Net mass transfer of galactosylceramide facilitated by glycolipid transfer protein from pig brain: a monolayer study

A net mass transfer of galactosylceramide (GalCer) and galactosyldiacylglycerol (GalDG) is catalyzed by the glycolipid transfer protein from pig brain. GalCer and GalDG are transferred from a monolayer to phosphatidylcholine vesicles in the subphase or from a glycolipid monolayer to a phosphatidylcholine monolayer. No transfer of phosphatidylcholine is measured under these conditions. It is found that the glycolipid transfer protein functions as a carrier and that glycolipid is bound to less than 50% of the transfer protein. The presence of lipid-free proteins fits with the proposed mechanism of net mass transfer. The glycolipid transfer is influenced by the fluidity of the lipid interface and by the matrix lipid of the interface. GalCer transfer is stimulated in the presence of GalDG.     

Glycolipid transfer protein from pig brain transfers glycolipids with beta-linked sugars but not with alpha-linked sugars at the sugar-lipid linkage

The glycolipid transfer protein purified from pig brain facilitates the transfer of various glycosphingolipids and glyceroglycolipids (Yamada, K., Abe, A. and Sasaki, T. (1985) J. Biol. Chem. 260, 4615-4621). In this paper, the transfer of Man beta 1----4Glc beta 1-Cer and Man alpha 1----4Man beta 1-Cer isolated from a bivalve, Corbicula japonica, the transfer of 3-[Glc alpha 1-]-sn-1,2-diacylglycerol and 3-[Glc alpha 1----2Glc alpha 1-]-sn-1,2-diacylglycerol prepared from Streptococcus lactis, and the transfer of 3-[Glc beta 1-]-rac-1,2-dipalmitylglycerol have been investigated. The transfer of these lipids from liposomes to mitochondria was assayed by the decrease of these lipids in the donor liposomes. These lipids were determined by chromatographic isolation of the lipids, acid hydrolysis of the isolated lipids, and subsequent determination of glucose in the hydrolysate. The glycolipid transfer protein facilitated the transfer of ManGlcCer and ManManGlcCer. The transfer protein did not facilitate the transfer of Glc alpha-diacylglycerol or Glc alpha Glc alpha-diacylglycerol. However, the transfer of Glc beta-dipalmitylglycerol was facilitated by the protein. These results strongly suggest that the glycolipid transfer protein has the specificity to the presence of beta-linked glucose or galactose directly linked to either ceramide or diacylglycerol.     

Purification and some properties of the glycolipid transfer protein from pig brain

A glycolipid-specific lipid transfer protein has been purified to apparent homogeneity from pig brain post-mitochondrial supernatant. The purified protein was obtained after about 6,000-fold purification at a yield of 19%. Evidence for the homogeneity of the purified protein includes the following: (i) a single band in acidic gel electrophoresis, in sodium dodecyl sulfate-gel electrophoresis, (ii) a single band in analytical gel isoelectric focusing, (iii) exact correspondence between the glycolipid transfer activity and stained protein absorbance in the acidic gel electrophoresis, and (iv) coincidence between the transfer activity and protein absorption at 280 nm in gel filtration through Ultrogel AcA 54. The protein has an isoelectric point of about 8.3 and a molecular weight of 22,000, as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A molecular weight of 15,000 was calculated from AcA 54 gel filtration. The amino acid composition has been determined. The protein binds [3H]galactosylceramide but not [3H]phosphatidylcholine. Under the conditions used, 1 mol of the transfer protein bound about 0.13 mol of [3H]galactosylceramide. The glycolipid transfer protein-[3H]galactosylceramide complex was isolated by a Sephadex G-75 chromatography. An incubation of the complex with liposomes resulted in the transfer of [3H]galactosylceramide from the complex to the acceptor liposomes. The result indicates that the complex functions as an intermediate in the glycolipid transfer reaction. The protein facilitates the transfer of [3H]galactosylceramide from donor liposomes to acceptor liposomes lacking in glycolipid as well as to acceptor liposomes containing galactosylceramide.     

Probing for preferential interactions among sphingolipids in bilayer vesicles using the glycolipid transfer protein.

