The lipidic particle as an intermediate structure in membrane fusion processes and bilayer to hexagonal HII transitions

Small unilamellar vesicles comprised of a mixture of phosphatidylethanolamine/phosphatidylcholine/cholesterol (3 : 1 : 2) fuse to form large multilamellar vesicles on increasing the temperature from 0 to 50°C. This event is associated with the appearance of lipidic particles at the fusion sites, consistent with a role as intermediary structures during the fusion process. Further, for phosphatidylcholine/cardiolipin (1 : 1) liposomes in the presence of Mn²⁺ a direct relationship between lipidic particles and the hexagonal (H_II) phase is demonstrated which suggests that lipidic particles can also occur as intermediaries between bilayer and hexagonal (H_II) structures.     

Rapid transbilayer movement of phosphatidylcholine in unsaturated phosphatidylethanolamine containing model membranes

Aqueous dispersions of egg phosphatidylethanolamine/18 : 1_c, 18 : 1_c-phosphatidylcholine/cholesterol/18 : 1_c, 18 : 1_c-phosphatidic acid (50 : 16 : 30 : 4) undergo a temperature-dependent transition from extended bilayers to structures characterized by isotropic ³¹P-NMR signals and visualized by freeze-fracturing as lipidic particles associated with the bilayer. This transition is accompanied by a 3-fold increase in the phosphatidylcholine pool which can be exchanged by phospholipid exchange protein demonstrating a direct relation between the occurrence of non-bilayer lipid structures and an increased transbilayer movement of phosphatidylcholine.     

Physicochemical characterization of 1,2-diphytanoyl-sn-glycero-3-phosphocholine in model membrane systems

We report here on a series of studies aimed at characterization of the structural and dynamical properties of the synthetic lipid diphytanoyl phosphatidylcholine, in multilamellar dispersions and vesicle suspensions.This lipid exhibits no detectable gel to liquid crystalline phase transition over a large temperature range (?120°C to +120°C).Examination of proton nuclear magnetic resonance (NMR) free induction decays obtained from multilayer dispersions of diphytanoyl phosphatidylcholine provided an estimate of the methylene proton order parameter. The estimated magnitude of 0.21 is comparable to those determined for other phospholipids.Sonication of aqueous dispersions of diphytanoyl phosphatidylcholine led to formation of bilayer vesicles as determined by the measurement of the outer/inner choline methyl proton resonances, vesicle sizes in electron micrographs, and comparison of proton NMR linewidths between multilayer and sonicated dispersions. Ultracentrifugation studies of diphytanoyl phosphatidylcholine vesicles in H²O and ²H₂O media yielded a value of 1.013 ± 0.026 ml/g for the partial specific volume of this lipid.We have measured spin lattice relaxation rates for the methyl and methylenemethyne protons of the hydrocarbon chains of diphytanoyl phosphatidylcholine in bilayer vesicles over a range of temperatures and at two NMR frequencies (100 and 220 MHz). The observed relaxation rates for the methylene protons in this system were approximately twice those previously reported for dipalmitoyl phosphatidylcholine at comparable temperatures and resonance frequencies, whereas the relaxation rates measured for the methyl protons were greater than those of the straight chain lipid by an order of magnitude.Measurement of the spin lattice relaxation rates of the hydrocarbon protons of the diphytanoyl phosphatidylcholine in a 10 mol% mixture of the branched-chain lipid in a deuterated host lipid, diperdeuteropalmitoyl phosphatidylcholine, showed a discontinuity in the temperature dependence of the proton NMR longitudinal relaxation rates of the branched-chain lipid in the region of the gel to liquid crystalline phase transition temperature of the deuterated dipalmitoyl phosphatidylcholine host lipid. This result may be taken as evidence of lateral phase separation of a liquid cyrstalline phase enriched in diphytanoyl phosphatidylcholine from a gel phase enriched in diperdeuteropalmitoyl phosphatidylcholine at temperatures below the phase transition temperature of deuterated host lipid. This conclusion is supported by the observation of an abrupt change in the hydrocarbon methylene linewidth (at 100 MHz) of 10 mol% diphytanoyl phosphatidylcholine in diperdeuteropalmitoyl phosphatidylcholine over the temperature range where lateral phase separation is taking place according to differential thermograms.     

