Role of oxidative stress in organophosphate insecticide toxicity - Short review

The objective of this paper is to present a short review of the state of knowledge regarding oxidative stress and its role in toxicity of organophosphate insecticides. The information has been obtained by searching the relevant literature using chemical abstracts, PubMed, scopus, medline and other data bases. The significance of the problem has been elucidated. Organophosphate insecticides (OP), apart from inhibition of cholinesterase and presence of cholinergic effects, oxidative stress and hyperglycemia has been reported by many authors as one of the adverse effects in poisoning by OP in both humans and animals. Oxidative stress induced by organophosphate leads to disturbances in the function of different organs and tissues. In subchronic or chronic OP exposition induction of oxidative stress has been reported, by many authors, as the main mechanism of its toxicity. Data were categorized according to animal studies (in vitro and in vivo) and clinical studies. On the basis of relevant literature it is concluded, that determination of oxidative stress parameters can be useful for monitoring people exposed to OP professionally. Supplementation with natural or synthetic antioxidant may be beneficial in OP poisoning, however the rat models of OP poisoning used in those studies do not completely reflect clinical situation. For this reason the clinical trials are needed to explore effectiveness of these antioxidants in protection against toxicity of OP.     

Effects of dichlorvos on lipid peroxidation in mice on subacute and subchronic periods

In this study, the effects of dichlorvos to lipid peroxidation were investigated at subacute and subchronic periods. Dichlorvos was given with drinking water to Swiss Albino male mice in three dosage levels as 10, 20, and 40 mg/kg b.w. Lipid peroxidation was evaluated by determining the malondialdehyde (MDA) levels in plasma, determining the superoxide dismutase and catalase activities in erythrocytes. The analysis of these enzymes was done in blood samples collected from mice on the days 15 and 45. The results showed that MDA levels increased in dichlorvos treated groups. Actually MDA levels in control and dichlorvos treated groups were determined (as nmol/ml) 10.49, 13.83, 14.30, and 14.50, respectively, at subacute period; 7.77, 8.15, 10.88, and 12.33, respectively, at subchronic period. Catalase activity in erythrocytes decreased at subacute and subchronic periods in dichlorvos treated groups. At subacute period CAT activities were determined (as k/mg Hb) in control and dichlorvos treated groups, 563.45, 532.11, 524.76, and 497.08, respectively; 660.53, 588.84, 525.85, and 512.01, respectively, at subchronic period. When subacute and subchronic periods were compared with each other; it was shown that SOD and CAT activities increased at subchronic period.     

Effects of thiacloprid, deltamethrin and their combination on oxidative stress in lymphoid organs, polymorphonuclear leukocytes and plasma of rats

Deltamethrin and thiachloprid are an α-cyano class pyrethroid and neonicotinoid insecticide, respectively. Recently, a pesticide combining deltamethrin and thiacloprid has also been released. In the present study, the acute and subacute toxic effects of deltamethrin, thiachlopride, and a combination of these insecticides, on the lymphoid organs (spleen, thymus and bone marrow), polymorphonuclear leukocytes (PMNs) and plasma of rats, were determined to better understand mammalian antioxidant-oxidant and inflammatory system responses. For this purpose, rats were treated orally with different doses of thiacloprid (single acute dose of 112.5 mg/kg); subacute dose of 22.5 mg/kg/day for 30 days; deltamethrin (single acute dose of 15 mg/kg); subacute dose (3 mg/kg/day for 30 days), or a combination of these pesticides. Results were compared with those from a comparable dosing regimen with the known immunosuppressive drug cyclophosphamide. Pesticide treatments caused significant changes in the levels of liver and kidney injury markers. Antioxidant enzyme (catalase and glutathione peroxidase), glutathione and plasma antioxidant levels decreased but lipid peroxidation increased in all lymphoid organs and the plasma. Glutathione-S-transferase and especially DT-diaphorase activity, decreased after thiacloprid treatment. Myeloperoxidase activity, carbonyl content, lipid peroxidation and total nitrite levels increased in PMNs and plasma. When evaluated as a whole, the oxidative and inflammatory stresses seen in the pesticide combination groups were not much more pronounced than in the groups treated with a single pesticide. In terms of the evaluated biochemical parameters, the pesticides showed similar effects to cyclophosphamide.     

