The dual effect of oxidation on lipid bilayer structure

Identification of a critical lipid ratio in raft-like phases exposed to nitric oxide: An AFM study

Lipid rafts are discrete, heterogeneous domains of phospholipids, sphingolipids, and sterols that are present in the cell membrane. They are responsible for conducting cell signaling and maintaining lipid-protein functionality. Redox-stress-induced modifications to any of their components can severely alter the mechanics and dynamics of the membrane causing impairment to the lipid-protein functionality. Here, we report on the effect of sphingomyelin (SM) in controlling membrane permeability and its role as a regulatory lipid in the presence of nitric oxide (NO). Force spectroscopy and atomic force microscopy imaging of raft-like phases (referring here to the coexistence of "liquid-ordered" and "liquid-disordered" phases in model bilayer membranes) prepared from lipids: 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC):SM:cholesterol (CH) (at three ratios) showed that the adhesion forces to pull the tip out of the membrane increased with increasing SM concentration, indicating decreased membrane permeability. However, in the presence of NO radical (1 and 5 μM), the adhesion forces decreased depending on SM concentration. The membrane was found to be stable at the ratio POPC:SM:CH (2:1:1) even when exposed to 1 μM NO. We believe that this is a critical ratio needed by the raft-like phases to maintain homeostasis under stress conditions. The stability could be due to an interplay existing between SM and CH. However, at 5 μM NO, membrane deteriorations were detected. For POPC:SM:CH (2:2:1) ratio, NO displayed a pro-oxidant behavior and damaged the membrane at both radical concentrations. These changes were reflected by the differences in the height profiles of the raft-like phases observed by atomic force microscopy imaging. Malondialdehyde (a peroxidation product) detection suggests that lipids may have undergone lipid nitroxidation. The changes were instantaneous and independent of radical concentration and incubation time. Our study underlines the need for identifying appropriate ratios in the lipid rafts of the cell membranes to withstand redox imbalances caused by radicals such as NO.

Association of Alpha-Crystallin with Fiber Cell Plasma Membrane of the Eye Lens Accompanied by Light Scattering and Cataract Formation

α-crystallin is a major protein found in the mammalian eye lens that works as a molecular chaperone by preventing the aggregation of proteins and providing tolerance to stress in the eye lens. These functions of α-crystallin are significant for maintaining lens transparency. However, with age and cataract formation, the concentration of α-crystallin in the eye lens cytoplasm decreases with a corresponding increase in the membrane-bound α-crystallin, accompanied by increased light scattering. The purpose of this review is to summarize previous and recent findings of the role of the: (1) lens membrane components, i.e., the major phospholipids (PLs) and sphingolipids, cholesterol (Chol), cholesterol bilayer domains (CBDs), and the integral membrane proteins aquaporin-0 (AQP0; formally MIP26) and connexins, and (2) α-crystallin mutations and post-translational modifications (PTMs) in the association of α-crystallin to the eye lens's fiber cell plasma membrane, providing thorough insights into a molecular basis of such an association. Furthermore, this review highlights the current knowledge and need for further studies to understand the fundamental molecular processes involved in the association of α-crystallin to the lens membrane, potentially leading to new avenues for preventing cataract formation and progression.

Impact of cigarette smoke on physical-chemical and molecular proprieties of human skin in an ex vivo model

BACKGROUND

This is a study about the skin ageing exposome, focusing on the effect of cigarette smoke. Human living skin explants (HSE) were exposed to cigarette smoke (CS) of two cigarettes for 2 hours using a custom-made exposure chamber, the Pollubox^® . Effects on the surface physico-chemistry and molecular properties of the skin were analyzed and reported for the first time.

BASIC PROCEDURES

To this end, transcriptomic study followed by immunohistochemistry, MDA (Malondialdehyde Dosage), and surface physio-chemistry data: surface free energy determination, TEWL (Trans Epidermal Water Loss), skin pH and FT-IR (Fourier Transform-Infrared) spectroscopy of the explant were collected from untreated and treated HSE.

MAIN FINDINGS

Results showed a decrease of the total surface free energy of the treated HSE. This decrease reflected higher interactions with polar compounds from the environment and consequently a decrease of the surface hydrophobicity. Additionally, an increase of TEWL and skin pH was observed after treatment. The transcriptomic analysis showed downregulation of mitochondrial genes (PON2-NDUFA4L2-ATP1A1-ALDH2-PRODH) combined with an increase of MDA in CS-treated HSE.

CONCLUSIONS

CS-induced oxidation of lipids at HSE surface alters the skin barrier: interactions with polar products are enhanced and the lipid chain packing at the surface is modified. Consequently, skin permeability could increase which correlated with repression of CA9 and AQP1 genes. Beside activation of AHR-NRF2 pathway in CS-exposed HSE, our results suggested that mitochondrial functions were strongly impacted and oxidized lipids failed to be eliminated promoting skin barrier alteration. A mitophagy activity was suggested through the confirmation of PINK1 accumulation in the epidermis by immunostaining.

Cold Atmospheric Plasma Stimulates Clathrin-Dependent Endocytosis to Repair Oxidised Membrane and Enhance Uptake of Nanomaterial in Glioblastoma Multiforme Cells

Cold atmospheric plasma (CAP) enhances uptake and accumulation of nanoparticles and promotes synergistic cytotoxicity against cancer cells. However, the mechanisms are not well understood. In this study, we investigate the enhanced uptake of theranostic nanomaterials by CAP. Numerical modelling of the uptake of gold nanoparticle into U373MG Glioblastoma multiforme (GBM) cells predicts that CAP may introduce a new uptake route. We demonstrate that cell membrane repair pathways play the main role in this stimulated new uptake route, following non-toxic doses of dielectric barrier discharge CAP. CAP treatment induces cellular membrane damage, mainly via lipid peroxidation as a result of reactive oxygen species (ROS) generation. Membranes rich in peroxidised lipids are then trafficked into cells via membrane repairing endocytosis. We confirm that the enhanced uptake of nanomaterials is clathrin-dependent using chemical inhibitors and silencing of gene expression. Therefore, CAP-stimulated membrane repair increases endocytosis and accelerates the uptake of gold nanoparticles into U373MG cells after CAP treatment. We demonstrate the utility of CAP to model membrane oxidative damage in cells and characterise a previously unreported mechanism of membrane repair to trigger nanomaterial uptake. This knowledge will underpin the development of new delivery strategies for theranostic nanoparticles into cancer cells.

