Nitric oxide plays a role as second messenger in the ultraviolet-B irradiated green alga <Emphasis Type="Italic">Chlorella pyrenoidosa</Emphasis>

Nitric oxide (NO) stimulated the activity of plasma membrane H⁺-ATPase, 5′-nucleotidase, peroxidase, ascorbate peroxidase and glutathione reductase in ultraviolet B (UV-B) irradiated Chlorella pyrenoidosa. It also boosted the activity of nitrogen-metabolism enzymes such as nitrate reductase, nitrite reductase, glutamine synthetase, which were inhibited by UV-B irradiation. The chlorophyll fluorescence ratio (Fv/Fm) of the UV-B irradiated algae and decreased continuously after the cells were transferred to UV-B irradiation. A continuing decrease of the Fv/Fm was observed even after the cells were transferred to photosynthetically active radiation (PAR). After adaptation for 8 h under PAR (after treatment with nitric oxide), Fv/Fm recovered to 55 % of normal levels — without NO the value approached zero. Exogenous NO stopped the decay of chlorophyll and thylakoid membrane in cells exposed to UV-B irradiation. NO plays probably a key role in damage induced by UV-B irradiation in green algae.     

UV-B induced changes in antioxidant enzymes and their isoforms in cucumber (Cucumis sativus L.) cotyledons

To assess the role of antioxidant defense system on exposure to ultra-violet-B (UV-B) radiation, the activities of antioxidant enzymes superoxide dismutase (SOD), ascorbic acid peroxidase (APX), glutathione reductase (GR) and guaiacol peroxidase (GPX), as well as the level of antioxidants ascorbic acid (AA) and alpha-tocopherol were monitored in cucumber (Cucumis sativus L. var long green) cotyledons. UV-B enhanced the activity of antioxidant enzymes as well as AA content, but decreased the level of alpha-tocopherol. Significant increase was observed in the activities of SOD and GPX. Analysis of isoforms of antioxidant enzymes by native-PAGE and activity staining revealed three isoforms of GPX in unexposed dark-grown cotyledons (control), and their intensity was enhanced by UV-B exposure. In addition, four new isoforms of GPX were observed in cotyledons after UV-B exposure. Although no new isoforms were observed for the other antioxidant enzymes, the activities of their existing isoforms were enhanced by UV-B.     

Ascorbic Acid and Heterocyst Development in the Blue–Green Alga Amabaema ambigua

The nitrogen–fixing blue–green alga Anabaena ambigua was grown in a medium which contained either ammonium chloride as nitrogen source or molecular nitrogen. In the latter case the alga produced heterocysts. The material was analysed for ascorbic acid, dehydro-ascorbic acid and diketogulonic acid. The amount of a,scorbic acid was found always to he higher in the alga grown with molecular nitrogen. When the alga grown with combined nitrogen was transferred to the medium lacking it, there was an increase in the ascorbic acid content. Conversely, when material cultured on the nitrogen–free medium was suspended in the medium containing ammonium chloride, there was a decrease in the cellular ascorbic acid. Esogenously added ascorbic acid, up to 0.5 mg per ml, increased the heterocyst frequency to nearly three times that of the control. D–isoascorbic acid, an analogue of ascorbic acid, also showed an enhancement of heterocyst production. Algal extracts were fractionated by poiyacrylamide electro–phoresis, and the presence of ascorbic acid oxidase was detected on the gels. Two bands, with Rf values 0.34 and 1.0, were found to give positive test: for the enzyme. The total enzyme activity was 16.7 % higher in cells grown with molecular nitrogen than in those grown with combined nitrogen. The exact location of the enzyme in the alga ist not known although the heterocysts were earlier shown to contain ascorbic acid. Cytochemical tests, however, indicated strong per–oxidase activity in the heterocysts.     

