In vitro effect of AlCl3 on human erythrocytes: Changes in membrane morphology and functionality

Aluminum belongs to a group of potential toxic elements capable of penetrating the human body. In this paper, the effect of aluminum concentrations on red blood cell membranes using different fluorescent probes able to localize in various parts of the phospholipid bilayer (TMA-DPH, laurdan and pyrene) were studied. Our results confirm that human erythrocytes exposed to aluminum undergo physico-chemical modifications at the membrane level. A decrease in fluorescence anisotropy of TMA-DPH and in the polarity of the lipid bilayer with a concomitant shift toward a gel phase was observed, and the pyrene excimerization coefficient (k_ex) increased.Furthermore, the presence of aluminum induced lipid peroxidation and reduced the activity of erythrocyte antioxidant enzymes (SOD, CAT and GSHPx). Al-induced morphological changes on the erythrocyte membrane surface were monitored using atomic force microscopy. These results provide further information on the target of action of different aluminum amounts.     

Interaction of tetrasubstituted cationic aluminum phthalocyanine with artificial and natural membranes

A study of the properties of water-soluble tetrasubstituted cationic aluminum phthalocyanine (AlPcN₄) revealed efficient binding of this photosensitizer to phospholipid membranes as compared with tetrasulfonated aluminum and zinc phthalocyanine complexes. This also manifested itself in enhanced photodynamic activity of AlPcN₄ as measured by the photosensitized damage of gramicidin channels in a planar bilayer lipid membrane. The largest difference in the photodynamic activity of cationic and anionic phthalocyanines was observed in a membrane containing negatively charged lipids, thereby pointing to significant contribution of electrostatic interactions to the binding of photosensitizers to a membrane. Fluoride anions suppressed the photodynamic activity and binding to membrane of both tetraanionic and tetracationic aluminum phthalocyanines, which supports our hypothesis that interaction of charged metallophthalocyanines with phospholipid membranes is mostly determined by coordination of the central metal atom with the phosphate group of lipid.     

Lipid peroxidation is an early symptom triggered by aluminum, but not the primary cause of elongation inhibition in pea roots

Pea (Pisum sativum) roots were treated with aluminum in a calcium solution, and lipid peroxidation was investigated histochemically and biochemically, as well as other events caused by aluminum exposure. Histochemical stainings were observed to distribute similarly on the entire surface of the root apex for three events (aluminum accumulation, lipid peroxidation, and callose production), but the loss of plasma membrane integrity (detected by Evans blue uptake) was localized exclusively at the periphery of the cracks on the surface of root apex. The enhancement of four events (aluminum accumulation, lipid peroxidation, callose production, and root elongation inhibition) displayed similar aluminum dose dependencies and occurred by 4 h. The loss of membrane integrity, however, was enhanced at lower aluminum concentrations and after longer aluminum exposure (8 h). The addition of butylated hydroxyanisole (a lipophilic antioxidant) during aluminum treatment completely prevented lipid peroxidation and callose production by 40%, but did not prevent or slow the other events. Thus lipid peroxidation is a relatively early symptom induced by the accumulation of aluminum and appears to cause, in part, callose production, but not the root elongation inhibition; by comparison, the loss of plasma membrane integrity is a relatively late symptom caused by cracks in the root due to the inhibition of root elongation.     

Effect of oxidative stress on plasma membrane fluidity of THP-1 induced macrophages

Plasma membrane is one of the preferential targets of reactive oxygen species which cause lipid peroxidation. This process modifies membrane properties such as membrane fluidity, a very important physical feature known to modulate membrane protein localization and function. The aim of this study is to evaluate the effect of oxidative stress on plasma membrane fluidity regionalization of single living THP-1 macrophages. These cells were oxidized with H₂O₂ at different concentrations, and plasma membrane fluidity was analyzed by two-photon microscopy in combination with the environment-sensitive probe Laurdan. Results show a significant H₂O₂ concentration dependent increase in the frequency of rigid lipid regions, mainly attributable to lipid rafts, at the expense of the intermediate fluidity regions. A novel statistical analysis evaluated changes in size and number of lipid raft domains under oxidative stress conditions, as lipid rafts are platforms aiding cell signaling and are thought to have relevant roles in macrophage functions. It is shown that H₂O₂ causes an increase in the number, but not the size, of raft domains. As macrophages are highly resistant to H₂O₂, these new raft domains might be involved in cell survival pathways.     