The potential contribution of miRNA-200-3p to the fatty acid metabolism by regulating AjEHHADH during aestivation in sea cucumber

The sea cucumber (Apostichopus japonicus) has become a good model organism for studying environmentally-induced aestivation by a marine invertebrate more recently. In the present study, we hypothesized that miRNA-200-3p may contribute to establish rapid biological control to regulate fatty acid metabolism during a estivation. The peroxisomal bi-functional enzyme (EHHADH) is a crucial participant of the classical peroxisomal fatty acid β-oxidation pathway, the relative mRNA transcripts and protein expressions of EHHADH were analyzed in intestine from sea cucumbers experienced long-term aestivation. Both mRNA transcripts and protein expressions of EHHADH in intestine decreased significantly during deep-aestivation as compared with non-aestivation controls. Analysis of the 3′ UTR of AjEHHADH showed the presence of a conserved binding site for miR-200-3p. Level of miR-200-3p showed an inverse correlation with EHHADH mRNA transcripts and protein levels in intestine, implicating miR-200-3p may directly targeted AjEHHADH by inducing the degradation of AjEHHADH mRNA in the aestivating sea cucumber, further dual-luciferase reporter assay validated the predicted role of miRNA-200-3p in regulating AjEHHADH. In order to further understand their regulatory mechanism, we conducted the functional experiment in vivo. The overexpression of miR-200-3p in sea cucumber significantly decreased mRNA and protein expression levels of AjEHHADH. Taken together, these findings suggested the potential contribution of miRNA-200-3p to the fatty acid metabolism by regulating AjEHHADH during aestivation in sea cucumber.

Pathophysiology of mitochondrial lipid oxidation: Role of 4-hydroxynonenal (4-HNE) and other bioactive lipids in mitochondria

Mitochondrial lipids are essential for maintaining the integrity of mitochondrial membranes and the proper functions of mitochondria. As the "powerhouse" of a cell, mitochondria are also the major cellular source of reactive oxygen species (ROS). Oxidative stress occurs when the antioxidant system is overwhelmed by overproduction of ROS. Polyunsaturated fatty acids in mitochondrial membranes are primary targets for ROS attack, which may lead to lipid peroxidation (LPO) and generation of reactive lipids, such as 4-hydroxynonenal. When mitochondrial lipids are oxidized, the integrity and function of mitochondria may be compromised and this may eventually lead to mitochondrial dysfunction, which has been associated with many human diseases including cancer, cardiovascular diseases, diabetes, and neurodegenerative diseases. How mitochondrial lipids are oxidized and the underlying molecular mechanisms and pathophysiological consequences associated with mitochondrial LPO remain poorly defined. Oxidation of the mitochondria-specific phospholipid cardiolipin and generation of bioactive lipids through mitochondrial LPO has been increasingly recognized as an important event orchestrating apoptosis, metabolic reprogramming of energy production, mitophagy, and immune responses. In this review, we focus on the current understanding of how mitochondrial LPO and generation of bioactive lipid mediators in mitochondria are involved in the modulation of mitochondrial functions in the context of relevant human diseases associated with oxidative stress.

Plasma membrane order and fluidity are diversely triggered by elicitors of plant defence

Although plants are exposed to a great number of pathogens, they usually defend themselves by triggering mechanisms able to limit disease development. Alongside signalling events common to most such incompatible interactions, modifications of plasma membrane (PM) physical properties could be new players in the cell transduction cascade. Different pairs of elicitors (cryptogein, oligogalacturonides, and flagellin) and plant cells (tobacco and Arabidopsis) were used to address the issue of possible modifications of plant PM biophysical properties induced by elicitors and their links to other events of the defence signalling cascade. We observed an increase of PM order whatever the elicitor/plant cell pair used, provided that a signalling cascade was induced. Such membrane modification is dependent on the NADPH oxidase-mediated reactive oxygen species production. Moreover, cryptogein, which is the sole elicitor able to trap sterols, is also the only one able to trigger an increase in PM fluidity. The use of cryptogein variants with altered sterol-binding properties confirms the strong correlation between sterol removal from the PM and PM fluidity enhancement. These results propose PM dynamics as a player in early signalling processes triggered by elicitors of plant defence.

Mitochondrial Dysfunction in Cancer and Neurodegenerative Diseases: Spotlight on Fatty Acid Oxidation and Lipoperoxidation Products

In several human diseases, such as cancer and neurodegenerative diseases, the levels of reactive oxygen species (ROS), produced mainly by mitochondrial oxidative phosphorylation, is increased. In cancer cells, the increase of ROS production has been associated with mtDNA mutations that, in turn, seem to be functional in the alterations of the bioenergetics and the biosynthetic state of cancer cells. Moreover, ROS overproduction can enhance the peroxidation of fatty acids in mitochondrial membranes. In particular, the peroxidation of mitochondrial phospholipid cardiolipin leads to the formation of reactive aldehydes, such as 4-hydroxynonenal (HNE) and malondialdehyde (MDA), which are able to react with proteins and DNA. Covalent modifications of mitochondrial proteins by the products of lipid peroxidation (LPO) in the course of oxidative cell stress are involved in the mitochondrial dysfunctions observed in cancer and neurodegenerative diseases. Such modifications appear to affect negatively mitochondrial integrity and function, in particular energy metabolism, adenosine triphosphate (ATP) production, antioxidant defenses and stress responses. In neurodegenerative diseases, indirect confirmation for the pathogenetic relevance of LPO-dependent modifications of mitochondrial proteins comes from the disease phenotypes associated with their genetic alterations.

Tumor cell survival pathways activated by photodynamic therapy: a molecular basis for pharmacological inhibition strategies

Photodynamic therapy (PDT) has emerged as a promising alternative to conventional cancer therapies such as surgery, chemotherapy, and radiotherapy. PDT comprises the administration of a photosensitizer, its accumulation in tumor tissue, and subsequent irradiation of the photosensitizer-loaded tumor, leading to the localized photoproduction of reactive oxygen species (ROS). The resulting oxidative damage ultimately culminates in tumor cell death, vascular shutdown, induction of an antitumor immune response, and the consequent destruction of the tumor. However, the ROS produced by PDT also triggers a stress response that, as part of a cell survival mechanism, helps cancer cells to cope with the PDT-induced oxidative stress and cell damage. These survival pathways are mediated by the transcription factors activator protein 1 (AP-1), nuclear factor E2-related factor 2 (NRF2), hypoxia-inducible factor 1 (HIF-1), nuclear factor κB (NF-κB), and those that mediate the proteotoxic stress response. The survival pathways are believed to render some types of cancer recalcitrant to PDT and alter the tumor microenvironment in favor of tumor survival. In this review, the molecular mechanisms are elucidated that occur post-PDT to mediate cancer cell survival, on the basis of which pharmacological interventions are proposed. Specifically, pharmaceutical inhibitors of the molecular regulators of each survival pathway are addressed. The ultimate aim is to facilitate the development of adjuvant intervention strategies to improve PDT efficacy in recalcitrant solid tumors.

