Lipid peroxidation and hemolysis induced by lactoperoxidase and thyroid hormones

Inhibition of lactoperoxidase-catalyzed 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and tyrosine oxidation by tyrosine-containing random amino acid copolymers

Oxidation of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) by lactoperoxidase was found to be inhibited by tyrosine-containing random amino acid copolymers but not by tyrosine. Both electrostatic effects and polymer size were found to be important by comparison of negatively and positively charged copolymers of varying lengths, with poly(Glu, Tyr)4:1 ([E 4Y 1] approximately 40) as the strongest competitive inhibitor (EC 50 approximately 20 nM). This polymer did not form dityrosine in the presence of lactoperoxidase (LPO) and peroxide. Furthermore, incubation with tert-butyl hydroperoxide, as opposed to hydrogen peroxide, resulted in a peculiar long lag phase of the reaction between the redox intermediate compound II and [E 4Y 1] approximately 40, indicating a very tight association between enzyme and inhibitor. We propose that interactions between multiple positively charged areas on the surface of LPO and the polymer are required for optimal inhibition.

Oxidative damage and antioxidant enzyme activities in experimental hypothyroidism

Free radicals are now well known to damage cellular components. To investigate whether age and thyroid level affect peroxidation speed, we examined the levels of malondialdehyde and antioxidant enzyme activities in different age groups of hypothyroid rats. Hypothyroidism was induced in 30- and 60-day-old Wistar Albino rats by the i.p. administration of propylthiouracil (10 mg kg(-1) body weight) for 15 days. While malondialdehyde levels of 30- or 60-day-old hypothyroid rats were increased in liver, they were decreased in the tissues of the heart and thyroid. While glucose-6-phosphate dehydrogenase activity levels did not change in heart, brain and liver tissues of 30-day-old rats, they increased in brain and heart tissues of 60-day-old experimental groups, but decreased in the liver. Catalase activities decreased in the liver and heart of rats with hypothyroidism, but increased in erythrocytes. In control groups while malondialdehyde levels increased in brain, heart and thymus with regard to age, they decreased in plasma. Glucose-6-phosphate dehydrogenase and catalase activities were not affected by age in tissues of the thymus, thyroid and brain, but they were decreased in the heart tissue. The changes in the levels of lipid peroxidation and antioxidant enzyme activities which were determined in different tissues of hypothyroid rats indicate a cause for functional disorder of these tissues. Moreover, there may be changes depending on age at lipid peroxidation and antioxidant enzyme activity levels.

Thyronines and probucol inhibition of human capillary endothelial cell-induced low density lipoprotein oxidation

Oxidized lipoproteins have been implicated as important factors in the pathogenicity of atherosclerosis. Thus, antioxidants play a significant role in inhibiting a critical step in atheroma progression. Previously, we demonstrated that thyronine analogs inhibit Cu(2+)-induced low density lipoprotein (LDL) oxidation. In the present study, we examined the effect of thyronine analogs on endothelial cell (EC)-induced LDL oxidation. LDL was incubated with or without EC in the presence or absence of various concentrations of thyronine, vitamin C, or probucol at 37 degrees in a humidified atmosphere (95% air, 5% CO2). Thyronine analogs, probucol, and vitamin C inhibited EC-induced LDL oxidation in a concentration-dependent manner. The concentration of each agent (microM) producing 50% inhibition (IC50) of EC-induced LDL oxidation for thiobarbituric acid reactive substances (TBARS) and electrophoretic mobility, respectively, was as follows: 0.294 and 0.417 for levothyroxine (L-T4); 0.200 and 0.299 for L-triiodothyronine (L-T3); 0.125 and 0.264 for dextro-thyroxine (D-T4); 0.203 and 0.304 for reversed triiodothyronine (rT3); 1.02 and 1.44 for probucol; and 13.6 and 14.9 for vitamin C. Thyroid binding globulin (TBG) inhibited EC-induced LDL oxidation; further, thyronines bound to TBG exhibited more antioxidant activity than unbound thyronines. Pretreatment of EC with any of the thyronines decreased the ability of EC to oxidize LDL. Also, our results showed that a synergistic interaction exists between vitamin C and T4 in the inhibition of EC-induced LDL oxidation. The T4 and TBG concentrations that inhibited LDL oxidation were in the physiological range. We conclude that T4, like the pharmacological agent probucol, reduces oxidative modification of LDL and thus may act as a natural inhibitor of atherogenesis.

Lipid peroxidation and homocysteine induced toxicity

Homocysteine induced toxicity has been examined in cultures of human umbilical vein endothelial cells. The toxic effects of the amino acid alone and the amino acid plus Cu2+ could be prevented by catalase and decreased by desferal, when either was present in the culture medium. When desferal was allowed to accumulate intracellularly, no significant protection from homocysteine induced toxicity was observed. Even though lipid peroxidation accompanied the toxicity induced by homocysteine and homocysteine plus Cu2+, inhibition of lipid peroxidation in either case had no effect on cell viability. The significance of these results is discussed.