Role of electrostatics in the binding of charged metallophthalocyanines to neutral and charged phospholipid membranes

Binding of the cationic tetra(tributylammoniomethyl)-substituted hydroxoaluminum phthalocyanine (AlPcN₄) to bilayer lipid membranes was studied by fluorescence correlation spectroscopy (FCS) and intramembrane field compensation (IFC) methods. With neutral phosphatidylcholine membranes, AlPcN₄ appeared to bind more effectively than the negatively charged tetrasulfonated aluminum phthalocyanine (AlPcS₄), which was attributed to the enhancement of the coordination interaction of aluminum with the phosphate moiety of phosphatidylcholine by the electric field created by positively charged groups of AlPcN₄. The inhibitory effect of fluoride ions on the membrane binding of both AlPcN₄ and AlPcS₄ supported the essential role of aluminum-phosphate coordination in the interaction of these phthalocyanines with phospholipids. The presence of negative or positive charges on the surface of lipid membranes modulated the binding of AlPcN₄ and AlPcS₄ in accord with the character (attraction or repulsion) of the electrostatic interaction, thus showing the significant contribution of the latter to the phthalocyanine adsorption on lipid bilayers. The data on the photodynamic activity of AlPcN₄ and AlPcS₄ as measured by sensitized photoinactivation of gramicidin channels in bilayer lipid membranes correlated well with the binding data obtained by FCS and IFC techniques. The reduced photodynamic activity of AlPcN₄ with neutral membranes violating this correlation was attributed to the concentration quenching of singlet excited states as proved by the data on the AlPcN₄ fluorescence quenching.     

The effect of H<Subscript>3</Subscript>O<Superscript>+</Superscript> on the membrane morphology and hydrogen bonding of a phospholipid bilayer

At the 2017 meeting of the Australian Society for Biophysics, we presented the combined results from two recent studies showing how hydronium ions (H₃O⁺) modulate the structure and ion permeability of phospholipid bilayers. In the first study, the impact of H₃O⁺ on lipid packing had been identified using tethered bilayer lipid membranes in conjunction with electrical impedance spectroscopy and neutron reflectometry. The increased presence of H₃O⁺ (i.e. lower pH) led to a significant reduction in membrane conductivity and increased membrane thickness. A first-order explanation for the effect was assigned to alterations in the steric packing of the membrane lipids. Changes in packing were described by a critical packing parameter (CPP) related to the interfacial area and volume and shape of the membrane lipids. We proposed that increasing the concentraton of H₃O⁺ resulted in stronger hydrogen bonding between the phosphate oxygens at the water–lipid interface leading to a reduced area per lipid and slightly increased membrane thickness. At the meeting, a molecular model for these pH effects based on the result of our second study was presented. Multiple μs-long, unrestrained molecular dynamic (MD) simulations of a phosphatidylcholine lipid bilayer were carried out and showed a concentration dependent reduction in the area per lipid and an increase in bilayer thickness, in agreement with experimental data. Further, H₃O⁺ preferentially accumulated at the water–lipid interface, suggesting the localised pH at the membrane surface is much lower than the bulk bathing solution. Another significant finding was that the hydrogen bonds formed by H₃O⁺ ions with lipid headgroup oxygens are, on average, shorter in length and longer-lived than the ones formed in bulk water. In addition, the H₃O⁺ ions resided for longer periods in association with the carbonyl oxygens than with either phosphate oxygen in lipids. In summary, the MD simulations support a model where the hydrogen bonding capacity of H₃O⁺ for carbonyl and phosphate oxygens is the origin of the pH-induced changes in lipid packing in phospholipid membranes. These molecular-level studies are an important step towards a better understanding of the effect of pH on biological membranes.     

