Stopped flow kinetics of pyrene transfer between human high density lipoproteins

Pyrene-labeled lipids as tools in membrane biophysics and cell biology

Pyrene is one of the most frequently used lipid-linked fluorophores. Its most characteristic features are a long excited state lifetime and (local) concentration-dependent formation of excimers. Pyrene is also hydrophobic and thus does not significantly distort the conformation of the labeled lipid molecule. These characteristics make pyrene lipids well-suited for studies on a variety of biophysical phenomena like lateral diffusion, inter- or transbilayer movement of lipids and lateral organization of membranes. Pyrene lipids have also been widely employed to determine protein binding to membranes, lipid conformation and the activity of lipolytic enzymes. In cell biology, pyrene lipids are promising tools for studies on lipid trafficking and metabolism, as well as for microscopic mapping of membrane properties. The main disadvantage of pyrene lipids is the relatively large size of the fluorophore. Another disadvantage is that they require UV-excitation, which is not feasible with all microscopes.

Hydrolysis of phospholipids by purified milk lipoprotein lipase. Effect of apoprotein CII, CIII, A and E, and synthetic fragments

Different pyrene-labeled phospholipid monolayer vesicles were used as substrates for the bovine milk lipoprotein lipase activity. The effects of synthetic fragments of apoprotein C II were measured on the hydrolysis of 1-myristoyl-2[9(1pyrenyl)-nonanoyl] phosphatidylcholine in vesicles: The activating capacity of fragments 30-78 and 43-78, 50-78 and 55-78, compared to entire apo CII, were similar to that obtained with hydrolysable triglycerides. Our study shows that the longer the carboxy terminal fragment is, the higher is the activation. The phospholipid hydrolysis activity represents in the presence of apo C II, 36% of the triglycerides hydrolysis activity. Phospholipid hydrolysis is less dependent on activator than triglycerides hydrolysis (100% and 300% of increase with apo CII for phosphatidyl-choline and triglycerides respectively). The ratio hydrolysis without apo C II/hydrolysis with apo CII was different when other phospholipids than myrystoyl-phospatidylcholine were assayed: phosphatidyl-serine, ethanolamine, -choline, -glycerol, or diglycerides and butanoylglycerols. Fragment CIII(1) (1-40) which did not bind to lipids, had no inhibitory effect. The entire sugar moiety and the first 40 amino acids are not required for the total inhibition of LPL. Inhibition was also obtained with Apo A I, A II,C I and fragments of apo E.

Synthesis and intermembrane transfer of pyrene-labelled liponucleotides: ceramide phosphothymidines

Phospholipid conjugates of 3'-azido-3'-deoxythymidine (AZT) show activity against human immunodeficiency virus (HIV) in vitro. Here we report on the synthesis and characterization of two pyrene containing conjugates: 2-N-(4-(pyren-1-yl)butanoyl)ceramide 5'-phosphothymidine (Pbs-Cer-P-T) (XII) and 2-N-(10-(pyren-1-yl)decanoyl)ceramide 5'-phosphothymidine (Pds-Cer-P-T) (XIII). These fluorescent labelled conjugates served as model compounds to study incorporation of sphingoliponucleotides into membranes. The complex compounds were prepared by condensation of 3'-acetylthymidine and labelled ceramides using the phosphite triester coupling procedure. UV absorption, fluorimetry as well as 1H-, 31P-, 13C-NMR analyses were used for structure confirmation of the synthesized substances. When incorporated into small unilamellar 1-palmitoyl-2-oleoyl-glycerophosphatidyl-choline (POPC) vesicles and incubated with unlabelled acceptor POPC vesicles, the compounds (XII) and (XIII) exhibited spontaneous transfer. Kinetic data suggest that transfer from donor to acceptor vesicles occurred via the intervening aqueous phase. The non-specific lipid transfer protein from bovine liver stimulated the transfer of Pds-Cer-P-T between phospholipid vesicles in a concentration dependent manner.

