Lipid-induced recognition of a conformational determinant (residues 65 to 83) in myelin basic protein

The precipitation by antibodies to intact myelin basic protein (BP) and to synthetic peptides containing a sequence based on the region 65 to 83 of bovine BP, S82, S81, S79, and S24, of intact BP in solution or bound to lipid vesicles was compared, using 125I-BP or 14C-DPPC-labeled lipid-BP vesicles. The antipeptide antibodies were shown earlier to recognize conformational determinants which are not expressed in the intact protein in solution. Several anti-BP antibodies precipitated more of the BP free in solution than when bound to lipid vesicles, suggesting that some of the determinants recognized by these antibodies were either sequestered in the bilayer or were altered in conformation. In contrast, one anti-peptide antisera, which had a high titer for the conformational determinant in two of these peptides, S82 and S81, precipitated the protein to a significant degree when it was bound to PG vesicles, even though it did not react with the intact protein in solution. These results indicated that PG was able to confer on the protein the unique peptide conformation recognized by this antibody. PS was less effective, and other lipids were ineffective at conferring this conformation on the protein, supporting earlier results which showed that the conformation of the protein is influenced by the lipid composition of its environment. None of the other anti-peptide antibodies studied bound to the protein either in solution or in lipid vesicles. These results indicate that the lipid environment can sequester or alter the conformation of some antigenic determinants, preventing recognition by some anti-BP antibodies, and can expose or generate other conformational determinants, allowing recognition by an anti-peptide antiserum.

Lipid-induced recognition of a conformational determinant (residues 65 to 83) in myelin basic protein

The precipitation by antibodies to intact myelin basic protein (BP) and to synthetic peptides containing a sequence based on the region 65 to 83 of bovine BP, S82, S81, S79, and S24, of intact BP in solution or bound to lipid vesicles was compared, using 125I-BP or 14C-DPPC-labeled lipid-BP vesicles. The antipeptide antibodies were shown earlier to recognize conformational determinants which are not expressed in the intact protein in solution. Several anti-BP antibodies precipitated more of the BP free in solution than when bound to lipid vesicles, suggesting that some of the determinants recognized by these antibodies were either sequestered in the bilayer or were altered in conformation. In contrast, one anti-peptide antisera, which had a high titer for the conformational determinant in two of these peptides, S82 and S81, precipitated the protein to a significant degree when it was bound to PG vesicles, even though it did not react with the intact protein in solution. These results indicated that PG was able to confer on the protein the unique peptide conformation recognized by this antibody. PS was less effective, and other lipids were ineffective at conferring this conformation on the protein, supporting earlier results which showed that the conformation of the protein is influenced by the lipid composition of its environment. None of the other anti-peptide antibodies studied bound to the protein either in solution or in lipid vesicles. These results indicate that the lipid environment can sequester or alter the conformation of some antigenic determinants, preventing recognition by some anti-BP antibodies, and can expose or generate other conformational determinants, allowing recognition by an anti-peptide antiserum.

Increase in vesicle permeability mediated by myelin basic protein: effect of phosphorylation of basic protein

The two most basic charge isomers of myelin basic protein (BP), components 1 and 2 (C1 and C2), which presumably differ in the degree of deamidation, were purified from bovine BP by cation-exchange chromatography. Two additional specific types of posttranslational modifications were introduced into the purified isomers: (1) C-terminal arginine deficient derivatives of C1 and C2 were prepared by incubating the isomers with a carboxypeptidase, and (2) phosphorylated derivatives of C1 (1.6 and 1.7 mol of phosphate/mol of protein) were prepared by incubating C1 with the protein kinase from rabbit muscle. The ability of these charge isomers to increase the permeability of multilamellar vesicles composed of phosphatidylserine/phosphatidylcholine (1:11.5 w/w) and sphingomyelin/cholesterol/phosphatidic acid (1:1:0.2 w/w/w) was measured by monitoring the release of a water-soluble spin-label (tempocholine chloride) from the vesicles. The increase in vesicle permeability caused by BP was taken as a measure of the degree of perturbation of the bilayer by the protein, most likely by penetration partly into the bilayer. All classes of charge isomers (naturally occurring or generated in vitro) were more effective at increasing vesicle permeability than was poly(L-lysine), a polycation that only interacts electrostatically with the bilayer. Although C1 and C2 and their C-terminal-deficient derivatives did not differ in the amount of marker released, the phosphorylated derivative of C1 caused a smaller increase in vesicle permeability than did the other isomers, suggesting that phosphorylation had altered the ability of the protein to perturb the bilayer.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of myelin basic protein and polylysine with synthetic species of cerebroside sulfate