Oxidative Damage to Biomimetic Membrane Systems: In Situ Fe(II)/Ascorbate Initiated Oxidation and Incorporation of Synthetic Oxidized Phospholipids

Damage to cellular membranes from oxidative stress has been implicated in aging related diseases. We report the effects of oxidative damage on the structure and properties of biomimetic phospholipid membrane systems. Two oxidation methods were used, in situ oxidation initiated using Fe(II) and ascorbate, and the incorporation of a synthetic "oxidized" phospholipid, PoxnoPC, into biomimetic membranes. The biomimetic systems employed included multibilayer stacks, tethered bilayers, and phospholipid monolayers studied using a combination of reflectometry, attenuated total reflection infrared spectroscopy, electrochemical impedance spectroscopy, and neutron diffraction. We show that oxidation with Fe(II) and ascorbate caused an increase in the order of the membrane, attributed to cross-linking of the phospholipids, and a change in the electrical permeability of the membrane, but no significant impact on the thickness or completeness of the membrane. Incorporation of PoxnoPC, on the other hand, had a larger impact on the structure of the membrane. Inversion of the aldehyde-terminated truncated sn-2 chain of PoxnoPC into the head group region was observed, along with a slight decrease in the thickness and order of the membrane.

Eicosapentaenoic acid inhibits glucose-induced membrane cholesterol crystalline domain formation through a potent antioxidant mechanism

Lipid oxidation leads to endothelial dysfunction, inflammation, and foam cell formation during atherogenesis. Glucose also contributes to lipid oxidation and promotes pathologic changes in membrane structural organization, including the development of cholesterol crystalline domains. In this study, we tested the comparative effects of eicosapentaenoic acid (EPA), an omega-3 fatty acid indicated for the treatment of very high triglyceride (TG) levels, and other TG-lowering agents (fenofibrate, niacin, and gemfibrozil) on lipid oxidation in human low-density lipoprotein (LDL) as well as membrane lipid vesicles prepared in the presence of glucose (200 mg/dL). We also examined the antioxidant effects of EPA in combination with atorvastatin o-hydroxy (active) metabolite (ATM). Glucose-induced changes in membrane structural organization were measured using small angle x-ray scattering approaches and correlated with changes in lipid hydroperoxide (LOOH) levels. EPA was found to inhibit LDL oxidation in a dose-dependent manner (1.0-10.0 µM) and was distinguished from the other TG-lowering agents, which had no significant effect as compared to vehicle treatment alone. Similar effects were observed in membrane lipid vesicles exposed to hyperglycemic conditions. The antioxidant activity of EPA, as observed in glucose-treated vesicles, was significantly enhanced in combination with ATM. Glucose treatment produced highly-ordered, membrane-restricted, cholesterol crystalline domains, which correlated with increased LOOH levels. Of the agents tested in this study, only EPA inhibited glucose-induced cholesterol domain formation. These data demonstrate that EPA, at pharmacologic levels, inhibits hyperglycemia-induced changes in membrane lipid structural organization through a potent antioxidant mechanism associated with its distinct, physicochemical interactions with the membrane bilayer.

Mitochondria as a source and target of lipid peroxidation products in healthy and diseased heart

The heart is a highly oxidative organ in which cardiomyocyte turnover is virtually absent, making it particularly vulnerable to accumulation of lipid peroxidation products (LPP) formed as a result of oxidative damage. Reactive oxygen and nitrogen species are the most common electrophiles formed during lipid peroxidation and lead to the formation of both stable and unstable LPP. Of the LPP formed, highly reactive aldehydes are a well-recognized causative factor in ageing and age-associated diseases, including cardiovascular disease and diabetes. Recent studies have identified that the mitochondria are both a primary source and target of LPP, with specific emphasis on aldehydes in cardiomyocytes and how these affect the electron transport system and Ca(2+) balance. Numerous studies have found that there are functional consequences in the heart following exposure to specific aldehydes (acrolein, trans-2-hexanal, 4-hydroxynonenal and acetaldehyde). Because these LPP are known to form in heart failure, cardiac ischaemia-reperfusion injury and diabetes, they may have an underappreciated role in the pathophysiology of these disease processes. Lipid peroxidation products are involved in the transcriptional regulation of endogenous anti-oxidant systems. Recent evidence demonstrates that transient increases in LPP may be beneficial in cardioprotection by contributing to mitohormesis (i.e. induction of anti-oxidant systems) in cardiomyocytes. Thus, exploitation of the cardioprotective actions of the LPP may represent a novel therapeutic strategy for future treatment of heart disease.

Photo-activated phase separation in giant vesicles made from different lipid mixtures

Using giant unilamellar vesicles (GUVs) made from POPC, DPPC, cholesterol and a small amount of a porphyrin-based photosensitizer that we name PE-porph, we investigated the response of the lipid bilayer under visible light, focusing in the formation of domains during the lipid oxidation induced by singlet oxygen. This reactive species is generated by light excitation of PE-porf in the vicinity of the membrane, and thus promotes formation of hydroperoxides when unsaturated lipids and cholesterol are present. Using optical microscopy we determined the lipid compositions under which GUVs initially in the homogeneous phase displayed Lo-Ld phase separation following irradiation. Such an effect is attributed to the in situ formation of both hydroperoxized POPC and cholesterol. The boundary line separating homogeneous Lo phase and phase coexistence regions in the phase diagram is displaced vertically towards the higher cholesterol content in respect to ternary diagram of POPC:DPPC:cholesterol mixtures in the absence of oxidized species. Phase separated domains emerge from sub-micrometer initial sizes to evolve over hours into large Lo-Ld domains completely separated in the lipid membrane. This study provides not only a new tool to explore the kinetics of domain formation in mixtures of lipid membranes, but may also have implications in biological signaling of redox misbalance.