Photodynamic activity and binding of sulfonated metallophthalocyanines to phospholipid membranes: Contribution of metal-phosphate coordination

Photosensitized efficacy of tetrasulfonated phthalocyanines of zinc, aluminum and nickel (ZnPcS₄, AlPcS₄ and NiPcS₄, respectively) as studied by gramicidin channel (gA) photoinactivation was compared with adsorption of the dyes on the surface of a bilayer lipid membrane as measured by the inner field compensation method. The adsorption of the negatively charged phthalocyanines on diphytanoylphosphatidylcholine (DPhPC) membranes led to formation of a negative boundary potential difference between the membrane/water interfaces. Good correlation was shown between the photodynamic activity and the membrane binding of the three metallophthalocyanines. ZnPcS₄ appeared to be the most potent of these photosensitizers, while NiPcS₄ was completely ineffective. All of these phthalocyanines displayed no binding and negligible gA photoinactivation with membranes formed of glycerol monooleate (GMO), whereas Rose Bengal exhibited significant binding and photodynamic efficacy with GMO membranes. Gramicidin photoinactivation in the presence of AlPcS₄, being insensitive to the ionic strength of the bathing solution, was inhibited by fluoride and attenuated by phosphate ions. A blue shift of the fluorescence peak position of ZnPcS₄ dissolved in ethanol was elicited by phosphate, similarly to fluoride, which was indicative of the coordination interaction of these ions with the central metal atom of the phthalocyanine macrocycle. This interaction was enhanced in the medium modeling the water-membrane interface. The results obtained imply that binding of tetrasulfonated metallophthalocyanines to phospholipid membranes is determined primarily by metal-phosphate coordination.     

DMSO Induces Dehydration near Lipid Membrane Surfaces

Dimethyl sulfoxide (DMSO) has been broadly used in biology as a cosolvent, a cryoprotectant, and an enhancer of membrane permeability, leading to the general assumption that DMSO-induced structural changes in cell membranes and their hydration water play important functional roles. Although the effects of DMSO on the membrane structure and the headgroup dehydration have been extensively studied, the mechanism by which DMSO invokes its effect on lipid membranes and the direct role of water in this process are unresolved. By directly probing the translational water diffusivity near unconfined lipid vesicle surfaces, the lipid headgroup mobility, and the repeat distances in multilamellar vesicles, we found that DMSO exclusively weakens the surface water network near the lipid membrane at a bulk DMSO mole fraction (X_DMSO) of <0.1, regardless of the lipid composition and the lipid phase. Specifically, DMSO was found to effectively destabilize the hydration water structure at the lipid membrane surface at X_DMSO <0.1, lower the energetic barrier to dehydrate this surface water, whose displacement otherwise requires a higher activation energy, consequently yielding compressed interbilayer distances in multilamellar vesicles at equilibrium with unaltered bilayer thicknesses. At X_DMSO >0.1, DMSO enters the lipid interface and restricts the lipid headgroup motion. We postulate that DMSO acts as an efficient cryoprotectant even at low concentrations by exclusively disrupting the water network near the lipid membrane surface, weakening the cohesion between water and adhesion of water to the lipid headgroups, and so mitigating the stress induced by the volume change of water during freeze-thaw.     

Mitigation of Acute Aluminum Toxicity by Sodium Selenite and N-Acetylcysteine in Adult Male Rats