The effect of myelin basic protein on the myelin lipid cerebroside sulfate was studied by differential scanning calorimetry and use of the fatty acid spin label, 16-S-SL, in order to determine (i) the effect of basic protein on the metastable phase behavior experienced by this lipid, and (ii) to determine if basic protein perturbs the lipid packing as it does with some acidic phospholipids. The effects of basic protein on the thermodynamic parameters of the lipid phase transition were compared with those of polylysine which has an ordering effect on acidic phospholipids as a result of its electrostatic interactions with the lipid head groups. Different synthetic species of cerebroside sulfate of varying fatty acid chain length and with and without a hydroxy fatty acid were used. The non-hydroxy fatty acid forms of cerebroside sulfate undergo a transition from a metastable to a more ordered stable state while the hydroxy fatty acid forms remain in the metastable state at the cation concentration used in this study (0.01 M Na+ or K+). The non-hydroxy fatty acid forms were still able to go into a stable state in the presence of both basic protein and polylysine. At low concentrations, basic protein increased the rate of the transition to the stable state, while polylysine decreased it for the longest chain length form studied. However, at high concentrations, basic protein probably prevented formation of the stable state. The hydroxy fatty acid forms did not go into the stable state in the presence of basic protein and polylysine. It is argued that the increased rate of formation of the stable state in the presence of basic protein and decreased rate in the presence of polylysine are consistent with interdigitation of the lipid acyl chains in the stable state. Basic protein also had a small perturbing effect on the lipid. It decreased the total enthalpy of the lipid phase transition. When added to the non-hydroxy fatty acid forms it increased the temperature of the liquid crystalline to metastable phase transition and decreased the temperature of the stable to liquid crystalline phase transition. It significantly decreased the transition temperature of the hydroxy fatty acid forms but only a portion of the lipid was affected. In contrast, polylysine increased the transition temperature of the metastable and stable states of all forms of cerebroside sulfate but had a greater effect on the non-hydroxy fatty acids forms than on the hydroxy fatty acid forms.(ABSTRACT TRUNCATED AT 400 WORDS)

Phase transitions and fatty acid spin label behavior in interdigitated lipid phases induced by glycerol and polymyxin

Glycerol and polymyxin have been shown by X-ray diffraction to induce interdigitated bilayers in phosphatidylcholine (PC) and phosphatidylglycerol (PG), respectively (McDaniel, R.V., et al. (1983) Biochim. Biophys. Acta 731, 97-108; Ranck, J.-L. and Tocanne, J.-F. (1982) FEBS Lett. 143, 175-178). In the present study we have investigated the phase behavior of PC and PG in the presence of glycerol and polymyxin by differential scanning calorimetry and the use of fatty acid spin labels. Interdigitation causes a large increase in the order parameter of a fatty acid spin labeled near the terminal methyl, 16-doxylstearate, so that it was similar to that of a fatty acid labeled much closer to the polar head group region, 5-doxylstearate. Thus interdigitation abolishes the fluidity gradient found in a non-interdigitated bilayer. 16-Doxylstearate may be useful in detecting interdigitation of lipid bilayers caused by other substances. The different samples all went through two transitions on heating or cooling, or both. However, use of the fatty acid spin label showed that the molecular events during these transitions varies for different samples. The results suggested that PC-glycerol freezes from the liquid-crystalline phase into a non-interdigitated gel phase. This subsequently becomes interdigitated upon lowering the temperature a few degrees, in a low enthalpy transition. PG-polymyxin shows a similar behavior except that the enthalpy of the non-interdigitated gel to interdigitated phase transition is greater and the transition is reversible on heating. Thus on heating PG-polymyxin first goes through a transition from the interdigitated phase to a non-interdigitated gel phase and then, in a separate transition, to the liquid-crystalline phase. This occurs because the fatty acid chains in the presence of polymyxin become too disordered with increase in temperature to maintain the interdigitated state. PG-glycerol goes into the interdigitated state less readily than the other mixtures. If cooled rapidly, PG-glycerol freezes into a metastable phase which is more disordered than the interdigitated phase. It goes into the interdigitated phase in an exothermic transition on heating. An increase in fatty acid chain length causes greater steric hindrance to interdigitation but also increases the stabilizing energy gained by interdigitation.