Mitochondria induce oxidative stress, generation of reactive oxygen species and redox state unbalance of the eye lens leading to human cataract formation: disruption of redox lens organization by phospholipid hydroperoxides as a common basis for cataract disease

The aging eye appears to be at considerable risk from oxidative stress. Lipid peroxidation (LPO) is one of the mechanisms of cataractogenesis, initiated by enhanced promotion of oxygen free radicals in the eye fluids and tissues and impaired enzymatic and non-enzymatic antioxidant defenses of the crystalline lens. The present study proposes that mitochondria are one of the major sources of reactive oxygen species (ROS) in mammalian and human lens epithelial cells and that therapies that protect mitochondria in lens epithelial cells from damage and reduce damaging ROS generation may potentially ameliorate the effects of free radical-induced oxidation that occur in aging ocular tissues and in human cataract diseases. It has been found that rather than complete removal of oxidants by the high levels of protective enzyme activities such as superoxide dismutase (SOD), catalase, lipid peroxidases in transparent lenses, the lens conversely, possess a balance between peroxidants and antioxidants in a way that normal lens tends to generate oxidants diffusing from lenticular tissues, shifting the redox status of the lens to become more oxidizing during both morphogenesis and aging. Release of the oxidants (O(2)(-)·, H(2)O(2) , OH·, and lipid hydroperoxides) by the intact lenses in the absence of respiratory inhibitors indicates that these metabolites are normal physiological products inversely related to the lens life-span potential (maturity of cataract) generated through the metal-ion catalyzed redox-coupled pro-oxidant activation of the lens reductants (ascorbic acid, glutathione). The membrane-bound phospholipid (PL) hydroperoxides escape detoxification by the lens enzymatic reduction. The lens cells containing these species would be vulnerable to peroxidative attack which trigger the PL hydroperoxide-dependent chain propagation of LPO and other damages in membrane (lipid and protein alterations). The increased concentrations of primary LPO products (diene conjugates, lipid hydroperoxides) and end fluorescent LPO products were detected in the lipid moiety of the aqueous humor samples obtained from patients with cataract as compared to normal donors. Since LPO is clinically important in many of the pathological effects and aging, new therapeutic modalities, such as patented N-acetylcarnosine prodrug lubricant eye drops, should treat the incessant infliction of damage to the lens cells and biomolecules by reactive lipid peroxides and oxygen species and "refashion" the affected lens membranes in the lack of important metabolic detoxification of PL peroxides. Combined in ophthalmic formulations with N-acetylcarnosine, mitochondria-targeted antioxidants are promising to become investigated as a potential tool for treating a number of ROS-related ocular diseases, including human cataracts.

Lipids and the ocular lens

The unusually high levels of saturation and thus order contribute to the uniqueness of human lens membranes. In addition, and unlike in most biomembranes, most of the lens lipids are associated with proteins, thus reducing their mobility. The major phospholipid of the human lens is dihydrosphingomyelin. Found in significant quantities only in primate lenses, particularly human ones, this lipid is so extremely stable that it was reported to be the only lipid remaining in a frozen mammoth 40,000 years after its death. Unusually high levels of cholesterol add peculiarity to the composition of lens membranes. Beyond the lateral segregation of lipids into dynamic domains known as rafts, the high abundance of cholesterol in the human lens leads to the formation of patches of pure cholesterol. Changes in human lens lipid composition with age and disease as well as differences among species are greater than those observed for any other biomembrane. The relationships among lens membrane composition, structure, and lipid conformation reviewed in this article are unique to the mammalian lens and offer exciting insights into lens membrane function. This review focuses on findings reported over the last two decades that demonstrate the uniqueness of mammalian lens membranes regarding their morphology and composition. Because the membranes of human lenses do undergo the most dramatic changes with age and cataractogenesis, the final sections of this review address our current knowledge of the unusual composition and organization of adult human lens membranes with and without opacification. Finally, the questions that still remain to be answered are presented.

Lipid peroxidation of membrane phospholipids generates hydroxy-alkenals and oxidized phospholipids active in physiological and/or pathological conditions

Polyunsaturated fatty acids (PUFAs) and their metabolites have a variety of physiological roles including: energy provision, membrane structure, cell signaling and regulation of gene expression. Lipids containing polyunsaturated fatty acids are susceptible to free radical-initiated oxidation and can participate in chain reactions that increase damage to biomolecules. Lipid peroxidation, which leads to lipid hydroperoxide formation often, occurs in response to oxidative stress. Hydroperoxides are usually reduced to their corresponding alcohols by glutathione peroxidases. However, these enzymes are decreased in certain diseases resulting in a temporary increase of lipid hydroperoxides that favors their degradation into several compounds, including hydroxy-alkenals. The best known of these are: 4-hydroxy-2-nonenal (4-HNE) and 4-hydroxy-2-hexenal (4-HHE), which derive from lipid peroxidation of n-6 and n-3 fatty acids, respectively. Compared to free radicals, these aldehydes are relatively stable and can diffuse within or even escape from the cell and attack targets far from the site of the original event. These aldehydes exhibit great reactivity with biomolecules, such as proteins, DNA, and phospholipids, generating a variety of intra and intermolecular covalent adducts. At the membrane level, proteins and amino lipids can be covalently modified by lipid peroxidation products (hydoxy-alkenals). These aldehydes can also act as bioactive molecules in physiological and/or pathological conditions. In addition this review is intended to provide an appropriate synopsis of identified effects of hydroxy-alkenals and oxidized phospholipids on cell signaling, from their intracellular production, to their action as intracellular messenger, up to their influence on transcription factors and gene expression.

