Glutathione peroxidase from bovine lens: a selenoenzyme

Purification and molecular properties of ascorbate peroxidase from bovine eye

Ascorbate peroxidase (APX) is a hydrogen peroxide-scavenging peroxidase which uses ascorbate (AsA) as the specific electron donor. APX has not been isolated in mammals. Ocular tissue contains AsA at high concentrations, and we detected APX activity in bovine retinal pigment epithelium (RPE) and choroid. We purified APX from bovine RPE and choroid by four chromatographic steps. The purified APX was a monomeric hemoprotein with a molecular mass of 43 kDa. The amino acid sequence of the amino-terminal region of the purified APX showed a high degree of homology to that of plants. The primary product of the APX reaction was identified as the monodehydroascorbate radical. The APX showed high specificity for AsA as an electron donor. This is the first isolation and characterization of APX from mammals, and its role in the protection against active species of oxygen in ocular tissue is discussed.

The contribution of GSH peroxidase-1, catalase and GSH to the degradation of H2O2 by the mouse lens

Utilizing cultured lenses from normal and homozygous glutathione peroxidase (GSHPx-1) knockout mice and inhibitors for GSSG Reductase (GSSG Red), 1,3-bis(2-chlorethyl)-1-nitrosourea (BCNU) and catalase (Cat), 3-aminotriazole (3-AT), the ability to degrade H2O2 was examined at two H2O2 concentrations, 300 microM and 80 microM. It was found that GSHPx-1 contributed about 15% to the H2O2 degradation. The Cat contribution was concentration dependent being about 30% at 300 microM H2O2 and approximately 8% to 15% at 80 microM H2O2. GSH loss measured as nonprotein thiol (NP-SH) was shown to be linked to most of the remaining H2O2 degradation accounting for about 54% to 72% of the H2O2 degradation at 300 microM and 80 microM, respectively. However, based on evaluation of the ability of GSH to nonenzymatically degrade H2O2, it can only account for about 36% at 300 microM and 19% at 80 microM H2O2 of the observed lens H2O2 degradation. It is, therefore, concluded that lens GSH must be involved in other reactions either directly or indirectly related to H2O2 degradation.

Purification and characterization of glutathione peroxidase from human blood platelets. Age-related changes in the enzyme

Platelet glutathione peroxidase (GPx) is known to play a pivotal role in controlling the level of lipid hydroperoxides, especially those resulting from the 12-lipoxygenase activity. GPx was purified from the cell cytosol by more than 700-fold using an exchange chromatography, FPLP, gel filtration and covalent fixation. Isoelectric focusing revealed a peak activity at pH 5.1. The molecular mass of the enzyme was found between 90 and 100 kDa by gel filtration, and was approximating at 23 kDa by SDS-PAGE. A polyclonal antibody raised against commercial bovine erythrocyte GPx recognized the human platelet enzyme. It is concluded that human platelet GPx is likely a homotetramer of 92 kDa as described for most other sources. We have also found that the decreased platelet GPx activity observed in platelets from elderly people is associated with a lower content of the immunoreactive enzyme.

Effects of inhaled ethylene oxide on the lens glutathione redox cycle in rats

The effects of chronic ethylene oxide (EtO) inhalation on the lens glutathione redox cycle were investigated. When Wistar male rats were exposed to 500 ppm EtO for 6 h a day, 3 times a week for 13 weeks, glutathione reductase decreased significantly in the lens while glutathione peroxidase did not. Glutathione reductase activity decreased time dependently, by as much as 81% after 13 weeks. In spite of changes in the glutathione redox cycle, reduced and oxidized glutathione levels were not affected. Our results raise the possibility that EtO inhalation may produce a cataract via changes in the glutathione redox cycle.

Non-selenium glutathione peroxidase without glutathione S-transferase activity from bovine ciliary body

A glutathione peroxidase was purified from bovine ciliary body by ammonium sulfate fractionation. Sephacryl S-300 gel filtration, diethylaminoethyl (DEAE)-cellulose chromatography and hydroxyapatite chromatography. The purified enzyme has an apparent mw of 112 kDa by gel filtration and 29 kDa by SDS-polyacrylamide gel electrophoresis. The enzyme therefore is composed of four identical subunits. The ciliary enzyme is active with H2O2 (25), cumene hydroperoxide (170), t-butyl hydroperoxide (22), triphenylcarbinyl hydroperoxide (12), linoleic hydroperoxide (34) and 5-phenylpentenyl hydroperoxide (22): the numbers after substrates are K'm in microM. Glutathione is essential for the reaction; L-cysteine, dithiothreitol and 2-mercaptoethanol are inactive. Mercaptosuccinate (10 microM) inhibits the enzyme competitively (Ki = 7 microM) when cumene hydroperoxide is substrate, and uncompetitively (Ki = 10 microM) when H2O2 is substrate. No selenium was found in the enzyme by the fluorometric assay with 2.3-diaminonaphthalene. The enzyme demonstrates no glutathione S-transferase activity when tested with 1-chloro-2,4-dinitrobenzene, and several other compounds. A partial sequence of the enzyme shows some similarities both to Se-glutathione peroxidases and a glutathione S-transferase isozyme.