Intramembrane Polarity by Electron Spin Echo Spectroscopy of Labeled Lipids

The association of water (D₂O) with phospholipid membranes was studied by using pulsed-electron spin resonance techniques. We measured the deuterium electron spin echo modulation of spin-labeled phospholipids by D₂O in membranes of dipalmitoyl phosphatidylcholine with and without 50 mol% of cholesterol. The Fourier transform of the relaxation-corrected two-pulse echo decay curve reveals peaks, at one and two times the deuterium NMR frequency, that arise from the dipolar hyperfine interaction of the deuterium nucleus with the unpaired electron spin of the nitroxide-labeled lipid. For phosphatidylcholine spin-labeled at different positions down the sn-2 chain, the amplitude of the deuterium signal decreases toward the center of the membrane, and is reduced to zero from the C-12 atom position onward. At chain positions C-5 and C-7 closer to the phospholipid headgroups, the amplitude of the deuterium signal is greater in the presence of cholesterol than in its absence. These results are in good agreement with more indirect measurements of the transmembrane polarity profile that are based on the ¹⁴N-hyperfine splittings in the conventional continuous-wave electron spin resonance spectrum.     

The interaction between water and the polar head in inverted phosphatidylcholine micelles A 2H and 31P relaxation study

²H and ³¹P spin-lattice relaxation times (T₁) were studied for invented egg phosphatidylcholine micelles in CCl₄ as functions of ²H₂O concentration. When the ²H₂O/phosphatidylcholine mole ratio changed from 1.0 to 18.0, T₁ of ³¹P increased by about 2.6 fold, whereas T₁ of ²H increased by about 50 fold. A quantitative analysis of the deuterium T₁ data showed that there is only one water molecule tightly bound to the polar head, and it is in rapid exchange with the rest of the water molecules. The activation energy for the deuterium T₁ was 7.1 ± 0.8 kcal/mol (30 ± 3 kJ/mol), and was independent of the ²H₂O concentration.     

Ketone-body utilization and lipid synthesis by developing rat brain—a comparison between in vivo and in vitro experiments

The distribution of ketone bodies between oxidation and lipid synthesis was analysed in homogenates of developing rat brain. The capacity for lipid synthesis of homogenized or minced brain preparations was compared with rates of lipid synthesis in vivo, assessed by incorporation of ³H from ³H₂O into fatty acids and cholesterol. Brain homogenates of suckling rats (but not those of adults) incorporated label from [3-¹⁴C]ketone bodies into lipids, but this process was slow as compared to ¹⁴CO₂ production (< 5%) and much slower than the total rate of ketone-body utilization (< 0.5%). Study of ³H₂O incorporation demonstrated that the rates of lipogenesis and cholesterogenesis are at least one order of magnitude higher in vivo than in vitro. Maximal rates of ³H incorporation into fatty acids (3 μmol/g brain . h) and into cholesterol (0.6 μmol/g brain . h) were found during the third postnatal week. Adult rats still incorporated ³H into brain fatty acids at an appreciable rate (1 μmol/g brain . h), whereas cholesterogenesis was very low. It is concluded that in vitro measurements of lipid synthesis severely underestimate the rates that occur in developing rat brain in vivo. The high rate of ³H incorporation into lipids by developing and adult rat brain as compared to the amounts of these lipids present in the brain suggests an important contribution of endogenous lipid synthesis during brain development and an appreciable rate of fatty acid turnover during brain growth, but also in the adult brain.     