Inhibition of low density lipoprotein oxidation by thyronines and probucol

Oxidation of low density lipoproteins (LDL) results in increased macrophage uptake of LDL which may contribute to the formation of macrophage-derived foam cells in the early atherosclerotic lesion. In this study we show that thyroxine (T4), its optical antipodes, certain desiodo analogs and probucol inhibited cupric sulfate-catalyzed oxidation of human LDL in a concentration-dependent manner as assessed by measuring the electrophoretic mobility, thiobarbituric acid reactive substances (TBARS) and LDL degradation in mouse macrophages. In Cu(2+)-catalyzed LDL oxidation at 24 hr, the TBARS level was 80 nmol/mg LDL protein/24-hr incubation. The concentrations (microM) of each agent producing 50% inhibition in the formation of oxidized LDL (IC50) for TBARS, electrophoretic mobility and macrophage degradation, respectively, were 1.13, 1.27 and 1.30 for reversed triiodothyronine; 1.33, 1.80 and 1.27 for triiodothyronine; 1.33, 1.37 and 1.37 for racemic thyroxine, DL-T4; 1.10, 1.40 and 1.50 for L-T4; 1.13, 1.33 and 1.23 for D-T4; and 1.47, 1.63 and 1.37 for probucol. No differences in inhibitory potency were observed when rT3, T3, the optical antipodes of T4 and the hydrophobic antioxidant drug probucol were compared. In air-induced LDL oxidation, TBARS was 16.1 nmol/mg LDL protein/6-hr incubation. The IC50 concentrations (microM) for TBARS and diene conjugation, respectively, were 0.187 and 0.336 for D-T4; 0.205 and 0.243 for L-T4 and 1.30 and 3.02 for probucol. With air-induced LDL oxidation conditions, the L-T4 concentrations included the physiological range, and thyroid-binding globulin did not modify the inhibitory effect of the endogenous enantiomer, L-T4. Putative uptake of this stereoisomer into LDL inhibited oxidation of these lipoproteins. Since concentrations of these thyronines which blocked air-induced LDL oxidation were in the physiological range, we conclude that thyronines, like the pharmacological agent probucol, limit the oxidative modification of LDL and thus may serve as natural inhibitors of atherogenesis.

Susceptibility of glucose-6-phosphate dehydrogenase deficient red cells to primaquine, primaquine enantiomers, and its two putative metabolites. II. Effect on red blood cell membrane, lipid peroxidation, MC-540 staining, and scanning electron microscopic studies

The effects of primaquine (PQ), its enantiomers [(+)PQ,(-)PQ] and hydroxy metabolites [5-hydroxyprimaquine (5HPQ) and 6-desmethyl-5-hydroxyprimaquine (6D5HPQ)] on cell membranes of glucose-6-phosphate dehydrogenase (G-6-PD) deficient red cells were studied in vitro. There was no significant effect of PQ on the malonyldialdehyde (MDA) content of normal and heterozygous red cells, but it caused a significant increase in MDA in G-6-PD deficient red cells (P less than 0.05). There was no noticeable difference between the effects of the two enantiomers on this variable (P greater than 0.05). Compared to PQ, the hydroxy metabolites produced a significantly greater increase in MDA in all the groups studied (P less than 0.001). Of the two hydroxy metabolites, 6D5HPQ was more toxic than 5HPQ. Staining with MC540 showed that exposure to PQ, its enantiomers and two putative metabolites produced significant fluorescence, indicating that the drug produces marked alterations in membrane fluidity. Although the fluorescence was seen both in normal and heterozygous cells, the effect was marked in hemizygous deficient red cells (P less than 0.001). Scanning electron microscopic (SEM) studies revealed that PQ enantiomers had a stomatocytic effect on red cells of normal, heterozygous and hemizygous G-6-PD deficient red cells, whereas the putative metabolites had an echinocytic effect. The effects were most pronounced in G-6-PD deficient red cells.