A new fluorometric method for measuring the action of C apolipoproteins on milk lipoprotein lipase

Monolayer vesicles containing pyrene-labelled nonanoyltriglyceride (1-2 ditetradecyl 3-pyrene nonanoyl glyceride) were used as a substrate to measure bovine milk lipoprotein lipase activity. The activation of lipoprotein lipase by synthetic fragments of apolipoprotein C II and apo C III was measured. Fragments 30-78 and 43-78 had actions similar to that of the entire apo C II. Fragments 50-78 and 55-78 were 50% active, fragment 60-78 was 10% active and fragment 66-78 was inactive. Thus the activating capacity depended on the length of the carboxyterminal fragment. Replacing tyrosine 62 in apo C II by glycine removed all lipoprotein lipase activating capacity, while making Tyr 62 less accessible for binding to lipids and enzyme decreased apo C II activating capacity. Apo C III1 inhibited both basal lipoprotein lipase activity (no apo C II) and lipoprotein lipase activated by apo C II. Apo C III, fragment A (1-40) which did not bind lipids, had no inhibitory effect, while fragment B(41-79) had the same effect as whole apo C III,. Apo AI, AII and C I also inhibited lipoprotein lipase. The fluorometric assay is easy to perform, and suitable for metabolic studies such as fatty-acid exchanges between lipoproteins, as it produces no alteration in the reaction products. It also avoids the use of a radio-labelled substrate.

Effect of the apolipoprotein C-II/C-III1 ratio on the capacity of purified milk lipoprotein lipase to hydrolyse triglycerides in monolayer vesicles

The effect of the apolipoprotein C-II/C-III1 ratio on the capacity of purified bovine milk lipoprotein lipase to hydrolyse triglycerides was measured in a controlled model of pyrene-labeled nonanoyltriglycerides (1-2 ditetradecyl 3-pyrene nonanoyl glyceride) monolayer vesicles. Monolayer was composed of triglycerides, a non-hydrolysable phospholipid ether and cholesterol, a model system where the quality of the interface can be controlled. LPL released fatty acids from pyrene-triglycerides which were transferred from the lipoprotein structure to albumin. This transfer induces a decrease in the excimer production and in the excimer fluorescence intensity. Apolipoprotein C-II and C-III0 and C-III1 were purified from apolipoprotein VLDL. The 2 fragments, C-III1 A (peptide 1-40) and C-III1 B (peptide 41-79), were obtained after thrombin cleavage. Apolipoproteins C-III0 and C-III1 had a similar inhibitory effect on LPL. Inhibition with apo C-III0 or apo C-III1 was 85% of full LPL activity without inhibitor: Apo C-III1 B inhibited 62% of basal activity. It was 27% less effective than apo C-III1. Fragment C-III1 A did not inhibit LPL. The effect of change in both apo C-II (0-0.6 microM) and apo C-III1 (0-1.0 microM) on triglyceride hydrolysis shows the importance of the apo C-II/C-III1 ratio for the release of free fatty acids from triglycerides by LPL. The activating effect of apo C-II in the absence of the apo C-III inhibitor was maximal at 0.06 microM. No further activation was obtained between 0.06 and 0.30 microM. Higher concentrations decreased LPL activity. Apo C-III1 (0.1 microM) decreased the maximum activation by apo C-II from 0.0196 to 0.063 nmol/min/nmol LPL. Higher concentrations of apo C-III1 (0.1-0.5 microM) required higher apo C-II concentrations (0.30 microM instead of 0.06 microM) for maximal activation than when apo C-III1 was absent. The activity of the enzyme without apo C-II was decreased by 65% by 0.12 microM apo C-III1. Increasing the apo C-II/apo C-III1 ratio from 0.1 to 1, increased the activation of the enzyme by a given apo C-II concentration. Moreover, for a given apo C-II/C-III1 ratio, the LPL activation increased with the apo C-II concentration (between 0 and 0.010 microM), until a plateau was reached. This is important, as the change in the C-II/C-III1 ratio is not the only factor affecting LPL activity, and inhibition by apo C-III1 also depends on the overall quantity of apolipoproteins. Extrapolation of these results suggests that hyperlipoproteinemia seems to be more likely due to overproduction of VLDL, than to a decrease in lipoprotein lipase activity.