Effects of gender and temperature on oxidative stress enzymes in Nile tilapia Oreochromis niloticus exposed to paraquat

Many classes of environmental pollutants can enhance the intracellular formation of reactive oxygen species, which can conduce to the damage of macromolecules and changes in oxidant defences levels in fish. In the present study it was analysed the hepatic levels of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione S-transferase (GST) in males and females of Nile tilapia Oreochromis niloticus exposed to paraquat (PQ), at 17 and 27 °C. Tilapia were exposed to a sublethal concentration of PQ (0.5 mg L⁻¹) during 45 days. Condition factor and hepatosomatic index of males and females exposed to PQ were significantly higher when compared with the control group, except in females at 17 °C. SOD and GST activities were higher in males and females exposed to PQ than in the control group at 17 and 27 °C. The levels of both enzyme activities revealed that they are sex-dependent with males exposed to PQ showing higher SOD activity (5.05 ± 0.13 and 4.84 ± 0.23 U/g protein, respectively at 17 and 27 °C) than females (4.21 ± 0.07 and 3.87 ± 0.27 U/g protein, respectively at the same temperature). Similar results were observed in GST activity. A GR activity significantly higher (9.09 ± 0.44 and 7.97 ± 1.08 U/g protein at 17 and 27 °C, respectively) was observed in PQ-exposed females, but not in exposed males. Fish exposed to PQ showed higher values of SOD and GST activities than the control group at both temperatures. These results are gender-dependent, while GR activity was higher only in PQ-exposed females. No significant differences were found for SOD, GST and GR activities between fish exposed to 17 and 27 °C, although males and females showed higher values at 17 °C. In short, this work advanced new knowledge on influence of gender in same biochemical parameters in tilapia exposed to PQ and demonstrated that their effects could be observed at different temperatures.     

Modulation of ethion-induced hepatotoxicity and oxidative stress by vitamin E supplementation in male Wistar rats

Organophosphorus insecticides (OPIs) may induce oxidative stress leading to generation of free radicals and alteration in antioxidant system of animals. Many studies reported that enzymatic and non-enzymatic antioxidant may play protective role against OPIs induced toxicity in human and rats. The aim of present study was to investigate the possible protective role of vitamin E on ethion-induced hepatotoxicity in rats using qualitative, quantitative and biochemical approaches. Adult male albino rats of Wistar strain were randomly divided into four groups; each group consists of six animals. Animals were treated for a period of 28 days. Group I (control group received corn oil); Group II [ethion treated (2.7 mg/kg bw/day)]; Group III (vitamin E treated (50 mg/kg of bw/day)]; Group IV (ethion + vitamin E treated). Animals were sacrificed after 7, 14, 21 and 28 days by decapitation and liver tissue was used for the measurement of proteins, lipid peroxidation (LPO), reduced glutathione (GSH) content and activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) glutathione reductase (GR) and glutathione-S-transferase (GST). Erythrocytes were analyzed for acetyl cholinesterase activity. The result of this study shows that in vivo administration of ethion caused a significant induction of oxidative damage in liver tissue as evidenced by increased level of LPO and decreased GSH content. Ethion toxicity also led to a significant increase in the activities of SOD, CAT, GPx and GST in liver tissue. In addition, decrease in GR activity was observed in ethion administered rats compared to control. Histopathological findings revealed that exposure to ethion caused damage in liver tissue. However, simultaneous supplementation with vitamin E restored these parameters partially. In conclusion, the results of the current study revealed that ethion-induced toxicity caused lipid peroxidation, alterations in the antioxidant enzymes and histopathological changes in liver. Supplementation of vitamin E exhibited protective effect by inhibiting ethion-induced toxicity in liver and erythrocytes.     