Immune lysis of lipid vesicles containing myelin basic protein or glycolipid antigens by multiple sclerosis and normal sera

We have compared the reactivity of sera from 34 multiple sclerosis (MS) patients and 32 normal (N) individuals with lipid vesicles containing myelin basic protein (BP) and several glycolipids reconstituted into a membrane environment. The ability of the sera to cause complement-mediated lysis of lipid vesicles containing these antigens was determined by measuring the release of a water-soluble spin label, tempocholine chloride, from the height of its electron spin resonance spectrum. Only 4 MS sera caused lysis of BP-containing vesicles which was comparable to that produced by specific antibody to BP. A number of both MS and N sera caused significant lysis of vesicles containing GM1 ganglioside or digalactosyldiglyceride. A few MS and N sera also caused significant lysis of vesicles containing GM2, GT1 and GD1a gangliosides. However, in no case was there a statistically significant difference between the mean lysis produced by MS and N sera. There was some overlap between the specific MS and N sera reactive to vesicles containing BP, GM1, GM2, and DGDG while a completely different group of MS and N sera were reactive to GT1 and GD1a gangliosides. This suggested that there was either antigenic cross reactivity between the two groups of glycolipids or two different origins of the immune response to the two groups of antigens. It was concluded that antibody-dependent complement fixation by these particular antigens, in the kind of lipid environment used, is not characteristic of or specific to MS.

Changes in the composition of two molecular species of ethanolamine plasmalogen in normal human myelin during development

Changes in the ratio of two molecular species of ethanolamine plasmalogen, PI-LE-1 and PI-PE-2, of human central nervous system myelin during development were measured by a TLC procedure. The ratio was found to decrease sharply with age up to 6 months as a result of an increase in the amount of PI-PE-2, believed to be a unique myelin lipid with 18:1 chains in both positions of the glycerol. The ratio continued to decrease gradually with age and did not reach the adult level until an age of 17 years.

Comparison of metastable phase behavior of the complexes of dipalmitoyl phosphatidylglycerol with Mg+2 and myelin basic protein

Both divalent cations and myelin basic protein (BP) induce metastable phase behavior in phosphatidylglycerol (PG). The stable phase of the divalent cation--PG complex is dehydrated cylinders or sheets of lipid in which adjacent bilayers are probably bridged by the divalent cation. The structure of the stable phase of the BP-PG complex is unknown. To help understand the metastable phase behavior in the BP-PG complex, it was compared with that of the MG+2-PG complex using fatty-acid spin labels and differential scanning calorimetry. The stable state of the BP-PG complex melts at a similar temperature and with a similar enthalpy as the Na+-salt form of the lipid, while the stable state of the Mg+2-PG complex melts 35 degrees higher than and with an enthalpy 63% greater than the Na+-salt form. BP causes pronounced motional restriction of a fatty acid spin labeled near the terminal methyl group in the metastable and stable states of its complex with PG. Computer resolution of spectra of fatty-acid spin labels in the Mg+2-PG complex revealed that the spectra characteristic of the stable state were exchange broadened, showing that the probes are not soluble in the stable dehydrated state of this complex. The much different behavior of the BP-PG complex suggests that its stable phase is not dehydrated, that the probes are not displaced from the complex, and that their behavior reflects the ability of hydrophobic residues of basic protein to penetrate into the bilayer while its basic residues bind electrostatically to the lipid polar head groups.

