Involvement Of Superoxide Radicals Research Articles

The involvement of superoxide radicals in medium pressure UV derived inactivation

Today, two types of lamp systems dominate the UV disinfection industry: low-pressure (LP) UV lamps and medium-pressure (MP) polychromatic lamps. Both lamp types have their advantages and disadvantages in microorganism inactivation, with LP lamps being cheaper, having longer life, and working at lower temperature, hence reducing fouling, and MP lamps showing better inactivation per germicidal dose for certain microorganisms. Bacterium-based biosensors were used to compare LP and MP irradiation. These biosensors were Escherichia coli bacteria carrying the lux operon genes under the control of different stress-responding promoters, where activation of the specific promoter is manifested as bioluminescence. MP irradiation, considerably more than LP irradiation, resulted in activation of the superoxide dismutase expression, indicating the formation of superoxide radicals inside the cells. Accordingly, pre-exposure (immunization) of the bacteria to an activator that produces superoxide radicals resulted in lower inactivation and increased resistance to MP irradiation, but not to LP irradiation. This study shows that the difference in germicidal efficiency may result from the production of intracellular superoxide radicals by MP irradiation, at wavelengths other than 254 nm, as emitted by LP lamps.

Roles of reactive oxygen species in UVA-induced oxidation of 5,6-dihydroxyindole-2-carboxylic acid-melanin as studied by differential spectrophotometric method.

Eumelanin photoprotects pigmented tissues from ultraviolet (UV) damage. However, UVA-induced tanning seems to result from the photooxidation of preexisting melanin and does not contribute to photoprotection. We investigated the mechanism of UVA-induced degradation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-melanin taking advantage of its solubility in a neutral buffer and using a differential spectrophotometric method to detect subtle changes in its structure. Our methodology is suitable for examining the effects of various agents that interact with reactive oxygen species (ROS) to determine how ROS is involved in the UVA-induced oxidative modifications. The results show that UVA radiation induces the oxidation of DHICA to indole-5,6-quinone-2-carboxylic acid in eumelanin, which is then cleaved to form a photodegraded, pyrrolic moiety and finally to form free pyrrole-2,3,5-tricarboxylic acid. The possible involvement of superoxide radical and singlet oxygen in the oxidation was suggested. The generation and quenching of singlet oxygen by DHICA-melanin was confirmed by direct measurements of singlet oxygen phosphorescence.

Trans-sulfuration Pathway Seleno-amino Acids Are Mediators of Selenomethionine Toxicity in Saccharomyces cerevisiae*

Toxicity of selenomethionine, an organic derivative of selenium widely used as supplement in human diets, was studied in the model organism Saccharomyces cerevisiae. Several DNA repair-deficient strains hypersensitive to selenide displayed wild-type growth rate properties in the presence of selenomethionine indicating that selenide and selenomethionine exert their toxicity via distinct mechanisms. Cytotoxicity of selenomethionine decreased when the extracellular concentration of methionine or S-adenosylmethionine was increased. This protection resulted from competition between the S- and Se-compounds along the downstream metabolic pathways inside the cell. By comparing the sensitivity to selenomethionine of mutants impaired in the sulfur amino acid pathway, we excluded a toxic effect of Se-adenosylmethionine, Se-adenosylhomocysteine, or of any compound in the methionine salvage pathway. Instead, we found that selenomethionine toxicity is mediated by the trans-sulfuration pathway amino acids selenohomocysteine and/or selenocysteine. Involvement of superoxide radicals in selenomethionine toxicity in vivo is suggested by the hypersensitivity of a Δsod1 mutant strain, increased resistance afforded by the superoxide scavenger manganese, and inactivation of aconitase. In parallel, we showed that, in vitro, the complete oxidation of the selenol function of selenocysteine or selenohomocysteine by dioxygen is achieved within a few minutes at neutral pH and produces superoxide radicals. These results establish a link between superoxide production and trans-sulfuration pathway seleno-amino acids and emphasize the importance of the selenol function in the mechanism of organic selenium toxicity.