Evaluation of the antioxidant properties of thyroid hormones and propylthiouracil in the brain-homogenate autoxidation system and in the free radical-mediated oxidation of erythrocyte membranes

The antioxidant capacity of thyroid hormones and the antithyroid drug propylthiouracil was studied in three model systems, namely, autoxidation of rat brain homogenates and oxidation of rat erythrocyte plasma membranes (EPM) induced by either 2,2'-azobis-(2-amidinopropane) (AAP) thermolysis or by gamma irradiation. Thyroid hormones significantly inhibited the development of lipid peroxidation in these systems at micromolar concentrations, as assessed either by visible light emission, thiobarbituric acid reactive substances accumulation or oxygen uptake. This behaviour was not observed when L-3,3',5-triiodothyronine (T3) and L-thyroxine (T4) were assayed at nanomolar concentrations. In EPM exposed to AAP or gamma irradiation, propylthiouracil inhibited the induced lipid peroxidation, with Q1/2 values of 112-150 microM. It is concluded that the antioxidant capacity of thyroid hormones found in vitro may not be of relevance in physiological conditions, which exhibit variations of T3 and T4 levels in the nanomolar range. On the other hand, the behaviour of propylthiouracil as an inhibitor of EPM lipid peroxidation is observed at concentrations close to the therapeutic levels, thus representing a possible complementary action to its antithyroid activity.

Prooxidant and antioxidant effects of ascorbate on tBuOOH-induced erythrocyte membrane damage

1. t-Butylhydroperoxide (tBuOOH) a lipoperoxide analog, causes rapid and considerable sulphydryl (SH) oxidation but almost no lipid peroxidation in red blood cell membranes (ghosts) containing no detectable haemoglobin. 2. tBuOOH, in the presence of ascorbate, produces significant lipid peroxidation the level of which is proportional to the ascorbate concentration. The initiation of lipid peroxidation is thought to occur by the reactive tBuO (butoxyl) species via the reductive decomposition of tBuOOH by ascorbate. 3. Ascorbate protects ghost membranes from the tBuOOH-induced SH oxidation in a dose-dependent fashion. 4. There is no parallelism between lipid peroxidation and SH oxidation in these systems. This suggests that the two processes occur independently of each other. 5. These findings indicate that, simultaneously, ascorbate can have both a protective and a prooxidant action in different membrane components under the same oxidative stress.

Binding of iron to human red blood cell membranes

The binding of Fe3+ to red cell membranes was studied in a system in which lipid peroxidation was proportional to Fe3+ concentration. Binding of Fe3+ was evaluated by labeling with 59FeCl3 and measurement of NMR water-proton relaxation times. Labeling with 59Fe showed that 95% of the Fe3+ was membrane bound at 100 microM FeCl3 in a 1.5 mg protein/ml membrane suspension. Both spin-lattice (T1) and spin-spin (T2) relaxation times decreased with increasing Fe3+ concentration. Addition of red cell membrane suspensions largely prevents the Fe3+ effect on relaxation times. Charge transfer to Fe3+ may occur at the membrane binding site with resultant decrease in the Fe3+ effect on water-proton relaxation times. These studies support the hypothesis that Fe3+ binds to the membrane and generates free radicals at the binding site.

The nature of oxidants and antioxidant systems in the inhibition of mutation and cancer

We briefly review current concepts with regard to the nature of oxygen-derived oxidants in biological systems. Of these substances, hydroxyl radicals derived from hydrogen peroxide seem most likely to be involved in the various stages of carcinogenesis. Hydrogen peroxide detoxification, primarily through glutathione activity, is essential in preventing hydroxyl-radical formation. Transition metals such as iron play a central role in this latter process. Alterations in cellular macromolecules are most likely to take place if hydroxyl-radical formation is directed toward specific intramolecular sites by appropriately sequestered metals. For this reason, repair and turnover events are apt to be more important protective devices than are the actions of molecules which scavenge hydroxyl radicals. Although many cellular constituents are potential targets in free-radical and oxidant attacks leading to carcinogenesis, nucleic acids have been most extensively studied in this connection. On the basis of these investigations, it is a facile conclusion that oxidants might be involved in the early events of carcinogenesis as well as in transformation or promotion. The literature on antioxidants in chemoprevention in animals is supportive of such a role. However, other biochemical effects of antioxidants should raise a note of caution in the interpretation of animal experiments.

Iron-mediated oxidative stress in erythrocytes

Erythrocytes subjected extracellularly to iron-mediated oxidant stress undergo haemoglobin oxidation and membrane damage, which can be modulated by maintaining the energy requirements of the cells. The results presented here suggest that a balance exists between the oxidation state of the haemoglobin and the oxidative deterioration of the membrane lipids, which is dependent on the metabolic state of the erythrocytes. These findings have important implications for thalassaemic erythrocytes that may be exposed to excess plasma iron levels, in which excessive membrane-bound iron in the form of haemichromes is a characteristic feature and in which cellular ATP levels are lowered.

Modulation of iron-mediated oxidant damage in erythrocytes by cellular energy levels

In this work we have investigated the effects of iron-induced oxidative stress on erythrocytes and their membranes, the importance of haemoglobin oxidation and of the maintenance of the metabolic properties of the cells. The results show that by maintaining the energy requirements of the erythrocyte, methaemoglobin production is minimised under conditions of iron-stress. However, in this situation, the membranes of the erythrocytes become more susceptible to the oxidative damage and increased lipid peroxidation ensues.