We have investigated the intervesicular transfer of galactosylceramide between unilamellar bilayer vesicles composed of differing sphingomyelin and phosphatidylcholine molar ratios. To monitor glycolipid transfer from donor to acceptor vesicles, we used a fluorescence resonance energy transfer assay involving anthrylvinyl-labeled galactosylceramide (AV-GalCer) and perylenoyl-labeled triglyceride. The transfer was mediated by glycolipid transfer protein (GLTP), purified from bovine brain and specific for glycolipids. The initial transfer rate and the total accessible pool of glycolipid in the donor vesicles were both measured. An increase in the sphingomyelin content of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) vesicles decreased the transfer rate in a nonlinear fashion. Decreased transfer rates were clearly evident at sphingomyelin mole fractions of 0.22 or higher. The pool of AV-GalCer available for GLTP-mediated transfer also was smaller in vesicles containing high sphingomyelin content. In contrast, AV-GalCer was more readily transferred from vesicles composed of POPC and different disaturated phosphatidylcholines. Our results show that GLTP acts as a sensitive probe for detecting interactions of glycosphingolipids with neighboring lipids and that the lateral mixing of glycolipids is probably affected by the matrix lipid composition. The compositionally driven changes in lipid interactions, sensed by GLTP, occur in membranes that are either macroscopically fluid-phase or gel/fluid-phase mixtures. Gaining insights into how changes in membrane sphingolipid composition alter accessibility to soluble proteins with affinity for membrane glycolipids is likely to help increase our understanding of how sphingolipid-enriched microdomains (i.e., "rafts" and caveolae) are formed and maintained in cells.     

Glycolipid-binding proteins

Proteins which bind glycolipids with high specificity are tentatively divided into two groups. One group consists of activator proteins involved in the catabolism of glycolipids by acid lysosomal hydrolases. Two activator proteins, GM2-activator and sphingolipid activator protein-1, are critically appraised on their glycolipid-binding properties and on their activity to facilitate the transfer of glycolipids. These proteins are glycoproteins localized in the lysosomes. Their molecular weights are in a range of 21 000-27 000, and isoelectric points are 4-5. Glycolipid transfer protein (GLTP) is included in the other group. GLTP purified from pig brain has a molecular weight of about 20 000 and an isoelectric point of 8.3. GLTP facilitates the transfer of various glycosphingolipids and glyceroglycolipids between membranes. The protein does not facilitate the transfer of phospholipids or cholesterol. GLTP binds galactosylceramide. The galactosylceramide-GLTP complex participates in the transfer reaction as the intermediate. Each protein in both groups binds glycolipids with a characteristic specificity to the sugar moiety. A stoichiometry of 1 mol of lipid per mol of protein has been found in all three proteins. Proteins in both groups seem to have a hydrophobic region on their surface, since all three proteins have been efficiently purified by hydrophobic chromatography.     

Glycosphingolipid specificity of the human sulfatide activator protein

The interaction of the sulfatide activator protein with different glycosphingolipids have been studied in detail. The following findings were made. 1. The sulfatide activator protein forms water-soluble complexes with sulfatides [Fischer, G. and Jatzkewitz, H. (1977) Hoppe-Seyler's Z. Physiol. Chem. 356, 6588-6591] and various other glycospingolipids. 2. In the absence of degrading enzymes the activator protein acts in vitro as a glycosphingolipid transfer protein, transporting glycosphingolipids from donor to acceptor liposomes. Lipids having less than three hexoses, e.g. galactosylceramide, sulfatide and ganglioside GM3 were transferred at very slow rates, whereas complex lipids such as gangliosides GM2, GM1 and GD1a were transferred much faster than the former. The transfer rate increased with increasing length of the carbohydrate chain of the lipid molecules. 3. Both the acyl residue in the ceramide moiety and the nature of the carbohydrate chain are significant for recognition of the glycosphingolipids by the sulfatide activator protein. Apparently, both residues serve as an anchor and the longer they are the better they are recognized by the protein. 4. In the absence of activator protein, degradation rates of sulfatide derivatives by arylsulfatase A, and of ganglioside GM1 derivatives by beta-galactosidase, increase with decreasing length of acyl residues in their hydrophobic ceramide moiety. Addition of activator protein stimulates the degradation of only those GM1 and sulfatide derivatives that have long-chain fatty acids in their hydrophobic ceramide anchor.     