The objective of this study is to investigate the toxic effects of aluminum and the potential alleviation of selenite and N-acetylcysteine (NAC) on this toxicity. Acute aluminum toxicity was induced by intraperitoneal (i.p.) injection of AlCl₃ (30 mg Al³⁺/kg) for four consecutive days. Al³⁺ damaged the synthetic capability and regeneration power of liver cells and induced inflammation. It also damaged the kidney and disturbed the lipid profile enhancing the total cholesterol level and LDL-cholesterol level increasing the risks of atherosclerosis. Al³⁺ reduced the cellular antioxidant milieu typified by the decrease in reduced glutathione, vitamin E, and four antioxidant enzymes and induced lipid peroxidation (LPO). Selenite at 1 mg Se/kg and NAC at 150 mg/kg injected either simultaneously with or after Al³⁺ mitigated most of these damaging effects probably by the virtue of scavenging the free radicals, binding aluminum and stimulating its excretion and reducing its bioavailability, bolstering the endogenous antioxidant defense systems, stabilizing the cell membrane, and preventing LPO. The beneficial effects of selenite and NAC against aluminum toxicity were also confirmed by the light and electron histopathology study. There were no significant differences between the two regimens used (protection and therapeutic) in the current study probably due to the short time of exposure, and the abrogation of Al³⁺ toxicity offered by selenite was better than that provided by NAC on the histopathology level.     

S100A10‐Mediated Translocation of Annexin‐A2 to SNARE Proteins in Adrenergic Chromaffin Cells Undergoing Exocytosis

In neuroendocrine cells, annexin‐A2 is implicated as a promoter of monosialotetrahexosylganglioside (GM1)‐containing lipid microdomains that are required for calcium‐regulated exocytosis. As soluble N‐ethylmaleimide‐sensitive factor attachment protein receptors (SNAREs) require a specific lipid environment to mediate granule docking and fusion, we investigated whether annexin‐A2‐induced lipid microdomains might be linked to the SNAREs present at the plasma membrane. Stimulation of adrenergic chromaffin cells induces the translocation of cytosolic annexin‐A2 to the plasma membrane, where it colocalizes with SNAP‐25 and S100A10. Cross‐linking experiments performed in stimulated chromaffin cells indicate that annexin‐A2 directly interacts with S100A10 to form a tetramer at the plasma membrane. Here, we demonstrate that S100A10 can interact with vesicle‐associated membrane protein 2 (VAMP2) and show that VAMP2 is present at the plasma membrane in resting adrenergic chromaffin cells. Tetanus toxin that cleaves VAMP2 solubilizes S100A10 from the plasma membrane and inhibits the translocation of annexin‐A2 to the plasma membrane. Immunogold labelling of plasma membrane sheets combined with spatial point pattern analysis confirmed that S100A10 is present in VAMP2 microdomains at the plasma membrane and that annexin‐A2 is observed close to S100A10 and to syntaxin in stimulated chromaffin cells. In addition, these results showed that the formation of phosphatidylinositol (4,5)‐bisphosphate (PIP₂) microdomains colocalized with S100A10 in the vicinity of docked granules, suggesting a functional interplay between annexin‐A2‐mediated lipid microdomains and SNAREs during exocytosis.     

Effects of aluminum on membrane fluidity of the mycorrhizal fungus Amanita muscaria

The effect of aluminum on the ordering and dynamics of lipid molecules in plasmalemma was studied by electron paramagnetic resonance via the membrane-inserted reporter spin label molecules (methyl ester of 5-doxyl-hexadecanoic acid) in situ in mycelia of the ectomycorrhizal fungus Amanita muscaria . It was found, first, that the plasmalemma is structured into coexistent regions of lipids with different ordering and dynamics, and second, that aluminum stress is accompanied by the corresponding relative decrease of the proportion of the less ordered membrane domains. This effect is opposite to that found previously for Lactarius piperatus , where the membrane responded by an increase of the portion of the less ordered membrane domains. Such qualitatively diverse effects of aluminum at the membrane level is of interest as it coincides with the opposite effect on growth, i. e. inhibition of Amanita muscaria and stimulation of Lactarius piperatus .     

