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Biomedical subjects

A Sevanian

Publications and source records attributed to A Sevanian.

At least 109 records · Page 6Linked to original sources
PubMed1988

Hydrogen peroxide localization in ocular tissue: an electron microscopic cytochemical study.

A reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H)-dependent hydrogen peroxide-generating activity of rat ocular tissue was investigated cytochemically utilizing the cerium method. NADPH or NADH oxidation resulted in electron-dense fine granular deposits on the corneal epithelial and endothelial plasma membranes, around the photoreceptors of the retina, and on the processes of the retinal pigmented epithelium. Control specimens incubated in a substrate-free medium did not show any such deposits. These results suggest that, under normal conditions, there is generation of oxidants, such as hydrogen peroxide, in the ocular structures, and that such oxidants can be visualized by cytochemical techniques.

Animals↗
PubMed1988

Immunohistochemical localization of glutathione peroxidase in ocular tissue.

In this study enzyme glutathione peroxidase was localized in ocular tissue of Lewis rats using the peroxidase antiperoxidase immunohistochemical technique. Antisera to glutathione peroxidase was raised in a rabbit. Immunoreactive glutathione peroxidase was found to be distributed predominantly in the corneal epithelium and endothelium, choroid, inner segment of photoreceptors and retinal pigmented epithelium. Its presence in these sites suggests an adaptation to oxidative stress where this antioxidant enzyme along with other antioxidant systems serves to prevent damage.

Animals↗
PubMed1987

Treatment of experimental granulomatous uveitis by lipoxygenase and cyclo-oxygenase inhibitors.

Lens-induced granulomatous uveitis was produced in brown Norway rats. To determine the role of the lipoxygenase and cyclo-oxygenase products of arachidonic acid in the modulation of granulomatous uveitis, one group of experimental animals was treated with nafazatrom, while another group was treated with nordihydroguaiaretic acid (lipoxygenase inhibitors). Both resulted in significant attenuation of granulomatous inflammation, with reduced giant-cell infiltration into the choroid. In contrast, indomethacin (a cyclo-oxygenase inhibitor) therapy resulted in increased giant-cell formation. These findings suggest that the lipoxygenase and cyclo-oxygenase products of arachidonic acid play a role in the development of granulomatous uveitis and that such inflammation can be selectively suppressed by lipoxygenase inhibitors.

Animals↗
PubMed1987

Retinal lipid peroxidation in experimental uveitis.

Lewis rats with retinal S-antigen-induced uveitis were treated with the enzymatic antioxidants superoxide dismutase and catalase or with the hydroxyl radical scavenger dimethyl thiourea for one week. The enucleated eyes of these animals and of normal saline-treated control rats were studied for the presence of lipid peroxidation products in the retina; In addition, choroidal thickness was measured by morphometric analysis to determine the severity of choroidal inflammation. Control animals showed severe chorioretinal inflammation and presence of lipid peroxidation products. Intraocular inflammation was less severe and formation of conjugated dienes and malonaldehyde was significantly inhibited in animals treated with the enzyme antioxidants or dimethyl thiourea. These results suggest that free radicals generated during intraocular inflammation can lead to retinal damage by peroxidation of its lipid cell membranes.

Animals↗
PubMed1987

Cholesterol autoxidation in phospholipid membrane bilayers.

Lipid peroxidation in unilamellar liposomes of known cholesterol-phospholipid composition was monitored under conditions of autoxidation or as induced by a superoxide radical generating system, gamma-irradiation or cumene hydroperoxide. Formation of cholesterol oxidation products was indexed to the level of lipid peroxidation. The major cholesterol oxidation products identified were 7-keto-cholesterol, isomeric cholesterol 5,6-epoxides, isomeric 7-hydroperoxides and isomeric 3,7-cholestane diols. Other commonly encountered products included 3,5-cholestadiene-7-one and cholestane-3 beta, 5 alpha, 6 beta-triol. Superoxide-dependent peroxidation required iron and produced a gradual increase in 7-keto-cholesterol and cholesterol epoxides. Cholesterol oxidation was greatest in liposomes containing high proportions of unsaturated phospholipid to cholesterol (4:1 molar ratio), intermediate with low phospholipid to cholesterol ratios (2:1) and least in liposomes prepared with dipalmitoylphosphatidylcholine and cholesterol. This relationship held regardless of the oxidizing conditions used. Cumene hydroperoxide-dependent lipid peroxidation and/or more prolonged oxidations with other oxidizing systems yielded a variety of products where cholesterol-5 beta,6 beta-epoxide, 7-ketocholesterol and the 7-hydroperoxides were most consistently elevated. Oxyradical initiation of lipid peroxidation produced a pattern of cholesterol oxidation products distinguishable from the pattern derived by cumene hydroperoxide-dependent peroxidation. Our findings indicate that cholesterol autoxidation in biological membranes is modeled by the peroxide-induced oxidation of liposomes bearing unsaturated fatty acids and suggest that a number of cholesterol oxidation products are derived from peroxide-dependent propagation reactions occurring in biomembranes.