Nitric oxide is involved in the regulation of ascorbate and glutathione metabolism in Agropyron cristatum leaves under water stress

This study investigated the regulation of ascorbate and glutathione metabolism by nitric oxide in Agropyron cristatum leaves under water stress. The activities of ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), L-galactono-1,4-lactone dehydrogenase (GalLDH) and γ-glutamylcysteine synthetase (γ-ECS), and the contents of NO, reduced ascorbic acid (AsA), reduced glutathione (GSH), total ascorbate and total glutathione increased under water stress. These increases were suppressed by pretreatments with NO synthesis inhibitors N ^G-nitro-L-arginine methyl ester (L-NAME) and 4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). However, application of L-NAME and cPTIO to plants sufficiently supplied with water did not affect the activities of above mentioned enzymes and the contents of NO and above mentioned antioxidants. Pretreatments with L-NAME and cPTIO increased the malondialdehyde (MDA) content and electrolyte leakage of plants under water stress. Our results suggested that water stress-induced NO is a signal that leads to the upregulation of ascorbate and glutathione metabolism and has important role for acquisition of water stress tolerance.     

Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin

Ascorbic acid enhances NO bioactivity in patients with vascular disease through unclear mechanism(s). We investigated the role of intracellular ascorbic acid in endothelium-derived NO bioactivity. Incubation of porcine aortic endothelial cells (PAECs) with ascorbic acid produced time- and dose-dependent intracellular ascorbic acid accumulation that enhanced NO bioactivity by 70% measured as A23187-induced cGMP accumulation. This effect was due to enhanced NO production because ascorbate stimulated both PAEC nitrogen oxide (NO(2)(-) + NO(3)(-)) production and l-arginine to l-citrulline conversion by 59 and 72%, respectively, without altering the cGMP response to authentic NO. Ascorbic acid also stimulated the catalytic activity of eNOS derived from either PAEC membrane fractions or baculovirus-infected Sf9 cells. Ascorbic acid enhanced bovine eNOS V(max) by approximately 50% without altering the K(m) for l-arginine. The effect of ascorbate was tetrahydrobiopterin (BH(4))-dependent, because ascorbate was ineffective with BH(4) concentrations >10 microm or in PAECs treated with sepiapterin to increase intracellular BH(4). The effect of ascorbic acid was also specific because A23187-stimulated cGMP accumulation in PAECs was insensitive to intracellular glutathione manipulation and only ascorbic acid, not glutathione, increased the intracellular concentration of BH(4). These data suggest that ascorbic acid enhances NO bioactivity in a BH(4)-dependent manner by increasing intracellular BH(4) content.     

Ascorbate enhances iNOS activity by increasing tetrahydrobiopterin in RAW 264.7 cells

Studies on the effect of ascorbic acid on inducible nitric oxide synthase (iNOS) activity are few and diverse, likely to be dependent on the species of cells. We investigated a role of ascorbic acid in iNOS induction and nitric oxide (NO) generation in mouse macrophage cell line RAW 264.7. Although interferon- (IFN-) gamma alone produced NO end products, ascorbic acid enhanced NO production only when cells were synergistically stimulated with IFN-gamma plus Escherichia coli lipopolysaccharide (LPS). Ascorbate neither enhanced nor decreased the expression of iNOS protein in RAW 264.7 cells, in contrast to the reports that ascorbic acid augments iNOS induction in a mouse macrophage-like cell line J774.1 and that ascorbate suppresses iNOS induction in rat skeletal muscle endothelial cells. Intracellular levels of tetrahydrobiopterin (BH4), a cofactor for iNOS, were increased by ascorbate in RAW 264.7 cells. However, ascorbate did not increase GTP cyclohydrolase I mRNA, the main enzyme at the critical steps in the BH4 synthetic pathway, expression levels and activity. Sepiapterin, which supplies BH4 via salvage pathway, more efficiently enhanced NO production if ascorbate was added. These data suggest that enhanced activation of iNOS by ascorbic acid is mediated by increasing the stability of BH4 in RAW 264.7 cells.     

一氧化氮对增强的UV_B胁迫下螺旋藻生物损伤的减缓作用

为了探讨一氧化氮对增强的UV-B胁迫下螺旋藻生物学特性的影响,通过色素含量、蛋白质含量和生物量3个方面的变化证实了0.5mmol/L的一氧化氮(Nitric oxide,NO)供体硝普钠(Sodium nitroprusside,SNP)对增强UV-B胁迫下的螺旋藻(Spirulina platensis)794细胞生物损伤有明显的减缓作用。实验结果显示,NO能够显著诱导增强的UV-B胁迫下螺旋藻细胞内蛋白质含量、脯氨酸含量的提高,促进正常生长条件下螺旋藻(Spirulina platensis)794细胞内抗氧化物质GSH含量的增多,但外源NO又可以降低增强UV-B胁迫下螺旋藻细胞中GSH含量的增加。说明NO对增强UV-B胁迫下的螺旋藻794细胞有保护作用,可以减轻UV-B胁迫对螺旋藻(S.platensis)细胞引起的生物损伤。首次研究报道了增强UV-B胁迫下NO信号分子对蓝细菌———螺旋藻细胞生物损伤调节能力的影响,为进一步探讨NO信号及其与其它信号分子之间相互作用、相互关联来调节细胞的生理生化过程,以减缓UV-B胁迫下的生物损伤机理奠定了基础。     