Membrane interactions in small fast-tumbling bicelles as studied by 31P NMR

Small fast-tumbling bicelles are ideal for studies of membrane interactions at molecular level; they allow analysis of lipid properties using solution-state NMR. In the present study we used ³¹P NMR relaxation to obtain detailed information on lipid head-group dynamics. We explored the effect of two topologically different membrane-interacting peptides on bicelles containing either dimyristoylphosphocholine (DMPC), or a mixture of DMPC and dimyristoylphosphoglycerol (DMPG), and dihexanoylphosphocholine (DHPC). KALP21 is a model transmembrane peptide, designed to span a DMPC bilayer and dynorphin B is a membrane surface active neuropeptide. KALP21 causes significant increase in bicelle size, as evidenced by both dynamic light scattering and ³¹P T₂ relaxation measurements. The effect of dynorphin B on bicelle size is more modest, although significant effects on T₂ relaxation are observed at higher temperatures. A comparison of ³¹P T₁ values for the lipids with and without the peptides showed that dynorphin B has a greater effect on lipid head-group dynamics than KALP21, especially at elevated temperatures. From the field-dependence of T₁ relaxation data, a correlation time describing the overall lipid motion was derived. Results indicate that the positively charged dynorphin B decreases the mobility of the lipid molecules  – in particular for the negatively charged DMPG – while KALP21 has a more modest influence. Our results demonstrate that while a transmembrane peptide has severe effects on overall bilayer properties, the surface bound peptide has a more dramatic effect in reducing lipid head-group mobility. These observations may be of general importance for understanding peptide–membrane interactions.     

Intrinsic protein-lipid interactions. Infrared spectroscopic studies of gramicidin A, bacteriorhodopsin and Ca2+-ATPase in biomembranes and reconstituted systems

Infrared spectroscopy has been applied to the study of a number of aqueous systems of model and natural biomembranes. The absorption bands arising from water and buffer solutions were eliminated by means of an infrared spectrometer data station. Spectra were examined using H₂O and ²H₂O aqueous buffer systems. Pure lecithin-water systems, and various model biomembranes containing cholesterol, gramicidin A, bacteriorhodopsin or Ca²⁺-ATPase were examined. The infrared spectra of the reconstituted biomembranes were compared with those of the corresponding natural biomembranes, i.e. the purple membrane of Halobacterium halobium and also sarcoplasmic reticulum membranes, respectively.Changes in lipid chain conformation caused by the various intrinsic molecules incorporated within the model lipid bilayer structures were monitored by studying the shifts in frequency (cm^?1) of the CH₂ symmetric and asymmetric absorption bands arising from the lipid chains. The effect of gramicidin A and also the intrinsic proteins, as indicated by the shift of band frequencies, are quite different from that of cholesterol at temperatures above the main lipid transition temperature t_c. Cholesterol causes a reduction in gauche isomers which increases with concentration of cholesterol within the lipid bilayer. Whilst gramicidin A and the intrinsic proteins at low concentration cause a reduction of gauche isomers, at higher concentrations of these molecules, however, there is little difference in gauche isomer content when the intrinsic molecule is present compared with that of the fluid lipid alone. These results are considered and compared with previously published studies using deuterium nuclear magnetic resonance spectroscopy on similar model biomembrane systems. Below the lipid t_c value, all the intrinsic molecules produce an increase in gauche isomers presumably by disturbing the lipid chain packing in the crystalline lipid arrangement.Information about the polypeptide structure within gramicidin A. the reconstituted proteins and also the proteins in the natural biomembranes was obtained by examining the region of the infrared spectrum between 1600 and 1700 cm^?1 associated with the amide I and amide II bands. An examination of the infrared band frequencies of the different systems in this region leads to the conclusions: (1) that gramicidin A within a phospholipid bilayer structure probably has a single helix rather than a double helix structure; (2) that there are differences in band widths of the reconstituted Ca²⁺-ATPase and bacteriorhodopsin compared with the spectra of the corresponding sarcoplasmic reticulum and purple membrane; (3) different membrane proteins adopt different conformations as evinced by a comparison of the spectra of the sarcoplasmic reticulum and purple membrane; (4) the polypeptide arrangement in the purple membrane is mainly helical but the abnormal frequency of the amide I band suggests that some distortion of the helix occurs: and (5) the sarcoplasmic reticulum membrane contains unordered as well as α-helix polypeptide arrangements.     

