Phospholipids: synthesis, sorting, subcellular traffic - the yeast approach

Most of the enzymes and genes required for lipid biosynthesis and degradation in the budding yeast Saccharomyces cerevisiae have now been identified and the global mechanisms that regulate their activity are being established. Synthesis of phospholipids is restricted to specific subcellular compartments, and the lipids migrate from their site of formation to their final destination. In addition to synthesis, remodelling and degradation of phospholipids controls the content of the lipid portion of cellular membranes, while highly specific phospholipases catalyse the release of lipid-based second messengers. In this review, we describe the current understanding of the organization and regulation of phospholipid metabolism in yeast, and discuss the mechanisms that have been proposed for intracellular lipid transport.

Lateral microheterogeneity of diphenylhexatriene-labeled choline phospholipids in the erythrocyte ghost membrane as determined by time-resolved fluorescence spectroscopy

Choline phospholipids are the major constituents of the outer layer of the erythrocyte membrane. To investigate their lateral membrane organization we determined the fluorescence lifetime properties of diphenylhexatriene analogues of phosphatidylcholine, choline plasmalogen, (the respective enolether derivative), and sphingomyelin inserted into the outer layer of hemoglobin-free ghosts. Fluorescence lifetimes were recorded by time-resolved phase and modulation fluorometry and analyzed in terms of Continuous Lorentzian distributions. To assess the influence of membrane proteins on the fluorescence lifetime of the labeled lipids in the biomembrane, lipid vesicles were used as controls. In general, the lifetime distributions in the ghost membranes are broad compared to vesicles. Phosphatidylcholine and sphingomyelin exhibit very similar lifetime distributions in contrast to an increased plasmalogen lifetime heterogeneity in both systems. Orientational effects of side chain mobilities on the observed lifetimes can be excluded. Fluorescence anisotropies revealed identical values for all three labeled phospholipids in the biomembrane.

Fluorescent inhibitors for the qualitative and quantitative analysis of lipolytic enzymes

We report on the determination of active enzyme components in pure and crude lipases, using fluorescent inhibitors for covalent modification and visualization of the enzymatically active proteins. Lipase-specific compounds are triacylglycerol analogs, namely 1,2(2, 3)-di-O-alkylglyceroalkylphosphonic acid-p-nitrophenyl esters, containing a fluorescent substituent bound to the omega-end of an alkyl chain. Inhibitors derived from single-chain alcohols, such as p-nitrophenyl esters of fluorescent alkyl phosphonates, react with lipases and esterases. The p-nitrophenyl ester bond is susceptible toward nucleophilic attack by the active serine of the lipolytic enzyme. This reaction is stoichiometric, specific, and irreversible. Stable lipid-protein complexes are formed which can be analyzed on the basis of their fluorescent signal. From fluorescence intensity the moles of active serine (enzyme) were accurately determined. A lipase-specific inhibitor was used for the analysis of a commercial lipase preparation from Rhizomucor miehei. After incubation of the enzyme with the fluorescent lipid, a single fluorescence band was observed after SDS-gel electrophoresis, indicating the presence of a single lipase in the crude enzyme material. A linear correlation was obtained between fluorescence intensity and the amount of enzyme. Using a combination of different inhibitors, we were able to discriminate between lipases and esterases.

Characterization and Function in Vivo of Two Novel Phospholipases B/Lysophospholipases fromSaccharomyces cerevisiae

The yeast genome contains two genes, designated as PLB2 and PLB3, that are 67% and 62% identical, respectively, to PLB1, which codes for a phospholipase B/lysophospholipase in yeast (Lee, S. K., Patton, J. L., Fido, M., Hines, L. K., Kohlwein, S. D., Paltauf, F., Henry, S. A., and Levin, D. E. (1994) J. Biol. Chem. 269, 19725-19730). Deletion and overexpression studies and in vivo and in vitro activity measurements suggest that both genes indeed code for phospholipases B/lysophospholipases. In cell free extracts of a plb1 plb2 plb3 triple mutant, no phospholipase B activity was detectable. Upon overexpression of PLB2 in a plb1 plb3 mutant background, phospholipase B activity was detectable in the plasma membrane, periplasmic space extracts and the culture supernatant. Similar to Plb1p, Plb2p appears to accept all major phospholipid classes, with a preference for acidic phospholipids including phosphatidylinositol 3',4'-bisphosphate and phosphatidic acid. Consistent with a function as an extracellular lysophospholipase, PLB2 overexpression conferred resistance to lyso-phosphatidylcholine. Deletion of Plb2p function had no effect on glycerophosphoinositol or glycerophosphocholine release in vivo, in contrast to a deletion of Plb3p function, which resulted in a 50% reduction of phosphatidylinositol breakdown and glycerophosphoinositol release from the cells. In vitro, Plb3p hydrolyzes only phosphatidylinositol and phosphatidylserine and, to a lesser extent, their lyso-analogs. Plb3p activity in a plb1 plb2 mutant background was observed in periplasmic space extracts. Both Plb3p and Plb2p display transacylase activity in vitro, in the presence or absence, respectively, of detergent.

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of the lipid molecular species composition of yeast subcellular membranes reveals acyl chain-based sorting/remodeling of distinct molecular species en route to the plasma membrane

Nano-electrospray ionization tandem mass spectrometry (nano-ESI-MS/MS) was employed to determine qualitative differences in the lipid molecular species composition of a comprehensive set of organellar membranes, isolated from a single culture of Saccharomyces cerevisiae cells. Remarkable differences in the acyl chain composition of biosynthetically related phospholipid classes were observed. Acyl chain saturation was lowest in phosphatidylcholine (15.4%) and phosphatidylethanolamine (PE; 16.2%), followed by phosphatidylserine (PS; 29.4%), and highest in phosphatidylinositol (53.1%). The lipid molecular species profiles of the various membranes were generally similar, with a deviation from a calculated average profile of approximately +/- 20%. Nevertheless, clear distinctions between the molecular species profiles of different membranes were observed, suggesting that lipid sorting mechanisms are operating at the level of individual molecular species to maintain the specific lipid composition of a given membrane. Most notably, the plasma membrane is enriched in saturated species of PS and PE. The nature of the sorting mechanism that determines the lipid composition of the plasma membrane was investigated further. The accumulation of monounsaturated species of PS at the expense of diunsaturated species in the plasma membrane of wild-type cells was reversed in elo3Delta mutant cells, which synthesize C24 fatty acid-substituted sphingolipids instead of the normal C26 fatty acid-substituted species. This observation suggests that acyl chain-based sorting and/or remodeling mechanisms are operating to maintain the specific lipid molecular species composition of the yeast plasma membrane.