Effects of oxidizing and reducing agents on ovine pulmonary artery responses to nitric oxide donors, sodium nitroprusside and 3-morpholino-sydnonimine

Nitrovasodilators-sodium nitroprusside (SNP; 10(-9)-10(-4) M) and 3-morpholino-sydnonimine (SIN-1; 10(-9)-10(-4) M) produced concentration-dependent relaxation of the fourth generation sheep pulmonary artery, preconstricted with 5-hydroxytryptamine (1 microM). Oxidizing agents [oxidized glutathione (GSSG, 1 mM) and CuSO4 (5 and 20 microM)] and reducing agents [dithiothreitol (DTT, 0.1 mM), ascorbic acid (1 mM) and reduced glutathione (GSH, 1 mM)] caused opposite effects on nitric oxide (NO)-induced vasodilation in the artery. Ascorbic acid and GSH potentiated the NO responses, while GSSG and CuSO4 inhibited relaxation caused by the nitrovasodilators. DTT, however, reduced the relaxant potency and efficacy of SNP and SIN-1. Pretreatment of the pulmonary artery strips with DTT (0.1 mM) inhibited SNP (10 microM)-induced Na(+)-K(+)-ATPase activity, while ascorbic acid (1 mM) and GSH (1 mM) had no effect either on basal or SNP (10 microM)-stimulated 86Rb uptake, an index of Na(+)-K(+)-ATPase activity, in ovine pulmonary artery. The results suggest that reducing agents like ascorbic acid may have beneficial effect in improving the vascular function under oxidative stress.     

The effect of peroxynitrite and some antioxidants on the rate of osmotic hemolysis of bovine erythrocytes

Bovine erythrocytes treated with peroxynitrite (ONOO(-)), a cytotoxic species formed in vivo via the reaction of nitric oxide (NO(.)) and the superoxide anion (O2(-.)), show an increased rate of hemolysis on sudden osmotic stress. The increase in the rate was peroxynitrite concentration dependent. In the presence of some antioxidants (uric acid, ascorbic acid, glutathione, melatonin and albumin), this effect was significantly lower, with ascorbic acid as the most efficient antioxidant.     

Nitric Oxide Alleviates Oxidative Damage Induced by Enhanced Ultraviolet-B Radiation in Cyanobacterium

To study the role of nitric oxide (NO) on enhanced ultraviolet-B (UV-B) radiation (280–320 nm)-induced damage of Cyanobacterium, the growth, pigment content, and antioxidative activity of Spirulina platensis-794 cells were investigated under enhanced UV-B radiation and under different chemical treatments with or without UV-B radiation for 6 h. The changes in chlorophyll-a, malondialdehyde content, and biomass confirmed that 0.5 mM sodium nitroprusside (SNP), a donor of nitric oxide (NO), could markedly alleviate the damage caused by enhanced UV-B. Specifically, the biomass and the chlorophyll-a content in S. platensis-794 cells decreased 40% and 42%, respectively under enhanced UV-B stress alone, but they only decreased 10% and 18% in the cells treated with UV-B irradiation and 0.5 mM SNP. Further experiments suggested that NO treatment significantly increased the activities of superoxide dismutase (SOD) and catalase (CAT), and decreased the accumulation of O ₂ ⁻ in enhanced UV-B-irradiated cells. SOD and CAT activity increased 0.95- and 6.73-fold, respectively. The accumulation of reduced glutathione (GSH) increased during treatment with 0.5 mM SNP in normal S. platensis cells, but SNP treatment could inhibit the increase of GSH in enhanced UV-B-stressed S. platensis cells. Thus, these results suggest that NO can strongly alleviate oxidative damage caused by UV-B stress by increasing the activities of SOD, peroxidase, CAT, and the accumulation of GSH, and by eliminating O ₂ ⁻ in S. platensis-794 cells. In addition, the difference of NO origin between plants and cyanobacteria are discussed.     