Superoxide radical level and superoxide dismutase activity changes in maturing mammalian spermatozoa.

Mammalian spermatozoa, which like other living cells are susceptible to oxygen toxicity, generate increasing quantities of superoxide radical as epididymal maturation proceeds. This is associated with a concomitant reduction in the activity of superoxide dismutase. A possible involvement of superoxide radicals in bringing about some of the reported oxidative changes occurring during epididymal maturation of mammalian spermatozoa is discussed. It is suggested that superoxide radical may be involved in regulating membrane changes accompanying sperm maturation.

Oxygen Uptake and Involvement of Superoxide Radicals upon Photolysis of Ketones in Air‐saturated Aqueous Alcohol, Formate, Amine or Ascorbic Acid Solutions

The photolysis of acetophenone, benzophenone, 4-carboxybenzophenone and benzil was studied in air-saturated aqueous solution in the presence of alcohols. The overall reaction is an oxidation of 2-propanol to acetone. The quantum yield of oxygen uptake (Phi(-O(2))) increases with increasing 2-propanol concentration up to 0.9. The photoreaction can also be initiated by quenching of the ketone triplet state by ascorbic acid, formate or an amine e.g. triethylamine. Subsequent reactions of the involved radicals with oxygen yield the superoxide radical and eventually hydrogen peroxide. For the ketones in the presence of 3-30 mM ascorbic acid or triethylamine Phi(-O(2)) = 0.3-0.9. The specific properties of ketones, including 4-methoxyacetophenone and 2-acetonaphthone, the radicals involved and the pH and concentration dependences of Phi(-O(2)) are discussed.

RE: REACTIVE OXYGEN SPECIES PRODUCTION BY THE SPERMATOZOA OF PATIENTS WITH IDIOPATHIC INFERTILITY

No AccessJournal of UrologyClinical Urology: Letters to the Editor1 May 1999RE: REACTIVE OXYGEN SPECIES PRODUCTION BY THE SPERMATOZOA OF PATIENTS WITH IDIOPATHIC INFERTILITYRELATIONSHIP TO SEMINAL PLASMA ANTIOXIDANTS I. Alkan, F. Simsek, G. Haklar, E. Kervancioglu, H. Ozveri, S. Yalcin, and A. Akdas I. AlkanI. Alkan More articles by this author , F. SimsekF. Simsek More articles by this author , G. HaklarG. Haklar More articles by this author , E. KervanciogluE. Kervancioglu More articles by this author , H. OzveriH. Ozveri More articles by this author , S. YalcinS. Yalcin More articles by this author , and A. AkdasA. Akdas More articles by this author View All Author Informationhttps://doi.org/10.1016/S0022-5347(05)68980-XAboutFull TextPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookLinked InTwitterEmail "RE: REACTIVE OXYGEN SPECIES PRODUCTION BY THE SPERMATOZOA OF PATIENTS WITH IDIOPATHIC INFERTILITY." The Journal of Urology, 161(5), p. 1584 References 1 : Differential contribution of leucocytes and spermatozoa to the generation of reactive oxygen species in the ejaculates of oligozoospermic patients and fertile donors. J. Reprod. Fertil.1992; 94: 451. Google Scholar 2 : Origin of reactive oxygen in human semen: spermatozoa or leucocytes?. J. Reprod. Fertil.1992; 94: 463. Google Scholar 3 : Analysis of relationship between reactive oxygen species production and leucocyte infiltration in fractions of human semen separated on Percoll gradients. Int. J. Androl.1990; 13: 433. Google Scholar 4 : The beneficial effect of EDTA on development of mouse one-cell embryos in chemically defined medium. Dev. Biol.1977; 61: 378. Google Scholar 5 : The effect of iron and iron chelators on the in-vitro block to development of the mouse preimplantation embryo: BAT6 a new medium for improved culture of mouse embryos in vitro. Hum. Reprod.1990; 5: 997. Google Scholar 6 : Involvement of superoxide radicals in the mouse two-cell block. Mol. Reprod. Dev.1991; 28: 356. Google Scholar 7 : Oxidative stress on mouse embryo development in vitro. Free Radic. Biol. Med.1992; 13: 47. Google Scholar 8 : How does transferrin overcome the in vitro block to development of the mouse pre-implantation embryo?. J. Reprod. Fertil.1992; 96: 41. Google Scholar © 1999 by American Urological Association, Inc.FiguresReferencesRelatedDetails Volume 161Issue 5May 1999Page: 1584 Advertisement Copyright & Permissions© 1999 by American Urological Association, Inc.MetricsAuthor Information I. Alkan More articles by this author F. Simsek More articles by this author G. Haklar More articles by this author E. Kervancioglu More articles by this author H. Ozveri More articles by this author S. Yalcin More articles by this author A. Akdas More articles by this author Expand All Advertisement PDF DownloadLoading ...