Molecular dynamics of microbial lipases as determined from their intrinsic tryptophan fluorescence

We have studied the intrinsic tryptophan fluorescence of the lipases from Chromobacterium viscosum (CVL), Pseudomonas species (PSL), and Rhizopus oryzae (ROL) in aqueous buffer, zwitterionic detergent micelles, and isopropanol-water mixtures. It was the purpose of this study to obtain information about biophysical properties of the respective enzymes under conditions that modulate enzyme activities and stereoselectivities to a significant extent. According to their decay-associated emission spectra, CVL tryptophans are located in the hydrophobic interior of the protein. In contrast, the PSL and ROL tryptophans are probably confined to the core and the surface of the lipase. From the tryptophan lifetime distributions it can be concluded that the conformation of CVL is not much affected by detergent or organic solvent (isopropanol). Accordingly, CVL is enzymatically active in these systems and most active in the presence of isopropanol. In contrast, ROL and PSL show high conformational mobility, depending on the solvent, because their lifetime distributions are very different in the presence and absence of detergent or isopropanol. Time-resolved anisotropy studies provided evidence that the lipases exhibit very high internal molecular flexibility. This peculiar feature of lipases is perhaps the key to the great differences in activity and stereoselectivity observed in different reaction media. Furthermore, information about self-association of the lipases in different solvents could be obtained. PSL, but not CVL and ROL, forms aggregates in water. Lipase aggregation can be reversed by the addition of detergent or isopropanol, which competes for the hydrophobic surface domains of this protein. This dissociation could efficiently contribute to the increase in lipase activity in the presence of a detergent or isopropanol.

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.

Interactions of fluorescent triacylglycerol analogs covalently bound to the active site of a lipase from Rhizopus oryzae

Fluorescent triacylglycerol analogs were synthesized as covalent inhibitors of lipase activity. The respective 1(3), 2-O-dialkylglycero-3(1)-alkyl-phosphonic acid p-nitrophenyl esters contain a fluorescent pyrenealkyl chain and a long-chain alkyl residue bound to the sn-2 and sn-1(3) positions of glycerol, respectively. The phosphonic acid p-nitrophenyl ester bond is susceptible to nucleophilic substitution by the active serine residue in the catalytic triad of a lipase, leading to inactivation of the enzyme. The fluorescent dialkylglycerophosphonates contain two chiral centers, the sn-2 carbon of glycerol and the phosphorus atom. The (1-O-hexadecyl-2-O-pyrenedecyl-sn-glycero)-O-(p-nitrophenyl)-n-hex yl- phosphonate, first peak during HPLC separation and the (3-O-hexadecyl-2-O-pyrenedecyl-sn-glycero)-O-(p-nitrophenyl)-n-hex yl- phosphonate, second peak during HPLC separation were found to be potent lipase inhibitors. After incubation of an equimolar amount of these isomers with lipase from Rhizopus oryzae complete inactivation was observed. Stable conjugates containing a 1 : 1 molar ratio of lipid to protein were formed. The spatial proximity of the fluorescently labeled sn-2 alkyl chain of the inhibitor and tryptophan residues of the lipase was assessed by fluorescence resonance energy transfer. The extent of tryptophan fluorescence quenching and the concomitant increase in pyrene fluorescence upon excitation of lipase tryptophans was found to be similar for the above-mentioned isomers. Thus, the (labeled) sn-2 alkyl chains of a triacylglycerol analog are likely to interact with the same binding site of the R. oryzae lipase, irrespective of their steric configuration. However, it was shown that the extent of resonance energy transfer is strongly influenced by the reaction medium, indicating conformational changes of the lipase in different environments.

Redundant systems of phosphatidic acid biosynthesis via acylation of glycerol-3-phosphate or dihydroxyacetone phosphate in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae lipid particles harbor two acyltransferases, Gat1p and Slc1p, which catalyze subsequent steps of acylation required for the formation of phosphatidic acid. Both enzymes are also components of the endoplasmic reticulum, but this compartment contains additional acyltransferase(s) involved in the biosynthesis of phosphatidic acid (K. Athenstaedt and G. Daum, J. Bacteriol. 179:7611-7616, 1997). Using the gat1 mutant strain TTA1, we show here that Gat1p present in both subcellular fractions accepts glycerol-3-phosphate and dihydroxyacetone phosphate as a substrate. Similarly, the additional acyltransferase(s) present in the endoplasmic reticulum can acylate both precursors. In contrast, yeast mitochondria harbor an enzyme(s) that significantly prefers dihydroxyacetone phosphate as a substrate for acylation, suggesting that at least one additional independent acyltransferase is present in this organelle. Surprisingly, enzymatic activity of 1-acyldihydroxyacetone phosphate reductase, which is required for the conversion of 1-acyldihydroxyacetone phosphate to 1-acylglycerol-3-phosphate (lysophosphatidic acid), is detectable only in lipid particles and the endoplasmic reticulum and not in mitochondria. In vivo labeling of wild-type cells with [2-3H, U-14C]glycerol revealed that both glycerol-3-phosphate and dihydroxyacetone phosphate can be incorporated as a backbone of glycerolipids. In the gat1 mutant and the 1-acylglycerol-3-phosphate acyltransferase slc1 mutant, the dihydroxyacetone phosphate pathway of phosphatidic acid biosynthesis is slightly preferred as compared to the wild type. Thus, mutations of the major acyltransferases Gat1p and Slc1p lead to an increased contribution of mitochondrial acyltransferase(s) to glycerolipid synthesis due to their substrate preference for dihydroxyacetone phosphate.

