Superoxide dismutase-rich bacteria. Paradoxical increase in oxidant toxicity

Superoxide dismutase is considered important in protection of aerobes against oxidant damage, and increased tolerance to oxidant stress is associated with induction of this enzyme. However, the importance of superoxide dismutase in this tolerance is not clear because conditions which promote the synthesis of superoxide dismutase likewise affect other antioxidant enzymes and substances. To clarify the role of superoxide dismutase per se in organismal defense against oxidant-generating drugs, we employed Escherichia coli transformed with multiple copies of the gene for bacterial iron superoxide dismutase. These bacteria have greater than ten times the superoxide dismutase activity of wild-type E. coli but, importantly, are normal in other oxidant defense parameters including catalase, peroxidases, glutathione, and glutathione reductase. High superoxide dismutase and control bacteria were exposed to the O2- -generating drug paraquat and to elevated pO2. We find; high superoxide dismutase E. coli are more readily killed by paraquat under aerobic, but not anaerobic, conditions. During exposure to paraquat, high superoxide dismutase E. coli accumulate more H2O2. Coincidentally, the reduced glutathione content of high superoxide dismutase E. coli declines more than in control E. coli. E. coli with high superoxide dismutase activity are also more readily killed by hyperoxia. Interestingly, the susceptibility of the parental and high superoxide dismutase E. coli to killing by exogenous H2O2 is not significantly different. Thus, under these experimental conditions, greatly enhanced superoxide dismutase activity accelerates H2O2 formation. The increased H2O2 probably accounts for the exaggerated sensitivity of high superoxide dismutase bacteria to oxidant-generating drugs. These results support the concept that the product of superoxide dismutase, H2O2, is at least as hazardous as the substrate, O2-. We conclude that effective organismal defense against reactive oxygen species may require balanced increments in antioxidant enzymes and cannot necessarily be improved by increases in the activity of single enzymes.     

Toxicity of 1-methyl-4-phenylpyridinium derivatives in Escherichia coli.

Several derivatives of 1-methyl-4-phenylpyridinium (MPP+), i.e., 1-methyl-4-(4'-nitrophenyl)pyridinium (1), 1-methyl-4-(4'-cyanophenyl)pyridinium (2), 1-methyl-4-(3'-nitrophenyl)pyridinium (3), 1-methyl-4-(4'-chlorophenyl)pyridinium (4), 1-methyl-4-(4'-acetamidophenyl)pyridinium (5), and 1-methyl-4-(4'-aminophenyl)pyridinium (6), were synthesized in order to compare their toxicity with that of paraquat (PQ2+) in Escherichia coli. Addition of compounds 1, 2, and 3 to aerobic E. coli cell suspensions caused extracellular ferricytochrome c reduction, which was inhibited by superoxide dismutase in the same manner as that in the case of PQ2+. The rate of the ferricytochrome c (cyt. c) reduction was in the order of PQ2+ greater than 1 greater than 2 greater than 3, which is the same as that of the redox potentials of these compounds. On the other hand, MPP+, 4, 5, and 6, which have more negative potentials, had no effect on the cyt. c reduction. Compound 1 inhibited the growth of E. coli under aerobic conditions, but not under anaerobic conditions. The results show that compound 1 can act as a mediator for production of superoxide (O2-.), which seriously injures E. coli cells. However, though compounds 2 and 3 catalyzed the production of O2-. in E. coli cells, their activity of O2-. production was much lower than that of compound 1 or PQ2+. Thus, compound 3 had no effect on growth or survival of E. coli at 1 mM, while compounds 2 and 4 had both bacteriostatic and bacteriocidal effects which were independent of dioxygen (O2). The results show that the toxic mechanism is different from that of compound 1. MPP+, 5, and 6 had no effect on growth of E. coli. This paper shows that compound 1 is a novel enhancer of intracellular superoxide production, though the mechanism of toxicity of compounds 2 and 4 is not clear yet. The results suggest that the redox potential is a crucial factor for manifestation of the activity.     