Partial synthesis and physical properties of cerebroside sulfate containing palmitic acid or alpha-hydroxy palmitic acid

Chromatographically pure galactosylceramide I3-sulfate (cerebroside sulfate (CBS)) containing palmitic acid or D-2-hydroxypalmitic acid has been prepared by the acylation of galactosylsphingosine I3-sulfate obtained from the saponification of bovine brain sulfatides. Optically pure D-2-hydroxypalmitic acid was obtained by adapting literature methods for the synthesis of the racemic acid and its resolution. The thermotropic behavior of the two synthetic CBSs were compared to each other and to the corresponding components in natural CBS, obtained by fractionation of bovine brain sulfatides, in order to determine the contribution of the hydroxy fatty acid to intermolecular hydrogen bonding between molecules of the lipid. The gel to liquid crystalline phase transition temperature (Tc) of the hydroxy fatty acid (HFA) synthetic form is 53.2 degrees C, 3 degrees higher than that of the non-hydroxy fatty acid (NFA) form at low concentrations of Na+ or K+. A similar difference was found for the HFA and NFA forms of natural CBS. The enthalpy of the NFA synthetic form is 8.5 kcal/mol, about 30% greater than that of the HFA form. The difference in Tc between the NFA and HFA forms is abolished as the Na+ or K+ concentration increases but the difference in enthalpy persists. Increasing cation concentration, over the range 0.01-2 M, increases Tc more than for an acidic phospholipid, phosphatidylglycerol, probably due to increased intermolecular hydrogen bonding as the charged sulfate is shielded. K+ causes a 3-4 degrees C greater increase in Tc relative to that produced by Na+ while K+ and Na+ have similar effects on phosphatidylglycerol.

Modulation of bovine milk galactosyltransferase activity by lipids

The effect of lipids singly and in combination on the ability of galactosyltransferase to transfer galactose to N-acetyl-D-glucosamine-forming lactosamine and to glucose forming lactose has been studied. Lecithins, as egg phosphatidylcholine (PC), or saturated as dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine stimulated the activity of the enzyme to form lactosamine to different extents. Egg PC produced the greatest stimulation of all the lecithins tested. Egg phosphatidic acid (PA) inhibited the activity of the enzyme at very low concentrations of lipid. In mixed vesicles of gel phase or liquid crystalline phase lecithins and egg PA, the acidic lipid was able to overcome the stimulation produced by the lecithins. The dominant effect of the head group was demonstrated by the effects of gel phase dimyristoylphosphatidic acid (DMPA). In mixtures with PC, DMPA also was able to inhibit the enzyme for lactosamine synthesis but higher concentrations of the gel phase DMPA were required for inhibition compared to the liquid crystalline PA. Although the head group appeared to dominate the inhibition, the nature of the acyl chains of the lipid played a secondary role at least. Other acid lipids, phosphatidylserine (PS) and phosphatidylinositol (PI) were much less effective than PA. PS alone inhibited the activity of the enzyme. However, in mixed lipids (PS and egg PC), PS was unable to reverse the stimulation produced by PC while PC was able to reverse the inhibition produced by PS. PI alone had no effect on the enzyme activity. In mixtures with egg PC, the stimulating effect of PC was dominant. In the lactose synthetase reaction, the effect of lipids was similar to that of the lactosamine synthetase, i.e. PC stimulated and PA inhibited activity and in mixtures of PC and PA, the inhibitory effect of PA was dominant.

Immune lysis of reconstituted myelin basic protein--lipid vesicles and myelin vesicles

Complement-mediated lysis of reconstituted lipid-myelin basic protein (BP) vesicles and myelin vesicles due to antibody raised against BP and isolated myelin is measured by determination of the amount of a water-soluble spin label, tempocholine chloride, released from the vesicles. The response is shown to be antigen-specific, antibody-dependent, and complement mediated. The relative response to different anti-BP antibody samples is similar to that determined by radioimmunoassay procedures. In contrast to immunoassays with BP in aqueous solution, this method measures immune recognition of the protein in either a synthetic or a natural membranous environment. This is important because this protein has been shown to have a different conformation when bound to lipid bilayers than in aqueous solution and its conformation depends on lipid composition. It is also a more rapid method because no separation of spin label still trapped in the vesicles and that released due to immune lysis is required. In synthetic membranes consisting of sphingomyelin, cholesterol, and an acidic lipid, either phosphatidylglycerol, phosphatidic acid, or phosphatidylserine, the response was greatest when the acidic lipid was phosphatidic acid. The response did not depend significantly on the antigen concentration expressed as molar ratio of BP to sphingomyelin, over the range 0.15:600 to 2:600, although it decreased at molar ratios less than 0.15:600. The antigen density required for immune lysis of vesicles containing this protein antigen is similar to that reported elsewhere for lipid antigens, although the time required for maximal lysis was greater. Both anti-BP and anti-myelin antibodies caused a greater specific complement-mediated response with synthetic vesicles than with myelin vesicles, which may be due to the different lipid and/or protein composition of myelin. Response was also obtained with the myelin vesicles, however, indicating that some determinants of BP can be recognized on the surface of the bilayer in isolated myelin by anti-BP.