Stereoselectivity of Mucorales lipases toward triradylglycerols--a simple solution to a complex problem

The lipases from Rhizopus and Rhizomucor are members of the family of Mucorales lipases. Although they display high sequence homology, their stereoselectivity toward triradylglycerols (sn-2 substituted triacylglycerols) varies. Four different triradylglycerols were investigated, which were classified into two groups: flexible substrates with rotatable O'-C1' ether or ester bonds adjacent to C2 of glycerol and rigid substrates with a rigid N'-C1' amide bond or a phenyl ring in sn-2. Although Rhizopus lipase shows opposite stereopreference for flexible and rigid substrates (hydrolysis in sn-1 and sn-3, respectively), Rhizomucor lipase hydrolyzes both groups of triradylglycerols preferably in sn-1. To explain these experimental observations, computer-aided molecular modeling was applied to study the molecular basis of stereoselectivity. A generalized model for both lipases of the Mucorales family highlights the residues mediating stereoselectivity: (1) L258, the C-terminal neighbor of the catalytic histidine, and (2) G266, which is located in a loop contacting the glycerol backbone of a bound substrate. Interactions with triradylglycerol substrates are dominated by van der Waals contacts. Stereoselectivity can be predicted by analyzing the value of a single substrate torsion angle that discriminates between sn-1 and sn-3 stereopreference for all substrates and lipases investigated here. This simple model can be easily applied in enzyme and substrate engineering to predict Mucorales lipase variants and synthetic substrates with desired stereoselectivity.

Rational design of Rhizopus oryzae lipase with modified stereoselectivity toward triradylglycerols

The binding site of sn-1(3)-regioselective Rhizopus oryzae lipase (ROL) has been engineered to change the stereoselectivity of hydrolysis of triacylglycerol substrates and analogs. Two types of prochiral triradylglycerols were considered: 'flexible' substrates with ether, benzylether or ester groups, and 'rigid' substrates with amide or phenyl groups, respectively, in the sn-2 position. The molecular basis of sn-1(3) stereoselectivity of ROL was investigated by modeling the interactions between substrates and ROL, and the model was confirmed by experimental determination of the stereoselectivity of wild-type and mutated ROL. For the substrates, the following rules were derived: (i) stereopreference of ROL toward triradylglycerols depends on the substrate structure. Substrates with 'flexible' sn-2 substituents are preferably hydrolyzed at sn-1, 'rigid' substrates at sn-3. (ii) Stereopreference of ROL toward triradylglycerols can be predicted by analyzing the geometry of the substrate docked to ROL: if the torsion angle phiO3-C3 of glycerol is more than 150 degrees, the substrate will preferably be hydrolyzed in sn-1, otherwise in sn-3. For ROL, the following rules were derived: (i) residue 258 affects stereoselectivity by steric interactions with the sn-2 substituent rather than polar interactions. To a lower extent, stereoselectivity is influenced by mutations further apart (L254) from residue 258. (ii) With 'rigid' substrates, increasing the size of the binding site (mutations L258A and L258S) shifts stereoselectivity of hydrolysis toward sn-1, decreasing its size (L258F and L258F/L254F) toward sn-3.

Inhibition of microbial lipases with stereoisomeric triradylglycerol analog phosphonates

1,2(2,3)-Diradylglycero O-(p-nitrophenyl) n-hexylphosphonates were synthesized, with the diradylglycerol moiety being di-O-octylglycerol, 1-O-hexadecyl-2-O-pyrenedecanylglycerol, or 1-O-octyl-2-oleoyl-glycerol, and tested for their ability to inactivate lipases from Chromobacterium viscosum (CVL) and Rhizopus oryzae (ROL). The experimental data indicate the formation of stable, covalent 1:1 enzyme-inhibitor adducts with the di-O-alkylglycero phosphonates. The differences in reactivity of diastereomeric phosphonates with opposite configuration at the glycerol backbone was less expressed with both enzymes tested as compared to the influence of the stereochemistry at the phosphorus. Both lipases exhibited the same preference for the chirality at the phosphorus that was independent from the absolute configuration at the glycerol backbone. However, with CVL and ROL the inhibitors with the active site serine-directed phosphonate linked at position sn-1 of the glycerol moiety reacted significantly faster than the corresponding sn-3 analogs, reflecting the sn-1 stereopreference of the enzymes towards triacylglycerol analogs with a sn-2 O-alkyl substituent. In contrast, the phosphonates based on the 1-O-octyl-2-oleoylglycerol did not significantly inactivate CVL. Unexpectedly, these substances were hydrolyzed in the presence of lipase.

Characterization of a non-specific lipid transfer protein associated with the peroxisomal membrane of the yeast, Saccharomyces cerevisiae

A lipid transfer protein with a broad substrate specificity is associated with the peroxisomal membrane of the yeast Saccharomyces cerevisiae. The protein catalyzes in vitro the transfer of various phospholipids, phosphatidylinositol and phosphatidylserine being translocated at the highest rates. The transfer protein can be released from peroxisomal membranes by treatment with 0.25 M KCl and highly enriched using conventional chromatographic techniques. It is inactivated by heat, detergents, divalent cations and proteinases. During various steps of purification this lipid transfer protein co-fractionated with peroxisomal acyl-CoA oxidase (Pox1p). In a pox1 disruptant peroxisomal lipid transfer activity was still present, although at a reduced level. The peroxisomal lipid transfer protein from the pox1 mutant exhibited different chromatographic properties as compared to the wild-type strain suggesting that acyl-CoA oxidase and the peroxisomal lipid transfer protein may from a complex.

Identification of a novel, Ca(2+)-dependent phospholipase D with preference for phosphatidylserine and phosphatidylethanolamine in Saccharomyces cerevisiae

A membrane-bound phospholipase D (PLD) from Saccharomyces cerevisiae was solubilized from mitochondrial and plasma membranes and partially purified. The enzyme has an apparent molecular weight of approximately 60 kDa, is strictly Ca(2+)-dependent and preferentially hydrolyses phosphatidylserine and phosphatidylethanolamine. Enzyme activity is significantly increased in membranes from cells grown on a non-fermentable carbon source. The Ca(2+)-dependent PLD is distinct from PLD encoded by the SPO14IPLD1 gene. The 195 kDa SPO14IPLD1 gene product is specific for PtdCho, Ca(2+)-independent and is activated by PIP2. Furthermore, Pld1p has transphosphatidylation activity in the presence of ethanol and thus resembles the prototypic PLD activity found in mammalian cells and plants. In contrast, the Ca(2+)-dependent PLD described here is not affected by PIP2 and does not catalyze transphosphatidylation. Thus, the Ca(2+)-dependent PLD characterized in this study appears to be a member of a novel family of phospholipases D.