Cholesterol↗
PubMed1987

Role of oxygen radicals in experimental allergic uveitis.

The experimental autoimmune disease elicited by a large dose of retinal S antigen in guinea pigs is characterized by massive necrotizing uveitis and retinitis. Treatment of these animals with the antioxidants superoxide dismutase, catalase, and sodium benzoate resulted in marked reduction of uveal inflammation. The attenuated inflammation was characterized by a relatively well-preserved retina and retinal pigment epithelium along with a reduction of subretinal exudate and vitreous inflammation. These findings suggest that reactive oxygen metabolites may play a role in the destruction of ocular tissue and amplification of the inflammatory process in experimental uveitis.

Animals↗
PubMed1986

Uric acid-iron ion complexes. A new aspect of the antioxidant functions of uric acid.

In order to survive in an oxygen environment, aerobic organisms have developed numerous mechanisms to protect against oxygen radicals and singlet oxygen. One such mechanism, which appears to have attained particular significance during primate evolution, is the direct scavenging of oxygen radicals, singlet oxygen, oxo-haem oxidants and hydroperoxyl radicals by uric acid. In the present paper we demonstrate that another important 'antioxidant' property of uric acid is the ability to form stable co-ordination complexes with iron ions. Formation of urate-Fe3+ complexes dramatically inhibits Fe3+-catalysed ascorbate oxidation, as well as lipid peroxidation in liposomes and rat liver microsomal fraction. In contrast with antioxidant scavenger reactions, the inhibition of ascorbate oxidation and lipid peroxidation provided by urate's ability to bind iron ions does not involve urate oxidation. Association constants (Ka) for urate-iron ion complexes were determined by fluorescence-quenching techniques. The Ka for a 1:1 urate-Fe3+ complex was found to be 2.4 X 10(5), whereas the Ka for a 1:1 urate-Fe2+ complex was determined to be 1.9 X 10(4). Our experiments also revealed that urate can form a 2:1 complex with Fe3+ with an association constant for the second urate molecule (K'a) of approx. 4.5 X 10(5). From these data we estimate an overall stability constant (Ks approximately equal to Ka X K'a) for urate-Fe3+ complexes of approx. 1.1 X 10(11). Polarographic measurements revealed that (upon binding) urate decreases the reduction potential for the Fe2+/Fe3+ half-reaction from -0.77 V to -0.67 V. Thus urate slightly diminishes the oxidizing potential of Fe3+. The present results provide a mechanistic explanation for our previous report that urate protects ascorbate from oxidation in human blood. The almost saturating concentration of urate normally found in human plasma (up to 0.6 mM) represents 5-10 times the plasma ascorbate concentration, and is orders of magnitude higher than the 'free' iron ion concentration. These considerations point to the physiological significance of our findings.

Animals↗
PubMed1986

Catalytic properties and inhibition of hepatic cholesterol-epoxide hydrolase.

Hepatic cholesterol-epoxide hydrolase is a microsomal enzyme which appears to be catalytically distinct from the epoxide hydrolase responsible for the catabolism of a wide variety of aromatic and aliphatic epoxides. The diastereomeric forms of cholesterol epoxide, cholesterol 5 alpha,6 alpha-, and cholesterol 5 beta,6 beta-epoxides are converted to cholestane-3 beta,5 alpha,6 beta-triol with equal facility. Kinetic analysis of cholesterol-epoxide hydrolase demonstrated that both diastereomers bind to a common catalytic site. Apparent Km values of 3.69 and 4.42 microM were derived for cholesterol 5 alpha,6 alpha- and cholesterol 5 beta,6 beta-epoxide, respectively. In addition, enzyme activity with both diastereomers was product-inhibited by cholestanetriol through a competitive mechanism with the apparent Ki for cholestanetriol being 10.8 and 6.8 microM against cholesterol alpha- and beta-epoxides, respectively. This inhibitory effect of cholestanetriol may account for the difference observed in the hydration rates for the cholesterol epoxide isomers when they are incubated together in the presence of liver microsomes. Inhibitors of epoxide hydrolase were studied, and three oxidation products were found to be particularly effective against cholesterol-epoxide hydrolase while producing no significant inhibition of styrene-epoxide hydrolase. These inhibitors were 7-ketocholesterol, 6-ketocholestanol, and 7-ketocholestanol, the latter displaying an apparent Ki lower than the Km for either cholesterol epoxide isomer. None of the xenobiotic epoxide hydrolase inhibitors or activators studied affected cholesterol-epoxide hydrolase activity.

Animals↗
PubMed1986

Effect of iron chelation on severity of ocular inflammation in an animal model.