Peroxidation of polyunsaturated phosphatidyl-choline lipids during electroformation

Giant unilamellar vesicles (GUVs) have been utilized both as model systems to study the physico-chemical properties of biomembranes and as host materials for investigating biological processes in microbioreactors. GUVs are commonly formed by an electroformation technique. However, there is a concern that the electric fields applied during electroformation can peroxidize lipid acyl chains, thereby altering the phospholipid composition and material properties of the synthesized vesicles. Here in this paper, we report the effect of electroformation on the extent of peroxidation of a number of polyunsaturated phosphatidyl-choline lipids (PULs). Specifically, we detected peroxidation byproducts (malonaldehydes and conjugated dienes) of the following lipids utilizing UV/Vis spectroscopy: dilinoleoyl phosphatidyl-choline (DLPC) (di-18:2 PC), dilinolenoyl phosphatidyl-choline (DNPC) (di-18:3 PC), diarachidonoyl phosphatidyl-choline (DAPC) (di-20:4 PC), and didocosaheexaenoyl phosphatidyl-choline (DHA) (di-22:6 PC). The results indicate that PC PULs lipids are prone to peroxidation, with increasing unsaturation levels leading to higher levels of peroxidation byproducts. The levels of peroxidation byproducts of DAPC were found to depend linearly on the strength of the electric field, indicating that the observed effects were due to the applied electric field. Lipid peroxidation can affect a number of important membrane properties, including domain formation and mechanical stability. Thus, alteration of the chemical composition of polyunsaturated lipids (PULs) by the electroformation technique can potentially complicate the interpretation of experimental studies that utilize GUVs composed of PULs.

Lipid Peroxidation Induces Cholesterol Domain Formation in Model Membranes

Numerous reports have established that lipid peroxidation contributes to cell injury by altering the basic physical properties and structural organization of membrane components. Oxidative modification of polyunsaturated phospholipids has been shown, in particular, to alter the intermolecular packing, thermodynamic, and phase parameters of the membrane bilayer. In this study, the effects of oxidative stress on membrane phospholipid and sterol organization were measured using small angle x-ray diffraction approaches. Model membranes enriched in dilinoleoylphosphatidylcholine were prepared at various concentrations of cholesterol and subjected to lipid peroxidation at physiologic conditions. At cholesterol-to-phospholipid mole ratios (C/P) as low as 0.4, lipid peroxidation induced the formation of discrete, membrane-restricted cholesterol domains having a unit cell periodicity or d-space value of 34 A. The formation of cholesterol domains correlated directly with lipid hydroperoxide levels and was inhibited by treatment with vitamin E. In the absence of oxidative stress, similar cholesterol domains were observed only at C/P ratios of 1.0 or higher. In addition to changes in sterol organization, lipid peroxidation also caused reproducible changes in overall membrane structure, including a 10 A reduction in the width of the surrounding, sterol-poor membrane bilayer. These data provided direct evidence that lipid peroxidation alters the essential organization and structure of membrane lipids in a manner that may contribute to changes in membrane function during aging and oxidative stress-related disorders.

3D molecular modeling, free radical modulating and immune cells signaling activities of the novel peptidomimetic L-glutamyl-histamine: possible immunostimulating role

An original representative of the patented by author family of histamine-containing peptidomimetics L-glutamyl-histamine (L-Glu-Hist) was synthesized and characterized as a biologically active compound with a role of cytokine mimic leading to cellular responses of improved specificity. The study assesses the ability of L-Glu-Hist to affect molecular modeling, modulate free radical activity and influence immune cell signaling. The energy-minimized 3D conformations of L-Glu-Hist derived from its chemical structure resulted in stabilization for Fe2+ chelating complexes. L-Glu-Hist accelerated the decrease of ferrous iron in the ferrous sulfate solution in a concentration-dependent mode and showed the ferroxidase-like activity at concentrations less than 3 mM in the phenanthroline assay, whereas in the concentration range 3-20 mM L-Glu-Hist restricted the availability of Fe2+ to phenanthroline due to binding of ferrous ions in chelating complexes. L-Glu-Hist showed stimulatory effect on phosphatidylcholine liposomal peroxidation (LPO) catalyzed by the superoxide anion radical (O2*-)-generating system (Fe2+ + ascorbate) at low (less or about 1 mM) L-Glu-Hist concentrations and both revealed the inhibitory effect on LPO in this system of high (approximately 10 mM) L-Glu-Hist concentration. The stimulation of LPO by L-Glu-Hist was related to the ability of peptidomimetic in small (approximately 0.05 mM) concentrations to release O2*- free radicals as determined by the superoxide dismutase-inhibitable cytochrome c reduction assay. O2*- release by L-Glu-Hist might result from its ferroxidase-like activity, while inhibition of LPO by L-Glu-Hist was caused by its chelating activity to Fe2+ ions, prevention of free radical generation and lipid hydroperoxide-degrading ability of 5-20 mM L-Glu-Hist. L-Glu-Hist released O2*- in concentrations which stimulated [3H]-thymidine incorporation into DNA and proliferation of mouse spleen lymphocytes and mononuclear cells from human blood. L-Glu-Hist modulates the ability of oxygen free radicals to act as signaling agents at low concentrations, influencing gene expression. The structural peptide-like analogues of L-Glu-Hist such as L-Glu-Trp, carcinine (beta-alanylhistamine), but not L-Pro-Glu-Trp were active in stimulating thymidine incorporation and in inducing proliferation of mononuclear cells as compared to mitogen concanavalin A at doses 2.5-25.0 microg/ml. Our data provide evidence that L-Glu-Hist may act as a very fast, specific and sensitive trigger for lymphocyte proliferation and immunoregulation. The cited abilities and further obtained in vivo results make Immudilin ((INCI: glutamylamidoethyl imidazole, aqueous solution), L-Glu-Hist) a useful immunoregulatory agent.

Immunostimulating activities of the novel peptidomimetic L-glutamyl-histamine

An original representative of histamine-containing peptidomimetics L-glutamyl-histamine (L-Glu-Hist) was synthesized and characterized as a cytokine mimic leading to cellular responses of improved specificity. The energy-minimized 3-D conformations of L-Glu-Hist derived from its chemical structure resulted in stabilization for Fe(2+) chelating complexes. L-Glu-Hist accelerated the decrease of ferrous iron in the ferrous sulphate solution in a concentration-dependent mode and showed the ferroxidase-like activity at concentrations less than 3 mm in the phenanthroline assay, whereas in the concentration range 3-20 mm L-Glu-Hist restricted the availability of Fe(2+) to phenanthroline due to binding of ferrous ions in chelating complexes. L-Glu-Hist showed a stimulatory effect on phosphatidylcholine liposomal peroxidation (LPO) catalysed by the superoxide anion radical (O(2) (*))-generating system (Fe(2+)+ ascorbate) at low (less or about 1 mm) L-Glu-Hist concentrations and both revealed the inhibitory effect on LPO in this system of high ( approximately 10 mm) L-Glu-Hist concentration. L-Glu-Hist released O(2) (*) in concentrations which stimulated [(3)H]-thymidine incorporation into DNA and proliferation of mouse spleen lymphocytes and mononuclear cells from human blood. The structural peptide-like analogues of L-Glu-Hist such as L-Glu-Trp, carcinine (beta-alanylhistamine), but not L-Pro-Glu-Trp were active in stimulating thymidine incorporation and in inducing proliferation of mononuclear cells compared to mitogen concanavalin A at doses 2.5-25.0 microg/ml. Our data provide evidence that L-Glu-Hist may act as a very fast and sensitive trigger for lymphocyte proliferation and immunoregulation.