Modulation of substrate selectivity in plasma lipid transfer protein reaction over structural variation of lipid particle

The modulation of substrate selectivity of human plasma LTP reaction is the subject of the present investigation. The moderate selectivity by a factor of 5 to 6 was observed in the LTP-catalyzed transfer of cholesteryl ester over triacylglycerol between plasma lipoproteins. On the other hand, the transfer of cholesteryl ester by LTP was highly selective over the negligible transfer of triacylglycerol, by a factor of 60 to 500, between the microemulsions with LDL size, regardless of the activators such as human and pig apolipoprotein (apo) A-I, human apo C-III and apo E that bound to the surface of the emulsion in equilibrium. The presence of free cholesterol in these microemulsions reduced slightly the rate of cholesteryl ester transfer but had no effect on triacylglycerol transfer. Other surface-active reagents such as cholic acid, Triton X-100 and Tween-20, did not have an effect on the triacylglycerol transfer either. Triacylglycerol transfer by LTP became measurable between such lipid particles as prepared by co-sonication of lipid with pig apo A-I and isolated as the mixed-microemulsions in the density of LDL and HDL. In these conditions, the substrate selectivity for cholesteryl ester over triacylglycerol was a factor of 6 to 16 mimicking the ratio in plasma lipoproteins. The conformation of pig apo A-I estimated by circular dichroism showed that its apparent helical content was further more induced when apo A-I was integrated into the mixed-microemulsion by co-sonication than the lipid-bound apo A-I in equilibrium. Apo A-I, thus integrated into lipid particles, was highly resistant to the denaturation by guanidine hydrochloride while the lipid-bound apo A-I in equilibrium was denatured as readily as the lipid-free protein. Thus, triacylglycerol transfer by LTP was induced by structural modulation of substrate-carrying lipid particles such as higher integration of apolipoproteins.

Selective transfer of cholesteryl ester over triglyceride by human plasma lipid transfer protein between apolipoprotein-activated lipid microemulsions

The substrate-specific rate of the human plasma lipid transfer protein (LTP) reaction was studied using pyrene-labeled substrate lipid analogues as probes for various lipids, by monitoring the ratio of the fluorescence intensities of their excimers to those of their monomers as an indicator of pyrene concentration in the microenvironment. Transfer of cholesteryl ester (CE) and triglyceride (TG) was demonstrated between human high-density lipoproteins, between low-density lipoproteins, and between these two lipoprotein, and the specific fractional transfer rate of CE was always higher than that of TG by a factor of 2.4-7.9. On the other hand, the transfer by LTP of CE, TG, and phosphatidylcholine (PC) was also demonstrated between lipid microemulsions having an average diameter of 25-26 nm using the same probes, but only when the emulsions were activated by apolipoproteins A-I, A-II, E, or C-III. The maximally activated rates of the transfer of CE and TG were the same when measured between the emulsions with cores composed exclusively of either lipid. The specific fractional transfer rate of pyrene-CE, however, was inversely proportional to the percentage of CE in the TG core of the emulsions, and the initial transfer of TG was almost completely inhibited by the presence of small percentages of CE in the TG core. Thus, the transfer of CE between the emulsions is highly selective over that of TG by orders of magnitude, much more selective than the reaction between any natural plasma lipoproteins, but this selectivity is not a rate-limiting step of the overall LTP reaction. The maximally activated LTP-catalyzed transfer rate of PC between the emulsions was somewhat higher than that of CE or TG and was not affected by the composition of the core lipids of the emulsion, TG or CE. When an excess amount of LTP was incubated with emulsion containing a small percentage of pyrene-CE in the TG core in the absence of the acceptor particles, excimer fluorescence rapidly decreased to the base line, and this change was suppressed when pyrene-CE was diluted with CE in the core. This result may indicate that LTP selectively disrupts pyrene-CE excimer formation on the basis of its selective interaction with the CE molecule over TG in the emulsion system as a putative background mechanism for the selective transfer of CE.