Initiation of lipid peroxidation in biological systems

The direct oxidation of PUFA by triplet oxygen is spin forbidden. The data reviewed indicate that lipid peroxidation is initiated by nonenzymatic and enzymatic reactions. One of the first steps in the initiation of lipid peroxidation in animal tissues is by the generation of a superoxide radical (see Figure 16), or its protonated molecule, the perhydroxyl radical. The latter could directly initiate PUFA peroxidation. Hydrogen peroxide which is produced by superoxide dismutation or by direct enzymatic production (amine oxidase, glucose oxidase, etc.) has a very crucial role in the initiation of lipid peroxidation. Hydrogen peroxide reduction by reduced transition metal generates hydroxyl radicals which oxidize every biological molecule. Hydrogen peroxide also activates myoglobin, hemoglobin, and other heme proteins to a compound containing iron at a higher oxidation state, Fe(IV) or Fe(V), which initiates lipid peroxidation even on membranes. Complexed iron could also be activated by O2- or by H2O2 to ferryl iron compound, which is supposed to initiate PUFA peroxidation. The presence of hydrogen peroxide, especially hydroperoxides, activates enzymes such as cyclooxygenase and lipoxygenase. These enzymes produce hydroperoxides and other physiological active compounds known as eicosanoids. Lipid peroxidation could also be initiated by other free radicals. The control of superoxide and perhydroxyl radical is done by SOD (a) (see Figure 16). Hydrogen peroxide is controlled in tissues by glutathione-peroxidase, which also affects the level of hydroperoxides (b). Hydrogen peroxide is decomposed also by catalase (b). Caeruloplasmin in extracellular fluids prevents the formation of free reduced iron ions which could decompose hydrogen peroxide to hydroxyl radical (c). Hydroxyl radical attacks on target lipid molecules could be prevented by hydroxyl radical scavengers, such as mannitol, glucose, and formate (d). Reduced compounds and antioxidants (ascorbic acid, alpha-tocopherol, polyphenols, etc.) (e) prevent initiation of lipid peroxidation by activated heme proteins, ferryl ion, and cyclo- and lipoxygenase. In addition, cyclooxygenase is inhibited by aspirin and nonsteroid drugs, such as indomethacin (f). The classical soybean lipoxygenase inhibitors are antioxidants, such as nordihydroguaiaretic acid (NDGA) and others, and the substrate analog 5,8,11,14 eicosatetraynoic acid (ETYA), which also inhibit cyclooxygenase (g). In food, lipoxygenase is inhibited by blanching. Initiation of lipid peroxidation was derived also by free radicals, such as NO2. or CCl3OO. This process could be controlled by antioxidants (e).(ABSTRACT TRUNCATED AT 400 WORDS)

t-butyl hydroperoxide-induced perturbations of human erythrocytes as a model for oxidant stress

Erythrocytes were incubated with t-butyl hydroperoxide in the presence and absence of hemoglobin as a model system for oxidative stress and the alterations in the structure and integrity of the membranes were investigated. The results showed that in the presence of hemoglobin a significant modification in the membrane surface charge was induced but no such alteration was observed in peroxidized hemoglobin-free membranes. As increased hemoglobin oxidation occurred in the erythrocytes, membrane lipid peroxidation diminished, suggesting a protective role for methemoglobin in t-butyl hydroperoxide-induced lipid peroxidation. Electrophoresis on polyacrylamide gels showed modification of the cytoplasmic protein region but no high molecular weight aggregates formed at the concentrations of the hydroperoxide used in this work. The results suggest that the t-butyl hydroperoxide/normal erythrocyte system seems to be an instructive model for membrane perturbations characteristic of oxidative disorders.

Glutathione depletion in isolated hepatocytes: its relation to lipid peroxidation and cell damage

The effects of glutathione depletion in isolated hepatocytes have been studied. A list of compounds which depleted glutathione and induced lipid peroxidation and cell lysis is given. The effects of halogenated acetamides were studied in more detail and results of studies on the interaction of iodoacetamide with cellular constituents are presented. A single metabolite of iodoacetamide, tentatively identified as the glutathione conjugate, was excreted from the cells while less than one percent of the "parent compound" was retained, tightly bound to macromolecules. This bound component could not be associated with the cellular damage. Methionine, cysteine and alpha-tocopherol, as wellas paracetamol and ethylmorphine, were found to prevent lipid peroxidation and lysis. It is concluded that GSH deficiency per se can lead to lipid peroxidation and that this reaction caused the observed hepatocellular lysis.