Interaction of cholesterol,phospholipid and apoprotein in high density lipoprotein recombinants

To examine the effect of incorporation of cholesterol into high density lipoprotein (HDL) recombinants, multilamellar liposomes of ³H cholesterol/¹⁴C dimyristoyl phosphatidylcholine were incubated with the total apoprotein (apoHDL) and principal apoproteins (apoA-1 and apoA-2) of human plasma high density lipoprotein. Soluble recombinants were separated from unreacted liposomes by centrifugation and examined by differential scanning calorimetry and negative stain electron microscopy. At 27°C, liposomes containing up to approx. 0.1 mol cholesterol/mol dimyristoyl phosphatidylcholine (DMPC) were readily solubilized by apoHDL, apoA-1 or apoA-2. However, the incorporation of DMPC and apoprotein into lipoprotein complexes was markedly reduced when liposomes containing a higher proportion of cholesterol were used. For recombinants prepared from apoHDL, apoA-1 or apoA-2, the equilibrium cholesterol content of complexes was approx. 45% that of the unreacted liposomes. Electron microscopy showed that for all cholesterol concentrations, HDL recombinants were predominantly lipid bilayer discs, approx. $\text{160 × 55}\text{A}\text{?}$. Differential scanning calorimetry of cholesterol containing recombinants of DMPC/cholesterol/apoHDL or DMPC/cholesterol/apoA-1 showed, with increasing cholesterol content, a linear decrease in the enthalpy of the DMPC gel to liquid crystalline transition, extrapolating to zero enthalpy at 0.15 cholesterol/DMPC. The enthalpy values were markedly reduced compared to control liposomes, where the phospholipid transition extrapolated to zero enthalpy at approx. 0.45 cholesterol/DMPC. The calorimetric and solubility studies suggest that in high density lipoprotein recombinants cholesterol is excluded from 55% of DMPC molecules bound in a non-melting state by apoprotein.     

Biochemical aspects of the visual process XXXII. Movement of sodium ions through bilayers composed of retinal and rod outer segment lipids

The leakage of Na⁺ from sonicated liposomes, composed of rod outer segment lipids, retinal lipids and a 4 : 1 phosphatidylcholine/phosphatidylserine mixture, has been studied. Both retinal and rod outer segment lipid liposomes lose Na⁺ faster than Ca²⁺ which indicates that the observed leakage occurs from closed liposomal structures.Liposomes from rod outer segment lipids are extremely leaky, losing sodium about 10 times as fast as retinal lipid liposomes and twice as fast as the phosphatidylcholine/phosphatidylserine liposomes.This high permeability of rod outer segment lipid liposomes, as compared to retinal lipid liposomes, is probably due to both the higher degree of unsaturation of the fatty acid chains and their lower cholesterol content. In the rod outer segment lipid extract 48% of the fatty acid chains consists of docosahexaenoic acid (C_22:6) against only 24% in retinal lipid extract. Rod outer segment lipids contain 4.0% cholesterol against 12.3% in retinal lipids.The sodium leakage from rod outer segment lipid liposomes is little affected by the presence of 5 mM calcium in the external dialysis medium, but with the two other types of liposomes significant decreases in permeability of about 20% are observed.The results are discussed in connection with the role of cations in visual excitation.     

The lateral diffusion coefficient of lecithin molecules in single bilayer vesicles studied by 14N NMR

The ¹⁴N nuclear relaxation times T₁ and T₂ in egg yolk phosphatidylcholine have been observed in single bilayer vesicles dispersed in the media of different viscosities, ¹H₂O and ²H₂O. The lateral diffusion coefficient of lipid molecule D has been calculated according to the method reported earlier: D = 2.2 × 10^?8cm²s^?1 in ¹H₂O and 2.1 × 10^?8cm²s^?1 in ²H₂O at 20°C. They are in excellent agreement. This result gives a strong basis of usefulness of ¹⁴N NMR method in the evaluation of D without introducing any system perturbation.     