Kinetics of incorporation of porphyrins into small unilamellar vesicles

The kinetics of hematoporphyrin or deuteroporphyrin incorporation in egg phosphatidylcholine small unilamellar vesicles have been investigated by fluorescence stopped-flow measurements. The processes can be described by a fast equilibrium. The on-rate constant is nearly diffusion controlled regardless of the compound used and the pH. The affinity of these porphyrins for the vesicles is merely governed by the exit rate which depends on the structure of the porphyrin and on its charge determined by pH.

Activation of human plasma lipid transfer protein by apolipoproteins

Activation of human plasma lipid transfer protein (LTP) by apolipoproteins was studied. Pyrenelabeled cholesteryl ester was used as a probe substrate for the transfer reaction between lipid microemulsions, with a diameter of 26 nm, of triglyceride and phosphatidylcholine, and the reaction was monitored as a change in the ratio of the peaks of monomer and excimer in the fluorescence spectrum of pyrene. The transfer of pyrene-cholesteryl ester was hardly catalyzed by highly isolated LTP in the absence of apolipoprotein unless extreme overdose of LTP was given, regardless of the presence of bovine serum albumin. Human apolipoprotein (apo) A-I and apoA-II activated the LTP reaction in a dose-dependent manner. The activation was directly proportional to the titration of the surface of the substrate lipid emulsions by the apolipoproteins when the rate was plotted against the apolipoproteins bound to the surface. Human apoE also activated the LTP reaction in the same manner. The activation by human apoC-III was also proportional to the surface-bound protein, but the rate of the transfer was lower than those with other apolipoproteins. Displacement of apoA-I by apoC-III from the lipid emulsion surface, therefore, resulted in apparent deactivation of the LTP reaction. Thus, LTP requires apolipoproteins for its activation, and the activation seems proportional to the area of the surface of the lipid substrate particles modified by apolipoproteins. ApoA-I, -A-II, and -E are more potent activators than apoC-III for cholesteryl ester transfer.

Pyrene-labeled lipids: versatile probes of membrane dynamics in vitro and in living cells

Pyrene-labeled analogs of fatty acids have been studied as probes of lipid metabolism in vitro and in cultured cells. Procedures for the synthesis of complex pyrenyl lipids and the analytical methods for their separation and quantification are described. Pyrenyl-lipids have been used to quantify the relationship between lipid structure and the rates of spontaneous lipid transfer. Modifications of these methods have also been used to monitor protein-mediated lipid transfer, lipolysis and lipid translocation across bilayer membranes. According to several criteria, pyrene dodecanoic acid has been identified as a good analog of some naturally occurring fatty acids. Digital imaging microscopy has been used to monitor the rate of accumulation of pyrenyl lipids in living cells.

Spectrofluorometric measurements of the dispersion state of pyrenedodecanoic acid and its uptake by cultured cells and liposomes

Pyrene dodecanoic acid (P12), a medium-chain fatty acid to which the fluorescent probe pyrene is covalently linked, showed a considerable increase in fluorescence when the probe was introduced into a hydrophobic environment. Also, when closely packed in an aggregate, an energy transfer between two adjacent molecules of pyrene occurred, resulting in a shift of the peak of the emission spectrum from 378 nm ('monomeric') to 475 nm ('excimeric'). These two respective properties were utilized for the following: (a) A spectrofluorometric measurement of the critical micellar concentration (CMC) of the pyrene fatty acid, defined as the concentration at which the 475 nm emission peak appeared as a consequence of the aggregation of P12 molecules in aqueous solution to form micelles; the CMC of P12 was found to be in the range of 1 to 2 microM. (b) The penetration of P12, from an aqueous solution or dispersion, into unilamellar phospholipid vesicles was determined by monitoring the increase of the fluorescence at 378 nm. The fluorescence increase was time-dependent and proportional to the respective concentrations of P12 or phospholipid vesicles. Substituting the neutral phosphatidylcholine with the negatively-charged phosphatidylserine vesicles resulted in a slower rate as well as lesser total uptake of P12. (c) The uptake of P12 by cells was accompanied by an increase in the monomeric fluorescence emission intensity. Using cells in suspension, this could be followed continuously in a spectrofluorometer equipped with a recorder. The uptake was found to be time-dependent and proportional to P12 concentration.