Protective effects of Nigella sativa oil on propoxur-induced toxicity and oxidative stress in rat brain regions

Propoxur (PPr) is a widely used broad spectrum carbamate insecticide mainly used to control household pests. Because of the widespread use of pesticides for domestic and industrial applications, evaluation of their neurotoxic effects is of major concern to public health. The aim of the present study was to evaluate the possible protective effects of Nigella sativa oil (NSO), an antioxidant agent, against PPr-induced toxicity and oxidative stress in different brain regions of rats including cerebellum, cortex and hippocampus. In the present study, 32 male Sprague-Dawley rats were used and divided into four equal groups. Group 1 was allocated as the control group. Groups 2-4 were orally administered 1 ml/kg/bw/day NSO, 8.51 mg/kg/bw/day PPr or NSO plus PPr, respectively, for 30 days. Lipid peroxidation (LPO), protein carbonyl content (PCC) and acetylcholine esterase activity (AChE) were determined. Enzymatic antioxidant activities [superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione-S-transferase (GST)] and non-enzymatic antioxidants [reduced glutathione (GSH)] were determined. PPr treatment significantly increased the levels of LPO, PCC and oxidized glutathione (GSSG) in brain regions. On the contrary, levels of GSH and the activities of SOD, CAT, GSH-Px, GST and AChE were significantly decreased. NSO treatment to PPr intoxicated rats restored such biochemical parameters to within control levels except GST activity, emphasizing its antioxidant role. We conclude that NSO significantly reduces PPr-induced toxicity and oxidative stress in rat brain regions via a free radicals scavenging mechanism.     

Chlorpyrifos-induced oxidative stress and histological changes in retinas and kidney in rats: Protective role of ascorbic acid and alpha tocopherol

This study was undertaken to evaluate the antioxidant effect of vitamins C and E against oxidative stress, apoptosis and histological changes of kidney and retina in CPF-treated rats. Forty male Sprague-Dawley rats were divided into four groups including the control group, the group treated orally with a single dose of CPF (63 mg/kg b.w.), the group injected intramuscularly (i.m.) with vitamin C (250 mg/kg b.w.), and intraperitonealy (i.p.) with vitamin E (150 mg/kg b.w.) daily for 7 days and the group treated with CPF (single dose) and injected with vitamins (for 7 days). The results showed that CPF induced apoptosis and severe oxidative stress as indicated by the significant increase in MDA and sFasL concentration and the significant decrease in GSH concentration in serum. Co-administration of vitamins C and E ameliorate these toxic effects and improved the histological pictures of kidneys and retinas. It could be concluded that combined administration of vitamins C and E is useful in the routine therapy for the protection against tissue damage induced by CPF.     

Effects of Mancozeb and Metribuzin on in vitro proliferative responses and oxidative stress of human and rat spleen lymphocytes stimulated by mitogens

Pesticides have been shown to possess marked immunotropic activity. The aim of this work was to study the in vitro effects of different concentrations (1–100 μM) of Mancozeb (fungicide) and Metribuzin (herbicide), on the proliferative responses of human and rat spleen lymphocytes stimulated by concanavalin A (ConA, mitogen), the Th1- (IL-2, INFγ) and Th2- (IL-4) cytokine secretion and on the intracellular oxidative status. The results showed that Mancozeb significantly reduced ConA lymphocyte proliferation in a dose-dependent manner in both humans and rats. It also decreased IL-2, INFγ and IL-4 secretion with a a shift away to Th1 phenotype. Metribuzin at low concentrations (1–10 μM) resulted in activation of ConA stimulated lymphocyte proliferation and cytokine production in both human and rat spleen cells. However, at high concentrations (25–100 μM), Metribuzin induced a dose-dependent inhibition of lymphocyte proliferation and cytokines. Changes in intracellular levels of reduced Glutathione, hydroperoxides and carbonyl proteins and in the activities of catalase and SOD were observed after Mancozeb and Metribuzin exposure reflecting oxidative stress and DNA damage specially at high concentrations.In conclusion, Mancozeb and Metribuzin had significant immunomodulatory properties with oxidative stress induction at high concentrations.     