Analysis of Lipid Peroxidation and??Electron Microscopic Survey of??Maturation Stages during Human Cataractogenesis

Morphological and biophysical techniques described in this study have shown that membrane derangement occurs in human cataractous lenses. The data suggest that these disruptions were globules, vacuoles, multilamellar membranes and clusters of highly undulating membranes. Deleterious structural damage of the lens fibre cell plasma membranes serve as the primary light-scattering centres that cause the observed lens opacity. Nuclear cataract, a major cause of loss of lens transparency in the aging human, has been thought to be associated with oxidative damage, particularly at the site of the nuclear plasma membrane. Phospholipid molecules modified by oxygen accumulate in the lipid bilayer, change its geometry and impair lipid-lipid and protein-lipid interactions in lenticular fibre membranes. Lipid peroxidation (LPO) is a causative and pathogenic factor in cataract. Increased concentrations of primary molecular LPO products (diene conjugates, lipid hydroperoxides, oxy-derivatives of phospholipid fatty acids) and end-fluorescent LPO products have been detected in the lipid moieties of aqueous humour samples and human lenses obtained from patients with senile and complicated cataracts as compared with normal donors. In the present study, a rapid and simple high-performance liquid chromatographic (HPLC) assay for determination of imidazole-containing dipeptides in the aqueous humour of the eye was developed. The method was applied to determine the pharmacokinetic parameters and the time-course of N-acetylcarnosine and L-carnosine-related product in the eye, following a single dosage of topical ocular administration of peptide. Utilising data from pharmacokinetic studies and the specific purity of the N-acetylcarnosine (NAC) ingredient as a source of the pharmacological principle L-carnosine, we have created an ophthalmic time-release prodrug form including the US FDA-approved carboxymethylcellulose lubricant and other essential ingredients (Can-C, private label Nu-Eyes). This formulation increases the intraocular absorption of L-carnosine in the aqueous humour and optimises its specific antioxidant activity in vivo while reducing the toxic effects of lipid peroxides on the crystalline lens. L-carnosine that enters the aqueous humour can accumulate in the lens tissue for a reasonable period of time. The presence of L-carnosine in transparent crystalline lenses during normal aging was detected and its concentration in this case was about 25 microM. At different stages of cataract development, the level of L-carnosine drastically decreased, reaching about 5 microM in ripe human cataracts. However, administration of pure L-carnosine (1% solution) to the rabbit eye (instillation or subconjunctival injection) does not lead to accumulation of this natural compound in the aqueous humour at the time level over 30 minutes at a concentration exceeding that in placebo-treated matched eyes, and its effective concentration is exhausted more rapidly. Use of NAC prodrug eye drops optimises the clinical effects of L-carnosine in the treatment of ophthalmic disorders (such as prevention and reversal of cataracts in human and animal [canine] eyes). The data provided predict a clinical effect with NAC ophthalmic prodrug, and show that the magnitude and duration of this effect are directly related to the bioavailability of L-carnosine released from NAC in the aqueous humour of the anterior eye segment. The ophthalmic NAC drug shows promise in the treatment of a range of ophthalmic disorders that have a component of oxidative stress in their pathogenesis (including cataract, glaucoma, dry eye, vitreous floaters, inflammatory disorders, and corneal, retinal and systemic diseases [such as diabetes mellitus and its ophthalmic complications]). There is a need for further and better collaboration between Innovative Vision Products' cataract control and ophthalmic services, improved education of people affected by cataract, a commitment that N-acetylcarnosine eye drops will be the preferred treatment before orthodox cataract surgery is attempted, and consideration of outcomes and a possible role of the NAC drug cataract treatment as source of referral for orthodox surgical, ophthalmic and optometric services.

Lens lipids and maximum lifespan

Unlike in most organs, the lipid composition of lenses varies dramatically among species and with age. The focus of this study is to assess how these changes relate to lifespan. Studies on cataract suggest that the lens may serve as a window into the processes leading to accelerated mortality. As a first step toward elucidating cellular processes in the lens that may serve as markers for accelerated mortality, we examined the correlation between species-dependent and age-related lens lipid compositional differences and maximum life span. We included data from camels, which, even in old age, rarely develop cataracts although they live under adverse conditions. Camel lens lipids were mainly composed of sphingolipids (77%) and phosphatidylcholines (23%). Bovine lens lipid composition was comparable to a previous study, and both bovine lens sphingolipids, phosphatidylcholines and camel lens phosphatidylcholines content fit well (within the 95% confidence limits) in the curve obtained by plotting maximum life spans of other species with sphingolipids and phosphatidylcholines. Lifespan was directly related to lens sphingolipid content and indirectly related to lens phosphatidylcholine content. The camel lens sphingolipid value was significantly above the curve for other species. Except for the camel lens nucleus, lipid order and sphingolipid content were linearly related, p < 0.005 with a slope of 0.85+/-0.07, and intercept of 6.9+/-3.8. Lipid phase transition temperature and sphingolipid content were also linearly related, p = 0.01 with a slope of 0.20+/-0.07, and intercept of 21.7+/-5.3. Our data support the hypothesis that humans have adapted so that their lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing these membranes to stay clear for a relatively longer time than is the case in many other species. Age-related changes in human lens lipid composition may serve as a marker for oxidative stress and may reflect systemic oxidative insult, providing a window into the health of an individual.

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

Light scattering of human lens vesicles in vitro

In passing through the lens, light crosses thousands of cell membranes. To explore the possible contribution of lipids to the scattering properties of the lens, we have carried out in vitro studies with lipids extracted from human lenses 1-90 years of age. Sphingomyelin and human lens lipids were extruded into large unilamellar vesicles (LUVs). The intensity of light scattered by human lens LUVs increased with age and lipid hydrocarbon chain order. Hydrocarbon chain order also correlated with light scattering intensity by sphingomyelin LUVs. Light scattered by LUVs composed of sphingomyelin (1-30 mg ml(-1)) was 20 to 100 times more intense than that scattered by the same concentration of alpha-crystallin in aqueous media. Increased lipid hydrocarbon chain order as well as variations in the headgroup and interfacial region of bilayers resulting from lipid compositional changes can influence membrane light scattering properties. In vitro measurements suggest that the contribution to light scattering by lipids may be significant and should not be disregarded in the investigation of factors and components that lead to the increase in light scattering by human lenses with age and cataract.