Lucigenin chemiluminescence as a probe for measuring reactive oxygen species production in Escherichia coli

Addition of oxygen to whole cells of Escherichia coli suspended in the presence of the chemiluminescent probe bis-N-methylacridinium nitrate (lucigenin) resulted in a light emission increase of 200% of control. Addition of air to cells showed a chemiluminescent response far less than the response to oxygen. The redox cycling agents paraquat and menadione, which are known to increase intracellular production of O2- and H2O2, were also found to cause a measurable increase in lucigenin chemiluminescence in E. coli cells when added at concentrations of 1 and 0.1 mM, respectively. The oxygen-induced chemiluminescent response was not suppressed by extracellularly added superoxide dismutase or catalase. Further, the lucigenin-dependent chemiluminescent response of aerobically grown E. coli to oxygen was significantly greater than that of cells grown anaerobically. Heat-killed cells showed no increase in chemiluminescence on the addition of either oxygen, paraquat, or menadione. These results show that lucigenin may be used as a chemiluminescent probe to demonstrate continuous intracellular production of reactive oxygen metabolites in E. coli.     

Catalase and superoxide dismutase in Escherichia coli

We assessed the roles of intrabacterial catalase and superoxide dismutase in the resistance of Escherichia coli to killing by neutrophils. E. coli in which the synthesis of superoxide dismutase and catalase were induced by paraquat 10-fold and 5-fold, respectively, did not resist killing by neutrophils. When bacteria were allowed to recover from the toxicity of paraquat for 1 h on ice and for 30 min at 37 degrees C, they still failed to resist killing by neutrophils. Induction of the synthesis of catalase 9-fold by growth in the presence of phenazine methosulfate did not render E. coli resistant to killing by either neutrophils or by H2O2 itself. The lack of protection by intrabacterial catalase from killing by neutrophils could not be attributed to an impermeable bacterial membrane; the evolution of O2 from H2O2 was no less rapid in suspensions of E. coli than in lysates. The failure of intrabacterial catalase or superoxide dismutase to protect bacteria from killing by neutrophils might indicate either that the flux of O-2 and H2O2 in the phagosome is too great for the intrabacterial enzymes to alter or that the site of injury is at the bacterial surface.     

Difference in superoxide toxicity between 4,7-dicyanobenzofurazan and paraquat.

The O2-. production by aerobically cultured Escherichia coli in the presence of benzofurazan (1), 4,7-dimethylbenzofurazan (2), 4,7-dibromobenzofurazan (3), 4-bromo-6-cyanobenzofurazan (4), and 4,7-dicyanobenzofurazan (5) was examined by using the cytochrome c reduction method in order to elucidate the mechanism of cytotoxicity of benzofurazans. Adding compound 5 to E. coli cell suspension caused cytochrome c reduction, which was completely inhibited by superoxide dismutase. The rate of cytochrome c reduction was in the order of 1 = 2 = 3 less than 4 less than 5, which correlates well with that of the reduction potentials of these benzofurazans. Adding glucose to the E. coli cell suspension-compound 5-cytochrome c system accelerated the rate of cytochrome c reduction. The formation of 4,7-dicyanobenzofurazan anion radical in the cell suspension-compound 5-glucose system in the absence of O2 was followed by ESR spectroscopy. The ESR signal of the anion radical disappeared when O2 was added. Compound 5 was shown to have an approximately 10-fold greater increasing effect on the flux of O2-. by E. coli than paraquat (PQ) by the cytochrome c reduction method. The results were confirmed by the electrochemical method with an oxygen electrode. However, compound 5 had a bacteriostatic, but not lethal, effect, while PQ had both effects. The effect of compound 5 and PQ on lethality of E. coli showed a dramatic difference when E. coli was exposed to these two compounds and washed prior to testing the effects of that exposure. This difference probably arose because compound 5 readily leaked from the cells during dilution and plating. Also, the reduced form of compound 5 exits from the cells more readily than the reduced form of PQ and then generates O2-. in the medium by autoxidation. This suggests the importance of the intracellular production of O2-., rather than the extracellular production of O2-., for lethal effect.     

Induction of superoxide dismutases in Escherichia coli by manganese and iron.