Immune lysis of reconstituted myelin basic protein--lipid vesicles and myelin vesicles

Complement-mediated lysis of reconstituted lipid-myelin basic protein (BP) vesicles and myelin vesicles due to antibody raised against BP and isolated myelin is measured by determination of the amount of a water-soluble spin label, tempocholine chloride, released from the vesicles. The response is shown to be antigen-specific, antibody-dependent, and complement mediated. The relative response to different anti-BP antibody samples is similar to that determined by radioimmunoassay procedures. In contrast to immunoassays with BP in aqueous solution, this method measures immune recognition of the protein in either a synthetic or a natural membranous environment. This is important because this protein has been shown to have a different conformation when bound to lipid bilayers than in aqueous solution and its conformation depends on lipid composition. It is also a more rapid method because no separation of spin label still trapped in the vesicles and that released due to immune lysis is required. In synthetic membranes consisting of sphingomyelin, cholesterol, and an acidic lipid, either phosphatidylglycerol, phosphatidic acid, or phosphatidylserine, the response was greatest when the acidic lipid was phosphatidic acid. The response did not depend significantly on the antigen concentration expressed as molar ratio of BP to sphingomyelin, over the range 0.15:600 to 2:600, although it decreased at molar ratios less than 0.15:600. The antigen density required for immune lysis of vesicles containing this protein antigen is similar to that reported elsewhere for lipid antigens, although the time required for maximal lysis was greater. Both anti-BP and anti-myelin antibodies caused a greater specific complement-mediated response with synthetic vesicles than with myelin vesicles, which may be due to the different lipid and/or protein composition of myelin. Response was also obtained with the myelin vesicles, however, indicating that some determinants of BP can be recognized on the surface of the bilayer in isolated myelin by anti-BP.

The effect of linoleic acid and benzyl alcohol on the activity of glycosyltransferases of rat liver Golgi membranes and some soluble glycosyltransferases

The effects of the membrane perturbing reagents linoleic acid and benzyl alcohol on the activities of four rat liver Golgi membrane enzymes, N-acetylglucosaminyl-, N-acetylgalactosaminyl-, galactosyl-, and sialyltransferases and several soluble glycosyltransferases, bovine milk galactosyl- and N-acetylglucosaminyltransferases and porcine submaxillary N-acetylgalactosaminyltransferases have been studied. In rat liver Golgi membranes, linoleic acid inhibited the activities of N-acetylgalactosaminyl- and galactosyltransferases by 50% or greater, sialyltransferase by 10-15%, and N-acetylglucosaminyltransferase not at all. The isolated bovine milk N-acetylglucosaminyltransferase and porcine submaxillary N-acetylgalactosylaminyltransferase were not inhibited but bovine milk galactosyltransferase was inhibited by 95% or greater. The inhibition by linoleic acid on Golgi membrane galactosyltransferase appears to be a direct effect of the reagent on the enzyme. Incorporation of bovine milk galactosyltransferase into liposomes formed from saturated phospholipids, DMPC, DPPC, and DSPC (dimyristoyl-, dipalmitoyl-, and distearoylphosphatidylcholine) prevented inhibition of the enzyme activity suggesting that the lipid formed a barrier which did not allow linoleic acid access to the enzyme. The water soluble benzyl alcohol was more effective in inhibiting enzymes of the isolated rat liver Golgi complex. All four glycosyltransferases were inhibited, the N-acetylglucosaminyl- and N-acetylgalactosaminyltransferases by more than 95%. A higher concentration of benzyl alcohol was necessary to inhibit the galactosyltransferases than was required for the other Golgi enzymes. Benzyl alcohol also inhibited the isolated bovine milk N-acetylglucosaminyl- and galactosyltransferases 90% to 95%, respectively, but did not affect the isolated porcine submaxillary gland N-acetylgalactosaminyltransferase. Benzyl alcohol did not inhibit the milk galactosyltransferase incorporated into DMPC or DPPC liposomes but showed a complex effect on the activity of the enzyme incorporated into DSPC vesicles, a stimulation of activity at low concentrations followed by an inhibition. A lipid environment consisting of saturated lipids appears to present a barrier to inhibiting substances such as linoleic acid and benzyl alcohol, or lipid may stabilize the active conformation of the enzyme. The different effects of these reagents on four transferases of the Golgi complex suggest that the lipid environment around these enzymes may be different for each transferase.