Glutathione synthesis and glutathione redox pathways in naphthalene cataract of the rat

This investigation examined many parameters during the course of early development of naphthalene-induced cataract in a time span of 0 to 79 days of treatment. Feeding naphthalene daily to Black-Hooded rats resulted in gradual progressive development of cataract. The first faint opacities were detectable after 7 days. Free soluble total glutathione (oxidized and reduced) of these lenses was shown to gradually decrease to a maximum loss of about 20%, a value reached by day 30 of treatment. No activity loss of either enzyme required for glutathione synthesis (gamma-glutamylcysteine synthetase or glutathione synthetase) was observed in homogenates of naphthalene versus control lenses. There was also neither impairment of [35S]-L-cystine uptake nor of [35S]-glutathione synthetic capacity in lenses cultured from rats after 12, 24 or 36 days of naphthalene feeding when compared to control lenses. Hence, glutathione loss cannot be explained by a damaged glutathione synthesis system. Progressive activity loss of glutathione peroxidase and glutathione reductase was observed. The loss of glutathione peroxidase activity was especially remarkable. Thus, the defense system against oxidative damage is impaired and may be a significant factor in naphthalene-induced cataract of the rat.

Glutathione S-transferase of bovine iris and ciliary body: characterization of isoenzymes

Five forms of glutathione (GSH) S-transferase (GST) having catalytic activities towards a variety of xenobiotics were present in bovine iris-ciliary body. In contrast to that in lens, cornea, and retina, GST isoenzymes belonging to all the three classes (alpha, mu and pi) were present in iris as well as in the ciliary body. GST isoenzymes of iris-ciliary body had pI values of 8.7, 7.4, 7.0, 6.6, and 6.0. GST 8.7 and GST 7.4 were apparent homodimers of 27,000 and 22,500 Mr subunits, respectively. GST 8.7 cross-reacted only with antibodies raised against the alpha class GST of human liver and GST 7.4 cross-reacted with the antibodies raised against GST pi of human placenta. GST 7.0 and 6.6 were heterodimers of Mr 26,500 and 25,000 subunits and both these subunits cross-reacted with the antibodies raised against the mu class human GST. Iris-ciliary body contained both, GSH peroxidase I and GSH peroxidase II activities and in this respect also, they differ from lens, cornea, and retina each of which have only one of these two activities. The presence of several GST isoenzymes belonging to all the three major classes and both GSH peroxidase I and II activities in iris-ciliary body may be important for the detoxification of oxidants and xenobiotics in order to prevent their infiltration in aqueous humor.

Ascorbate peroxidase in bovine retinal pigment epithelium and choroid

Peroxidase, catalyzing hydrogen peroxide reduction concurrent with ascorbate oxidation, was demonstrated in the extract of retinal pigment epithelium and choroid. The peroxidase in the choroid, RPE, and retina are 236.1, 25.1, and 0.5 units/mg protein respectively. Ammonium sulfate fractionation and high pressure liquid chromatography showed that the peroxidase in the RPE-choroid is associated with a group of heme proteins with absorption maxima at 410 nm, and optimal activity at pH 4.5. The high peroxidase activity in the RPE-choroid explains the observation of dehydroascorbate in these tissues and indicates a possible role of this enzyme in the removal of H2O2.

Glutathione peroxidase, glutathione reductase and glutathione-S-transferase activities in the rhesus monkey lens as a function of age

The activities of glutathione peroxidase, glutathione reductase and glutathione-S-transferase were determined in lenses from rhesus monkeys (Macaca mulatta) as a function of age. The ages ranged from 137 day old embryos to a 34 year old. Glutathione peroxidase activity (units/g lens) occurred at a very low level in lenses of fetuses and neonates, but increased dramatically with age, peaking in the adult of about 12 to 20 years of age and declining thereafter. Glutathione reductase activity (units/g lens) decreased throughout juvenile life, leveling off when adulthood was reached (at least 6 years of age). Glutathione-S-transferase activity showed considerable age-related variation. Calculations show that glutathione reductase is rate-limiting in the glutathione redox pathway.

Characterization of glutathione peroxidase in frog retina

The properties and kinetic parameters of glutathione peroxidase were measured in retinal homogenates from frogs (R. pipiens) using a spectrophotometric assay in which the oxidation of glutathione is coupled to the oxidation of NADPH+ by exogenous glutathione reductase. The standard assay utilized 5.0 mM glutathione and 0.6 mM cumene hydroperoxide in 50 mM sodium phosphate buffer at pH 7.0. All solutions were bubbled with argon and all reactions were carried out under an atmosphere of argon. The enzyme activity was linear with protein concentration at different concentrations of both substrates. Determination of the pH optimum was complicated by a large increase in non-enzymatic oxidation of glutathione at alkaline pH. The highest ratio of enzymatic to non-enzymatic activity was at pH 7.0. Increasing glutathione concentration showed less effect on the spontaneous reaction than increasing the cumene hydroperoxide concentration. Glutathione peroxidase Km value for glutathione was 3.86 mM and for cumene hydroperoxide was 0.55 mM. Vmax for glutathione at 0.6 mM cumene hydroperoxide was 138 nmoles glutathione oxidized/min/mg protein, while at 5.0 mM glutathione the value for cumene hydroperoxide was 146 nmoles glutathione oxidized/min/mg protein. These studies demonstrate that glutathione peroxidase is active in the retina and establish the optimal experimental conditions for determination of the enzymatic activity.