Hydrolysis and esterification of acylglycerols and analogs in aqueous medium catalyzed by microbial lipases

The stereoselectivity of microbial lipases from Chromobacterium viscosum (CVL) and Rhizopus arrhizus (RAL) towards monoacylglycerols (rac-1(3)-oleoylglycerol and 2-oleoylglycerol), diacylglycerols (1,3-dioleoylglycerol and rac-1,2(2,3)-dioleoylglycerol) and 2-O-ether analogs (rac-1(3)-oleoyl-2-O-hexadecylglycerol and rac-1(3)-octanoyl-2-O-hexadecylglycerol) was determined. The results of the hydrolysis of 2-O-ether analogs confirmed the importance of the substituent at C-2 of acylglycerols in the stereoselective recognition by microbial lipases and also showed that acylation of mono- and diradylglycerols with oleic acid overlaps the hydrolysis reaction in aqueous medium. With the short-chain, water-soluble octanoic acid no significant esterification occurred. Using rac-1,2(2,3)-dioleoylglycerol as a substrate for the hydrolysis with RAL and CVL, the appearance of 1,3-dioleoylglycerol and of 1(3)-monooleoylglycerol was demonstrated. The possibility of chemical vs. enzyme-catalyzed isomerization of 1,2-dioleoylglycerol and of 2-oleoylglycerol is discussed.

New fluorogenic triacylglycerol analogs as substrates for the determination and chiral discrimination of lipase activities

A new type of fluorogenic and isomerically pure 1(3)-O-alkyl-2,3 (3,2)-diacyl glycerols was synthesized that can be used as substrate for the determination of lipase activities. These compounds contain a fluorescent pyrene acyl chain and, as a potent quencher of pyrene fluorescence, a trinitrophenylamino acyl residue. In their intact form, the fluorogens show only low fluorescence intensity. Upon lipase-induced or chemical hydrolysis of the substrates, however, the fluorophore and quencher separate from each other. This leads to a gradual increase in pyrene fluorescence, reflecting the time-dependent progress of lipolysis and, under substrate saturation conditions, lipase activity. This lipase assay is continuous and does not require separation of substrate and reaction products. Short- and long-chain homologues as well as optical isomers of the fluorogenic alkyldiacyl glycerols were hydrolyzed by pancreatic lipase, hepatic lipase, and lipo-protein lipase at highly different rates depending on the substrate or enzyme preparation and source (e.g., postheparin plasma or cultured cells). It is proposed that a useful set of enantiomeric and/or homologous substrates in combination with appropriate reaction media might be applied to the selective determination of a lipase in a mixture of lipases, e.g., hepatic and lipoprotein lipase in PHP, for medical diagnostics.

Enantiomeric perylene-glycerolipids as fluorogenic substrates for a dual wavelength assay of lipase activity and stereoselectivity

A new type of fluorogenic alkyldiacyl glycerols was synthesized and used as fluorogenic substrates for the analysis of lipase activities and stereoselectivities. These compounds contain perylene as a fluorophore and the trinitrophenylamino (TNP) residue as a quencher. Both substituents are covalently bound to the omega-ends of the sn-2 and sn-1 (3) acyl chains, respectively. Upon glycerolipid hydrolysis, the residues are separated from each other thus allowing determination of lipase activity by the continuous increase in fluorescence intensity which is caused by dequenching. Using enantiomeric pairs of these compounds, we were able to analyze lipase stereoselectivity depending on the reaction medium. Mixtures of enantiomeric fluorogenic alkyldiacyl glycerols, selectively labelled with pyrene or perylene as fluorophores, can be used for a dual-wavelength "stereoassay" of lipases. Since absorption and emission maxima of both labels are clearly separated, hydrolysis of the respective enantiomeric substrates can be determined simultaneously, and the difference in the rates of hydrolysis can be taken as a parameter for the stereopreference of a lipase. Hydrolysis rates measured with perylene-substituted lipids are generally lower than those obtained with the pyrene analogs. Thus, with a mixture of perylene and pyrene-substituted lipids, we observe a higher apparent stereoselectivity of lipases since we measure a combination of stereo- and substrate selectivity. In the presence of albumin, all microbial lipases tested so far exhibit stereopreference for the sn-1 glycerol position. In our assay, the apparent stereoselectivities are highest if in the presence of albumin, the sn-1 position carries pyrene and the sn-3 position is substituted with perylene. The lipase stereoselectivity assay described here requires the simultaneous measurement of the fluorescence intensities at two different wavelengths in a single cuvette and can thus be carried out using existing and cheap instrumentation that was developed for the fluorimetric analysis of Ca+2 concentrations.

Delay of copper-catalyzed oxidation of low density lipoprotein by in vitro enrichment with choline or ethanolamine plasmalogens

Low density lipoprotein (LDL) isolated from human serum of different donors was enriched with plasmalogens and their diacyl analogs in order to investigate a possible effect of these phospholipids on the rate of lipid peroxidation in this lipoprotein. LDL was incubated with either vesicles of choline plasmalogen or phosphatidylcholine in presence of lipoprotein- deficient serum, or with liposomes of ethanolamine plasmalogen or phosphatidylethanolamine together with the non-specific phospholipid transfer protein isolated from beef liver. After re-isolation of LDL by ultracentrifugation, a dose-dependent incorporation of the exogenous phospholipids was obtained. Enrichment of LDL with choline plasmalogen resulted in a delay of the copper-catalyzed oxidation of LDL from five different donors. LDL from two donors was also enriched with diacylglycerophosphocholine which led to a delay of oxidation, but the protective effect was smaller than with choline plasmalogen. Enrichment of LDL from two additional donors with ethanolamine plasmalogen resulted in the strongest protection against oxidation, whereas, diacylglycerophospho-ethanolamine had little effect. The delay of the copper-catalyzed LDL oxidation may be due to a direct antioxidative action of the plasmalogens, which are partially degraded during the lag phase of oxidation, or to an indirect effect caused by alteration of the LDL surface in the presence of an excess of glycerophospholipids.