Comparison of membrane structure, osmotic fragility, and morphology of multiple sclerosis and normal erythrocytes

Erythrocyte membranes from multiple sclerosis (MS) patients and normal individuals were studied by electron spin resonance spectroscopy, osmotic fragility tests, scanning electron microscopy (SEM) and fatty acid analysis of membrane lipids. There was no significant difference in the membrane fluidity between MS and normal erythrocytes using fatty acid spin labels with the nitroxide moiety on carbons 5, 12, or 16 from the carboxyl group. Linoleic acid, which has been reported to decrease the absolute electrophoretic mobility of only MS erythrocytes, increased the fluidity of MS and normal erythrocyte membranes to a similar extent. The osmotic fragility of MS erythrocytes obtained from outpatients was similar to normal control cells but the osmotic fragility of erythrocytes obtained from hospitalized MS patients was greater than normal. Scanning electron microscopy of MS erythrocytes revealed no gross abnormalities. Cells incubated with linoleic acid had transformed from discocytes into sphero-echinocytes with prominent membrane surface indentations but MS and normal erythrocytes appeared identical. Of the fatty acid content of the total lipid extract, erythrocytes from most, but not all, MS hospitalized patients and some patients with other demyelinating diseases had relatively less (P less than .001) 18:2 than the normal cells. These results indicate that at least some of the abnormalities reported in MS erythrocytes may only be found in hospitalized patients and may be due to other complications of the disease. They also indicate that the reported abnormal effects of linoleic acid on the electrophoretic mobility of MS erythrocytes may be caused by some other mechanism than an effect on the fluidity of the bilayer.

Comparison of two molecular species of ethanolamine plasmalogen in multiple sclerosis and normal myelin

A TLC procedure which resolves two molecular species of ethanolamine plasmalogen, Pl-PE-1 and Pl-PE-2, was used to compare the ratio of these was species in myelin isolated from normal appearing white matter from brains of 17 multiple sclerosis (MS) patients, 17 normal (N) individuals, 1 patient with subacute sclerosing panencephalitis (SSPE) and 1 patient with a non-demyelinating neurological disease (OND). One of these species (Pl-PE-2) has been reported to be unique to myelin and has primarily 18:1 in both the 1 and 2 positions of glycerol. The other species (Pl-PE-1) is also present in other membranes and has primarily a saturated chain in the 1 position and a polyunsaturated chain in the 2 position. The Pl-PE-1 to Pl-PE-2 ratio was quantitated by scanning the plates with a densitometer. The ratio was similar to normal in most of the MS samples, 0.88 +/- 0.09, but was much less th an normal in 4 of the MS samples and the SSPE sample. This is attributed to increased decomposition or hydrolysis of Pl-PE-1 relative to Pl-PE-2 at some stage, either during the disease process or due to post-mortem decomposition. Although the reason for the enhanced decomposition of Pl-PE-1 is not known it suggests that Pl-PE-2 is more stable chemically. This may be related to the unique occurrence of Pl-PE-2 in myelin.

Effect of pH and fatty acid chain length on the interaction of myelin basic protein with phosphatidylglycerol