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles

The effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles at a protein-to-lipid molar ratio of 1:60 have been investigated by employing the techniques of nanosecond emission anisotropy with 1,6-diphenyl-1,3,5-hexatriene as the fluorescent probe, enhancement by polar groups of the weakly allowed 0-0 vibronic transition in the fluorescence spectrum of pyrene, and Raman spectroscopy. The emission anisotropy results, which are found to be consistent with the wobble-in-cone model, show that the protein induces an increase in the order parameter, S, of the acyl chains of unilamellar vesicles below, at, and above their phase transition temperature, Tt, and it decreases strongly the diffusion rate, Dw, only below Tt. On the other hand, for multilamellar vesicles, the protein induces a decrease in S only at Tt and does not affect Dw. These effects are consistent with the observed changes in the degree of enhancement of the 0-0 vibronic transition of pyrene. Moreover, the protein broadens the thermal transition profile of multilamellar vesicles but sharpens dramatically that of unilamellar vesicles and fuses them without changing significantly the Tt in either case. On the other hand, the Raman data detect a decrease in the inter- and intramolecular order of the acyl chains of multilamellar vesicles below Tt and a decrease of only the former Tt. This disparity between the Raman and the nanosecond emission anisotropy data is discussed in terms of differences in the time scales of the two techniques and in the state of aggregation of the lipid-bound melittin. The data for the enhancement of the 0-0 vibronic transition of pyrene suggest that, for a melittin-to-lipid ratio of 1:60, the size or structure of channels formed in the bilayer by melittin does not allow the penetration of a neutral molecule the size of pyrene deeply into the bilayer.

Rates of hydration of fatty acids bound to unilamellar vesicles of phosphatidylcholine or to albumin

The rates of hydration of naturally occurring fatty acids bound to unilamellar vesicles of dimyristoylphosphatidylcholine were measured by following the rate of quenching of the inherent fluorescence of albumin. Rates of hydration of fatty acids bound to albumin could be estimated from the same data. The data show that these rates depend on the chain length and unsaturation of the fatty acid. Increasing chain length diminishes the rate of hydration whereas increasing unsaturation increases this rate. Rates of hydration of fatty acids bound to lipid vesicles appear to be rapid enough to account for intracellular movement between compartments in the absence of carrier proteins. It is uncertain whether this is true for hydration of fatty acids bound to albumin. Rates for this process are about 100-300 times slower vs. rates of hydration of fatty acids bound to lipid vesicles.

Equilibrium of apoproteins between high density lipoprotein and the aqueous phase: modelling of in vivo metabolism

There is a growing body of evidence that apoproteins of the high density lipoproteins (HDL) exchange between lipoprotein particles through the aqueous phase and that there is always a finite but physiologically important quantity of lipid-free apoprotein in plasma. We have studied the theoretical consequences of this concept on the catabolism of apoproteins and developed a testable model for studying the in vivo metabolism of HDL apoproteins. This model describes the putative modification of the apparent tissue distribution spaces of the apoprotein that would result from a change in the partitioning of this apoprotein between HDL and the aqueous phase. The main parameters predicted by the model include the tissue distribution spaces of free monomeric apoprotein, those of HDL-bound apoprotein and the partition coefficient of the apoprotein between HDL and the aqueous phase in blood. We have designed several ways of testing this model in vivo including the use of model synthetic apoproteins. This model can be generalized to a number of other binding systems.

