Chloroplast and microsomal membranes from the primary leaf of bean acquired increasing proportions of gel phase lipid as the tissue senesced. The lipid-phase transition temperature for microsomes rose from about 25 to 43 C and that for chloroplasts rose from below -30 C to about 52 C within 5 weeks of planting. This was accompanied by large increases (2-to 4-fold) in the sterol to phospholipid ratio of the membranes, which reflected breakdown of phospholipid. Changes in fatty acid saturation were of insufficient magnitude to account for the rise in transition temperature. AU of these senescence-related changes in chloroplast and microsomal membranes were also induced by treating young, 2-week-old-plants with 10 milligrams per liter paraquat. Within 48 hours of treatment, the transition temperature rose from 25 to 57 C for microsomes and from below -30 to 24 C for chloroplasts. The membranes sustained only small changes in fatty acid saturation, comparable to those incurred during natural senescence, and there was a selective loss ofphospholipid, resulting in augmented sterol to phospholipid ratios. Malondialdehyde, a product of lipid peroxidation, rose by 2-to 3-fold in both senescing and paraquat-treated leaves. Paraquat is known to form cation radicals that react with 02 to produce 02 and has been implicated as an agent of lipid peroxidation. Accordingly, these observations suggest that membrane deterioration during natural senescence may be due in part to free radical damage.
Wide-angle x-ray diffraction studies revealed that the lipid-phase transition temperature of multiple sclerosis (MS) myelin was about 20°C lower than that of normal myelin, indicating differences in the physical organization of the bilayer. The transition temperature of liposomes prepared from total lipid extracts of normal myelin was 12°C lower than that for corresponding intact myelin, demonstrating that the protein of normal myelin had a substantial ordering effect on the lipid bilayer. The transition temperature for liposomes of MS myelin lipid was essentially similar to that for isolated MS myelin. Because the protein/phospholipid ratio was higher in MS myelin, and no difference in degree of fatty acid saturation was observed, the inability of MS myelin protein to organize the lipid reflects a qualitative difference in the proteins.Multiple sclerosis (MS) is a demyelinating disease affecting primarily the white matter of the central nervous system (1, 2). Demyelination occurs in discrete plaques found throughout the white matter of the brain (3, 4). Consequently, the disease has been considered to be one with multiple foci of discrete demyelination.There remains the possibility, however, that there is a more generalized involvement of white matter in the pathology of MS. In a recent low-angle x-ray diffraction study in which long-chain fatty alcohols were incorporated into myelin, the presence of additional reflections in the MS samples implied that the alcohols formed regular structures in domains of disorganized bilayer (5). Boggs and Moscarello (6) have shown an increased protein/lipid content in MS myelin but were unable to detect any significant difference in membrane fluidity by using fatty acid spin labels or fluorescent probes. Recently, however, an x-ray diffraction study re-vealed that basic protein from MS myelin was much less effective in inducing lipid organization in egg phosphatidylglycerol vesicles in comparison with basic protein from normal myelin (7). As well, the formation of the characteristic multilayer arrangement has been shown to depend on the concentration of basic protein because sharp reflections were observed by x-ray diffraction only when the concentration of basic protein was 30% (wt/wt) in the basic proteinphosphatidylglycerol system (8), supporting an earlier report in which digestion by trypsin disrupted myelin structure (9).The present study focuses on alterations in biophysical and chemical changes in myelin due to the neuropathology of MS.
The biophysical properties of human myelin isolated from white matter of patients aged two months to 74 years were investigated using wide-angle X-ray diffraction. The myelin transition temperature increased from 13°C to 65°C as age increased from two months to 17 years, demonstrating an increase in the myelin lipid stability. Following this maturation period, the myelin transition temperature remained constant at 65'C until age 50. Beyond age 50, the transition temperature of myelin decreased by 13°C indicating that myelin stability decreased with aging. During this aging period, the levels of malondialdehyde and conjugated diene increased, indicating an increasing amount of lipid peroxidation. Although evidence is indirect, the results of this investigation strongly suggest that free radicals could be a primary factor in the acceleration of the aging processes in the human brain, MyelinLipid phase Aging Lipid peroxidation
Treatment of primary bean leaves with 10 mg/I paraquat induces the formation of gel phase lipid in microsomal and chloroplast membranes and enhances the activity of superoxide dismutase, but only if the leaves are exposed to light. These light‐dependent changes in membrane lipid phase properties show a close temporal correlation with enhanced O27 production by illuminated chloroplasts and the onset of lipid peroxidation. Malondialdehyde and ethane, which are both formed during lipid peroxidation, are produced in large amounts by paraquat‐treated leaves exposed to light, but not by those maintained in the dark. Electron spin resonance measurements indicate that production of O27 by illuminated chloroplasts is more than 2‐fold greater in the presence of paraquat than in its absence. The identity of the radical formed by illuminated chloroplasts in the presence of paraquat was confirmed by using the diagnostic spin trap 5,5′‐dimethyl‐l‐pyrroline‐l‐oxide and by establishing that its formation is sensitive to superoxide dismutase. The observations collectively indicate that paraquat‐mediated membrane deterioration is light‐dependent and attributable to enhanced O27‐production.
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