Another class of compounds that can be incorporated into heptane-extracted LDL consists of lipids that contain a polyisoprenyl side chain, such as retinyl palmitate and ubiquinone-10. Each of the reconstituted LDL preparations retained the ability to bind to the LDL receptor of human fibroblasts and thus to deliver its respective core lipid to cells. The current data establish that plasma LDL can be made to function as a carrier for a variety of hydrophobic compounds that contain either long chain unsaturated fatty acyl or polyisoprenyl groups. The preferential incorporation of such compounds into LDL as compared with compounds containing long chain saturated fatty acids suggests that the protein or phospholipid component of LDL may have a specific ability to interact with long chain hydrocarbons that contain one or more double bonds.