Interaction of a human plasma lipid transfer protein complex with lipid monolayers

Lipid transfer protein-mediated distribution of HDL-derived cholesteryl esters among plasma apo B-containing lipoprotein subpopulations

The substrate specificity of lipid transfer protein has been examined in whole plasma in vitro by following the redistribution of high density lipoprotein (HDL)-derived [3H]cholesteryl ester (CE) into apolipoprotein (apo) B-containing lipoproteins using density gradient ultracentrifugation. HDL-derived [3H]CEs were incubated with plasma or isolated lipoprotein classes (very low density lipoprotein, intermediate density lipoprotein, and low density lipoprotein [LDL] subpopulations from the HDL donor) with and without lipoprotein lipase for 0.5-6 hours at 37 degrees C. After incubation, lipoproteins were separated into 38 fractions after density gradient ultracentrifugation, and radioactivity, protein, and cholesterol were monitored across the profiles. These studies indicate that 1) lipid transfer protein activity varied among the individuals as well as within an individual; 2) the majority of the [3H]CE was associated with LDL; 3) in most individuals (71%), more HDL-derived [3H]CE distributed within the buoyant LDL density region; and 4) the distribution of HDL-derived [3H]CE was similar to the distribution of lipoprotein lipase-derived "remnant" particles within buoyant LDL. These in vitro studies support the hypothesis that HDL-derived [3H]CEs vary in their distribution among apo B-containing particles and that more HDL-derived [3H]CEs are transferred to lipoproteins within the buoyant LDL density range. Additional studies suggest that lipoprotein heterogeneity within this density range, such as the presence of remnant-like lipoproteins, may contribute to the selective distribution of HDL-derived [3H]CE into buoyant LDL.

Cholesteryl ester analogs inhibit cholesteryl ester but not triglyceride transfer catalyzed by the plasma cholesteryl ester-triglyceride transfer protein

A lipid transfer protein complex (LTC), purified from human plasma by immunoaffinity chromatography, catalyzed the interlipoprotein transfer of cholesteryl esters (CE) and triglycerides (TG). The CE transfer activity of LTC was governed by the structure of the CE. Incubation of LTC with long chain CE both activated and stabilized LTC. Short chain CE also enhanced the CE and TG transfer activity of LTC during the initial time of incubation. However, LTC's incubation with short chain CE induced a subsequent and time-dependent loss of CE transfer activity without concomitant loss of TG transfer activity. The data indicate that the CE and TG transfer activity of LTC can be regulated independently.

Purification and characterization of human plasma proteins that inhibit lipid transfer activities

A protein which inhibits cholesteryl ester and triacylglycerol transfer activities was purified from human lipoprotein-deficient plasma by chromatography on phenyl-Sepharose CL-4B, chromatofocusing, Bio-Gel A-0.5m and hydroxylapatite. The inhibitor is a sialoglycoprotein with molecular weight 32 000 and a relatively broad isoelectric region of 3.9-4.3. The inhibitor suppressed triacylglycerol and cholesteryl ester transfer activities to a similar extent. Apolipoprotein A-I, which was separated from the inhibitor by chromatofocusing chromatography, suppressed triacyglycerol transfer more than cholesteryl ester transfer. The percentage reduction of lipid transfer between lipoproteins by the inhibitor was independent of the concentration of transfer protein but was decreased at higher lipoprotein concentrations. The inhibition was not observed during lipid transfer between liposomes. These results indicate that the inhibitor interacts with substrates rather than with the transfer protein.

The effect of the sterol oxygen function on the interaction with phospholipids

The effect of cholesteryl ethers (namely cholesteryl methyl ether, cholesteryl ethyl ether, cholesteryl n-propyl ether, cholesteryl isopropyl ether, cholesteryl butyl ether, cholesteryl methoxymethyl ether, cholesteryl (2'-hydroxy)-3-ethyl ether) and cholesteryl ester (namely cholesteryl acetate) is tested on the interaction with phosphatidylcholines in liquid-crystalline and crystalline state. The interfacial properties of sterols are tested at the air-water interface. The cholesteryl ethers show a reduced interfacial stability with increasing hydrophobicity of the ether-linked moiety. The interaction between the sterol derivatives and phospholipids in mixed monolayers is indicated by measuring the deviation from the simple addivity rule (condensing effect). An interaction is found only for cholesteryl (2'-hydroxy)-3-ethyl ether, cholesteryl methyl ether and cholesteryl ethyl ether. These sterols also reduce the glucose permeability of liposomal membranes in this order. In this respect cholesteryl (2'-hydroxy)-3-ethyl ether is as effective as cholesterol. Cholesteryl methyl ether and cholesteryl ethyl ether show 62 and 33 percent of the effect observed with cholesterol. The effect of the sterol derivatives on the gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine is measured by differential scanning calorimetry. Cholesteryl methyl ether, cholesteryl ethyl ether, and cholesteryl (2'-hydroxy)-3-ethyl ether reduce the energy content of the phase transition nearly as effective as cholesterol, cholesteryl n-propyl ether has only a small effect. Although cholesteryl acetate, and cholesteryl methoxymethyl ether have no condensing or permeability-reducing effect, they have a considerable effect on the gel-to-liquid-crystalline phase transition. Cholesteryl isopropyl ether and cholesteryl butyl ether have no effect. It is concluded that a free 3 beta-hydroxy group is not a prerequisite to observe a sterol-like effect in membranes. However, the interfacial stability and the orientation of the sterol and oxygen moiety at the sterol 3-position are important.