Protective effects of garlic (Allium sativum) extract upon lindane-induced oxidative stress and related damages in testes and brain of male rats

The purpose of this study, carried out on male Wistar rats, was to evaluate the beneficial effects of garlic (Allium sativum) extract injections upon lindane-induced damages in testes, brain and thyroid function. Under our experimental conditions, lindane poisoning (in drinking water for 30 days, supplying about 50 mg/kg body weight per day) resulted in a decreased weight of testes, epididymides, prostate gland and seminal vesicles (−52%, −42%, −50% and −5%, respectively), a decrease of spermatozoa count and motility (−56%, −37%, respectively), an increased level of free thyroxin (+84%) and decreased levels of TSH and FSH in serum (−74%, −77%, respectively). In addition, lindane treatment triggered an oxidative stress in testes and brain as revealed by an increased level of lipids peroxidation (TBARS) (+96%,+92%), an increase of superoxide-dismutase activity in testes (+69%) and a decrease of glutathione-peroxidase and catalase activities in testes and brain (−52%, −34% and −49%, −45%, respectively). These lindane-induced changes were almost reversed to normal in animals injected with a garlic extract (an amount corresponding to 300 mg fresh garlic/kg/day), what confirms a beneficial effect of this vegetal source of anti-oxidants.     

Toxicological effects of oxyfluorfen on oxidative stress enzymes in tilapia Oreochromis niloticus

Several environmental pollutants enhance the intracellular formation of reactive oxygen species, and can lead to the damage of macromolecules and a decrease in oxidant defences levels in fish. The effects of the herbicide oxyfluorfen on the activities of antioxidant enzymes such as catalase, superoxide dismutase, glutathione reductase, and glutathione S-transferase were evaluated in freshwater fish Oreochromis niloticus. These were determined in tilapia liver exposed to sublethal concentrations (0.3 and 0.6 mg/L at 7, 14, and 21 days of exposure. This study also analyzed the effects of oxyfluorfen on the total fatty acid profile. The results showed that CAT activity was higher in tilapia exposed to oxyfluorfen at the sampling days, except at the highest concentration after 21 days. Similarly, the enhancing effect of the herbicide was observed on the GR activity. However, its effect was moderate at the highest dose. On the contrary, fish treated with oxyfluorfen at both doses displayed a decrease in the SOD activity. After 7 days of treatment at both concentrations tilapia showed a significant increase in GST levels, although the enzymatic activity decreased at 14 and 21 days of exposition when compared with the control. The major saturated fatty acids measured in tilapia liver were the palmitic acid (C16:0; 17.9%) and stearic acid (C18:0; 8.7%). The exposure to oxyfluorfen caused a significant increase of the oleic acid (C18:1), whereas the amount of nervonic acid (C24:1) increased at all sampling data. The results of the present study should be taken in account when using tilapia as an environmental indicator species in studies of xenobiotic biotransformation and biomarker response, as well as in monitoring programmes.     