Lipid oxidation deletes the nanodomain organization of artificial membranes

Nanoscopic domains with different crystal structures have been induced in closed artificial membranes and have been directly imaged by atomic force microscopy at a spatial resolution better than 0.3 nm. These observations provide experimental evidence to the hydrophobic mismatching theory of lateral phase separation phenomena. Under oxidant conditions, the lipid-lipid assembly reorganizes into a new steady-state structure with disappearance of specific nanodomains. This finding may contribute to understanding the mechanism of peroxidative damage to membrane properties. In fact, alterations of specific modes of molecular conformation and packing may lead to perturbation of specific properties.

Structural evidence for membrane lipid changes in human cataract

Lipid changes in relationship to cataractogenesis were studied with histochemical methods (topoptical reactions) of polarization microscopy. Frozen section of formaldehyde-fixed human lenses were used for these studies. Six lenses were transparent and 14 lenses presented early to confluent cortical opacities. Cell membrane lipids of transparent lenses showed 8.0 +/- 2.7 nm light retardation. In the early cataractous lenses the light retardation of cell membranes was 23.3 +/- 5.0 nm and that of the fusiform and globular lipids was 37.7 +/- 4.0 nm and 48.5 +/- 6.9 mn, respectively. In the non transparent cortical regions of cataractous lenses, membrane lipids were not observed. Similar to other cell membranes, normal lens membranes are composed of loosely organized lipids. In early cataract lipid density uniformly increased along the cell membranes at the clinically transparent areas, while at the areas with clinically evident fine opacities, small fusiform and globuler lipid drops were formed by even more dense lipids. Confluent cortical cataracts were associated with disappearance of membrane lipids. In our study our findings demonstrated intramembrane lipid changes associated with cataractogenesis.

Lipid fluidity of triacylglycerol-rich lipoproteins isolated from copper-deficient rats

Triacylglycerol-rich lipoproteins (TGRLP) were isolated from Cu-deficient and control rats. TGRLP from Cu-deficient rats appeared more fluid than those from controls as sensed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH). This high fluidity was related to a low cholesterol:phospholipid ratio and high triacylglycerol content in these lipoproteins. TGRLP from Cu-deficient rats were more susceptible to in vitro peroxidation than lipoproteins from control rats as shown by the rate of diene conjugation. The damage induced by the peroxidation resulted in a more ordered state of the lipid fraction especially in lipoproteins from Cu-deficient rats. Thus, after in vitro peroxidation, TGRLP from Cu-deficient rats were more rigid than those from controls. These results suggest that Cu deficiency induces modifications in physicochemical properties of TGRLP which could affect their metabolism.

Failure to withstand oxidative stress induced by phospholipid hydroperoxides as a possible cause of the lens opacities in systemic diseases and ageing

Lipid peroxidation (LPO) is a causative factor of cataract. The increased concentrations of primary molecular LPO products (diene conjugates, lipid hydroperoxides) and end fluorescent LPO products, were detected in the lipid moieties of the aqueous humor samples obtained from patients with senile and complicated cataracts as compared to normal donors. The degrees of lens clouding were assessed quantitatively by measuring the optical density indices and areas of equidensities using digital image analysis. Human cataractous lenses showed decreased activity of glutathione peroxidase (GPX, catalyzing reduction of organic hydroperoxides including hydroperoxides of lipids). The apparent Km for tert-butylhydroperoxide was 0.434 mM for human normal and cataractous lens GPX. When lenses were exposed for 1 h at 37 degrees C to linoleic acid hydroperoxide (LOOH, 0.5 mM) or egg phosphatidyl-choline hydroperoxide (PLOOH, 1 micro mol per 112 micro mol of phospholipid) in liposomes suspended in the incubation medium, normal, immature and mature human cataractous lenses showed a significant loss in the residual content of liberated LOOH to 62%, 38% or 17%, correspondingly, but little or no reduction was observed with PLOOH in liposomal membranes. Human, rabbit or mice transparent or immature cataractous lenses induced significantly more absorbance changes in conjugated diene, iodometric and TBA-reactive substance measurements when incubated with liposomal membranes which were decreased in the presence of free radical scavengers and antioxidant enzymes (EDTA, SOD, L-carnosine, chelated iron, catalase). Injection into the vitreous body of the rabbit eye of a suspension of liposomes prepared from phospholipids containing LPO products induced the development of posterior subcapsular cataract. Saturated liposomes did not cause clouding of the lens. This modelling of cataract was accompanied by accumulation of fluorescing LPO products in the vitreous body, aqueous humor and the lens and also by a fall in the concentration of GSH in the lens. The peroxidative damage to the lens cell membranes and biomolecules induced in the lack of reductive detoxification of phospholipid hydroperoxides is proposed as the triggering mechanism of cataractogenesis.

Nitric oxide potentiates hydrogen peroxide-induced killing of Escherichia coli

Previously, we reported that nitric oxide (NO) provides significant protection to mammalian cells from the cytotoxic effects of hydrogen peroxide (H2O2). Murine neutrophils and activated macrophages, however, produce NO, H2O2, and other reactive oxygen species to kill microorganisms, which suggests a paradox. In this study, we treated bacteria (Escherichia coli) with NO and H2O2 for 30 min and found that exposure to NO resulted in minimal toxicity, but greatly potentiated (up to 1,000-fold) H2O2-mediated killing, as evaluated by a clonogenic assay. The combination of NO/H2O2 induced DNA double strand breaks in the bacterial genome, as shown by field-inverted gel electrophoresis, and this increased DNA damage may correlate with cell killing. NO was also shown to alter cellular respiration and decrease the concentration of the antioxidant glutathione to a residual level of 15-20% in bacterial cells. The iron chelator desferrioxamine did not stop the action of NO on respiration and glutathione decrease, yet it prevented the NO/H2O2 synergistic cytotoxicity, implicating metal ions as critical participants in the NO/H2O2 cytocidal mechanism. Our results suggest a possible mechanism of modulation of H2O2-mediated toxicity, and we propose a new key role in the antimicrobial macrophagic response for NO.