A protein purified from pig brain accelerates the intermembranous translocation of mono- and dihexosylceramides,but not the translocation of phospholipids

By the use of an assay that measures the transfer of [³H]galactosylceramide from donor to acceptor liposomes, a protein has been purified 1683-fold from pig brain. The most purified fraction was purified to homogeneity as judged by electrophoresis on 15% polyacrylamide gel in the presence of sodium dodecyl sulfate. The protein has a molecular weight of 23000 as determined by the gel electrophoresis and 18500 as estimated by gel filtration through Sephadex G-75. The protein accelerates the transfer of labeled glycolipids at the following relative rates: 100 for glucosylceramide, 43 for lactosylceramide, 17 for galactosyldiglyceride, and 15 for galactosylceramide. The lipid-transfer stimulated by the protein is specific to glycolipids; the protein does not accelerate the transfer of labeled phosphatidylcholine and phosphatidylethanolamine from donor to acceptor liposomes.     

Expression machinery of GM4: the excess amounts of GM3/GM4S synthase (ST3GAL5) are necessary for GM4 synthesis in mammalian cells

The ganglioside GM4 is a sialic acid-containing glycosphingolipid mainly expressed in mammalian brain and erythrocytes. GM4 is synthesized by the sialylation of galactosylceramide (GalCer), while the ganglioside GM3 is synthesized by the sialylation of lactosylceramide (LacCer). Recently, the enzyme GM3 synthase was found to be responsible for the synthesis of GM4 in vitro and in vivo, yet the mechanism behind GM4 expression in cells remains unclear. In this study, we attempted to establish GM4-reconstituted cells to reveal the regulation of GM4 synthesis. Interestingly, GM4 was not detected in RPMI 1846 cells expressing LacCer, GalCer, and GM3. Similarly, GM4 was not detected in CHO-K1 cells, even when such cells expressing LacCer and GM3 were stably transfected with the GalCer synthase (GalCerS) gene. GM4 became detectable only when the GM3/GM4 synthase (GM3/GM4S, ST3GAL5) gene was overexpressed in either RPMI 1846 or CHO-K1/GalCerS cells. A mutant of the B16 melanoma cell line, GM-95, lacks GlcCer and LacCer, due to an absence of GlcCer synthase, but carries endogenous LacCer synthase and GM3/GM4S. GalCer became detectable after transfection of GalCerS into GM95 cells, but the GM95/GalCerS reconstituted cells did not express GM4, indicating that competition between the substrates LacCer and GalCer for GM3/GM4S does not cause the failure of GM4 synthesis. These results suggest that the expression machinery of GM4 under physiological conditions is independent from that of GM3.     

3-O-acetyl-sphingosine-series myelin glycolipids: characterization of novel 3-O-acetyl-sphingosine galactosylceramide

Several glycosphingolipids, less polar than galactosylceramide (GalCer), have been purified from rat brain and designated as fast migrating cerebrosides (FMCs). They co-appear with GalCer during myelinogenesis, reach a peak concentration at postnatal day 25-30 and are derivatives of GalCer. Extensive structural analysis of the partially methylated alditol acetates, mass-spectrometry, and (1)H- and (13)C-nuclear magnetic resonance (NMR) spectroscopy unequivocally established the structure of two of these FMCs as 3-O-acetyl-sphingosine GalCer with non-hydroxy and hydroxy fatty N-acylation respectively. That is, an acetyl group is linked at the C3-OH of the sphingosine base of GalCer. In addition, NMR spectroscopy of all of the purified FMCs indicates that they contain a 3-O-acetyl group linked with sphingosine and thus delineates a novel series. Several lines of evidence indicate that FMCs are myelin constituents. FMCs, enriched in both central nervous system (CNS) and peripheral nervous system (PNS) myelin, are concentrated in spinal cord and white matter that are composed of myelinated nerve fibers. There is N-acylation with alpha-hydroxy and C18 and C24 fatty acids, all characteristic of myelin components. They disappear along with GalCer in the murine genetic dysmyelinating disorders, jimpy and quaking, and in a knockout mutant which is devoid of GalCer. In addition, a decrease in FMC and GalCer concentration has been found in Krabbé's disease, a human genetic dysmyelinating disorder.     