Kinetic study of aroxyl radical-scavenging action of vitamin E in membranes of egg yolk phosphatidylcholine vesicles

Vitamin E is localized in membranes and functions as an efficient inhibitor of lipid peroxidation in biological systems. In this study, we measured the reaction rates of vitamin E (α-, β-, γ-, δ-tocopherols, TocH) and tocol with aroxyl radical (ArO) as model lipid peroxyl radicals in membranes by stopped-flow spectrophotometry. Egg yolk phosphatidylcholine (EYPC) vesicles were used as a membrane model. EYPC vesicles were prepared in the aqueous methanol solution (MeOH:H₂O = 7:3, v/v) that gave the lowest turbidity in samples. The second-order rate constants (k_s) for α-TocH in MeOH/H₂O solution with EYPC vesicles were apparently 3.45 × 10⁵ M⁻¹ s⁻¹, which was about 8 times higher than that (4.50 × 10⁴ M⁻¹ s⁻¹) in MeOH/H₂O solution without EYPC vesicles. The corrected k_s of α-TocH in vesicles, which was calculated assuming that the concentration of α-TocH was 133 times higher in membranes of 10 mM EYPC vesicles than in the bulk MeOH/H₂O solution, was 2.60 × 10³ M⁻¹ s⁻¹, which was one-seventeenth that in MeOH/H₂O solution because of the lower mobility of α-TocH in membranes. Similar analyses were performed for other vitamin E analogues. The k_s of vitamin E in membranes increased in the order of tocol < δ-TocH < γ-TocH ∼ β-TocH < α-TocH. There was not much difference in the ratios of reaction rates in vesicles and MeOH/H₂O solution among vitamin E analogues [k_s(vesicle)/k_s (MeOH/H₂O) = 7.7, 10.0, 9.5, 7.4, and 5.1 for α-, β-, γ-, δ-TocH, and tocol, respectively], but their reported ratios in solutions of micelles and ethanol were quite different [k_s(micelle)/k_s(EtOH) = 100, 47, 41, 15, and 6.3 for α-, β-, γ-, δ-TocH, and tocol, respectively]. These results indicate that the reaction sites of vitamin E analogues were similar in vesicle membranes but depended on hydrophobicity in micelle membranes, which increased in the order of tocol < δ-TocH < γ-TocH ∼ β-TocH < α-TocH.     

Catalase purification from rat liver with iron-chelated poly(hydroxyethyl methacrylate-N-methacryloyl-(l)-glutamic acid) cryogel discs

In this study, iron-chelated poly(hydroxyethyl methacrylate-N-methacryloyl-(l)-glutamic acid) (PHEMAGA/Fe³⁺) cryogel discs were prepared. The PHEMAGA/Fe³⁺ cryogel discs were characterized by elemental analysis, scanning electron microscopy, Fourier transform infrared spectroscopy, swelling tests, and surface area measurements. The PHEMAGA/Fe³⁺ cryogel discs had large pores ranging from 10 to 100?µm with a swelling degree of 9.36?g H₂O/g cryogel. Effects of pH, temperature, initial catalase concentration, and flow rate on adsorption capacity of the PHEMAGA/Fe³⁺ cryogel discs were investigated. Maximum catalase adsorption capacity (62.6?mg/g) was obtained at pH 7.0, 25°C, and 3?mg/ml initial catalase concentration. The PHEMAGA/Fe³⁺ cryogel discs were also tested for the purification of catalase from rat liver. After tissue homogenization, purification of catalase was performed using the PHEMAGA/Fe³⁺ cryogel discs and catalase was obtained with a yield of 54.34 and 16.67 purification fold.     

Resistance of Ca2+-ATPase to dilution by excess phospholipid in reconstituted vesicles

Ca²⁺-ATPase and other membrane proteins of the sarcoplasmic reticulum membrane from rabbit skeletal muscle have been reconstituted into lipid vesicles with increasing amounts of phosphatidylcholine. The protein composition and phospholipid concentration of these vesicles were analyzed by determining the density of the reconstituted membrane vesicles on linear H₂O-²H₂O gradients, in a constant concentration of sucrose. In all combinations of the Ca²⁺-ATPase with a weight excess of phosphatidylcholine, the reconstituted vesicles had a phospholipid-to-protein ratio similar to that of the native sarcoplasmic reticulum membrane, even though both solubilization and mixing had occurred. These vesicles of low phospholipid and high protein content exhibited all the original Ca²⁺-ATPase activity and ATP-stimulated calcium transport. The Ca²⁺-ATPase, and the calcium-binding proteins to a lesser extent, may order the lipid in such a manner so as to maintain the initial stoichiometry of lipid to protein observed in the native sarcoplasmic reticulum membrane.     