Nitric oxide production mediated by nitrate reductase in the green alga Chlamydomonas reinhardtii: an alternative NO production pathway in photosynthetic organisms

Biological activity of nitric oxide (NO) production was investigated in the unicellular green alga Chlamydomonas reinhardtii. An NO specific electrode detected a rapid increase in signal when nitrite (NO(2)(-)) was added into a suspension of C. reinhardtii intact cells in the dark. The addition of KCN or the NO quencher bovine hemoglobin completely abolished the signal, verifying that the nitrite-dependent increase in signal is due to enzymatic NO production. L-arginine, the substrate for NO synthase, did not induce detectable NO production and the NOS inhibitor N(omega)-nitro-L-arginine showed no inhibitory effect on the nitrite-dependent production of NO. Illuminating cells showed a significant suppressive effect on NO production. When the photosynthetic electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea was present in the suspension, C. reinhardtii cells produced NO after the addition of nitrite even under illumination. Kinetic and microscopic observations, using the intracellular fluorescent NO probe 4,5-diaminofluorescein-2 diacetate, both demonstrated that NO was produced within the cells in response to the addition of nitrite. The Chlamydomonas mutant cc-2929, which lacks nitrate reductase (NR) activity, did not display any of the responses observed in the wild-type cells. The results presented here provide direct in vivo evidence to confirm that NR is involved in the nitrite-dependent NO production in the green alga.     

UV-B radiation and selenium affected energy availability in green alga <Emphasis Type="Italic">Zygnema</Emphasis>

Green alga Zygnema was exposed to three concentrations of selenium and two levels of UV-B radiation. The combined effects of both treatments on energy availability; photochemical quantum yield and respiratory potential were studied. Our findings show that traces of selenium enhance metabolic process connected with photochemical quantum yield and mitochondrial respiration. Surprisingly, selenium does not diminish the effects of UV-B radiation; on the contrary, the combined action of UV-B radiation and traces of selenium leads to pronounced negative effects on photochemical quantum yield and the respiratory potential. Selenium is involved in the activation of energy resources in green alga Zygnema. The importance of selenium for activity of the mitochondria is possibly an evolutionary recollection from an endosymbiotic bacterium.     

Silicon and Nitric Oxide-Mediated Regulation of Growth Attributes,Metabolites and Antioxidant Defense System of Radish (<i>Raphanus sativus</i> L.) under Arsenic Stress

Arsenic (As) contaminated food chains have emerged as a serious public concern for humans and animals and are known to affect the cultivation of edible crops throughout the world. Therefore, the present study was designed to investigate the individual as well as the combined effects of exogenous silicon (Si) and sodium nitroprusside (SNP), a nitric oxide (NO) donor, on plant growth, metabolites, and antioxidant defense systems of radish (Raphanus sativus L.) plants under three different concentrations of As stress, i.e., 0.3, 0.5, and 0.7 mM in a pot experiment. The results showed that As stress reduced the growth parameters of radish plants by increasing the level of oxidative stress markers, i.e., malondialdehyde and hydrogen peroxide. However, foliar application of Si (2 mM) and pretreatment with SNP (100 µM) alone as well as in combination with Si improved the plant growth parameters, i.e., root length, fresh and dry weight of plants under As stress. Furthermore, As stress also reduced protein, and metabolites contents (flavonoids, phenolic and anthocyanin). Activities of antioxidative enzymes such as catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD), and polyphenol oxidase (PPO), as well as the content of non-enzymatic antioxidants (glutathione and ascorbic acid) decreased under As stress. In most of the parameters in radish, As III concentration showed maximum reduction, as compared to As I and II concentrations. However, the individual and combined application of Si and NO significantly alleviated the As-mediated oxidative stress in radish plants by increasing the protein, and metabolites content. Enhancement in the activities of CAT, APX, POD and PPO enzymes were recorded. Contents of glutathione and ascorbic acid were also enhanced in response to co-application of Si and NO under As stress. Results obtained were more pronounced when Si and NO were applied in combination under As stress, as compared to their individual application. In short, the current study highlights that Si and NO synergistically regulate plant growth through lowering the As-mediated oxidative stress by upregulating the metabolites content, activity of antioxidative enzymes and non-enzymatic antioxidants in radish plants.