Transfer of polycyclic aromatic hydrocarbons between model membranes: relation to carcinogenicity

Polynuclear aromatic hydrocarbons (PAH), some of which are potent carcinogens, are common environmental pollutants. The transport processes for these hydrophobic compounds into cells and between intracellular membranes are diverse and are not well understood. A common mechanism of transport is by spontaneous desorption and transfer through the aqueous phase. From the partitioning parameters, we have inferred that the rate limiting step involves solvation of the transfer species in the interfacial water at the phospholipid surface. Transfer of 10 PAH (pyrene, 3,4-benzophenanthrene, triphenylene, chrysene, 1,2-benzanthracene, 1,1'-binaphthyl, 9-phenylanthracene, 2,2'-binaphthyl, m-tetraphenyl and 1,3,5-triphenylbenzene) out of phosphatidylcholine vesicles has been examined. Our results show that the molecular volume of the PAH is a rate-determining factor. Moreover, high performance liquid chromatography (HPLC) data confirms the hypothesis that the rate of transfer is correlated with the size of the molecule and with the partitioning of the molecule between a polar and hydrocarbon phase. The kinetics and characteristics of the spontaneous transfer of carcinogens are likely to have a major impact on the competitive processes of PAH metabolism within cells.

Kinetics of spontaneous and plasma-stimulated sphingomyelin transfer

The mechanism of transfer of a pyrene-labeled sphingomyelin (PySM) between different lipid compartments was studied by a fluorescence technique. The first-order kinetics are independent of donor and acceptor concentration and the identity of the acceptor; the rates are accelerated by 'structure-breaking' solutes and inhibited by 'structure-making' solutes. These observations are consistent with the transfer of PySM occurring via the aqueous phase that separates the donor and acceptor compartments. We have partially purified a plasma factor that stimulates the transfer rate. Our in vitro results suggest that both spontaneous and stimulated transfer might contribute to the redistribution of sphingomyelin in vivo,

Glycoprotein-protein interaction examined by kinetic studies of pyrene transfer

The transfer of pyrene between alpha 1-acid glycoprotein, acethylcholinesterase and sonicated liposomes was used to monitor glycoprotein-protein interaction on the lipid bilayer. When a density solution of glycoprotein or protein labeled with pyrene was mixed with unlabeled suspension of free-phospholipid liposomes, or suspensions containing the complexes of glycoprotein-lipid, protein-lipid, or glycoprotein-protein-lipid, pyrene excimer fluorescence increased with a half-time of approximately 30--50 msec. Since the increase in excimer fluorescence indicates an increase in the microscope concentrations of pyrene, the observed fluorescence change reflects pyrene transfer. The half-times for the increase in excimer fluorescence were determined in the presence of glycoprotein and protein in the liposomes. On the basis of the determined half-times it was concluded that both, glycoprotein and protein are bound on the lipid bilayer. Our data also suggest that the thickness of the lipid bilayer is significantly changed in this case. The observation suggests strongly that the limiting step in the transfer of pyrene is not the dissociation of pyrene, but the uptake of the pyrene monomers by the lipid phase.

Kinetics and mechanism of association of human plasma apolipoproteins with dimyristoylphosphatidylcholine: effect of protein structure and lipid clusters on reaction rates

We have used a series of lipid-associating proteins with similar pI's and with molecular weights between 2280 and 28000 to study the mechanism of lipid-protein association. All of these polypeptides spontaneously associate with dimyristoylphosphatidylcholine (DMPC) to give quasi-discrete products. The reaction of the apoproteins with unsaturated lecithins is slow, if it occurs at all. Our data support the Kanehisa-Tsong cluster model [Kanehisa & Tsong (1978) J. Am. Chem. Soc. 100, 424] of lipid permeability in many of its qualitative aspects. These are (a) that the rate of lipid- protein association increases with decreasing polypeptide molecular weight, (b) that there is a small temperature dependence for the rate of association of small peptides with DMPC but with large polypeptides the temperature at which association with lipid is rapid is confined to the solid leads to fluid transition temperature (Tc) of DMPC, and (c) that the rate is asymmetric about Tc, with the change in the rate with respect to temperature below Tc being greater than at T greater than Tc. In addition, we have shown that unfolded monomeric proteins with a large number of exposed hydrophobic residues associate with DMPC faster than self-associated and/or folded proteins. Our data suggest that the association of some of the apoproteins with phospholipids is subject to kinetic control.