Inversion of lipase stereospecificity for fluorogenic alkyldiacyl glycerols. Effect of substrate solubilization

We synthesized enantiomeric 1-O-alkyl-2,3-diacyl-sn-glycerol and 3-O-alkyl-1,2-diacyl-sn-glycerol containing pyrene as a fluorescent reporter and the trinitrophenylamino residue as a fluorescence quencher; both reporter groups were covalently bound to the omega end of the acyl chains at positions sn-2 and sn-3(1), respectively. The fluorescence of the intact substrate molecules was very low. Chemical or enzymic release of the fatty acyl chains lead to fluorescence dequenching. The rate of lipolysis could be measured from the time-dependent increase in fluorescence intensity. We used the respective substrates for the continuous determination of activity and stereopreference of four different microbial lipases from Chromobacterium viscosum, Candida rugosa, Pseudomonas sp., Rhizopus arrhizus, as well as cutinase from Fusarium solani and lipoprotein lipase from bovine milk. The stereopreference of the lipases depended, in general, on how the substrate was solubilized in the reaction medium. All lipases under investigation preferentially hydrolysed the sn-1 acyl ester bond, if the lipid analog was dispersed in albumin-containing Tris/HCl buffer in the absence of detergent or organic solvent. In mixtures of 1:1 (by vol.) water/ethanol, the enzymes showed higher activity toward the sn-3 acyl ester bond, except for lipoprotein lipase which preferred the sn-1 acyl isomer under all conditions tested. Different stereopreferences were observed with the different lipases if the substrate was solubilized by amphiphiles (micelles of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate). C. rugosa lipase and F. solani cutinase showed high stereopreference for the sn-3 acyl ester, whereas Pseudomonas sp. lipase and C. viscosum lipase hydrolysed both enantiomers at similar rates. From spectroscopic studies, it can be inferred that the conformation of the fluorescent lipids is probably similar in water, mixtures of water and organic solvents, and in micelles. The possible effects of reaction conditions on substrate accessibility and enzyme conformation on stereoselectivity of the respective lipases are discussed.

Mas37p, a novel receptor subunit for protein import into mitochondria

By screening a collection of Saccharomyces cerevisiae mutants temperature sensitive for growth on a nonfermentable carbon source, we have isolated a gene (termed MAS37) which encodes a novel receptor for protein import into mitochondria. Mas37p is a 37-kD outer membrane protein with two putative membrane-spanning regions. Inactivation of the MAS37 gene renders cells temperature-sensitive for respiration-driven growth, inhibits import of precursors into isolated mitochondria, and is synthetically lethal with a deletion of one of the genes encoding the import receptors Mas70p or Mas20p. Inactivation of Mas37p with specific antibodies inhibits import of different precursors to different extents; the precursor specificity of Mas37p resembles that of the previously described import receptor Mas70p. Mas70p and Mas37p form a 1:1 complex in detergent extracts of mitochondria and overexpression of one protein enhances that of the other. We suggest that the Mas37p/Mas70p heterodimer functions as a receptor for protein import into yeast mitochondria and that the mitochondrial receptor system consists of hetero-oligomeric subcomplexes with distinct binding activities, but overlapping precursor specificities.

Characterization of a microsomal subfraction associated with mitochondria of the yeast, Saccharomyces cerevisiae. Involvement in synthesis and import of phospholipids into mitochondria

In the yeast, Saccharomyces cerevisiae, similar to higher eukaryotes most phospholipids are synthesized in microsomes. Mitochondria contribute to the cellular biosynthesis of phospholipids insofar as they harbor phosphatidylethanolamine decarboxylase, and enzymes of phosphatidylglycerol and cardiolipin synthesis. In this paper we present evidence that certain enzymes of phospholipid biosynthesis, namely phosphatidylserine and phosphatidylinositol synthase, are enriched in a special microsomal fraction associated with mitochondria, which we named MAM. This fraction was isolated and characterized with respect to marker enzymes, protein and phospholipid composition, and enzymes of phospholipid synthesis. According to these analyses MAMs are a specialized subfraction of the endoplasmic reticulum, which is distinct from other microsomal subfractions. Phosphatidylserine synthesized in MAMs can be readily imported into mitochondria and converted to phosphatidylethanolamine. Reassociation of MAMs with purified mitochondria led to reconstitution of the import of phosphatidylserine into mitochondria. Organelle contact is suggested as a possible mechanism of this process.

Export of steryl esters from lipid particles and release of free sterols in the yeast, Saccharomyces cerevisiae

Fatty acyl esters of the yeast specific sterol, ergosterol, are exclusively stored in lipid particles. Under conditions of sterol deficiency, e.g., in the presence of terbinafine, an inhibitor of fungal squalene epoxidase, steryl esters are hydrolyzed, and sterols are set free for membrane formation. Lipid particles do not contain steryl-ester hydrolase activity themselves; the highest specific activity of this enzyme is found in the plasma membrane. Therefore, steryl esters have to be exported from lipid particles to their site of hydrolytic cleavage. This process of translocation and metabolic conversion was studied in vivo. Addition of nocodazole to terbinafine-treated cells did not disturb the mobilization of steryl esters, indicating that this process is not mediated by microtubuli-dependent vesicle flux. Under the influence of inhibitors of cellular energy production (azide and fluoride) and protein biosynthesis (cycloheximide) mobilization of steryl esters came to an halt. These results support the view that ongoing membrane proliferation may be a driving force for the release of sterols from steryl esters of lipid particles.

Transfer of phospholipase A-resistant pyrene-dialkyl-glycerophosphocholine to plasma lipoproteins: differences between Lp[a] and LDL

1-O-Hexadecyl-2-O-pyrenedecanyl-sn-glycero-3-phosphocholine, a non-hydrolyzable fluorescent diether analog of phosphatidylcholine (PC), was synthesized as a probe for studying phospholipid transfer to different lipoprotein classes with potential phospholipase activities. After incubation of total human plasma with the new probe at 37 degrees C for 4.5 h, a characteristic partition between the main lipoprotein fractions was observed. The fluorescent lipid was not degraded under these conditions and, therefore, served as a measure for choline glycerophospholipid distribution between plasma lipoproteins. In low density lipoprotein (LDL) and high density lipoprotein-3 (HDL3) the fluorescent PC analog showed only monomer fluorescence, whereas in Lp[a] and HDL2 monomer and excimer fluorescence were observed, indicating that the fluorescent phosphatidylcholine analog was incorporated into the respective lipoproteins to a different extent. According to the increased pyrene excimer fluorescence in Lp[a] compared with LDL the labeled phosphatidylcholine must be enriched and/or clustered in Lp[a]. Data from phospholipid and total fluorescence analyses are compatible with the assumption of higher label concentration in Lp[a]. On the other hand, transfer rates for serum protein-catalyzed lipid transport into isolated Lp[a] were slower as compared to LDL. It is suggested that slower lipid transfer to Lp[a] under these conditions is due to the decreased lipid mobility in the Lp[a] surface, whereas the higher extent of label partition into Lp[a] as observed in total plasma might be due to the higher affinity of apolipoproteins for phosphatidylcholine in Lp[a] (Sommer, A., et al. 1992. J. Biol. Chem. 267: 24217-24222). The use of a fluorescent dialkyl- instead of diacyl-glycerophosphocholine for transfer studies was mandatory, as we found that lipoproteins contained phospholipase A2 activity toward long-chain phosphatidylcholine. The lipoprotein-associated phospholipase A2 was three times more active in Lp[a] than in LDL. The degradation products formed by the phospholipase, fatty acids, and lyso-PC may add to the high atherogenic potential of Lp[a].