The basic protein of myelin binds electrostatically to acidic lipids but has several hydrophobic segments which may penetrate into the lipid bilayer. Calorimetric and spin-label evidence suggests that below the phase transition temperature, Tc, several phase states occur in the complex of phosphatidylglycerol with basic protein, possibly due to differences in the degree of penetration of the protein and/or interdigitation of the lipid acyl chains. One of these states is a metastable state which starts to melt 10 degrees C below the Tc of the pure lipid and then refreezes, with release of heat, into a stable state. The stable state melts near the Tc of the pure lipid but restricts the motion of fatty acid spin-labeled near the terminal methyl much more than does the pure lipid. The relationship between the rate of conversion to the stable state and the degree of penetration of the protein at varying pH, in the range 4--8, and the lipid acyl chain length, in the range 14 to 18 carbons, was investigated. Altering the pH in this range affects protonation of the histidines of the protein but has no effect on the lipid at pH 4 and above. The rate of conversion of the sample to both the metastable state and the stable state decreased with increase in pH for phosphatidylglycerol with all lipid chain lengths. It also decreased with decreasing chain length at constant pH. This suggested that the lipid could refreeze into the stable state more readily if a smaller proportion of the total bilayer thickness was occupied by the hydrophobic segments of the protein. The consistency of these results with the concept of penetration of portions of the protein partway into the bilayer lends support to this hypothesis.

Antibody precipitation of lipid vesicles containing myelin proteins: dependence on lipid composition

Antibody binding to human CNS myelin basic protein and to rabbit sciatic nerve myelin P-2 in their lipid-bound and water-soluble conformations has been investigated. 125I-labeled basic protein or P-2 was bound to the surface of liposomes (vesicles) of different acidic lipids, phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylglycerol (PG), and cerebroside sulfate (CBS). The antibody was prepared against aqueous solutions of basic protein and P-2. Antibody binding to the proteins in liposomes was measured by precipitation of the liposomes by using a double antibody radioimmunoassay. The amount of 125I-basic protein precipitated was lest when the protein was bound to PA and increased in the order PA less than PS less than PG less than CBS less than PE approximately equal to basic protein in solution, suggesting that the antigenic determinants were lest exposed or most altered for PA and most exposed for PE. This agreed fairly well with previously published biophysical studies that suggested that hydrophobic segments of the protein penetrated into the lipid bilayer and that this penetration decreased in the order PA approximately equal to PG greater than PS greater than CBS greater than or equal to PE. The amount of 125I-P-2 precipitated was least for PA and CBS and increased in the order PA approximately equal to CBS less than PS less than PG less than PE approximately equal to P-2 in solution. However, the differences were less than for basic protein and the effect of CBS was different for the 2 proteins. Less is known about the conformation of P-2 in these lipids but it is known that lipids increase its disease-inducing activity. These results indicate that interaction with lipid may sequester or alter the conformation of antigenic determinants such that antibody binding decreases.

Antibody precipitation of lipid vesicles containing myelin proteins: dependence on lipid composition

Antibody binding to human CNS myelin basic protein and to rabbit sciatic nerve myelin P-2 in their lipid-bound and water-soluble conformations has been investigated. 125I-labeled basic protein or P-2 was bound to the surface of liposomes (vesicles) of different acidic lipids, phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylglycerol (PG), and cerebroside sulfate (CBS). The antibody was prepared against aqueous solutions of basic protein and P-2. Antibody binding to the proteins in liposomes was measured by precipitation of the liposomes by using a double antibody radioimmunoassay. The amount of 125I-basic protein precipitated was lest when the protein was bound to PA and increased in the order PA less than PS less than PG less than CBS less than PE approximately equal to basic protein in solution, suggesting that the antigenic determinants were lest exposed or most altered for PA and most exposed for PE. This agreed fairly well with previously published biophysical studies that suggested that hydrophobic segments of the protein penetrated into the lipid bilayer and that this penetration decreased in the order PA approximately equal to PG greater than PS greater than CBS greater than or equal to PE. The amount of 125I-P-2 precipitated was least for PA and CBS and increased in the order PA approximately equal to CBS less than PS less than PG less than PE approximately equal to P-2 in solution. However, the differences were less than for basic protein and the effect of CBS was different for the 2 proteins. Less is known about the conformation of P-2 in these lipids but it is known that lipids increase its disease-inducing activity. These results indicate that interaction with lipid may sequester or alter the conformation of antigenic determinants such that antibody binding decreases.