Neutron scattering studies of photosynthetic membranes in aqueous dispersion

A method for structural analysis of biological membranes using neutron scattering from suspensions is described and applied to photosynthetic membranes from bacteria. The variation of scattering density across the membrane is analysed using small-angle scattering and contrast variation with H₂O/²H₂O mixtures. Effects due to membrane curvature and scattering density variation in the plane of the membrane are evaluated. Thickness parameters (D) are obtained from the small-angle scattering data, which are the one-dimensional analogues of radii of gyration. The formalism of contrast variation is used to describe the change of intensity and thickness parameter with H₂O/²H₂O mixture. The results are expressed in terms of a thickness parameter at infinite contrast, which is directly related to the physical thickness of the membrane, and a measure of the variation of the scattering density across the membrane, produced, for example, by the higher scattering densities of the polar surfaces relative to a hydrocarbon interior of the membrane. Asymmetry in the membrane scattering density is also evaluated.The results for photosynthetic membranes demonstrate a lipid hydrocarbon core in the membrane. About two-thirds of the protein is closely associated with the lipid layer, and no substantial amounts of protein project more than short distances from the lipid layer. There is a contribution to the variation in scattering density across the membrane that cannot be attributed to lipid, and may involve scattering density heterogeneity within the protein, giving a high proportion of hydrophobic protein segments at the interior of the membrane that have lower scattering densities than the hydrophilic segments at the surfaces of the membrane. The membrane scattering density is not markedly asymmetric. Several alternative structures previously proposed for photosynthetic membranes are incompatible with these results.     

Isolation, Composition, and Structure of Membrane of Listeria monocytogenes

The plasma membrane of Listeria monocytogenes strain 42 was prepared by osmotic lysis of protoplasts with tris(hydroxymethyl)aminomethane (Tris) buffer, pH 8.2, containing MgCl₂ and glucose, followed by washing with NaCl and MgCl₂ in Tris buffer. Electron microscopy showed that the preparation was not contaminated with cytoplasmic material. The membrane preparation was composed of 55 to 60% protein, 1.5% ribonucleic acid, 0.1% deoxyribonucleic acid, 1.3 to 2.3% carbohydrate, 0.17 to 0.38% amino sugar, 0.2 to 0.4% rhamnose, 3.5 to 4.0% phosphorus, 10.5 to 12.0% nitrogen, and 30 to 35% lipid. Amino acid composition of the washed membrane showed some variation from that of the whole cells. Sulfur-containing amino acids were not present in the membrane hydrolysate. The membrane carbohydrate contained glucose, galactose, ribose, and arabinose. The membrane lipid was 80 to 85% phospholipid and 15 to 20% neutral lipid. The lipid contained 2.3 to 3.0% phosphorus, 2.5 to 3.0% carbohydrate, and a very small amount of nitrogen (0.2 to 0.3%). The phospholipid was of the phosphatidyl glycerol type. Electron micrographs of the washed membrane showed three layers. The outer and inner layers varied in thickness from 25 to 37 A and the middle layer from 20 to 25 A. The total thickness varied between 85 and 100 A. These preparations contained many vesicles which stained heavily with lead citrate. Some vesicles were also attached to the protoplast ghosts in the form of extrusions or intrusions, or both. Membrane preparations obtained by lysis of protoplasts in the absence of MgCl₂ were fragmented and contained less lipid (20 to 22%) and ribonucleic acid (0.3 to 0.5%) than preparations prepared with MgCl₂.     

FATTY ACID AND LIPID COMPOSITION OF THE ACIDOPHILIC GREEN ALGA CHLAMYDOMONAS SP.1

The lipid and fatty acid compositions of Chlamydomonas sp. isolated from a volcanic acidic lake and C. reinhardtii were compared, and the effects of pH of the medium on lipid and fatty acid components of Chlamydomonas sp. were studied. The fatty acids in polar lipids from Chlamydomonas sp. were more saturated than those of C. reinhardtii. The relative percentage of triacylglycerol to the total lipid content in Chlamydomonas sp. grown in medium at pH 1 was higher than that in other cells grown at higher pH. A probable explanation might be that Chlamydomonas sp. has two low pH adaptation mechanisms. One mechanism is the saturation of fatty acids in membrane lipids to decrease membrane lipid fluidity, and the other is the accumulation of triacylglycerol, as a storage lipid, to prevent the osmotic imbalance caused by high concentrations of H₂SO₄.     