Mechanism of cholesterol exchange between phospholipid vesicles

The kinetics of cholesterol exchange between two populations of small unilamellar vesicles has been investigated. There is no change in the initial rate of this exchange process over a 100-fold change in the acceptor vesicle concentration at a constant donor concentration. These results are not consistent with a collision-dependent exchange mechanism. In support of transfer via the aqueous phase, the inclusion of a negatively charged lipid into the vesicles did not affect the exchange rate. Evidence for a water-soluble pool of cholesterol that had partitioned ut of the vesicle was obtained. Finally, cholesterol exchange was observed when donor and acceptor membranes were separated by a barrier through which neither could pass. These data together support our contention that the exchange of cholesterol between these vesicles involves a water-soluble intermediate.

Kinetics of soluble lipid monomer diffusion between vesicles

The fluorescent phospholipid 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]phosphatidylcholine (C12-NBD-PC) was used to study the kinetics of lipid transfer between phospholipid vesicles. A model based on lipid transfer resulting from the diffusion of soluble monomers was found to accurately predict the kinetics of this transfer process. From these studies, we conclude that (i) C12-NBD-PC transfer between vesicles results from the diffusion of soluble monomers and not from vesicle collision, (ii) the rate at which a lipid molecule enters or leaves a bilayer is dependent upon both its molecular structure and the characteristics of the donor and acceptor bilayers, and (iii) under the appropriate conditions, either the rate of lipid association or dissociation from the bilayer or a combination of both may determine the rate of transfer.

Mechanism of cholesterol and phosphatidylcholine exchange or transfer between unilamellar vesicles

The mechanism of cholesterol and phosphatidylcholine exchange has been investigated by following the transfer of radiolabeled cholesterol and phosphatidylcholine from negatively charged, unilamellar cholesterol-egg yolk phosphatidylcholine donor vesicles to neutral acceptor vesicles of similar composition. Vesicles were incubated in the absence of protein and were stable to fusion over the course of the experiment. At intervals, donor and acceptor vesicles were separated by passage through a column of DEAE-Sepharose; less than 1% of the charged and 80-95% of the neutral vesicles were recovered in the eluate. Over 12 h at 37 degrees C, 90% of the donor vesicle [4-14C]cholesterol was transferred to the acceptor vesicles in a first-order process whose half-time was 2.3 +/- 0.3 h. This indicates that transfer of cholesterol molecules from the inner to outer monolayer of the vesicle bilayer is not rate limiting in exchange. In contrast to cholesterol exchange, the half-time for 1-palmitoyl-2-oleoyl[1-14C]phosphatidylcholine exchange was 48 +/- 5 h so that more than six molecules of cholesterol were transferred for each molecule of phosphatidylcholine. The interfacial flux of cholesterol from the donor bilayer is 5.3 x 10(-15) mol cm-2 s-1 (approximately 3 molecules/min for an average vesicle) and is similar to fluxes observed in other systems where phosphatidylcholine or cholesterol ester exchange is catalyzed by an exchange protein. When the acceptor vesicle concentration was increased 20-fold in cholesterol exchange experiments or 9-fold in phosphatidylcholine exchange experiments, the rate of label transfer was not affected. The activation energy of cholesterol exchange between 15 and 37 degrees C was 73 +/- 5 kJ mol-1. Transfer of cholesterol across a dialysis membrane is shown to be a slow process whose rate may be predicted by application of Fick's first law of diffusion. These results are only consistent with a mechanism of lipid exchange in which cholesterol and phosphatidylcholine diffuse through the aqueous phase; the experimental activation energy is associated with desorption of lipid from the donor bilayer into the aqueous phase.