Stereoselectivity of microbial lipases. The substitution at position sn-2 of triacylglycerol analogs influences the stereoselectivity of different microbial lipases

In the present study, the stereoselectivity of purified lipases from Candida rugosa, Chromobacterium viscosum, Pseudomonas species and Rhizopus arrhizus towards triacylglycerols in comparison to various structural analogs were investigated. Different triacylglycerol analogs with distinct polarities at position sn-2 of the glycerol backbone (1,3-diacyl-2-X-glycerol, where 2-X = 2-acyloxy, 2-alkyloxy, 2-deoxy-2-alkyl, or 2-deoxy-2-phenyl) were synthesized. Substrate hydrophobicity and steric requirement was modified by variation of the alkyl and acyl chain length. Hydrolysis of these substrates demonstrated that minor structural variations at C2 of triacylglycerol strongly affect the stereoselectivity of the lipases tested. It was noteworthy that the variation of substrate structure did not only affect the quantity of stereoselectivity expressed as percentage enantiomeric excess, but also resulted in a reversal of stereopreference in some cases. Replacement of the acylester in position 2 of glycerol by a non-ester-linked aliphatic moiety shifted the preference of Chromobacterium viscosum lipase from sn-3 to sn-1. Lipases from Chromobacterium viscosum. Pseudomonas species and Rhizopus arrhizus exhibited sn-3 preference with 2-deoxy-2-phenyl analogs, while towards substrates with a 2-deoxy-2-alkyl moiety sn-1 stereobias was recorded. Candida rugosa lipase was rather insensitive to substrate variations concerning the polarity at position 2 of the glycerol backbone. However, variation of the acyl chain length significantly influenced stereoselectivity of this lipase.

Import of phosphatidylinositol and phosphatidylcholine into mitochondria of the yeast, Saccharomyces cerevisiae

An in vitro assay was designed to study the import of 3H-labeled phosphatidylinositol and phosphatidylcholine, respectively, from unilamellar vesicles into isolated mitochondria of the yeast, Saccharomyces cerevisiae. Both phospholipids reached the inner mitochondrial membrane. During import they were detected in contact sites between the outer and the inner mitochondrial membrane, supporting the notion that these zones are sites of intramitochondrial phospholipid transport. The uncoupler CCCP, the antibiotic adriamycin, and energy depletion caused by oligomycin and apyrase did not inhibit the transport of phosphatidylinositol and phosphatidylcholine into mitochondria.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.

Role of sn-2 acyl group of phosphatidylcholine in determining the positional specificity of lecithin-cholesterol acyltransferase

Although human plasma lecithin-cholesterol acyltransferase (LCAT) is believed to be specific for the sn-2 position of phosphatidylcholine (PC), our recent studies showed that it derives a significant percent of acyl groups from the sn-1 position of certain PC species. To understand the physicochemical basis for this altered positional specificity, we determined the effect of sn-2 acyl group of PC on the enzyme activity and utilization of 16:0 from the sn-1 position by purified human and rat LCATs. Positional isomers of PC containing 16:0 at sn-2 were better substrates for human LCAT than the corresponding sn-1-16:0 isomers, whereas the reverse was true for rat LCAT. The positional specificity of human LCAT varied greatly depending on the nature of the acyl group at sn-2. The sn-1 contribution from various sn-1-16:0-2-acyl PCs for cholesteryl ester (CE) synthesis was 1.0% from 16:0-16:0, 1.4% from 16:0-20:5, 7.3% from 16:0-18:1, 47.0% from 16:0-20:3, 49.9% from 16:0-20:4, 54.9% from 16:0-22:6, and 72.3% from 16:0-18:0. There was a linear relationship between the percentage of 16:0 CE formed (from sn-1 position) and the acyl chain length at sn-2 position (r = 0.94). Rat LCAT also transferred some 16:0 from sn-1 position of 16:0-22:6, 16:0-20:3, and 16:0-18:0 PCs, but not from the other natural PCs tested. The phospholipase A activity of both LCATs in the presence of 16:0-20:4 PC showed the same positional specificity as CE synthesis, indicating that the specificity is determined at the formation of acyl-enzyme intermediate. These results show that the positional specificity of LCAT is influenced by the structure of PC, especially the chain length of the sn-2 acyl group.

Characterization of lipid particles of the yeast, Saccharomyces cerevisiae

Lipid particles of the yeast, Saccharomyces cerevisiae, were isolated to high purity and their components were analysed. The hydrophobic core of this organelle consists of triacylglycerols and steryl esters, which are almost exclusively located to that compartment. Lipid particles are stabilized by a surface membrane consisting of phospholipids and proteins. Electron microscopy confirmed the purity of the preparations and the proposed structure deduced from biochemical experiments. Major proteins of lipid particles have molecular weights of 72, 52, 43 and 34 kDa, respectively. The 43 kDa protein reacts with an antiserum against human apolipoprotein AII. In lipid particles of the yeast mutant strain S. cerevisiae erg6, which is deficient in sterol delta 24-methyltransferase, this protein is missing thereby identifying the protein and confirming our previous finding (Zinser et al., 1993) that sterol delta 24-methylation is associated with lipid particles. A possible involvement of surface proteins of lipid particles in the interaction with other organelles is discussed with respect to sterol translocation in yeast.

