Studies on phosphatidylcholine vesicles with thiocholesterol and a thiocholesterol-linked spin label incorporated in the vesicle wall

Human plasma lecithin:cholesterol acyltransferase. On the substrate efficiency of cholest-5-ene-3 beta-thiol as a fatty acyl acceptor

Lecithin:cholesterol acyltransferase (LCAT) is a plasma enzyme which catalyses cholesteryl ester formation from lecithin and cholesterol present in the surface of plasma lipoproteins. Sterol fatty acid acceptors have previously been shown to require the presence of a trans conformation of the A/B ring and a 3 beta-OH group. Our laboratory has, however, demonstrated that two thiol sites within LCAT can become fatty acylated following lecithin cleavage although this does not appear to be essential for catalysis. In order to assess the ability of LCAT to donate a fatty acid derived from the sn-2 position of lecithin and present as an acyl enzyme intermediate (linked via an oxyester bond to Ser-181) to a sulfhydryl residue, we evaluated the ability of cholest-5-ene-3 beta-thiol to act as a substrate for cholesterol ester formation by LCAT. Thiocholesterol was a good terminal fatty acyl acceptor when incorporated into synthetic proteoliposomes containing lecithin/thiocholesterol/apo A-I in the molar ratios of 250:15:0.8. The Km for thiocholesterol was 203.6 microM with a Vmax of 5.3 nmol thiocholesteryl ester formed/h per microgram. The Km for cholesterol when substituted for thiocholesterol in the proteoliposomes was 29.5 microM with a Vmax of 8.8 nmol cholesteryl ester formed/h per microgram. Thiocholesterol and cholesterol were shown to occupy the same catalytic site in LCAT. Thus, thiocholesterol exhibits approx. 10% of the substrate efficiency of cholesterol when incubated with pure human LCAT. We conclude that LCAT can transacylate a fatty acyl moiety from the sn-2 position of lecithin to the 3 beta-SH group of thiocholesterol forming a cholesteryl thioester. Although the 3 beta-SH group is not as good a terminal acceptor as the 3 beta-OH group of cholesterol, LCAT is clearly capable of transacylating a fatty acid esterified via an oxyester linkage to one containing a thioester.

Proxyl nitroxide of lithocholic acid: a potential spin probe for model membranes

A new steroidal proxyl (2,2,5,5-tetramethylpyrrolidine-N-oxyl) nitroxide (SPN), with the proxyl nitroxide moiety in the pendant side chain of the steroid, has been synthesized. Its localization in lipid bilayers was ascertained with the help of 1H NMR and 31P NMR experiments. The effects of the nitroxide group in SPN incorporated into the bilayer on 13C relaxation times are interpreted qualitatively in terms of localization of the nitroxide group within the bilayer structure. The nitroxide SPN was used to monitor changes in membrane fluidity and permeability induced by local anaesthetics, mepivacaine and xylocaine and the antikeratinizing agent, azelaic acid. The results conclusively proved the applicability of the new steroidal proxyl nitroxide (SPN) as a potential spin probe for spin labeling studies.

Thiolation of preformed liposomes with iminothiolane

Preformed phosphatidylethanolamine-containing liposomes were thiolated with 2-iminothiolane (Traut's reagent) and subsequently activated by mixed disulfide formation with 5,5'-dithiobis(2-nitrobenzoic acid). Up to 65% of amino groups of the outer liposomal lamella, corresponding to 230 SH-groups per vesicle, were modified. Covalent attachment of thiolated alpha-chymotrypsin to these thiol-liposomes via S-S bridges yielded a protein/lipid ratio of 3.6 X 10(-4) mol enzyme/mol lipid.

Role of membrane lipid asymmetry in aging

Recent advances in our understanding of the asymmetric distribution of lipids across nervous system membranes coupled with the application of biophysical techniques to examine transbilayer structure and function have led to the formulation of a new hypothesis. The author hopes that the insights presented herein will stimulate investigation into this developing new field. The theory provides an approach to correlation the accumulation of nervous tissue membrane peroxidative and cross-linking damage, the loss of transbilayer lipid asymmetry, and loss of transbilayer neuroendocrine, transport, secretory and immunoregulatory functions. Central to this scheme is the role of membrane lipid asymmetry in regulation to and/or coupling of transbilayer functions.