Purification and characterization of glycolipid transfer protein from bovine brain

Bovine brain contains a lipid transfer protein that is specific for neutral glycosphingolipids and gangliosides but does not stimulate phospholipid or neutral lipid intermembrane transfer (Brown, R.E., Stephenson, F.A., Markello, T., Barenholz, Y. and Thompson, T.E. (1985) Chem. Phys. Lipids 38, 79-93). This report describes a new procedure for purifying glycolipid transfer protein from bovine brain as well as a characterization of the resulting protein. Chief among the newly introduced approaches are dye-ligand and fast protein cation-exchange liquid chromatography. Other modifications include increasing the overall scale of purification, incorporating a pH precipitation step and adding different proteinase inhibitors. The resulting procedure simplifies and accelerates the purification process while yielding a homogeneous protein. The purified protein has a molecular weight near 23 kDa as estimated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Chromatofocusing reveals that glycolipid transfer protein activity co-elutes with the 23 kDa protein and has an isoelectric point near pH 9.0. A similar isoelectric point is observed using denaturing isoelectric focusing conditions. The protein's amino acid composition reveals high levels of amino acids with non-polar side chains (48%). Based on the findings reported here and on previously published data, bovine brain glycolipid transfer protein has been compared to other lipid transfer proteins as well as lysosomal sphingolipid activator proteins.     

Properties of a specific glycolipid transfer protein from bovine brain

A transfer protein specific for glycolipids has been isolated from bovine brain. As judged by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis, the protein is 68% pure and has a molecular weight of 20 000. Three different assays were employed to study the protein's specificity and glycolipid binding properties. The protein transferred several different neutral glycosphingolipids and ganglioside GM1 equally well, but failed to accelerate phosphatidylcholine or sphingomyelin intervesicular movement. The protein's ability to interact with glycolipids was strongly influenced by the physical properties of the matrix phospholipid in which the glycolipids reside. Both the phase state of the phospholipid matrix and bilayer curvature affected glycolipid intervesicular transfer rates. Protein binding to phospholipid vesicles containing either tritium-labeled or pyrene-labeled glucosylceramide could not be demonstrated by density gradient centrifugation or fluorescence energy transfer measurements, respectively. A specific association of the transfer protein for pyrene-labeled glucosylceramide was found when the fluorescence emission of the pyrene excimer-to-monomer ratio was measured suggesting that a portion of the fluorescent glycolipid was being sequestered from the phospholipid vesicles and was binding to the freely soluble protein.     

Point mutational analysis of the liganding site in human glycolipid transfer protein. Functionality of the complex

Mammalian glycolipid transfer proteins (GLTPs) facilitate the selective transfer of glycolipids between lipid vesicles in vitro. Recent structural determinations of the apo- and glycolipid-liganded forms of human GLTP have provided the first insights into the molecular architecture of the protein and its glycolipid binding site (Malinina, L., Malakhova, M. L., Brown, R. E., and Patel, D. J. (2004) Nature 430, 1048-1053). In the present study, we have evaluated the functional consequences of point mutation of the glycolipid liganding site of human GLTP within the context of a carrier-based mechanism of glycolipid intermembrane transfer. Different approaches were developed to rapidly and efficiently assess the uptake and release of glycolipid by GLTP. They included the use of glass-immobilized, glycolipid films to load GLTP with glycolipid and separation of GLTP/glycolipid complexes from vesicles containing glycolipid (galactosylceramide or lactosylceramide) or from monosialoganglioside dispersions by employing nickel-nitrilotriacetic acid-based affinity or gel filtration strategies. Point mutants of the sugar headgroup recognition center (Trp-96, Asp-48, Asn-52) and of the ceramide-accommodating hydrophobic tunnel (Phe-148, Phe-183, Leu-136) were analyzed for their ability to acquire and release glycolipid ligand. Two manifestations of point mutation within the liganding site were apparent: (i) impaired formation of the GLTP/glycolipid complex; (ii) impaired acquisition and release of bound glycolipid by GLTP. The results are consistent with a carrier-based mode of GLTP action to accomplish the intermembrane transfer of glycolipid. Also noteworthy was the inefficient release of glycolipid by wtGLTP into phosphatidylcholine acceptor vesicles, raising the possibility of a function other than intermembrane glycolipid transfer in vivo.     