Generation of glycerophospholipid molecular species in the yeast Saccharomyces cerevisiae. Fatty acid pattern of phospholipid classes and selective acyl turnover at sn-1 and sn-2 positions

Acyl chains linked to phospholipids of the yeast, Saccharomyces cerevisiae, are mainly C16:1 and C18:1 accompanied by minor amounts of C14:0, C16:0 and C18:0. In view of this rather simple fatty acid composition, the question arose whether in yeast, as in higher eukaryotes, fatty acyl groups were characteristically distributed among the sn-1 and sn-2 positions of distinct phospholipid classes. Analysis of fatty acids linked to the sn-1 and sn-2 positions of the major phospholipids showed that indeed saturated fatty acyl groups predominated in the sn-1 positions. While the percentage of saturated fatty acids was low (10%) in phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) from cells grown on rich medium, it was higher in phosphatidylserine (PtdSer) (25%) and highest in phosphatidylinositol (PtdIns) (41%). Oleate was mainly linked to position sn-2, while palmitoleate predominated in position sn-1. Striking differences in the fatty acid distribution of phospholipids that are metabolically closely related (e.g. PtdSer and PtdEtn, PtdEtn and PtdCho, and PtdIns and PtdSer) suggest that pathways must exist for the generation of distinct phospholipid molecular species within the different phospholipid classes. The highly selective incorporation of exogenous [14C]palmitic acid (90%) and [3H]oleic acid (99%) into the sn-2 position of PtdCho, and the preferential incorporation of these fatty acids into the sn-2 position of PtdEtn (70 and 90%, respectively, for palmitic and oleic acid) are compatible with the postulate that phospholipase A2-mediated deacylation followed by reacylation of the lysophospholipids is involved in the generation of phospholipid species in yeast.

Characterization, quantification and subcellular localization of inositol-containing sphingolipids of the yeast, Saccharomyces cerevisiae

In yeast, as in higher eukaryotic cells, sphingolipids are essential membrane components. The yeast, Saccharomyces cerevisiae, contains three classes of sphingolipids, inositolphosphorylceramide (InsPCer), mannosylinositolphosphorylceramide (ManInsPCer) and mannosyldiinositolphosphorylceramide (ManPIns2PCer). As a prerequisite to localize these sphingolipids in subcellular membranes, authentic standards of the respective lipids were isolaed and characterized using biochemical methods and electrospray ionization mass spectrometry. The complete set of yeast subcellular membranes was isolated at high purity, and sphingolipids were extracted. InsPCer, ManInsPCer, and ManPIns2PCer were separated by thin-layer chromatography, stained and densitometrically scanned along with the respective standards. These methods enable a complete overview of the subcellular distribution of yeast sphingolipids to be obtained, as far as is known, for the first time. InsPCer was highly enriched in Golgi and vacuolar membranes, whereas the largest amounts of ManInsPCer and ManPIns2PCer were found in the plasma membrane. The presence of inositol-containing sphingolipids in organelles of the protein-secretory pathway strongly supports the notion that protein secretion and intracellular trafficking of sphingolipids are linked processes.

The Saccharomyces cerevisiae PLB1 gene encodes a protein required for lysophospholipase and phospholipase B activity

Several enzymes with lysophospholipase/phospholipase B activity have been described from the budding yeast Saccharomyces cerevisiae. In vitro, these enzymes are capable of hydrolyzing all phospholipids that can be extracted from yeast cells. Two forms of the enzyme have been isolated from plasma membranes and a third from culture supernatants and the periplasmic space, but their biological roles have not been determined. These highly glycosylated enzymes were reported to have very similar catalytic properties but differed with respect to apparent molecular weight. We isolated a gene from S. cerevisiae, encoding a protein predicted to share 45% amino acid sequence identity with phospholipase B from Penicillium notatum. This yeast gene, designated PLB1, was mapped to the left arm of chromosome VIII. No residual lysophospholipase/phospholipase B activity was detected upon assay of extracts or culture supernatants of a plb1 delta mutant. Thus, either the PLB1 gene encodes all of the previously detected isoforms of phospholipase B or its gene product is required for their expression or activation. Deletion of PLB1 did not result in any apparent phenotypic defect, suggesting either that we failed to identify the growth conditions that would betray such a defect or that Plb1p is functionally redundant with another protein, whose activity has gone undetected. A plb1 delta mutant released wild-type levels of the soluble phosphatidylinositol metabolite glycerophosphoinositol into the growth medium but released greatly reduced levels of the corresponding phosphatidylcholine and phosphatidylethanolamine metabolites. These results indicate that PLB1 is principally responsible for the production of the deacylation products of phosphatidylcholine and phosphatidylethanolamine but not phosphatidylinositol.

Phase behavior of the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphochloline

The physicochemical properties of the antineoplastic etherphospholipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphochline were examined in the concentration range 1-35% (w/w) lipid, as a function of temperature (range -10 degrees C to 40 degrees C) and of different aqueous solvents by dynamic light scattering, small- and wide-angle X-ray scattering, differential scanning calorimetry and ultrasonic speed measurements. On cooling the lipid dispersion undergoes a phase transition near 6 degrees C, transforming slowly from a micellar into a lamellar gel phase with interdigitating hydrocarbon chains. The lamellar repeat distance is nearly constant over the hydration range 65-90% buffer (d = 5.09-5.14 nm). The size of the micelles in terms of the hydrodynamic radius is 3.8 +/- 0.1 nm, the polydispersity is low. Their average shape is spherical. The electron density distribution across the micelle gives 2.5 nm for the extension of the hydrocarbon chains and 1.5 nm for the polar moiety. The existence of micelles was verified up to a concentration of 35% lipid. Throughout this concentration range size and shape do not change significantly. The kinetics of formation of the low-temperature phase is slow on cooling, increasing with increasing concentration. Upon heating the phase behavior shows a hysteresis. The extended lamellar organizations start to break down into smaller aggregates near 3 degrees C. The micellar phase is reformed near 20 degrees C.