Growth of Escherichia coli B in simple media enriched with Mn(II) resulted in the elevation of the manganese-containing superoxide dismutase, whereas growth in such medium enriched with iron caused increased content of the iron-containing superoxide dismutase. Enrichment of the medium with Co(II), Cu(II), Mo(VI), Zn(II), or Ni(II) had no effect. The inductions of superoxide dismutase by Mn(II) or by Fe(II) were dioxygen dependent, but these metals did not affect the CN- -resistant respiration of E. coli B and did not influence the increase in the CN- -resistant respiration caused by paraquat. Mn(II) and paraquat acted synergistically in elevating the superoxide dismutase content, and Mn(II) reduced the growth inhibition imposed by paraquat, E. coli grown in the complex 3% Trypticase soy broth (BBL Microbiology Systems)-0.5% yeast extract-0.2% glucose medium contained more superoxide dismutase than did cells grown in the simple media and were less responsive to enrichment of the medium with Mn(II) or Fe(II). Nevertheless, in the presence of paraquat, inductions of superoxide dismutase by these metals could be seen even in the Trypticase-yeast extract-glucose medium. On the basis of these observations we propose that the apo-superoxide dismutases may act as autogenous repressors and that Mn(II) and Fe(II) increase the cell content of the corresponding enzymes by speeding the conversion of the apo- to the holoenzymes.     

Effects of overproduction of superoxide dismutase on the toxicity of paraquat toward Escherichia coli

Gross overproduction of the manganese-containing superoxide dismutase in Escherichia coli, by virtue of a multicopy plasmid bearing the sodA gene, decreases enumeration on paraquat-containing agar plates. This reflects growth inhibition, not lethality, since cells on these plates can be rescued by exclusion of dioxygen. Growth in liquid medium revealed that the control strain adapted to growth in the presence of paraquat more rapidly than did the overproducer. Glucose-6-phosphate dehydrogenase, taken as a representative of the superoxide-inducible soxR regulon, was induced during exposure to paraquat to a much greater extent in the control than in the superoxide dismutase-over-producing strain. These results support the view that overproduction of superoxide dismutase interferes with induction of the soxR regulon and thus prevents a balanced adaptation to the multiple aspects of the toxicity of aerobic paraquat.     

Degradation of oxidatively denatured proteins in Escherichia coli

When exposed to oxidative stress, by oxygen radicals or H2O2, E. coli exhibited decreased growth, decreased protein synthesis, and dose-dependent increases in protein degradation. The quinone menadione induced proteolysis when cells were incubated in air, but was not effective when cells were incubated without oxygen. Anaerobically grown cells also exhibited significantly lower proteolytic capacity than did cells that were grown aerobically. Xanthine plus xanthine oxidase (which generate O2- and H2O2) caused a stimulation of proteolysis which was inhibitable by catalase, but not by superoxide dismutase: Indicating that H2O2 was responsible for the increased protein degradation. Indeed, H2O2 alone was effective in inducing increased intracellular proteolysis. Two-dimensional polyacrylamide gel electrophoresis of [3H]leucine labeled E. coli revealed greater than 50% decreases in the concentrations of 10-15 cell proteins following H2O2 or menadione exposure, while several other proteins were less severely affected. To test for the presence of soluble proteases, we prepared cell-free extracts of E. coli and incubated them with radio-labeled protein substrates. E. coli extracts degraded casein and globin polypeptides at rapid rates but showed little activity with native proteins such as superoxide dismutase, hemoglobin, bovine serum albumin, or catalase. When these same proteins were denatured by exposure to oxygen radicals or H2O2, however, they became excellent substrates for degradation in E. coli extracts. Studies with albumin revealed correlations greater than 0.95 between the degree of oxidative denaturation and proteolytic susceptibility. Pretreatment of E. coli with menadione or H2O2 did not increase the proteolytic capacity of cell extracts; indicating that neither protease activation, nor protease induction were required.(ABSTRACT TRUNCATED AT 250 WORDS)     

Paraquat and Escherichia coli. Mechanism of production of extracellular superoxide radical.

Paraquat mediates a superoxide dismutase-inhibitable reduction of cytochrome c by suspensions of Escherichia coli B. Glucose was most effective in providing electrons for this cytochrome c reduction, but other nutrients could serve in this capacity, provided the cells were preconditioned by growth on these nutrients. Paraquat reduction depended upon a NADPH:paraquat diaphorase, present in the cytosol. Reduced paraquat could diffuse across the cell envelope and react with dioxygen, in the suspending medium, thus generating O2- in that compartment. Most of the paraquat reduced in the cell, under the conditions used, reoxidized in situ and most of the O2- production was thus intracellular. The partitioning of reduced paraquat between intracellular and extracellular compartments, prior to reaction with dioxygen, depended upon intracellular pO2 and any strategy which raised intracellular pO2 decreased the efflux of reduced paraquat and thus decreased extracellular O2- production. Extracellular O2- and H2O2 did contribute to cell damage in proportion to the amount produced. O2- appeared to be unable to cross the cell envelope in either direction and the only O2- which was effective in raising the rate of biosynthesis of the manganese-superoxide dismutase, was that generated within the cell.     