Effect of BASF 13-338, a Substituted Pyridazinone, on Lipid Metabolism in Leaf Tissue of Spinach, Pea, Linseed, and Wheat

A substituted pyridazinone (BASF 13-338) inhibited photosynthesis in spinach (Spinacia oleracea, Hybrid 102 Arthur Yates Ltd.) leaf discs and reduced the incorporation of [1-¹⁴C]acetate into trienoic acids of diacylgalactosylglycerol while causing radioactivity to accumulate in diacylgalac-tosylglycerol dienoic acids. Although BASF 13-338 inhibited photosynthesis in isolated spinach chloroplasts, it did not prevent dienoate desaturation. In discs, the labeling of fatty acids was affected by the inhibitor only in diacylgalactosylglycerol. Very little radioactivity was incorporated into trienes of phosphatidylcholine and the proportion of the label recovered in the fatty acids of phosphatidylcholine was not changed by BASF 13-338. The herbicides caused an increase in the proportion of the lipid ¹⁴C incorporated into diacylgalactosylglycerol and a decrease in labeling of phosphatidylcholine, whereas the proportion of ¹⁴C recovered in other lipids remained unchanged. Similar results were obtained with pea (Pisum sativum cv. Victory Freeze), linseed (Linum usitatissimum cv. Punjab), and wheat (Triticum aestivum cv. Karamu). With these species, a greater proportion of the label was incorporated into phosphatidylcholine and less into diacylgalactosylglycerol than with spinach. The data indicate that trienoate synthesis uses diacylgalactosylglycerol as substrate. BASF 13-338 appears to act at that step, and seems to cause in spinach a shift in polyenoate synthesis from the pathway involving microsomal phosphatidylcholine to the pathway operating inside the chloroplast.     

A high-resolution NMR study (1H, 13C, 31P) of the interaction of paramagnetic ions with phospholipids in aqueous dispersions

¹H-, ¹³C-and ³¹P-NMR spectra of egg-yolk phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidic acid (PA) and cosonicated mixtures of these phospholipids were obtained from ultrasonicatcd dispersions containing Pr³⁺, Eu³⁺, Gd³⁺ and Mn²⁺ ions.The differences in chemical shift values. °_n, between the “inner” and “outer” resonance signals for the different nuclei of the polar head group of egg-yolk phosphatidyl choline provide information about the average distances of the paramagnetic ion within the polar groups of the phospholipid molecules. In the Pr(²H₂O)³⁺_n/egg-yolk phosphatidylcholine system the ions are nearest to the phosphate and -CH₂CH₂ group, respectively but relatively far from the N(CH₃)₃ group of the polar head group of the lipid.The integral analysis of the¹ H-NMR spectra obtained from dispersions containing Pr³⁺ and Mn²⁺ ions enables us to calculate the number of the polar groups in both sides of the egg-yolk phosphatidylcholine bilayer, the size of the lipid vesicle and to give some features of the arrangement of the phospholipid molecules in cosonicated egg-yolk phosphatidylcliotine/ phosphatidytserine vesicles. At p²H 8.3 in PC/PS mixtures an extreme asymmetry is observed with PS preferentially in the outer side of the membrane. This side contains approximately three times more PS than PC molecules.Some comments are made concerning the quantitative integral analysis of proton-noise decoupled ³¹ P-NMR spectra as obtained from similar phospholipid mixtures by Michaelson et al. and Berden et at.     