Chemical versus Mechanical Perturbations on the Protonation State of Arginine in Complex Lipid Membranes: Insights from Microscopic pKa Calculations

Charged amino acids such as Arginine play important roles in many membrane-mediated biological processes such as voltage gating of ion channels and membrane translocation of cell penetration peptides. It is well established that local membrane deformation and formation of water defects are crucial to the stabilization of charged species in contact with the membrane, which suggests that mechanical properties of the membrane are relevant although a clear connection has not been established. As a quantitative measure, we study how changes in the composition and therefore mechanical properties of a lipid bilayer influence the pK_a of Arg in the membrane center using free energy simulations. Compared to previous studies in a single-component lipid bilayer containing saturated lipids or lipids with a modest degree of unsaturation, substantially larger pK_a shifts are observed in the presence of highly unsaturated lipid tails and cholesterol. Moreover, the underlying molecular mechanisms for the pK_a perturbation are distinct in different systems, with the unsaturated lipid tails mainly destabilizing the charged state of Arg and the cholesterol stabilizing the neutral state of Arg. The observed behaviors in both cases are at odds with predictions based on mechanical considerations at a mesoscopic level—highlighting that, while mechanical considerations are useful for stimulating hypothesis, their applicability to dissecting phenomena at the molecular-length scale is rather limited.     

Mechanical stresses in erythrocyte membranes (theoretical models)

The type of mechanical stresses that arise in erythrocyte membranes on exposure to catecholamines and steroid hormones is considered. Tensors of mechanical stresses and displacements were obtained for a membrane interacting with hormones. A possible mechanism of membrane rupture under mechanical stresses is discussed. Catecholamines and androgens increase the microviscosity of membranes, and alternating kink and stretching sites occur in the lipid membrane bilayer to produce a checker-wise pattern. The membrane becomes thinner in a stretching site (smectic A → smectic C transition). When tensile stresses increase further and exceed a certain critical value the membrane may rupture. It is possible that a gel phase L_β? → liquid crystalline phase L_α transition takes place in the stretching site of the lipid bilayer prior to disruption. The density of the lipid bilayer decreases in the process, pores form, and then cracks occur.     

Flooding-induced membrane damage, lipid oxidation and activated oxygen generation in corn leaves

Flooding effects on membrane permeability, lipid peroxidation and activated oxygen metabolism in corn (Zea mays L.) leaves were investigated to determine if activated oxygens are involved in corn flooding-injury. Potted corn plants were flooded at the 4-leaf stage in a controlled environment. A 7-day flooding treatment resulted in a significant increase in chlorophyll breakdown, lipid peroxidation (malondialdehye content), membrane permeability, and the production of superoxide (O ₂ ^- ) and hydrogen peroxide (H₂O₂) in corn leaves. The effects were much greater in older leaves than in younger ones. Spraying leaves with 8-hydroxyquinoline (an O ₂ ^- scavenger) and sodium benzoate (an .OH scavenger) reduced the oxidative damage and enhanced superoxide dismutase (SOD) activity. A short duration flooding treatment elevated the activities of SOD, catalase, ascorbate peroxidase (AP), and glutathione reductase (GR), while further flooding significantly reduced the enzyme activities but enhanced the concentrations of ascorbic acid and reduced form glutathione (GSH). It was noted that the decline in SOD activity was greater than that in H₂O₂ scavengers (AP and GR). The results suggested that O ₂ ^- induced lipid peroxidation and membrane damage, and that excessive accumulation of O ₂ ^- is due to the reduced activity of SOD under flooding stress.