Superoxide sensitivity of the Escherichia coli 6-phosphogluconate dehydratase.

The activity of 6-phosphogluconate dehydratase was significantly lower in extracts of aerobically grown Escherichia coli deficient in superoxide dismutase (sodAsodB) and in mutants lacking the inducible manganese-containing superoxide dismutase (sodA), exposed to the redox-cycling agent paraquat, than in the parental strain. Growth of these strains on a gluconate minimal medium was also impaired under these conditions. The enzyme was most susceptible to dioxygen in superoxide dismutase (SOD)-free extracts, and exogenous SOD afforded a concentration-dependent protection against inactivation. The amount of SOD necessary for full protection was comparable to the amount normally present in extracts of aerobic E. coli (7-36 units/mg protein), and the rate of reaction of O2- with the dehydratase was estimated to be approximately 2.0 x 10(8) M-1 s-1. The dehydratase was much less sensitive to O2 or H2O2 than to O2-. The virtual substrate, alpha-glycerophosphate, provided partial protection. Iron chelators, thiol-reactive reagents, and oxidants, including nitrite and diamide, inactivated the enzyme. Fluoride ions stabilized the dehydratase and blocked the effect of oxidants. The O2(-)-sensitive target site is proposed to be an iron-sulfur cluster which is readily destroyed by oxidation.     

Inactivation-reactivation of aconitase in Escherichia coli. A sensitive measure of superoxide radical.

The rapid inactivation of aconitase by O2-, previously seen to occur in vitro, was explored in vivo. A fraction of the aconitase in growing, aerobic, Escherichia coli is inactive at any instant but can be activated by imposition of anaerobic conditions. This reactivation occurred in the absence of protein synthesis and was inhibited by the ferrous chelator alpha,alpha'-dipyridyl. This fraction of inactive, but activatable, aconitase was increased by augmenting O2- production with paraquat, decreased by elevation of superoxide dismutase, and increased by inhibiting reactivation with alpha,alpha'-dipyridyl. The balance between inactive and active aconitase thus represented a pseudoequilibrium between inactivation by O2- and reactivation by restoration of Fe(II), and it provided, for the first time, a measure of the steady-state concentration of O2- within E. coli. On this basis, [O2-] was estimated to be approximately 20-40 pM in aerobic log phase E. coli containing wild type levels of superoxide dismutase and approximately 300 pM in a mutant strain lacking superoxide dismutase.     

Participation of beta-carotene in antioxidant defense of bacterial cells

The effect of recombinant beta-carotene on the resistance of E. coli culture to menadione and paraquat was studied. The presence of beta-carotene in the E. coli cell significantly prevented an increase in the activity of superoxide dismutase induced by redox mediators but had no effect on the culture growth. The findings suggested the involvement of beta-carotene in the cell defense against oxidative stress.     

Menadione-catalyzed O2- production by Escherichia coli cells: application of rapid chemiluminescent assay to antimicrobial susceptibility testing

This study proposes a novel chemiluminescent assay of bacterial activity. Luminol chemiluminescence (LC) was amplified on addition of menadione to Escherichia coli suspension, and it was effectively inhibited by addition of superoxide dismutase rather than catalase. This fact suggests that H2O2 produced from O2 by superoxide dismutase is decomposed by catalase of E. coli. NAD(P)H:menadione reductase activities in periplasm and cytosol corresponded to the amplification of menadione-catalyzed LC, and outer and cytoplasmic membranes were only slightly involved in the LC. The total activity and Vmax of NAD(P)H:menadione reductase in the cytoplasm were greater than those in the periplasm. A transient increase in menadione-catalyzed LC was observed in the exponential phase and the LC decreased in the stationary phase during growth of E. coli. Menadione-catalyzed LC was sensitive to antibiotic action. A decrease in menadione-catalyzed LC by the impairment of membrane functions and by the inhibition of protein synthesis was observed at 5 min and 3 hr, respectively. These findings suggest the possibility that menadione-catalyzed luminol chemiluminescent assay is applicable to rapid antimicrobial assay because LC is sensitive to the change in growth and cytotoxic events caused by antimicrobial agents.     