The low-resolution structure of nHDL reconstituted with DMPC with and without cholesterol reveals a mechanism for particle expansion

Small-angle neutron scattering (SANS) with contrast variation was used to obtain the low-resolution structure of nascent HDL (nHDL) reconstituted with dimyristoyl phosphatidylcholine (DMPC) in the absence and presence of cholesterol, [apoA1:DMPC (1:80, mol:mol) and apoA1:DMPC:cholesterol (1:86:9, mol:mol:mol)]. The overall shape of both particles is discoidal with the low-resolution structure of apoA1 visualized as an open, contorted, and out of plane conformation with three arms in nascent HDL/dimyristoyl phosphatidylcholine without cholesterol (nHDL_DMPC) and two arms in nascent HDL/dimyristoyl phosphatidylcholine with cholesterol (nHDL_DMPC+Chol). The low-resolution shape of the lipid phase in both nHDL_DMPC and nHDL_DMPC+Chol were oblate ellipsoids, and fit well within their respective protein shapes. Modeling studies indicate that apoA1 is folded onto itself in nHDL_DMPC, making a large hairpin, which was also confirmed independently by both cross-linking mass spectrometry and hydrogen-deuterium exchange (HDX) mass spectrometry analyses. In nHDL_DMPC+Chol, the lipid was expanded and no hairpin was visible. Importantly, despite the overall discoidal shape of the whole particle in both nHDL_DMPC and nHDL_DMPC+Chol, an open conformation (i.e., not a closed belt) of apoA1 is observed. Collectively, these data show that full length apoA1 retains an open architecture that is dictated by its lipid cargo. The lipid is likely predominantly organized as a bilayer with a micelle domain between the open apoA1 arms. The apoA1 configuration observed suggests a mechanism for accommodating changing lipid cargo by quantized expansion of hairpin structures.     

Lateral segregation of membrane lipids and formation of stable rod-shaped membrane projections in erythrocytes treated with long-chain alcohols

Human erythrocytes, incubated with sonicated dispersions of phosphatidylcholine, cholesterol and saturated straight-chain alcohols (C₁₆-C₁₈) develop stiff, rod-shaped, hemoglobin-containing membrane projections within 120 min. The number of these ‘rods’ varies (1–3 per cell), they reach a length of up to 14 μm (twice the cell diameter) and a thickness of 0.3–1.0 μm. ‘Rods’ may be separated from ‘residual cells’ by shear flow and centrifugation without severe hemolysis. Lipid analyses carried out on residual cells and rods indicate lateral segregation of the phospholipids of the outer leaf of the membrane lipid bilayer (phosphatidylcholine and sphingomyelin) and of the alcohol applied. Phosphatidylcholine accumulates in the residual cells, sphingomyelin and the alcohol in the rods. No differences in membrane protein patterns were observed between rods and residual cells. The rod-shape is dependent on the presence of the alcohol, extraction of the alcohol converts rods into hemoglobin-containing spheres without lysis. The formation of rods, which is indicative of a lateral phase separation, is discussed in terms of lipid-lipid interactions and with respect to parameters determining the shape of cells.     

Beneficial effects of combined treatment with niacin and chromium on the liver of hyperlipemic rats

Many studies have shown that niacin and Cr exert combined effects. Significant beneficial effects in serum lipid levels following Cr supplementation have been reported. Niacin decreases total plasma levels of cholesterol, triglycerides, and low-density lipoprotein cholesterol and increases high-density lipoprotein cholesterol. In this study, 12-mo-old female Swiss albino rats were used. They were randomly divided into four groups. The animals of group I (control) were fed with pellet chow. Group II was fed with pellet chow and treated with 250 μg/kg CrCl₃·6H₂O and 100 mg/kg niacin for 45 d, by the gavage technique. The rats of group III were fed with lipogenic diet consisting of 2% cholesterol 0.5% cholic acid, and 20% sunflower oil added to the pellet chow and given 3% alcoholic water for 60 d. Group IV was fed with the same lipogenic diet, and 15 d after, the experimental animals were made hyperlipemic; they were treated with 250 μg/kg CrCl₃·6H₂O and 100 mg/kg niacin by gavage technique for 45d. On d 60, liver and blood samples were taken from the animals. The sections were examined under light and electron microscopes. Serum total lipid and cholesterol levels were determined by spectrophotometric methods. The aim of the present study was *** DIRECT SUPPORT *** A02Q2015 00004     

Analysis of the hexagonal II phase and its relations to lipidic particles and the lamellar phase A freeze-fracture study