Molecular mechanism of cataractogenesis: III. Toxic metabolites of oxygen as initiators of lipid peroxidation and cataract

A free radical mechanism of cataractogenesis involving enzymatic and nonenzymatic reactions, is proposed. Supporting experimental evidence is briefly reviewed. H2O2, which is one of the toxic metabolites of oxygen, was significantly increased 2-3 fold in ocular humors in several experimental cataracts and in human senile cataract. Various cataractogenic agents were also found to increase H2O2 in ocular humors in vivo prior to cataract formation. Enzymatic defenses against O2-. and H2O2 provided by superoxide dismutase, catalase and glutathione peroxidase were impaired in cataracts. In some cataracts, catalase and superoxide dismutase were affected earlier. Malondialdehyde (MDA), a major breakdown product of lipid peroxides was significantly increased by 2-4-fold in human senile cataract, in cataracts induced in rabbit and rat, and in hereditary cataracts in mice. All the reactive species of O2 (O2-., H2O2, OH. and 1 delta gO2) may participate in initiating lipid peroxidation of lens in vitro. Various scavengers of these species were capable of preventing lenticular lipid peroxidation, amongst which OH. scavengers were found to be the most effective. Biological antioxidant, vitamin E afforded 44% prevention of lipid peroxidation in lens. The important observation was that vitamin E was therapeutically effective in about 50% of animals in arresting cataract induced in rabbit by 3-aminotriazole. In these rabbits, H2O2 and ascorbic acid of ocular humors and MDA of lens were close to normal. It is our working hypothesis that the carbonyl groups of MDA and amino groups of amino acids, proteins, nucleic acids and their bases, and phospholipids could interact in a cross-linking reaction producing high molecular weight aggregates by Schiff-base conjugate formation in addition to disulfide cross-linking of proteins, and finally resulting in cataract.

Glutathione peroxidase and reductase, glucose-6-phosphate dehydrogenase and catalase activities in multiple sclerosis

Our previous studies have demonstrated a decreased glutathione feroxidase (GSH-Px) activity of erythrocytes and leucocytes from multiple sclerosis (MS) patients. In the present communication these activities were compared with the activities of associated enzymes (glutathione reductase (GSSG-RD), glucose-6-phosphate dehydrogenase (G-6-PD) and catalase). All enzymic activities were compared between MS patients, other neurologic patients (ON patients) and normal control individuals. Compared to data of ON patients and normal controls, in MS the ratio of GSHPx/GSSGRD in lympho- and granulocytes was significantly decreased (2 alpha less than or equal to 0.05) by 35% and 51%, respectively. The significant correlation between GSSG-RD and the GSH-Px activity (2 alpha less than or equal to 0.05, r = 0.501) found in control lymphocytes was not present in MS lymphocytes. However, the lymphocyte GSH-Px activities of controls as well as of MS correlated with the corresponding serum selenium levels (2 alpha less than or equal to 0.05, r = 0.594 and 2 alpha less than or equal to 0.01, r = 0.967, respectively). The G-6-PD activity was insignificantly increased by 41% in MS lymphocytes compared to normal control. Catalase activity was unchanged in lymphocytes but decreased 50% in MS granulocytes compared to normal control. No significant differences were found between MS and the ON group. The catalase activity of MS erythrocytes was increased by 63% (2 alpha less than or equal to 0.05) in comparison with both the normal control and ON data.

Glutathione peroxidase, glutathione reductase and superoxide dismutase in the aging lens

Glutathione reductase (GR) and glutathione peroxidase (GPx) show in bovine lenses a decrease in specific activity; furthermore, the heat lability of both enzymes increases with age monitoring structural changes of the molecules. GR activity was correlated with type of cataract in human lenses. Its decrease is significantly connected with cortical opacities. Superoxide dismutase activity declines in aging and cataractous lenses. These results support the assumption that in old lens tissue the capacity of the antioxidative scavanger system is diminished.

Glutathione metabolism in lenses of dogs and rabbits: activities of five enzymes

Several biochemical parameters were examined in clear dog and rabbit lenses as functions of age, and in posterior subcapsular cataracts in the Alaskan malamute. Tabulated data include soluble protein, reduced sulfhydryl content of soluble protein, reduced glutathione, water, and activity of five enzymes of glutathione metabolism. The enzymes include the glutathione biosynthesis system consisting of gamma-glutamylcysteine synthetase and glutathione synthetase, as well as glutathione peroxidase, glutathione reductase and glutathione-S-transferase. Each enzyme, acting last in a sequential reaction of either two or three reactions, was in excess activity over the preceding enzyme(s) in every case but one. In the exception, the ratio of glutathione reductase to glutathione peroxidase activity was about 1:600 and 1:155 in the dog and rabbit lens, respectively.