Comparative studies on chlorpyrifos and methyl parathion induced oxidative stress in different parts of rat brain: Attenuation by antioxidant vitamins

Organophosphate (OP) pesticides are among the most widely used synthetic chemicals for controlling a wide variety of pests. Chlorpyrifos (o,o′-diethyl-o-3,5,6-trichloro-2-pyridyl phosphorothionate, CPF) is among the leading OP pesticides used extensively throughout the world including India while methyl parathion (o,o-di methyl-o-p-nitrophenyl phosphorothioate, MPT) another OP compound, widely used as insecticide and acaricide to control many biting or sucking pests of agricultural crops. Present study was carried out to compare the chronic toxicity of CPF and MPT, their potential to generate oxidative stress and ameliorating effects of antioxidant vitamins in brain of rats. Results of the present study clearly demonstrated that the oral exposure of CPF or MPT, generated oxidative stress in different parts of rat brain consequently accumulating malondialdehyde (MDA) and 4-hydroxynonanal (4HNE), the two major end products of lipid peroxidation, in all the three brain regions i.e. fore-, mid- and hind-brain. The levels of hydrogen peroxide (H₂O₂) were also increased in all the three brain regions when compared with control. CPF and MPT exposure caused decrease in the levels of reduced glutathione (GSH) and increase in the levels of oxidized glutathione (GSSG) in all the three brain regions. The increase in the levels of MDA, 4HNE, H₂O₂ and GSSG was less pronounced when CPF or MPT was given to the rats fed with a mixture of vitamin A, E and C. The present findings clearly show that oral intake of a mixture of vitamin A, E and C protects the rats from MPT or CPF induced oxidative stress and suggest that this treatment alleviates the toxicity of these pesticides to a greater extent.     

Evaluation of aspect of some oxidative stress parameters using vitamin E, proanthocyanidin and N-acetylcysteine against exposure to cyfluthrin in mice

A hundred and sixty female white mice, each weighing 35-40 g, were used in this study. The animals were assigned into eight groups as one control group and 7 experimental groups. Groups 2, 3 and 4 were administered N-acetylcysteine (NAC), proanthocyanidin and vitamin E alone, at doses of 100 mg/kg/body weight/day by intra-peritoneal, oral route and, intramuscular, respectively. Group 5 was administered a single dose of cyfluthrin (100 mg g/kg/body weight ∼1/3LD₅₀) by oral, whereas Groups 6, 7 and 8 were given cyfluthrin+NAC, cyfluthrin+proanthocyanidin and cyfluthrin+vitamin E, at the same dose, respectively. The administration of the drugs was initiated following the administration of cyfluthrin, and continued until the end of the seventh day of the study. Blood samples were collected from each group, 24 h, and 3, 7 and 9 days after the administration of cyfluthrin for the assessment of blood malondialdehyde (MDA) levels and superoxide dismutase (SOD) and catalase (CAT) activities. According to the data obtained, compared to the control group, increase in the plasma MDA level of the group administered cyfluthrin alone, and decrease in erythrocyte SOD activities in some periods and CAT activities in all periods were determined. On the other hand, especially, MDA levels and CAT activities were observed to move closer to values of the control group, in the groups that were administered NAC, proanthocyanidin and vitamin E in addition to cyfluthrin. In other words, in most periods, decrease in plasma MDA levels, and increase in erythrocyte CAT and SOD activities were observed in comparison to the group administered cyfluthrin alone. Statistical analyses demonstrated significant differences to exist between the groups on the third, seventh and ninth days with respect to plasma MDA levels, and the third and ninth days with respect to erythrocyte SOD and CAT activities (P < 0.05). However no significant difference was demonstrated in any of the periods in the groups that were administered NAC, proanthocyanidin and vitamin E alone in comparison to the control group (P > 0.05). In view of the parameters examined, animals were concluded to be affected by cyfluthrin and the administration of the three compounds at the indicated doses and for the indicated periods were considered to alleviate the adverse effects of cyfluthrin partly throughout the study period.     