Isolation and characterization of a mutant of Saccharomyces cerevisiae with pleiotropic deficiencies in transcriptional activation and repression

The isolation of the dep1 mutant of Saccharomyces cerevisiae is reported. The mutant was identified by its disability to regulate expression of structural genes involved in phospholipid biosynthesis, INO1, CHO1 and OPI3, in response to supplementation with soluble lipid precursors. Expression of the INO1, CHO1 and OPI3 genes was not fully derepressed in the absence of soluble lipid precursors, inositol and choline in the dep1 mutant, as compared to wild type. The mutant also exhibited incomplete repression of these same genes in the presence of inositol and choline. Repression by phosphate of the PHO5 gene was reduced in the mutant, as was derepression of this gene in the absence of phosphate. In addition, we show that expression of INO1 and OPI3 structural genes is strongly dependent on the growth phase both in wild-type and dep1 mutant strains. However, in the mutant, elevated basal steady-state mRNA levels were reached in the late stationary growth phase, independent of supplementation conditions. The dep1 mutation represents a new complementation group with respect to phospholipid synthesis and was mapped to a position of about 12 cM distal from the centromere on the left arm of chromosome I. Deficiencies in transcription activation and repression of metabolically unrelated genes, as well as reduced mating efficiency and lack of sporulation of homozygous diploid dep1/dep1 mutants indicate a pleiotropic regulatory function of the DEP1 gene product. Thus, Dep1p appears to be a new member of a class of transcriptional modulators, including Rpd1p/Sin3p/Ume4p/Sdi1p/Gam3p, Rpd3p, Spt10p and Spt21p.

Absorption of monoacylglycerols by small intestinal brush border membrane

The absorption of monoacylglycerol by small intestinal brush border membrane is a passive process, i.e., the movement of monoacylglycerol from small unilamellar phospholipid vesicles as donor particles through the aqueous medium and the incorporation into the outer monolayer of the lipid bilayer of the brush border membrane are passive processes involving diffusion of the lipid along a concentration gradient. Small unilamellar vesicles of egg phosphatidylcholine containing 1 mol% of radiolabeled hexadecylglycerol were used as donor, and rabbit small intestinal brush border membrane vesicles or intact enterocytes isolated from pig jejunum, as acceptor. Hexadecylglycerol was employed as a lipase-resistant model compound for monoacylglycerols. Both acceptor membranes behave similarly in terms of hexadecylglycerol absorption: the kinetics of hexadecylglycerol absorption are biphasic. The initial fast phase is due to the movement of hexadecylglycerol from the donor particle through the aqueous medium to the outer lipid monolayer of the acceptor membrane, and the second slow phase probably involves the flip-flop motion of hexadecylglycerol from the outer to the inner monolayer of the acceptor membrane. The values for the pseudo-first-order rate constants of the initial fast phase for hexadecylglycerol absorption are relatively large and primarily determined by the high solubility (cmc) of hexadecylglycerol in aqueous media. The pseudo-first-order rate constants depend linearly on the protein (lipid) concentration of the acceptor membrane, indicating that the on rate of the hexadecylglycerol into the brush border membrane is rate limiting. The mechanism of the hexadecylglycerol absorption involves mainly monomer diffusion and probably collision-induced transfer.

Conformation of phosphatidylserine in bilayers as studied by Fourier transform infrared spectroscopy

The 13C labeled lipid 1[1'-13C]DPPS-NH4+ and its metal salts were used to unambiguously assign all carbonyl vibrations in the infrared spectrum of phosphatidylserines. It is shown that the C=O stretching band at 1741 cm-1 of phosphatidylserines previously assigned to the sn-1 C = O vibration contains contributions from both the sn-1 and the sn-2 carbonyls. The C=O stretching band at frequencies between 1715 and 1730 cm-1 previously assigned to the sn-2 C=O vibration also contains contributions from both carbonyl groups. The frequency dependence observed with the ester carbonyls primarily reflects hydrogen bonding and the polarity of the immediate vicinity. Conformational changes are accounted for in terms of frequency shifts if the conformational change involves the disposition of the C=O groups and in turn the hydrogen bonding properties. The infrared spectra of phospholipids dispersed in aqueous medium in the liquid crystalline state are inconsistent with a simple phospholipid conformation, e.g., with a conformation as found in the single-crystal structure of 1,2-dimyristoyl-sn-phosphatidylcholine and 1,2-dilauroyl-rac-phosphatidylethanolamine. The spectra support the hypothesis proposed earlier (Hauser et al., Biochemistry, 1988) on the basis of existing single-crystal phospholipid structures and NMR evidence. The hypothesis states that several conformations are present in liquid crystalline phospholipid dispersions.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfer of pyrene-labelled diacyl-, alkylacyl-, and alkenylacyl-glycerophospholipids from vesicles to human blood platelets

The present study was aimed at investigating the spontaneous transfer of fluorescently labelled serine- and choline-glycerophospholipids from unilamellar vesicles to resting human blood platelets. The most effectively transferred phospholipids were pyrene-phosphatidylserine (PS) and the ether analogues of choline-glycerophospholipids, e.g., pyrene-alkylacyl- and pyrene-1'-alkenylacyl-glycerophosphocholines (plasmalogens). Transfer of pyrene-diacyl-glycerophosphocholine and pyrene-phosphatidic acid was almost not detectable under the same experimental conditions. The fast intermembrane PS-transfer could be explained by the very high degree of adsorption of PS donor vesicles to the platelet plasma membrane. The short halftime of transfer rate (12-14 min) and the high incorporation (1.08-2.16% of total platelet glycerophosphocholines) observed for ether choline-phospholipids in contrast to pyrene-PS (20 min, 0.8% of total platelet PS), could be interpreted in terms of their bulk membrane properties.

Apoprotein-phospholipid interactions in Lp(a)

Lipoprotein (a) (Lp(a)) and low-density lipoprotein (LDL) are structurally related to each other. Both exhibit identical phospholipid compositions and possess one molecule of apoprotein B-100 (apoB). Lp(a) contains, in addition, apoprotein (a) (apo(a)), which localizes to the particle surface and interacts with the apoB component by non-covalent and covalent forces. Protein-protein interaction is probably interrelated with protein-lipid interaction. Fluorescent analogs of phosphatidylcholine and sphingomyelin were inserted into the surface layer of LDL and Lp(a). The obtained fluorescence data reflecting mobility and distributional heterogeneity of the labeled lipids provided evidence that apo-proteins discriminate between choline phospholipids and preferentially associate with phosphatidylcholine. This effect is enhanced in Lp(a) because of the presence of apolipoprotein (a). Higher affinity for Lp(a) as compared with LDL was also observed with a fluorescent diether analog of phosphatidylcholine in native serum. In contrast, the time-dependent transfer of the same lipid into Lp(a) was slower compared with LDL, probably as a consequence of the more rigid surface of the former lipoprotein.