Transmembrane movement of phosphatidylglycerol and diacylglycerol sulfhydryl analogues

Transmembrane movement of phospholipids is a fundamental step in the process of biological membrane assembly and intracellular lipid sorting. To facilitate study of transmembrane movement, we have synthesized analogues of phosphatidylglycerol and diacylglycerol in which a sulfhydryl group replaces a hydroxyl group in the polar head group. A rapid, continuous assay for the movement of phospholipids across single-walled lipid vesicles was developed that exploits the reactivity of these analogues toward 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), a nonpenetrating, colorimetric, sulfhydryl reagent. In the reaction of DTNB with vesicles containing phosphatidylthioglycerol, a phosphatidylglycerol analogue, two kinetic phases were seen, which represent the reaction of DTNB with phosphatidylthioglycerol in the outer and inner leaflets of the bilayer. Analysis of the slow second phase indicated that the half-time for phosphatidylthioglycerol transbilayer movement was in excess of 8 days. In a similar experiment using dioleoylthioglycerol, a diacylglycerol analogue, the reaction was complete within 15 s. The large difference in translocation rates between these two lipids indicates that the primary barrier to transmembrane movement is the polar head group and implies that phospholipid translocation events in biological membranes may not be unlike those for molecules similar to the polar head groups alone.

The rapid transbilayer movement of thiocholesterol in small unilamellar phospholipid vesicles

Cholesterol is a major component of biological membranes, yet there is very little information concerning its distribution across the membrane. Recent experiments in our laboratory, using cholesterol oxidase, have demonstrated that cholesterol can undergo a rapid transbilayer movement in lecithin-cholesterol vesicles in a half-time of 1 min or less at 37 degrees C. In order to support this conclusion, we have sought other approaches to the measurement of this process. We now report our finding that the transbilayer movement of thiocholesterol in phospholipid vesicles occurs in a half-time of 1 min or less at 20 degrees C.

Transmembrane distribution of alpha-tocopherol in single-lamellar mixed lipid vesicles

A study of the molar ratio dependence of the incorporation of alpha-tocopherol into single-lamellar vesicles showed that the number of molecules which the bilayers can accommodate increased linearly with increasing alpha-tocopherol/phosphatidylcholine initial molar ratios till about 0.05, then approached a saturation limit. At 5 mol%, one alpha-tocopherol molecule per 60 phospholipids can be incorporated into the membranes. Up to this limit the distribution of alpha-tocopherol in the bilayers is uniform, while at initial molar ratios higher than 0.05 a disproportionation toward the inner monolayer of the vesicles is observed. The average outer/total ratio is found to be 0.27 +/- 0.03 at alpha-tocopherol/phosphatidylcholine molar ratios above 0.07 and is similar to asymmetrical distributions that have been reported in vesicles containing other one-chain amphiphiles (e.g., cholesterol). This large disproportionation is in contrast with the packing distribution of certain two-chain amphiphiles, and indicates that one of the driving forces for asymmetry formation in lipid bilayers might be dependent on the number of hydrocarbon chains per amphiphile molecule. A possible reason for the disproportionation effect observed in our experiments is the displacement of unsaturated phospholipids to the outer monolayer of the single-lamellar vesicles, by the more rigid isoprene units of alpha-tocopherol.