Primary structure of glycolipid transfer protein from pig brain

The amino acid sequence of a glycolipid transfer protein from pig brain was determined by automatic sequencing and fast atom bombardment mass spectroscopic analysis of peptides produced by chemical and enzymatic cleavage reactions. The protein consists of 208 residues, with N-acetylalanine as the N-terminal residue and valine as the C-terminal residue. It contains 3 cysteine residues. The primary structure of the glycolipid transfer protein from pig brain is as follows: acetyl-A-L-L-A-E-H-L-L-K-P-L-P-A-D-K15-Q-I-E-T- G-P-F-L-E-A-V-S-H-L-P30-P-F-F-D-C-L-G-S-P-V-F- T-P-I-K45-A-D-I-S-G-N-I-T-K-I-K-A-V-Y-D60-T-N- P-A-K-F-R-T-L-Q-N-I-L-E-V75-E-K-E-M-Y-G-A-E- W-P-K-V-G-A-T90-L-A-L-M-W-L-K-R-G-L-R-F-I-Q- V105-F-L-Q-S-I-C-D-G-E-R-D-E-N-H-P120-N-L-I-R- V-N-A-T-K-A-Y-E-M-A-L135-K-K-Y-H-G-W-I-V-Q- K-I-F-Q-A-A150-L-Y-A-A-P-Y-K-S-D-F-L-K-A-L- S165-K-G-Q-N-V-T-E-E-E-C-L-E-K-V-R180-L-F-L-V- N-Y-T-A-T-I-D-V-I-Y-E195-M-Y-T-K-M-N-A-E-L-N- Y-K-V-OH. The sequence does not have detectable homology with other lipid transfer proteins or lipid-binding proteins. The cysteine residue at position 35 is reactive to iodoacetamide under nondenaturing conditions.     

Neutral glycosphingolipid-dependent inactivation of coagulation factor Va by activated protein C and protein S.

To test whether neutral glycosphingolipids can serve as anticoagulant cofactors, the effects of incorporation of neutral glycosphingolipids into phospholipid vesicles on anticoagulant and procoagulant reactions were studied. Glucosylceramide (GlcCer), lactosylceramide (LacCer), and globotriaosylceramide (Gb(3)Cer) in vesicles containing phosphatidylserine (PS) and phosphatidylcholine (PC) dose dependently enhanced factor Va inactivation by the anticoagulant factors, activated protein C (APC) and protein S. Addition of GlcCer to PC/PS vesicles enhanced protein S-dependent APC cleavage in factor Va at Arg-506 by 13-fold, whereas PC/PS vesicles alone minimally affected protein S enhancement of this reaction. Incorporation into PC/PS vesicles of GlcCer, LacCer, or Gb(3)Cer, but not galactosylceramide or globotetraosylceramide, dose dependently prolonged factor Xa-1-stage clotting times of normal plasma in the presence of added APC without affecting baseline clotting times in the absence of APC, showing that certain neutral glycosphingolipids enhance anticoagulant but not procoagulant reactions in plasma. Thus, certain neutral glycosphingolipids (e.g. GlcCer, LacCer, and Gb(3)Cer) can enhance anticoagulant activity of APC/protein S by mechanisms that are distinctly different from those of phospholipids alone. We speculate that under some circumstances certain neutral glycosphingolipids either in lipoprotein particles or in cell membranes may help form antithrombotic microdomains that might enhance down-regulation of thrombin by APC in vivo.     

Unusual Gangliosidosis in Emu (Dromaius novaehollandiae)

Abstract: A previous study has demonstrated an unusual gangliosidosis in emu that is characterized by the accumulation of gangliosides in the brain tissues with GM3 and GM1 predominating. To provide insight into this unique disorder of emu gangliosidosis, the current study focused on analysis of neutral glycosphingolipids and gangliosides from brain and liver tissues of affected birds and healthy controls. We found not only that the total lipid-bound sialic acid content was increased three- and fourfold in the affected brain and liver, respectively, but also that the ganglioside pattern was rather complex as compared with the control. The absolute ganglioside sialic acid content was significantly increased in the diseased tissues, with the highest elevation levels of GM3 (14-fold) and GM1 (ninefold) in the affected brain. Relative increases in content of these monosialogangliosides were also significant. GM2 was only detected in the affected brain, but not in normal controls. The neutral glycosphingolipid fraction showed accumulation of many oligosylceramides, with six- and 5.5-fold increases in lactosylceramide levels for brain and liver, respectively. The level of myelin-associated galactosylceramide (GalCer) in the brain was decreased to only 41% of that in the healthy control, whereas no difference was found in liver tissues from both groups. Besides GalCer, the brain content of sulfatide (cerebroside-sulfate esters), another myelin-associated glycolipid, decreased to only 16% of the control. The loss of myelin-associated GalCer and sulfatide strongly suggests demyelination in the affected emu brain. Our overall data are consistent with the presence of a unique form of sphingolipidosis in the affected emus, perhaps with secondary demyelination, and suggest a metabolic disorder related to total sphingolipid activator deficiency.     