Metabolites of oxygen-free radicals generated by polymorphonuclear leukocytes and macrophages are believed to inflict the initial tissue damage in acute inflammations. Of the various oxygen products, hydroxyl radicals are known to be potent toxic agents, and their generation depends largely on the presence of free iron. Treatment of experimental uveitis in Lewis rats with an iron chelator, deferoxamine mesylate, resulted in marked reduction in choroidal inflammation and suppression of retinal damage. These findings suggest that in experimental uveitis the severity of ocular inflammation and tissue damage may be mediated by the iron-catalyzed generation of hydroxyl radicals, and deferoxamine may thus serve as an anti-inflammatory agent.

Animals↗
PubMed1986

Modulation of lens-induced uveitis by superoxide dismutase.

Recent studies on reactive oxygen metabolites have suggested that these products may be important mediators in the early tissue damage that develops from immunopathologic inflammations. The superoxide anion appears to be the principal product of the respiratory burst. In order to determine the effects of removing superoxide with superoxide dismutase on modulating the inflammation in experimental phacoanaphylactic endophthalmitis, the animals with this experimental disorder were treated with superoxide dismutase. The treatment resulted in significant reduction of choroidal inflammation, retinal edema and vasculitis, suggesting phlogogenic role of superoxide in the ocular immune-complex disease.

Animals↗
PubMed1986

Antiphlogistic effect of catalase on experimental phacoanaphylactic endophthalmitis.

Oxygen-derived free radicals generated by polymorphonuclear leukocytes have recently been implicated as important pathogenic mediators of tissue damage in immune complex-mediated acute inflammatory reactions. In order to study the role of oxygen radicals in experimentally induced phacoanaphylactic endophthalmitis, rats were immunized with soluble bovine lens protein and, following surgical disruption of the anterior lens capsule, subjected to treatment with catalase. Our data show that phacoanaphylactic endophthalmitis, as determined morphologically and by measurements of retinal edema, choroidal inflammation, retinal hemorrhage and necrotizing vasculitis, is greatly diminished in catalase-treated animals. This protective effect of catalase indicates that hydrogen peroxide (H2O2) and/or conversion products of H2O2 (e.g., hydroxyl radical, hypochlorous acid) may play an important role in the development of experimental phacoanaphylactic endophthalmitis.

Animals↗
PubMed1986

Modulation of lens-induced uveitis by dimethyl sulfoxide.

Experimental lens-induced uveitis was produced in Lewis rats by sensitization with bovine lens protein prior to disruption of the lens capsule. Treatment of these animals by intraperitoneal injection of dimethyl sulfoxide resulted in reduction in retinal vasculitis, hemorrhage and necrosis. Furthermore, morphometric analysis of choroidal inflammation revealed significant reduction of choroidal thickness in the treated animals. These findings suggest that hydroxyl radicals play a role in producing ocular tissue damage in the acute Arthus-type of ocular inflammation.

Animals↗
PubMed1985

Conservation of vitamin C by uric acid in blood.

Urate at physiological concentrations increased the stability of ascorbate in human serum approximately fivefold. These measurements were made by depleting human serum of urate with uricase. In model experiments (using phosphate buffer instead of serum), urate protected against iron-induced ascorbate oxidation and, to a lesser extent, pH-induced ascorbate oxidation. In both human serum and in phosphate buffer, urate exerted its protective effect without itself undergoing measurable oxidation as determined by spectrophotometric and high pressure liquid chromatography (HPLC) techniques. These experiments suggest an important physiological role for urate in preserving ascorbate in blood and other biological fluids.

Ascorbic Acid↗
PubMed1985

Phospholipase A2 dependent release of fatty acids from peroxidized membranes.

Unilamellar vesicles (liposomes) consisting of liver phosphatidylcholine and phosphatidylethanolamine were used as model membranes and subjected to lipid peroxidation. Following peroxidation samples were treated with phospholipase A2 from snake venom and subsequently analyzed for products of lipid peroxidation and of phospholipase A2 action. A significantly increased susceptibility to phospholipase A2 was noted for liposomes subjected to peroxidation, as compared to controls, which correlated with the extent of lipid peroxidation measured by the formation of thiobarbituric acid reacting products and conjugated dienes. Low levels of peroxides were detected in control liposomes and these peroxidases were rapidly cleaved by phospholipase A2, such that nearly 40% of the total peroxide content was associated with free fatty acids after l5 min incubation. Oxidized liposomes contained over seven fold the level of lipid peroxidation products, and the were also rapidly cleaved by phospholipase A2 where over 50% were recovered as free fatty acids following l5 min of exposure to phospholipase A2. Along with this high order of removal of oxidized fatty acids, a marked hydrolysis of intact fatty acids was also observed. The extent of fatty acid release was roughly correlated with the degree of fatty acid unsaturation. A substantial increase in the release of arachidonic acid was found when peroxidized membranes were analyzed.

Fatty Acids↗