An effect of colchicine on galactosyl- and sialyltransferases of rat liver Golgi membranes

Colchicine inhibited the activity of the galactosyl- and sialyltransferases of rat liver Golgi membranes. The sialyltransferase was more sensitive to the drug than galactosyltransferase since it was inhibited to a greater extent and at lower concentrations of colchicine than the galactosyltransferase. Two soluble enzymes, i.e. that from rat serum and that isolated from bovine milk, were not inhibited by colchicine. Even with very high concentrations of colchicine a marked stimulation of activity was observed. The data suggest that the inhibition observed in the Golgi membranes is in some way related to the arrangement of the enzymes in the lipid bilayer. In support of this hypothesis, the milk galactosyltransferase became very sensitive to colchicine after incorporation of the enzyme into lipid vesicles. The incorporation of colchicine into Golgi membranes was shown to decrease the order parameter as determined by electron spin resonance which reflects an increased fluidity of the Golgi membranes. A change in fluidity may be responsible for the inhibition of enzyme activity at least in part.

Intermolecular hydrogen bonding between lipids: influence on organization and function of lipids in membranes

Biological membranes have unique lipid compositions suggesting a specific role for many lipids. Evidence is reviewed concerning the intermolecular forces between glycero- and sphingolipids and cholesterol, the dependence of many of these interactions on the state of ionization of lipids, pH, ionic strength, and divalent cation concentration. The effect of intermolecular interactions between certain lipids on lipid clustering, interaction with cholesterol, on the conformation of proteins, and on transitions to the hexagonal phase is considered. Other forces which cause lipids phase separation or clustering are discussed. It is concluded that lipids are in dynamic equilibrium with their environment and can act as receptors for certain intra- or extra-cellular stimuli, which they can translate into a response by undergoing changes in fluidity, phase transitions, or phase separation.

Similar effect of proteolipid apoproteins from human myelin (lipophilin) and bovine white matter on the lipid phase transition

The proteolipid apoprotein from bovine white matter has been reported to increase the phase transition temperature of dimyristoyl phosphatidylcholine, in contrast to a proteolipid apoprotein fraction from human myelin, called lipophilin, which decreases the enthalpy without altering the phase transition temperature. Since these results lead to different conclusions concerning the structure and amount of boundary lipid surrounding these proteins, the effects of the two proteins on the phase transition of dimyristoyl phosphatidylcholine were compared. Neither protein has any effect on the phase transition temperature, regardless of the method of delipidation of the protein, the amount of residual lipid, the method of incorporation into vesicles, or heating rates used for differential scanning calorimetry. However, a higher melting component was observed when decomposition of the lipid to lysophosphatidylcholine had occurred. Addition of as little as 6% of the decomposition products of dimyristoyl phosphatidylcholine, lysodimyristoyl phosphatidylcholine and myristic acid, is enough to produce a higher-temperature peak. The intensity of this peak increases with increasing protein concentration similar to the reported result on the bovine white matter proteolipid. The question as to whether the protein-induced decrease in enthalpy is due to boundary lipid or entrapment of lipid in protein aggregates was also addressed by studying the appearance of the intramembranous protein particles by freeze-fracture electron microscopy at temperatures above and below the phase transition and between the premelt and main transitions. The protein is randomly dispersed above the phase transition. At low concentrations, below the phase transition, it clusters, forming particle-free and particle-rich areas, but does not aggregate. At higher concentrations it is randomly dispersed below the premelt and main transition but is clustered between the premelt and main transition. Since the protein is more randomly dispersed above the transition than below, the reduction in enthalpy of the freezing transition was compared to that of the melting transition and was found to be identical, suggesting that the withdrawal of lipid from the phase transition is probably not due to lipid entrapment but due to the formation of a boundary lipid interface between the protein and the bulk lipid.

Transmembrane orientation of lipophilin in phosphatidylcholine vesicles

Lipophilin, a hydrophobic myelin protein, was incorporated into phosphatidylcholine vesicles by dialysis from 2-chloroethanol which has been shown to produce single-layered lipid-protein vesicles. These vesicles were labeled with a nonpenetrating surface-labeling reagent, 4,4'-diisothiocyano-2,2'-ditritiostilbene disulfonic acid, ([3H]DIDS), in order to determine if the protein completely spans the bilayer. After labeling the vesicles, lipophilin was isolated. At least 88% of the protein ws labeled with [3H]DIDS. Dextran (mol wt 250,000-275,000) was converted to the dialdehyde form and reacted with lipophilin-PC vesicles. In this case greater than 90% of the protein was complexed to the dextran. The high degree of labeling obtained with both compounds was consistent with a model in which lipophilin was considered to span the bilayer completely.