Evidence for the presence of a glycosphingolipid-transfer protein in rat brain cytosol

Proteins in the postmicrosomal supernatant fraction of rat brain catalyzed the transfer of bovine brain galactocerebroside, sulfatide, and ganglioside GM1 from unilamellar liposomes to the rat erythrocytes or ghosts. The vesicles were made with egg yolk lecithin, cholesterol, 3H-labelled glycolipid, and a trace of [14C]triolein as a nonexchangeable marker. The routine assay of the glycosphingolipid transfer consisted of incubation of the donor liposomes with erythrocytes in the presence or absence of supernatant protein in physiological buffer at 37 degrees C for various time intervals. After the incubation, the erythrocytes were separated from the vesicles by centrifugation and the extent of protein-catalyzed transfer of labelled glycolipid in the membrane-bound total lipid fraction was determined by scintillation spectrometry. The fraction of [3H]glycosphingolipid transferred is represented by a change in the 3H/14C ratios at initial and subsequent time intervals. The glycosphingolipid transfer catalyzed by the supernatant protein was found to be logarithmic, whereas the protein-independent transfer was linear over a period of 3-4 h. The rate constant (K) and half time (t1/2) of the protein-catalyzed transfer reaction of cerebrosides and sulfatides were almost the same, while the transfer of ganglioside GM1 occurred at a slightly faster rate, probably owing to the greater aqueous solubility of this lipid. The transfer activity was also increased in a manner dependent on the amount of supernatant protein added up to 10 mg. The catalytic activity of the protein was lost when heated at 70 degrees C for 5 min. The pH optimum of the activity was around 7.4.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adaptive changes in sulfoglycolipids of kidney cell lines by culture in anisosmotic media

(1) The effects of osmolarity environments on renal glycolipid composition were examined using established renal cell lines. The profile of glycosphingolipids of Madin-Darby canine kidney cells (MDCK) in culture with anisosmotic media showed that a hyposomotic medium reduced the concentration of GalCer I3-sulfate and LacCer II3-sulfate. (2) The concentrations of sulfoglycolipids were increased by maintaining the culture in a hyperosmotic media prepared by the addition of various sodium salts to the control isosmotic medium, while the contents of most of the neutral glycolipids were reduced. The hyperosomotic medium supplemented with nonelectrolytes, mannitol, sucrose or urea, also increased the concentration of sulfoglycolipids. (3) Both sulfoglycolipids were increased linearly with gradual increases of sodium chloride in the medium. Hyperosmolarity produced by the addition of a nonelectrolyte, mannitol, also increased the levels of sulfoglycolipids. In both series of media, the most prominent accumulation was observed in LacCer II3-sulfate. (4) The incorporation of radioactive sulfate into sulfoglycolipids was elevated in cells adapted to high NaCl or mannitol. The increase of the label was observed not only in MDCK but also in three other established cell lines of renal tubular origin, JTC-12, LLC-PK1 and MDBK. (5) It was established, using the culture system of homogeneous cell lines, that the mechanism of increasing the amount of sulfoglycolipids is independent of the integral regulatory mechanism of animals and resides in the renal epithelial cell itself. These results suggest that by culture in hyperosmotic media, the elevated level of intracellular cations stimulated the activity of GalCer and LacCer sulfotransferase, inducing the increased expression of sulfoglycolipids.     

