A comparison of composition and fluidity of multiple sclerosis and normal myelin

Myelin was isolated from normal-appearing white matter from the brains of 11 multiple sclerosis (MS) patients, 11 normal individuals, and 1 patient with subacute sclerosing panencephalitis (SSPE). Ratios of protein, cholesterol, and phospholipid content, total fatty acid content, and total amino acid content were determined. The lipid bilayer fluidity of the myelin was measured using electron spin resonance and fluorescence polarization spectroscopy. The protein-to-phospholipid ratio was higher than normal, the cholesterol-to-protein ratio was lower than normal while the cholesterol-to-phospholipid ratio was normal in MS myelin. This suggested a relative increase of protein to lipid content in MS myelin. Of the fatty acid content of a total lipid extract, MS myelin had relatively more 16:0, less 18:3, and less 22:3, 24:1 than normal. The total amino acid content of MS myelin was altered in a way which suggested a decreased percentage of basic protein and an increased percentage of proteolipids. SSPE myelin had similar changes in amino acid content but not in protein-to-cholesterol or -phospholipid ratios or fatty acid content as MS myelin. There was no significant difference in myelin fluidity, however, between MS and normal myelin using fatty acid spin labels or fluorescent probes. A correlation was found between the fluidity and the cholesterol-to-phospholipid ratio but the MS and normal samples varied over a similar range. Although these alterations in lipid and protein composition had no effect on fluidity, they may nevertheless have serious consequences for myelin structure.

Dependence of boundary lipid on fatty acid chain length in phosphatidylcholine vesicles containing a hydrophobic protein from myelin proteolipid

Lipophilin, a hydrophobic protein fraction, purified and delipidated from the proteolipid of human myelin, possesses a layer of boundary lipid surrounding it when incorporated into lipid vesicles. The protein reduces the energy absorbed during the lipid phase transition, indicating that the boundary lipid does not go through the phase transition. The amount of boundary lipid was estimated by plotting the enthalpy of the transition against the protein to lipid mole ratio and extrapolating to deltaH = 0 for a number of synthetic phosphatidylcholines, to determine the ability of fatty acid chains of varying length to interact with the protein. The amount of boundary lipid was found to be similar, 21-25 molecules per molecule of lipophilin, for fatty acid chains of length 14-18 carbons but somewhat less, 16 molecules of lipid per molecule of protein, for a fatty acid chain length of 12 or for one with a trans double bond (18:1tr). No preferential interaction was observed with a lipid containing a particular fatty acid chain length when the protein was incorporated into a mixture of these lipids. These results suggest that the binding of lipids to the boundary layer of other membrane proteins and enzymes may not depend significantly on lipid fatty acid chain length.

Circular dichroism studies on lipid-protein complexes of a hydrophobic myelin protein

Circular dichroism spectra of lipophilin (a hydrophobic protein purified from human central nervous system myelin) were analyzed by the method of Chen et al. (1974) to obtain information on its secondary structure in aqueous and lipid environments. When introduced into phosphatidylcholine vesicles by dialysis from 2-chloroethanol, the protein possessed about 75% alpha helix. A new water-soluble form of lipophilin also containing over 70% alpha helix was obtained by a similar dialysis in the absence of lipid. This product had a higher helical content than two other water-soluble preparations derived by dialysis from phenol-acetic acid-urea. Interaction of all three aqueous forms of the protein with lysolecithin micelles resulted in increases in total helical content or in the average length of helical segments. The amount of beta sheet was at a minimum for lipophilin incorporated into vesicles, where the presence of lipid also provided some protection against thermal denaturation.

Effect of basic protein from human central nervous system myelin on lipid bilayer structure

The effect of myelin basic protein from normal human central nervous system on lipid organization has been investigated by studying model membranes containing the protein by differential scanning calorimetry or electron spin resonance spectroscopy. Basic protein was found to decrease the phase transition temperature of dipalmitoyl phosphatidylglycerol, phosphatidic acid, and phosphatidylserine. The protein had a greater effect on the freezing temperature, measured from the cooling scan, than on the melting temperature, measured from the heating scan. These results are consistent with partial penetration of parts of the protein into the hydrocarbon region of the bilayer in the liquid crystalline state and partial freezing out when the lipid has been cooled below its phase transition temperature. The effect of the protein on fatty acid chain packing was investigated by using a series of fatty acid spin labels with the nitroxide group located at different positions along the chain. If the protein has not yet penetrated, it increases the order throughout the bilayer in the gel phase, probably by decreasing the repulsion between the lipid polar head groups. Above the phase transition temperature, when parts of it are able to pentrate, it decreases the motion of the lipid fatty acid chains greatly near the polar head group region, but has little or no effect near the interior of the bilayer. Upon cooling again the protein still decreases the motion near the polar head group region but increases it greatly in the interior. Thus, the protein penetrates partway into the bilayer, distorts the packing of the lipid fatty acid chains, and prevents recrystallization, thus decreasing the phase transition temperature. The magnitude of the effect varied with the lipid and was greatest for phosphatidic acid and phosphatidylglycerol. It could be reversed upon cooling for phosphatidylglycerol but not phosphatidic acid. The protein was only observed to decrease the phase transition temperature of phosphatidylserine upon cooling. It had only a small effect on phosphatidylethanolamine and no effect on phosphatidylcholine. Thus, the protein may penetrate to a different extent into different lipids even if it binds to the polar head group region by electrostatic interactions.