Phases and phase transitions of the sphingolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Herein, we present a review of the LIPIDAT data subset referring to sphingolipids together with an analysis of these data. It includes data collected over a 40-year period and consists of 867 records obtained from 112 articles in 25 different journals. An analysis of these data has allowed us to identify trends in hydrated sphingolipid phase behavior reflecting differences in fatty acyl chain length, saturation and hydroxylation, head group type, and sphingoid base identity. Information on the mesomorphism of biologically-derived and dry sphingolipids is also presented. This review includes 161 references.

A review of the evidence that ultraviolet irradiation is a risk factor in cataractogenesis

There are two approaches to the question of whether solar radiation contributes to human cataract. The first, epidemiological studies, investigates correlations between man's environmental UV dose and cataract frequency. The second, animal models, investigates the effects of varying UV strengths and spectra on lens opacification in vivo or in vitro. While the latter approach typically provides for direct evidence, the data must still be extrapolated to human lenses. Results of physiological studies suggest that UV photons interact with proteins of the epithelial cell membranes, in particular tryptophan residues, transport ATPases and cytoskeletal proteins. One hypothesis is that damage to ion pumps and channels accumulates over the years as repair processes incompletely restore membrane function. Peroxidative damage is likely in view of the formation of UV-induced lipid peroxides in the lens epithelial membranes. Loss of homeostatic control of ions, particularly Ca++, leads to crystallin disorder in small regions of the underlying fiber cells. In our diabetic cataract studies, intracellular Ca++ electrodes detected large shifts in intracellular Ca++ before bulk-lens changes were apparent. Similar occurrences likely characterize UV cataract. Our lab is one of few studying lens physiology and how it is altered following transient exposures to UV-B and UV-A, both of which pass through the cornea. Some changes include: loss of epithelial cell GSH; elevated Ca++; loss of membrane voltage; impaired transport of Na+; increased permeability to ions and water; inhibition of critical enzymes; and a decrease in the rate of membrane synthesis.

The role of the lens epithelium in development of UV cataract

In view of renewed interest in the lens epithelium as the initiation site for cataract development, it seemed timely to review recent studies which appear to establish UV damage in the lens epithelium as the cause of UV cataract. While UV photons can and do interact with lens proteins in the cortex and nucleus, experimental results from cultured lenses and tissue cultured epithelial cells also demonstrate both mutagenic and cytotoxic effects in the epithelium. This minireview examines UV-induced changes in lens physiology that appear to follow epithelial cell damage, including inactivation of critical enzymes of transport and metabolic processes. Changes in membrane function include altered cation transport, increased permeability, and altered biosynthesis. One potential scenario for the propagation of damage from the epithelium to the underlying fiber cells includes calcium elevation, an early event in cataract development and critical to many physiological processes.

Spectral characterization of lipid peroxidation in rabbit lens membranes induced by hydrogen peroxide in the presence of Fe2+/Fe3+ cations: a site-specific catalyzed oxidation

The role of free-radical-induced lipid peroxidation (LPO) in relation to lens opacity is investigated using Fourier transform infrared spectroscopy. Phospholipids extracted from nuclear and cortical regions of the rabbit lens membranes are subjected to oxidative-damage induced by hydrogen peroxide and Fe2+/Fe3+ cations. Vibrational data suggest a homolytic decomposition of the unsaturated membrane hydrocarbon chains at cis-double bonds, as well as structural modifications at the carbonyl and phosphate-oxygen sites of the fiber cell membranes upon metal oxidation. This is also evident from a substantial induction of the carbonyl groups and a significant dephosphorylation of the phosphate groups in lens phospholipids. These covalent modifications and/or alterations of the carbonyl and phosphate groups, and specificity of certain vibrational modes only to iron oxidation, may serve as a diagnostic probe of the metal-catalyzed LPO in lens membranes. Despite covalent modifications of the hydrophilic part of the lens membranes, hydrocarbon chain region remains largely intact at physiological concentrations of hydrogen peroxide. However, at elevated concentrations of hydrogen peroxide, a substantial breakdown of the acyl chains occurs. Striking similarities observed between the spectral features of the oxidized rabbit lens phospholipids and those of the cataractous human lenses suggest that the mechanism and pathways of lipid oxidation in model animal membranes and in human lenses are similar. Differences in the nuclear or cortical regions are also evident upon metal oxidation. Nuclear lipids experience increased effects of the metal oxidation compared to cortical lipids. Both the nuclear or the cortical lipids indicate effective penetration of the bilayer water creating segregated membrane domains, possibly through breakdown of headgroup-specific lipid-water interactions. This could effectively alter the lens membrane permeability and fluidity, rendering it susceptible to a host of toxic oxidants present in the eye. These findings also demonstrate that LPO can lead to acyl chain degradation that may effectively derange the lens membrane function, which could be a contributing factor in cataractogenesis.

Fourier transform infrared study of the rod outer segment disk and plasma membranes of vertebrate retina

Phospholipid composition and structure of disk and plasma membranes purified from bovine rod outer segments (ROS) are examined using Fourier transform infrared spectroscopy. Vibrational data indicate that both disk and plasma membranes lack sphingophospholipids, in contrast to the lens membranes. The hydrocarbon chains of the disk lipids are unsaturated by a factor of 5 over the acyl chains of the plasma lipids. The plasma lipids with 3-fold higher cholesterol and 5-fold higher saturation melt at a higher temperature (26 degrees C) than the disk lipids which melt at 16 degrees C. The transition temperature decreases by more than 20 degrees C in going from disk lipids to disk membrane, indicating a large drop in the enthalpy of the ROS membrane-matrix, presumably due to enhanced rhodopsin-lipid interaction. The lipid composition predisposes the disk and plasma membranes to be fluid and structurally disordered (about 84%) around physiological temperature. The fluid phospholipid environment of the disk membrane (i.e., just a few degrees above subzero temperatures) is considered to be vital for the ROS photoreceptor function. The amide I band profile of rhodopsin indicates an extensive alpha-helical (53%) peptide chain, with little beta-sheet (21%) and beta-turns (18%) in ROS membranes. This structure and/or conformation is conserved between 0-60 degrees C even though disk and plasma lipids undergo a phase change. The H-D exchange data indicate that as much as 84% of the peptide residues of ROS membranes in partially bleached retinas is accessible to D2O solvent after 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)