     

Changes in ascorbate, glutathione, and related enzyme activities during thidiazuron-induced bud break of apple

The changes of ascorbic acid, dehydroascorbic acid, and glutathione content and related enzyme activities were studied in apple buds during dormancy and thidiazuron-induced bud break. An increase in ascorbic acid, reduced form of glutathione (GSH), total glutathione, total non-protein thiol (NPSH) and non-glutathione thiol (RSH) occurred as a result of induction by thidiazuron during bud break, whereas dehydroascorbic acid and oxidized glutathione (GSSG) decreased during the same period. Thidiazuron also enhanced the ratio of GSH/GSSG, and activities of ascorbate free radical reductase (AFR; EC 1.6.5.4), ascorbate peroxidase (EC 1.11.1.11). dehydroascorbate reductase (DHAR; EC 1.8.5.1) and glutathione reductase (GR; EC 1.6.4.2). The ascorbic acid content and the activities of AFR, ascorbate peroxidase, and DHAR peaked when buds were in the side green or green tip stage just prior to the start of rapid expansion, and declined thereafter. The GSH, NPSH, RSH, ratio of GSH/GSSG, and activities of GR increased steadily during bud development.     

Combined arginine and ascorbic acid treatment induces apoptosis in the hepatoma cell line HA22T/VGH and changes in redox status involving the pentose phosphate pathway and reactive oxygen and nitrogen species

Arginine is a physiological substrate for nitric oxide synthase to generate nitric oxide (NO), which can influence tumor cell survival, while ascorbic acid is selectively toxic for cancer cells. This study explored the effect of an arginine/ascorbic acid combination on human cancer cell lines. The hepatoma cell line HA22T/VGH was the most sensitive of the tested cells to combination treatment. A combination of 5.74 mM of arginine and 0.57 mM of ascorbic acid induced HA22T/VGH cell death through apoptosis and an increase in levels of reactive oxygen species and NO, as well as its stable products NO₂⁻ and NO₃⁻. The combination also reduced the activity of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and transaldolase in the pentose phosphate pathway, a major mechanism for producing NADPH, resulting in a marked decrease in intracellular NADPH levels. A dramatic decrease in intracellular glutathione (GSH) levels, a decrease in the mitochondrial membrane potential, ATP depletion and release of cytochrome c were also seen. Caspase-9 and caspase-3 were activated, apoptotic protein Bax expression increased and the expression of the anti-apoptotic proteins Bcl-2 and Bcl-xL decreased. These results suggest that this combination induced HA22T/VGH cell death by interfering with redox state regulation by a reduction in pentose phosphate pathway activity and increasing oxidative and nitrosative stress.     

Modification of analytical procedures for determining vitamin C enzyme (L-gulonolactone oxidase) activity in swine liver

Modifications of the analytical method to determine L-gulono-gamma-lactone oxidase (EC 1.1.8) enzyme activity were conducted in pig liver by evaluating the concentration of added substrate (L-gulono-gamma-lactone), glutathione, and various tissue sample-to-buffer ratios in the incubation mixture. Sampling different liver sites (lobes), the effect of different cooling temperatures of the liver immediately after collection, and the effect of tissue storage length on subsequent enzyme activity were evaluated. Our results demonstrated that 10 mM of substrate added to the reaction media maximized L-gulono-gamma-lactone oxidase enzyme activity, whereas increasing levels of glutathione did not greatly affect enzyme activity. High sample-to-buffer ratios resulted in higher L-gulono-gamma-lactone oxidase activities but sample analytical variations and background interferences were greater. A 1:4 tissue sample to buffer ratio (weight:weight) resulted in repeatable values, but the importance of maintaining the same ratio of the two components seems to be critical within an experiment. Expressing L-gulono-gamma-lactone oxidase enzyme activity on a liver protein rather than on a liver weight basis also resulted in more consistent results. No difference in liver L-gulono-gamma-lactone oxidase enzyme activities or ascorbic acid concentrations occurred between liver lobes. L-gulono-gamma-lactone oxidase enzyme activity from 0 to 90 day of storage was not affected when tissue samples were immediately frozen in liquid nitrogen, or placed on crushed ice. During a 90-day storage the oxidized form of ascorbic acid (dehydroascorbic acid) decreased (P < 0.01), the reduced (ascorbic acid) form increased (P < 0.01), while total ascorbic acid concentration remained constant.     