A fluorimetric determination of the activity of glycolipid transfer protein and some properties of the protein purified from pig brain

The fluorimetric method of Correa-Freire et al. (Correa-Freire, M.C., Barenholz, Y. and Thompson, T.E. (1982) Biochemistry 21, 1244-1248) to measure glucosylceramide transfer between phospholipid bilayers has been applied to the determination of the activity of glycolipid transfer protein purified from pig brain. The transfer of pyrene-labeled galactosylceramide (PyrGalCer) from donor to acceptor vesicles was measured by a decrease in the intensity ratio of eximer (E) to excited monomer (M). A sensitive determination of the glycolipid transfer activity is possible by the fluorimetric method without separation of the donor and acceptor vesicles. The newly developed fluorimetric assay of glycolipid transfer protein was used to study the effects of N-ethylmaleimide, HgCl2 and sugars on the transfer activity. The treatment with N-ethylmaleimide inactivated the activity to about 40%. The activity was almost completely inactivated by the treatment with HgCl2. Monosaccharides and methyl-alpha-D-glucoside had no inhibitory effect on the transfer activity. A marked and immediate drop of the E/M ratio was observed by the addition of glycolipid transfer protein to vesicles containing PyrGalCer at a protein-to-PyrGalCer molar ratio of 1.56:1. The result suggests a complex formation of glycolipid transfer protein with PyrGalCer.     

Mass and relative elution time profiling: two-dimensional analysis of sphingolipids in Alzheimer's disease brains

Current lipidomic profiling methods rely mainly on MS to identify unknown lipids within a complex sample. We describe a new approach, involving LC×MS/MS (liquid chromatography×tandem MS) analysis of sphingolipids based on both mass and hydrophobicity, and use this method to characterize the SM (sphingomyelin), ceramide and GalCer (galactosylceramide) content of hippocampus from AD (Alzheimer's disease) and control subjects. Using a mathematical relationship we exclude the influence of sphingolipid mass on retention time, and generate two-dimensional plots that facilitate accurate visualization and characterization of the different ceramide moieties within a given sphingolipid class, because related molecules align horizontally or vertically on the plots. Major brain GalCer species that differ in mass by only 0.04 Da were easily differentiated on the basis of their hydrophobicity. The importance of our method's capacity to define all of the major GalCer species in the brain samples is illustrated by the novel observation that the proportion of GalCer with hydroxylated fatty acids increased approximately 2-fold in the hippocampus of AD patients, compared with age- and gender-matched controls. This suggests activation of fatty acid hydroxylase in AD. Our method greatly improves the clarity of data obtained in a lipid profiling experiment and can be expanded to other lipid classes.     

Increased urinary excretion of glycosphingolipids in familial hypercholesterolemia

The content of glycosphingolipids (GSL) was studied in the urinary sediments (24-hr specimens) from seven normal subjects, a patient with Fabry's disease, and five homozygotes with familial hypercholesterolemia (FH). Normal urinary sediments contained very small amounts of GalCer, GlcCer, GaOse(2)Cer, LacCer, GbOse(3)Cer, and GbOse(4)Cer. In Fabry urinary sediment, the levels (nmole glucose/24 hr) of GaOse(2)Cer and of GbOse(3)Cer were 389 and 550, respectively. In urinary sediments from the FH subjects, the mean contents (nmol glucose/mg protein per 24 hr) of GlcCer, GalCer, and LacCer were 2.7, 1.9, and 15.8 times higher, respectively, than in normals. The mean contents ( micro g/mg protein per 24 hr) of total cholesterol and phospholipid in the urinary sediment of FH (1.1 and 224, respectively) and normals (0.8 and 220) were similar. The mean contents of GlcCer, GalCer, and LacCer, expressed in terms of the cholesterol content of urinary sediment (nmol glucose/ micro g cholesterol per 24 hr), were increased 3.4-, 1.6-, and 5.4-fold, respectively, in the FH homozygotes. Of the five FH homozygotes, only one, who had undergone a portacaval shunt and was also receiving lipid-lowering therapy, had a normal value of LacCer. The other four FH homozygotes had levels of LacCer that were 3- to 55-fold higher (nmol glucose/mg protein per 24 hr) and 5.5- to 7.3-fold higher (nmol glucose/ micro g cholesterol per 24 hr) than the mean of the normals. One homozygote underwent plasma exchange therapy that reduced both the baseline urinary (nmol glucose/24 hr) and plasma (nmol/100 ml) LacCer levels from 86 to 7 and from 1491 to 852, respectively. Eleven days after plasma exchange, the urinary LacCer levels approached pre-exchange levels (59 nmol glucose/24 hr). The data indicate that there is an abnormality of GSL metabolism associated with familial hyper-cholesterolemia and that the urinary excretion of GSL can be modified by plasma exchange therapy.-Chatterjee, S., C. S. Sekerke, and P. O. Kwiterovich, Jr. Increased urinary excretion of glycosphingolipids in familial hypercholesterolemia.