Effect of fenvalerate on oxidative stress biomarkers in the brackish water prawn Penaeus monodon

The toxicity of fenvalerate to the prawn Penaeus monodon was evaluated using biomarkers of stress. In a preliminary bioassay test, P. monodon was exposed to a series of fenvalerate concentrations, which showed 4, 6.5 and 8.5 μg L⁻¹ to be sublethal, median lethal and lethal, respectively. Sublethal effect of fenvalerate was further evaluated in hepatopancreas, muscle and gills of prawns with reference to oxidative stress biomarkers. Significant induction of lipid peroxidation and glutathione-S-transferase activity was found in hepatopancreas, muscle and gills of prawns exposed to fenvalerate when compared to control (P < 0.001, P < 0.05 and P < 0.05). On the contrary, the activities of Superoxide dismutase, catalase, glutathione peroxidase, vitamin C, vitamin E and glutathione were found to be reduced in the experimental group of prawns when compared to control. The results suggest that the animals were under oxidative stress when exposed to sublethal concentration of fenvalerate.     

Effect of sulfite-hydrosulfite and nitrite on thiol redox state, oxidative stress and sclerotial differentiation of filamentous phytopathogenic fungi

This study shows that Na₂S₂O₄, Na₂SO₃ and NaNO₂ decreased differentiation of filamentous phytopathogenic fungi representing the four main types of sclerotia at maximum growth non-inhibiting concentrations, and also increased and decreased the oxidized and the reduced components of their thiol redox state, respectively. Na₂S₂O₄, Na₂SO₃ and NaNO₂ also severely inhibited fungal growth at certain concentrations. These effects concurred by the increase of the high oxidative stress indicator lipid peroxidation and could be attributed to the oxidative cytotoxicity of sulfites and nitrite. The cytotoxic mechanism of the latter may be related to the acidic pH. Sulfites and nitrite can be used as potent fungicides against sclerotiogenic phytopathogenic fungi by acting as growth inhibiting cytotoxic oxidants and by sustaining fungi in their undifferentiated hyphal stage, in which they are more vulnerable to degradation by soil microorganisms.     

Chronic chlorpyrifos-induced oxidative changes in the testes and pituitary gland of Wistar rats: Ameliorative effects of vitamin C

Chlorpyrifos (CPF), a chlorinated organophosphate insecticide that is widely used in agriculture and public health, has been implicated in male reproductive toxicity. Apart from acetylcholinesterase inhibition, CPF has been shown to induce changes characteristic of oxidative stress. Therefore, the aim of the present study was to evaluate the effects of vitamin C on oxidative changes in the testes and pituitary gland of rats chronically exposed to CPF. Twenty adult male Wistar rats were divided into four groups of five animals each: Group I (S/oil) received soya oil (2 ml/kg); Group II (VC) was administered with vitamin C (100 mg/kg); Group III (CPF) was given CPF (10.6 mg/kg; ∼1/8th LD₅₀); Group IV (VC + CPF) was pretreated with vitamin C (100 mg/kg) and then given CPF (10.6 mg/kg), 30 min later. The regimens were administered orally by gavage once daily for 15 weeks. Thereafter, the rats were sacrificed and the testes and pituitary glands were evaluated for the concentration of malonaldehyde (MDA) and activities of superoxide dismutase (SOD) and catalase (CAT). The result shows that CPF increased MDA concentration and reduced activities of SOD and CAT, which were ameliorated by pretreatment with vitamin C.     

Subacute chlorpyrifos-induced oxidative stress in rat erythrocytes and the protective effects of catechin and quercetin

This study examined the effects of chlorpyrifos in the rat erythrocyte antioxidant system and evaluated the ameliorating effects of catechin and quercetin on the oxidative damage induced by chlorpyrifos. Sexually mature male Wistar rats were given chlorpyrifos (5.4 mg/kg, 1/25 of the oral LD₅₀), catechin (20 mg/kg), quercetin (20 mg/kg), catechin plus chlorpyrifos, and quercetin plus chlorpyrifos daily via gavage for four weeks. No statistical differences were found in the catechin-only and quercetin-only groups compared with the control group. By the end of the fourth week, chlorpyrifos alone increased the levels of malondialdehyde (MDA) and decreased superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) activities compared with the control group in rat erythrocytes. In the catechin-plus-chlorpyrifos and quercetin-plus-chlorpyrifos groups, there were statistically significantly decreased MDA levels and increased SOD, CAT, and GPx activities compared with the chlorpyrifos-only group. Thus, it appears that catechin and quercetin ameliorate chlorpyrifos-induced oxidative stress in rat erythrocytes in vivo.     

