β-Carotene as a Probe of Lipid Domains of Reconstituted Human Plasma Low-Density Lipoprotein: Induced Circular Dichroism

Mixtures of neutral lipids containing cholesteryl esters and β-carotene were used to reconstitute the lipid core of heptane-extracted low-density lipoproteins (LDL). The resulting preparations of reconstituted LDL, referred to as r-[cholesteryl ester + β-carotene] LDL, exhibited temperature-dependent circular dichroism (CD) in the visible region similar to that of native LDL. Since β-carotene lacks intrinsic optical asymmetry, the observed CD must be induced by environmental constraint. LDL reconstituted with cholesteryl oleate, cholesteryl linoleate, or cholesteryl linolenate in the presence of β-carotene exhibited thermotropic transitions at approximately 40, 35, and 30 °C, respectively. These temperatures are similar to the liquid-crystalline to isotropic liquid phase transition temperatures observed for pure cholesteryl ester model systems. The addition of triolein to r-[cholesteryl linoleate + β-carotene] LDL lowered the transition temperature and decreased the specific ellipticity of the visible CD bands. When triolein accounted for 70% or more of the total neutral lipid content of r-[cholesteryl linoleate + triolein + β-carotene] LDL, specific ellipticity was zero. The addition of low levels of unesterified cholesterol to r-[cholesteryl linoleate + β-carotene] LDL did not alter the transition temperature but did reduce the specific ellipticity by 40%. The current data indicate that the organization of the core of neutral lipids in reconstituted LDL resembles that of native LDL with respect to environmental constraint on the β-carotene molecule and that the helicity of the protein moiety resembles that of native LDL. In addition, the data are consistent with the possibility that the core cholesteryl esters of reconstituted LDL undergo phase transitions similar to their transitions in the free state.