UV-B辐射对蚕豆叶片气孔运动的间接效应与NO和H2O2有关

0.2 W.m-2的UV-B辐射不仅能诱导整体蚕豆叶片气孔导度和开度的显著降低,而且能明显降低蚕豆叶肉光合活性,但该强度的UV-B辐射却不能明显影响离体表皮条的气孔开度.说明0.2W.m-2的UV-B主要通过间接途径调控了蚕豆叶片气孔运动.借助药理学试验和激光扫描共聚焦显微镜技术,进一步对该间接效应过程中是否有NO和H2O2的参与进行了探讨.结果显示:NO专一性清除剂cPT IO和一氧化氮合酶(NO S)抑制剂L-NAM E均能有效地抑制UV-B辐射诱导的叶片气孔关闭和保卫细胞内源NO水平的升高;H2O2清除剂抗坏血酸(A SC)和过氧化氢酶(CAT)也能有效地逆转UV-B辐射诱导的气孔关闭和保卫细胞内源H2O2含量的升高.另外,外源NO或H2O2处理也能有效地诱导叶片气孔关闭.结果说明0.2W.m-2的UV-B辐射对蚕豆叶片气孔关闭的间接诱导与NO和H2O2有关.     

Glutathione depletion switches nitric oxide neurotrophic effects to cell death in midbrain cultures: implications for Parkinson's disease

Nitric oxide (NO) exerts neurotrophic and neurotoxic effects on dopamine (DA) function in primary midbrain cultures. We investigate herein the role of glutathione (GSH) homeostasis in the neurotrophic effects of NO. Fetal midbrain cultures were pretreated with GSH synthesis inhibitor, l ‐buthionine‐(S,R)‐sulfoximine (BSO), 24 h before the addition of NO donors (diethylamine/nitric oxide‐complexed sodium and S‐nitroso‐N‐acetylpenicillamine) at doses tested previously as neurotrophic. Under these conditions, the neurotrophic effects of NO disappeared and turned on highly toxic. Reduction of GSH levels to 50% of baseline induced cell death in response to neurotrophic doses of NO. Soluble guanylate cyclase (sGC) and cyclic GMP‐dependent protein kinase (PKG) inhibitors protected from cell death for up to 10 h after NO addition; the antioxidant ascorbic acid also protected from cell death but its efficacy decreased when it was added after NO treatment (40% protection 2 h after NO addition). The pattern of cell death was characterized by an increase in chromatin condensed cells with no DNA fragmentation and with breakdown of plasmatic membrane. The inhibition of RNA and protein synthesis and of caspase activity also protected from cell death. This study shows that alterations in GSH levels change the neurotrophic effects of NO in midbrain cultures into neurotoxic. Under these conditions, NO triggers a programmed cell death with markers of both apoptosis and necrosis characterized by an early step of free radicals production followed by a late requirement for signalling on the sGC/cGMP/PKG pathway.     

Dependence of UV-B-induced camptothecin production on nitrate reductase-mediated nitric oxide signaling in Camptotheca acuminata suspension cell cultures

UV-B irradiation induced production of secondary metabolites in plant cells. However, the mechanisms of UV-B-induced secondary metabolite production remained largely unknown. Here we report that UV-B treatment stimulated nitric oxide (NO) generation and camptothecin (CPT) production in Camptotheca acuminata cells. To investigate the origin of the UV-B-triggered NO and the role of NO in UV-B-induced CPT production, we assayed the responses of nitrate reductase (NR) and NO synthase (NOS) activities of the cells to UV-B exposure and examined the effects of NR and NOS inhibitors on CPT production in UV-B-treated cells. The data showed that UV-B irradiation enhanced NR activities in the cells. Pretreatment with NR inhibitors tungstate and okadaic acid not only suppressed the UV-B-triggered NR activities but also inhibited the UV-B-induced NO generation and CPT production in the cells. In contrast, UV-B irradiation had no effects on NOS activity of the cells and treatment of NOS inhibitor did not suppress UV-B-induced CAT production. Together, the results demonstrated that NR activity was essential for UV-B-triggered NO generation and that NR-mediated NO signaling was involved in UV-B-induced CPT production in C. acuminata cells.