Functional incorporation of synthetic glycolipids into cells

Synthetic glycolipids containing an alpha-mannoside group linked by a hydrophilic spacer arm to cholesterol were incorporated into bovine erythrocytes by exchange from glycolipid-containing liposomes. When the distance between the sugar and the cholesterol moieties was approximately 26 A, functional incorporation of these glycolipids could be easily detected, as revealed by the concanavalin A-mediated agglutination of these cells. Bovine erythrocytes are not themselves susceptible to concanavalin A-mediated agglutination. The minimal concentration of concanavalin A required for agglutination of modified erythrocytes, containing 9.15 x 10(6) glycolipid molecules per cell, was 4 microgram/ml. Under these conditions, only approximately 4% of the membrane-bound cholesterol had been exchanged for the synthetic glycolipid. The observed aggregation was reversible in the presence of alpha-methyl mannoside and did not occur when beta-galactosyl-containing glycolipids were used in place of their alpha-mannoside isomers. These studies demonstrate a technique of sugar incorporation into cell membranes which should be of great advantage in studies on the roles of cell surface sugars in biological recognition. Furthermore, they demonstrate that the sugars need only be a short distance (26 A) from the membrane in order to functionally bind concanavalin A.

Threshold effects on the lectin-mediated aggregation of synthetic glycolipid-containing liposomes

Cholesterol analogs containing sugar residues linked by spacer groups to the cholesterol O can be incorporated into egg yolk lecithin small unilamellar liposomes. The synthetic glycolipid analogs distribute evenly on both sides of the bilayer. These liposomes are aggregated by the appropriate lectin. For example, when the sugar residue is a beta-galactoside the liposomes are aggregated by ricin and when it is an alpha-mannoside they are aggregated by Con A. The lectin-mediated aggregation of these liposomes is reversed by the addition of the appropriate sugar. The rates but not the extents of aggregation of these liposomes are highly sensitive to the amount of glycolipid incorporated. Below approximately 5% glycolipid incorporation the rate of the lectin-mediated aggregation of these liposomes is exceedingly slow, whereas above this level rapid aggregation proceeds. At all concentrations studied the synthetic glycolipids are incorporated in a unimodal fashion so that the observed threshold effects cannot be based on possible differences in the manner in which the glycolipids are incorporated at different concentrations. This conclusion is based on 1) studies with galactose oxidase that show that the percentage of galactose oxidation in a liposome prepared from a galactosyl-containing glycolipid is independent of glycolipid concentration, and 2) studies on the aggregation of liposomes containing mixed glycolipids in which the glycolipids are shown to behave independently. The importance of a critical density of membrane-bound receptors in order for aggregation to occur is discussed.

A structural model for the cholesterol-phosphatidylcholine complexes in bilayer membranes

Based on the structural properties of phospholipid and cholesterol molecules, and making use of the known structural and motional effects of cholesterol and its analogs on phospholipid bilayers, a model for the cholesterol-phosphatidylcholine complex is proposed. In this model, the 3beta-hydroxyl group of cholesterol is assumed to engage in hydrogen bonding with the carbonyl oxygen of the fatty acyl groups in phospholipids. Some specific configurations of the saturated and unsaturated fatty acyl chains of the phospholipid are suggested to participate in van der Waals attractive interactions with the apha and beta surface of the steroid nucleus.

Asymmetric exchange of vesicle phospholipids catalyzed by the phosphatidylcholine exhange protein. Measurement of inside--outside transitions

Purified phosphatidylcholine exchange protein was used to exchange phosphatidylcholine between homogeneous single-walled phosphatidylcholine vesicles and human erythrocyte ghosts. When excess ghosts were present, it was found that only 70% of the vesicle phosphatidylcholine was available for exchange. This fraction corresponds closely to the amount of phosphatidycholine in the outer monolayer of these vesicles, indicating that only the outer surface of the vesicle is accessible to the exchange protein. Also, it was found that all phosphatidylcholine introduced into vesicles by the exchange protein was available for subsequent exchange. Using the exchange protein, asymmetrical vesicles were prepared in which the outer monolayer was either enriched or depleted in radioactive phosphatidylcholine as compared to the inner monolayer. Re-equilibration of the radioactivity between the two surfaces of the vesicle (flip-flop) could not be detected, even after 5 days at 37degrees. It is estimated that the half-time for flip-flop is in excess of 11 days at 37degrees. These results indicate that the properties of the exchange protein can be expolited to measure phosphatidylcholine flip-flop rates and possible phosphatidylcholine asymmetry in biological and model membranes, without altering the structure of the membrane.