Lipid phase separation induced by a hydrophobic protein in phosphatidylserine--phosphatidylcholine vesicles

Differential scanning calorimetry (DSC) was used to detect phase separation induced by hydrophobic myelin protein, lipophilin, in a mixture of phosphatidylserine (PS) and dipalmitoylphosphatidylcholine (DPPC). Preferential binding of PS to the boundary layer of lipophilin causes a decrease in the PS content of the remaining lamellar phase with a resultant shift in the phase-transition temperature to a higher temperature. The phase diagram for this mixture in the presence and absence of lipophilin is presented. From the phase diagram, it can be estimated that for an equimolar mixture of PS and DPPC, the boundary layer contains only PS, although for higher DPPC contents, some DPPC can also be found in the boundary layer. In the case where partial phase separation in induced in this mixture by Ca2+ alone, lipophilin increases the phase separation indicating that it also binds PS preferentially in the presence of Ca2+. Preferential binding of two other acidic lipids, phosphatidic acid and phosphatidyl-glycerol, to the boundary layer was also found, including a mixture where the acidic lipid was the higher melting component in the mixture.

Preparation and properties of vesicles of a purified myelin hydrophobic protein and phospholipid. A spin label study

Lipophilin, a hydrophobic protein purified from the proteolipid of normal hupid and protein in 2-chloro-ethanol followed by dialysis against buffer. This method resulted in homogeneous incorporation of the protein into lipid vesicles as judged by sedimentation on a sucrose gradient and freeze fracture electreter and the freeze fracture faces contained intramembrane particles. The effect of lipophilin on the organization of the lipid was studied by use of spin label probes. Two distinct components were present in the spectrum of fatty acid spin labels in the lipid-protein vesicles. One was immobilized presumably due to the presence of boundary lipid around the protein and the second component waicles and probably due to a lamellar phase but with a slightly greater order parameter. Lipophilin was found to increase the order parameter linearly with increasing concentration of protein incorporated into the vesicles. However, the phase transition temperature as measured from the 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) solubility parameter was unchanged.

Pressure effect on the membrane action of a nerve-blocking spin label

A reversible nerve-blocking spin label, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was used to study the nature of anesthetic-binding sites in nerve membranes as a function of pressure. The nerve-blocking effect of TEMPO is enhanced under pressure. At atmospheric pressure, TEMPO blocks nerve conduction by solubilizing in the apolar region of the nerve membrane. However, the nerve-conduction-block by TEMPO at 150 atm of helium was related to the binding of TEMPO to a pressure-induced high-affinity polar site in the nerve membrane. The new TEMPO-binding site could not be detected in lipid model membranes and, thus, the involvement of membrane protein in the new site was inferred. Pressure may induce a nerve membrane conformation change in the presence of TEMPO. The observation that under different pressure, a single anesthetic, i.e., TEMPO, was capable of blocking nerve conduction by binding to two different sites within the nerve membrane, supports the view that there are multiple anesthetic receptor sites, which differ in chemical composition and location within the nerve membrane. These sites, when occupied by different classes of anesthetics, produce the general phenomenon of nerve-conduction block. The enhancement of nerve-conduction block by pressure may be due to the increased concentration of TEMPO in the new site in the nerve membrane under pressure.

Structural characteristics of hydrated glycerol-and sphingo-lipids. A spin label study

(1) The structural organization and fluidity of a variety of lipid classes were studied by electron spin resonance spectroscopy of fully hydrated planar films labeled with 3-doxyl-cholestane and 5-doxyl-palmitate and of dispersions labeled with 5-doxyl-palmitate. The phase of each lipid was checked by use of a polarizing microscope.(2) Orientation in a lamellar phase was detected for phosphatidyl-choline, -ethanolamine, -inositol, phosphatidic acid, cardiolipin, ceramide, and sphingomyelin. In the presence of 33 or 50 mol % cholesterol, the orientation was improved in all of these lipids with the exception of cardiolipin and orientation was induced in phosphatidylserine, monoglycerides, sphingosine, and cerebrosides. Cholesterol fluidized the sphingolipids sufficiently to solubilize the spin labels and allow formation of oriented multibilayers by this method.(3) Anisotropic motion of 5-doxyl-palmitate was also seen in films and disperions of 1-monoglycerides and diglycerides and in dispersions of gangliosides, which are known to exist in hexagonal or micellar phases.(4) The contribution of the molecular structures to the stability of the lamellar phase is discussed.

Orientation and motion of amphiphilic spin labels in hexagonal lipid phases

The acyl chain of spin-labeled fatty acids intercalates between the lipid hydrocarbon chains in hexagonal and micellar phases with the carboxyl group anchored at the lipid water interface. The spectra are characteristic of anisotropic motion and cannot be distinguished from the spectra of these probes in lamellar dispersions. In the hexagonal and micellar phases the molecular motion of the spin label increases as it is moved further away from the carboxyl group, similar to the behavior in the lamellar phase (Jost et al. (1971) J. Mol. Biol. 59, 77-98). The similarity in packing of the acyl chains in the hexagonal and lamellar phases suggests that localized regions of hexagonal phase are compatible with a bilayer matrix.