Mechanism of the spontaneous transfer of phospholipids between bilayers

A fluorescent phospholipid, 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine, was used to study the mechanism of spontaneous phospholipid transfer between single-walled phospholipid vesicles. The half-time for transfer of this molecule between vesicles of dimyristoylphosphatidylcholine at 36 degrees C is 13 h if flip-flop is negligible or 24 h if flip-flop is faster than intervesicle exchange. The half-time is unaffected by the concentration of acceptor vesicles, which indicates that transfer of label takes place by diffusion of monomers or micelles through the aqueous phase rather than by collision of vesicles. These results are compared with previous studies of spontaneous lipid transfer.

Sensitization of isolated canine coronary arteries to calcium ions after exposure to cholesterol

The abundance of membrane cholesterol is an important determinant of the functional properties of biomembranes. To determine whether arterial smooth muscle acquires altered contractile properties in a high cholesterol environment, isolated canine coronary arteries were exposed to cholesterol in stable aqueous solution. Cholesterol, 10(-12) to 10(-10) M, was an efficacious vasoconstrictor, as maximum contractions equaled those obtained with 15 mM KCl. Antiadrenergic interventions, including chemical sympathectomy in vivo with 6-hydroxydopamine and alpha- and beta-adrenergic blockade with phentolamine and L-propranolol (both 10(-6 M), did not significantly attenuate the contractions. However, responses to cholesterol were abolished completely by (+/-)-verapamil (10(-6) M). Cholesterol in picomolar concentration enhanced the constrictor effects of CaCl2 and KCl, both in the presence and absence of alpha- and beta-adrenergic blockade. Increases in tone in response to graded elevations in the CaCl2 concentration (0-2 mM) were augmented up to 1.5-fold by 10(-12) M cholesterol (P less than 0.01). Results indicate that cholesterol sensitizes isolated coronary arteries to external Ca2+ by a nonadrenergic mechanism. The findings are consistent with the hypothesis that acquisition of membrane cholesterol may alter the contractile properties of coronary arterial smooth muscle, a phenomenon that could play a role in the pathophysiology of atherosclerotic heart disease.

A review of the unique features of HDL apoproteins

The human plasma high density lipoproteins (HDL) are a heterogeneous ensemble of five proteins associated with both neutral and polar lipids. The sequence of all five proteins are known. ApoA-I and apoA-II are the major protein components; apoC-I, apoC-II and apoC-III are the minor protein components. All these apoproteins spontaneously recombine with phospholipids to give stable lipid-protein complexes and freely exchange between the two major HDL subclasses, HDL2 and HDL3. In addition, ApoC-I, apoC-II, and apoC-III exchange between HDL and very low density lipoproteins. Furthermore, certain HDL apoproteins are activators for plasma enzymes that are important in lipid metabolism. ApoA-I and apoC-I activate lecithin/cholesterol acyltransferase; apoC-II is an activator of lipoprotein lipase. The regions of apoC-I and apoC-II that are involved in the activation of these enzymes have been localized with synthetic peptides. Studies of synthetic and native fragments of apoA-II, apoC-I, apoC-II, and apoC-III as well as model lipid-binding peptides have identified specific regions with structural features common to lipid-binding proteins. These special properties, which include helical potential, sequences with a critical amphipathic length, and high hydrophobicity of the nonpolar side of the amphipathic helix, are the determinants of HDL structure and metabolism.

Kinetics of rac-1-oleyl-2-[4-(3-pyrenyl)butanoyl]glycerol transfer between high density lipoproteins

The mechanism of transfer of diglyceride between high density lipoproteins (HDL) was investigated with a pyrene-containing analogue whose fluorescent properties depend on the microscopic concentration in the lipoprotein. Transfer rates were first order, rapid (3.5s-1), and invariant over a 100-fold range of HDL concentration and over a 10-fold range of rac-1-oleyl-2-[4-(3-pyrenyl)butanoyl]glycerol concentrations. Similar behavior of the probe was observed with HDL3, although the rate was 40% slower. These results support a mechanism in which rate-limiting dissociation of the diglyceride analogue from one HDL particle into the aqueous phase precedes rapid diffusion and subsequent uptake by another such particle.