Biochemical and histopathological studies to assess chronic toxicity of triazophos in blood, liver and brain tissue of rats

Triazophos, O,O-diethyl-1-H-1,2,4-triazol-3-yl phosphorothioate, (TZ) is an organophosphorus pesticide which is extensively used in agriculture for controlling insect pests. Except a FAO/WHO report no study has investigated its short-term toxicity with respect to its potential to cause biochemical and histopathological alterations. The present study was designed to identify the effect of TZ at different doses (1.64, 3.2 and 8.2 mg/kg) on the oxidative stress parameters in blood as well as organs involved in xenobiotic metabolism (liver and brain) following chronic exposure for 90 days. Moreover, the study also delineates the effect of TZ on the histo-architecture of these organs. The results indicated a dose dependent induction (p < 0.001) of oxidative stress, as evident by increased malondialdehyde (MDA) level and compromised antioxidant defense including glutathione S transferase (GST) activity, glutathione (GSH) content and ferric reducing ability of plasma (FRAP) in blood, and increased MDA level with concomitantly decreased GSH content in tissues, following chronic exposure to TZ. The ratio of MDA: FRAP in blood was found to be increased following chronic exposure to TZ and may serve as a suitable indicator of severity of oxidative damage. Onset of such biochemical alterations is one of the early adaptive responses to TZ exposure which leads to histopathological alterations in terms of diffuse fatty changes expanding from mid-zonal area to whole lobule in liver. However, increased oxidative stress did not bring any morphological alteration in brain. The present study concludes that induction of oxidative stress, leading to subsequent histopathological alterations in liver, is an important mechanism underlying the TZ induced chronic toxicity.     

Photochemical damage and comparative performance of superoxide dismutase and ascorbate peroxidase in sugarcane leaves exposed to paraquat-induced oxidative stress

The physiological responses of sugarcane (Saccharum officinarum L.) to oxidative stress induced by methyl viologen (paraquat) were examined with respect to photochemical activity, chlorophyll content, lipid peroxidation and superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. Thirty-day-old sugarcane plants were sprayed with 0, 2, 4, 6 and 8 mM methyl viologen (MV). Chlorophyll fluorescence was measured after 18 h and biochemical analyses were performed after 24 and 48 h. Concentrations of MV above 2 mM caused significant damage to photosystem II (PSII) activity. Potential and effective quantum efficiency of PSII and apparent electron transport rate were greatly reduced or practically abolished. Both chlorophyll and soluble protein contents steadily decreased with MV concentrations above 2 mM after 24 h of exposure, which became more pronounced after 48 h, achieving a 3-fold decrease. Insoluble protein contents were little affected by MV. Oxidative stress induced by MV was evidenced by increases in lipid peroxidation. Specific activity of SOD increased, even after 48 h of exposure to the highest concentrations of MV, but total activity on a fresh weight basis did not change significantly. Nondenaturing PAGE assayed with H₂O₂ and KCN showed that treatment with MV did not change Cu/Zn-SOD and Mn-SOD isoform activities. In contrast, APX specific activity increased at 2 mM MV but then dropped at higher doses. Oxidative damage induced by MV was inversely related to APX activity. It is suggested that the major MV-induced oxidative damages in sugarcane leaves were related to excess H₂O₂, probably in chloroplasts, caused by an imbalance between SOD and APX activities, in which APX was a limiting step. Reduced photochemical activity allowed the early detection of the ensuing oxidative stress.