Quinolinate synthetase: the oxygen-sensitive site of de novo NAD(P)+ biosynthesis.

The ability of niacin to relieve the growth-inhibiting effect of hyperoxia on Escherichia coli can be attributed to the dioxygen sensitivity of quinolinate synthetase. The activity of this enzyme within E. coli was diminished by exposure of the cells to 4.2 atm O2, while the activity in extracts was rapidly decreased by 0.2 atm O2. Neither catalase nor superoxide dismutase afforded detectable protection against the inactivating effect of O2, indicating that H2O2 and O2- were not significant intermediates in this process. Nevertheless, H2O2 at 1.0 mM did inactivate quinolinate synthetase, even under anaerobic conditions and in the absence of catalatic activity which might have generated O2. Addition of paraquat to aerobic cultures of E. coli caused an inactivation of quinolinate synthetase, which may be explained in terms of an increase in the production of H2O2. The O2-dependent inactivation of quinolinate synthetase in extracts was gradually reversed during anaerobic incubation and this reactivation was blocked by alpha, alpha'-dipyridyl or by 1,10-phenanthroline. The sequence of the quinolinate synthetase "A" protein contains a--cys-w-x-cys-y-z-cys--sequence, which is characteristic of (Fe-S)4-containing proteins. This sequence, together with the effect of the Fe(II)-chelating agents, suggests that the O2-sensitive site of quinolinate synthetase is an iron-sulfur cluster which is essential for the dehydration reaction catalyzed by the A protein.     

Overproduction of superoxide dismutase does not protect Escherichia coli from stringency-induced growth inhibition by 1mM paraquat

Two plasmid-containing Escherichia coli strains which overproduce manganese superoxide dismutase by 4- to 5-fold and iron superoxide dismutase by about 7-fold were not more resistant than parent strains to 1 mM paraquat (a known generator of superoxide) as measured by effects on growth, survival and induction of stringency. These results indicate that overproduction of superoxide dismutase does not mitigate the growth-inhibitory effects of 1 mM paraquat, including those which are expressed through induction of the stringency mechanism.     

Oxygen Toxicity and the Superoxide Dismutase

Oxygen caused an increase in the amount of superoxide dismutase in Escherichia coli B but not in Bacillus subtilis. E. coli B cells, induced by growth under 100% O(2), were much more resistant to the lethal effects of 20 atm of O(2) than were cells which contained the low uninduced level of this enzyme. In contrast, B. subtilis, which could not respond to O(2) by increasing its content of superoxide dismutase, remained equally sensitive to hyperbaric O(2) whether grown under 100% O(2) or areobically. The catalase in these organisms exhibited a reciprocal response to oxygen. Thus, the catalase of E. coli B was not induced by O(2), whereas that of B. subtilis was so induced. These results are consistent with the view that superoxide dismutase is an important component of the defenses of these organisms against the toxicity of oxygen, whereas their catalases are of secondary importance in this respect. The ability of streptonigrin to generate O(2) (-), by a cycle of reduction followed by spontaneous reoxidation, has been verified in vitro. It is further observed that E. coli B which contain the high induced level of superoxide dismutase were more resistant to the lethality of this antibiotic, in the presence of oxygen, than were E. coli B which contained the low uninduced level of this enzyme. This difference between induced and uninduced cells was eliminated by the removal of O(2). These results are consistent with the proposal that the enhanced lethality of streptonigrin under aerobic conditions may relate to its in vivo generation of O(2) (-) by a cycle of reduction and spontaneous reoxidation. In toto, these observations lend support to the hypothesis that O(2) (-) is an important agent of oxygen toxicity and that superoxide dismutase functions to blunt the threat posed by this reactive radical.     

alpha, beta-Dihydroxyisovalerate dehydratase. A superoxide-sensitive enzyme

Increasing the intracellular flux of O-2 by incubating aerobic Escherichia coli with paraquat or plumbagin markedly lowered the alpha, beta-dihydroxyisovalerate dehydratase activity detectable in extracts from these cells. This effect was not seen in the absence of dioxygen and was exacerbated by inhibiting protein biosynthesis with chloramphenicol. These effects of paraquat and of plumbagin were both time- and concentration-dependent. Transfer of E. coli from aerobic to anaerobic conditions caused a rebound of the dehydratase activity, in the continued presence of paraquat and of chloramphenicol, indicating the presence of a mechanism for reactivating this enzyme. The instability of the dehydratase activity in cell extracts was exacerbated by selective removal of superoxide dismutase, but not of catalase, by immunoprecipitation. Addition of exogenous superoxide dismutase reversed the effect of immunoprecipitation; whereas catalase or inactive superoxide dismutase were ineffective. We conclude that the dehydratase is inactivated by O-2. This could account for the bacteriostatic effects of dioxygen and of paraquat.     