Model systems of phosphatidylethanolamine (PE) and cardiolipin (DPG), as pure components and in binary mixtures with phosphatidylcholine (PC) have been morphologically analysed. The relation between the hexagonal_II (H_II) phase and lipidic particles as well as between the H_II phase and the lamellar phase has been studied. Moreover, the periodicity of the various H_II tubes was determined. (1) The periodicity of the H_II phase of cardiolipin is dependent on the cation involved. DPG-Ca exhibits the smallest tube to tube distance when compared to Mg²⁺ and Mn²⁺. Moreover, the DPG-Ca tubes are quite straight, in contrast to the Mg²⁺ and Mn²⁺ tubes, which appear to be frequently curved. (2) H_II tubes with two distinct diameters have been observed in H_II phase containing lipid mixtures. The thickness of the H_II tube is related to the composition of the tube. In the cardiolipin-lecithin system, structural separation of the pure cardiolipin H_II phase has been suggested with Mg²⁺ and Mn²⁺, but not with Ca²⁺. (3) Models for the H_II to lamellar phase transition and for the H_II phase to the lipidic particles are presented. (4) Lipidic particles are exclusively found in lipid model systems, which contain H_II phase favouring lipids. Morphological evidence is presented which suggests these lipidic particles represent inverted micelles. These observations include: (${}_{\mathrm{<mtext>i</mtext>}}$) there is a strong topological and quantitative relation between H_II tubes and lipidic particles, (${}_{\mathrm{<mtext>ii</mtext>}}$) lipidic particles occur densely packed in conglomerates without the presence of a smooth layer.     

Permeability of Boric Acid Across Lipid Bilayers and Factors Affecting It

Boron enters plant roots as undissociated boric acid (H₃BO₃). Significant differences in B uptake are frequently observed even when plants are grown under identical conditions. It has been theorized that these differences reflect species differences in permeability coefficient of H₃BO₃ across plasma membrane. The permeability coefficient of boric acid however, has not been experimentally determined across any artificial or plant membrane. In the experiments described here the permeability coefficient of boric acid in liposomes made of phosphatidylcholine was 4.9 × 10⁻⁶ cm sec⁻¹, which is in good agreement with the theoretical value. The permeability coefficient varied from 7 × 10⁻⁶ to 9.5 × 10⁻⁹ cm sec⁻¹ with changes in sterols (cholesterol), the type of phospholipid head group, the length of the fatty acyl chain, and the pH of the medium. In this study we also used Arabidopsis thaliana mutants which differ in lipid composition to study the effect of lipid composition on B uptake. The chs1-1 mutant which has lower proportion of sterols shows 30% higher B uptake compared with the wild type, while the act1-1 mutant which has an increased percentage of longer fatty acids, exhibited 35% lower uptake than the wild type. Lipid composition changes in each of the remaining mutants influenced B uptake to various extents. These data suggest that lipid composition of the plasma membrane can affect total B uptake. Received: 15 October 1999/Revised: 11 February 2000     

Contribution of acetoacetate to the synthesis of cholesterol and fatty acids in regions of developing rat brain in vivo

³H₂O and [3-¹⁴C]acetoacetate were injected i.p. into developing rats (5–50 days of age). After 2 h the brains were dissected into 6 parts. The incorporation of ³H and ¹⁴C into total fatty acids and into cholesterol in these 6 parts and in the spinal cord was measured. The data were analysed to evaluate the developmental patterns of the synthesis of fatty acids and cholesterol in various parts of the rat CNS and to compare the contribution of acetoacetate to these processes. Our results indicate (1) a large variation between CNS regions in the rates of lipid synthesis as well as in the developmental patterns; highest activities were found in the spinal cord during the third postnatal week, whereas the activities in cortical areas were much lower during all stages of development; (2) a constant ratio between the amounts of label incorporated into lipid fractions from [3-¹⁴C]acetoacetate and from ³H₂O, indicating that acetoacetate contributes to a similar extent to lipid synthesis in all parts of the developing rat CNS; (3) a similar preference in the use of acetoacetate for cholesterogenesis as compared to lipogenesis in all parts of the CNS of suckling rats; (4) a marked increase of this preference after weaning of the pups.