Location and motion of free cholesterol molecules in high density lipoprotein

Alterations in the physiochemical characteristics of low and high density lipoproteins after lipolysis with phospholipase A2. A spin-label study

Human low and high density lipoproteins (LDL and HDL, respectively) were treated with porcine pancreatic phospholipase A2 (PLA2) in the presence of albumin resulting in hydrolysis of 40-84% of the lipoproteins phospholipids. The resulting PLA2-treated LDL and HDL and concurrent control lipoproteins incubated without PLA2 were reisolated by ultracentrifugation and labelled with 5-doxyl- and 16-doxyl-stearic acid, and with a spin-labelled analogue of maleimide. Analysis of ESR spectra showed that phospholipid hydrolysis both of LDL and HDL resulted in an increase in order, micro-viscosity and polarity of lipid regions in the surface monolayer of the particles. In the temperature range from 3 degrees C to 50-60 degrees C, Arrhenius plots of a spectral parameter of LDL and HDL labelled with 5-doxyl-stearate exhibited alterations which suggest an increase in free cholesterol content near the surface of the lipoproteins after PLA2-treatment. ESR spectra of the maleimide analogue bound covalently to the protein moiety of the lipoproteins have demonstrated that, following phospholipid hydrolysis, the conformation of the apoproteins became more condensed, with more masked domains. The possible implications of the revealed alterations for enhanced delivery of LDL and HDL cholesterol to cells after phospholipolysis of the lipoproteins are discussed.

Comparative study of the lipid dynamics in the surface layer of porcine and human high density lipoprotein subclasses by spin labeling

In order to obtain information on the determinants of the lipid dynamics in the high density lipoproteins (HDL), we have compared the structural properties of human HDL subclasses with porcine HDL artificially subdivided into density subfractions corresponding to those of human HDL. Four different positional isomers of spin labeled fatty acids and spin labeled androstanol experienced more restricted motion in porcine HDL than in the human HDL2 and HDL3 subclasses. The differences in the spin label motion could not be accounted for on the basis of the differences in the chemical composition of the lipoproteins examined. They are, however, most probably due to the specific properties of the interactions between lipids and proteins that differ among the lipoproteins.

Influence of cholesterol on bilayers of ester- and ether-linked phospholipids. Permeability and 13C-nuclear magnetic resonance measurements

13C-NMR and permeability studies are described for sonicated vesicles of phosphatidylcholines bearing two 16-carbon saturated hydrocarbon chains with (a) one ether linkage at carbon 1 (3) or 2 of glycerol and one ester linkage at carbon 2 or 1 (3) of glycerol; (b) two ether linkages and (c) two ester linkages at carbons 1 (3) and 2 of glycerol. The results of 13C-NMR relaxation enhancement measurements using cholesterol enriched with 13C at the 4 position indicate that no significant relocation of the cholesterol molecules takes place in the bilayer when a methylene group is substituted for a carbonyl group in phosphatidylcholine. The 4-13C atom of cholesterol undergoes similar fast anisotropic motions in diester- and diether -phosphatidylcholine bilayers, as judged by spin-lattice relaxation time measurements in the liquid-crystalline phase; although the fast motions are unaltered, linewidth and spin-spin relaxation time measurements suggested some restriction of the slow motions of cholesterol molecules in bilayers from phosphatidylcholines containing an O-alkyl linkage at the sn-2 position instead of an acyl linkage. At temperatures above the gel to liquid-crystal phase transition, the kinetics of ionophore A23187-mediated 45Ca2+ efflux from vesicles prepared from each type of phosphatidylcholine molecule were the same; the kinetics of spontaneous carboxyfluorescein diffusion from diester- and diether -phosphatidylcholine vesicles were the same, whereas mixed ether/ester phosphatidylcholine molecules gave bilayers which are less permeable. The rate constants were reduced on cholesterol incorporation into the bilayers of each type of phosphatidylcholine molecule. The reductions were not statistically significant for 45Ca2+ release. The rate constants for carboxyfluorescein release were also reduced by cholesterol to the same extent in vesicles from diester-, diether -, and 1-ether, and 1-ether-2-ester-phosphatidylcholines; however, a smaller reduction was noted in bilayers from the 1-ester-2-ether analog. The results provide further evidence that there are no highly specific requirements for ester or ether linkages in phosphatidylcholine for cholesterol to reduce bilayer permeability. This is a reflection of the fact that in both diester- and diether -phosphatidylcholine bilayers, the 4-13C atom of cholesterol is located in the region of the acyl carboxyl group or the glyceryl ether oxygen atom.

Organization of unesterified cholesterol in high density lipoproteins probed by filipin

The initial rate of filipin association with unesterified cholesterol in high density lipoproteins (HDL) was measured by stopped-flow spectrophotometry to assess the roles played by apolipoproteins and phospholipids in modulating the surface exposure of cholesterol. The initial rate of filipin-unesterified cholesterol association was enhanced upon hydrolysis of the glycerophospholipids of human HDL3 by phospholipase A2. Rate enhancements were also observed following trypsin-catalyzed hydrolysis of apolipoprotein A-I in canine HDL and of apolipoproteins A-I and A-II in human HDL3. However, the initial rate of filipin-unesterified cholesterol association was not altered upon incubation of HDL3 with polymorphonuclear cells, which causes hydrolysis of apolipoprotein A-II but leaves apolipoprotein A-I intact. These results are consistent with the general structural model of HDL in which unesterified cholesterol, apolipoproteins and glycerophospholipids are presumed to be localized at the surface of the HDL particle. From these studies and from results indicating that the initial rate of filipin-unesterified association was enhanced in canine HDL hybrids in which 50% of the apolipoprotein A-I had been replaced by apolipoprotein A-II, we also conclude that apolipoprotein A-I in HDL is in closer proximity to unesterified cholesterol than apolipoprotein A-II. Thus, it appears that rapid kinetic measurements of filipin-cholesterol association may be useful in assessing the organization of unesterified cholesterol in serum lipoproteins.