Superoxide Dismutases of Escherichia coli: Intracellular Localization and Functions

Escherichia coli B contains two superoxide dismutases which differ with respect to their localization within the cell, the nature of their prosthetic metals, their responses to changes in (p)O(2), and their functions. One of these enzymes, which was liberated from the cells by osmotic shock and which was therefore presumed to be localized in the periplasmic space, is an iron-containing superoxide dismutase. The amount of this iron enzyme did not vary in response to changes in (p)O(2) during growth. In contrast, the other superoxide dismutase was not solubilized by osmotic shock, was a mangano-protein, and was found in greater amounts in cells which had been grown at high (p)O(2). E. coli, which had low levels of the iron-enzyme and high levels of the mangano-enzyme, as a consequence of growth in iron-deficient aerated medium, was killed by exposure to an exogenous flux of O(2) (-) which was generated either photochemically or enzymatically. The addition of bovine superoxide dismutase to the suspending medium protected these cells against this stress. On the other hand, E. coli, which had high levels of the iron-enzyme and low levels of the mangano-enzyme, as a consequence of growth in iron-rich anaerobic medium, was resistant to exogeneous O(2) (-). On the basis of these and of previously reported results (4a, Yost, F. J. and I. Fridovich, J. Biol. Chem., 1973, in press), it appears that the iron superoxide dismutase, of the periplasmic space, serves as a defense against exogenous O(2) (-), whereas the mangano-superoxide dismutase, in the matrix of these cells, serves to counter the toxicity of endogenous O(2) (-).     

Drosophila Cu,Zn superoxide dismutase gene confers resistance to paraquat in Escherichia coli

Superoxide dismutase (SOD) is known to protect organisms from reactive oxygen metabolites. We tested the hypothesis that the Drosophila Cu,Zn SOD is capable of protecting Escherichia coli from oxidative damage caused by the herbicide paraquat. The Cu,Zn Sod gene of Drosophila sechellia was subcloned into pET-20b(+) expression vector. Transformation of E. coli with the constructed vector resulted in an overexpression of this eukaryotic superoxide dismutase, as evidenced by dramatically increased levels of the Cu,Zn SOD polypeptide in bacterial cytosolic extracts. As well, the E. coli transformants showed resistance to paraquat-mediated inhibition of growth and survival. Paraquat is known to promote formation of the superoxide radical anion inside cells and thus the data have been interpreted as indicating that the cloned superoxide dismutase provides protection in E. coli against damage attributable to free radicals.     

蜡样芽胞杆菌M22铜锌超氧化物歧化酶基因的克隆及表达

采用PCR技术 ,从蜡样芽胞杆菌M2 2基因组DNA扩增到长 132 0bp的基因片段 .该片段含编码 179个氨基酸残基的开放阅读框 ,推定蛋白序列与报道的BacillusanthracisCu ,Zn SOD序列有96 %同源性 ,其中N端 16个氨基酸残基推定为信号肽序列 .将Cu ,Zn SOD编码区插入载体pET 2 2b(+ )构建表达载体pET 2 2b sodC ,转化E .coliBL2 1(DE3) ,IPTG诱导融合蛋白表达 .SDS PAGE显示 ,融合蛋白表观分子质量约 2 4kD ,占菌体裂解液中总蛋白的 2 1 3% .将此表达载体转入SOD缺陷型大肠杆菌PN132 ,赋予了该菌株对paraquat的抗性 .NBT光抑制法测定SOD活性显示 ,与PN132无SOD活性相比 ,重组子在IPTG诱导前SOD活性极低 (1 79U/mg) ,诱导后活性高达 6 9 76U/mg .非变性电泳结果显示 ,该酶活性不同程度地受到 5mmol LH2 O2 和 5mmol LKCN的抑制