Involvement of superoxide radical in the impaired endothelium-dependent relaxation of cavernous smooth muscle in hypercholesterolemic rabbits.

Involvement of the superoxide radical in impaired relaxation of penile cavernous smooth muscle in hypercholesterolemia was investigated. New Zealand White rabbits (n = 40) were randomly divided into control and treatment groups. The control group (n = 20) received a regular diet while the treatment group (n = 20) was fed a diet of 2% cholesterol for 8 weeks. Blood level of cholesterol in the cholesterol-fed group was significantly higher than that of the control group. The contraction responses of cavernous tissues to norepinephrine were not significantly different in the two groups. The relaxation responses to endothelium-dependent agents (acetylcholine, bradykinin) were significantly reduced in the hypercholesterolemic group compared with the control group. However, the relaxation responses to endothelium-independent agents (papaverine, verapamil) were not significantly different in the two groups. The production of superoxide radicals was significantly higher in the hypercholesterolemic group than in the control group (P < 0.01). The activity of superoxide dismutase (total SOD, Mn-SOD, Cu,Zn-SOD) increased significantly in the hypercholesterolemic group compared with the control group (P < 0.05). The activities of catalase and glutathione peroxidase also increased in the hypercholesterolemic group, but were not significantly higher than those of the control group. Therefore, production of the superoxide radicals in rabbit cavernous tissues increases in the state of hypercholesterolemia, which may lead to functional impairment of cavernous smooth muscle relaxation in response to endothelium-mediated stimuli.

Role of tissue antioxidant defence in thyroid cancers.

Reactive oxygen species (ROS), consisting mainly of superoxide, hydrogen peroxide and hydroxyl radical, have been implicated in many diseases including cancer. ROS have been known to play an important role in the initiation and promotion of multistage carcinogenesis. The cellular antioxidant defence plays a crucial role in neoplastic disease. However, very little is known about the tissue antioxidant defence in thyroid cancers. We therefore undertook a study to assess the role of ROS in the pathogenesis of thyroid cancers. Our samples consisted of post-operated thyroid tissues (normal, goiters, follicular adenomas, follicular carcinomas and papillary carcinomas). The parameters studied were lipid peroxidation (LP), antioxidant enzymes--superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx)--and non-protein thiols (GSH). Compared to normal thyroid no changes were seen in goiters. LP was significantly higher in adenomas (16%) and carcinomas (60-69%). SOD was decreased by 15% in adenomas while in carcinomas it increased by 9-12%. GPx was raised in carcinomas by 10-21%. Follicular carcinomas showed a 4% increase in CAT activity while GSH was raised in adenomas and papillary carcinomas by 17%. Thus, in adenomas (initial stage) involvement of superoxide radicals and in carcinomas (later stage) hydrogen peroxide and, possibly, hydroxyl radical involvement cannot be ruled out. These ROS may be responsible for elevated LP observed in adenomas and carcinomas.

Immunohistochemical localization of manganese superoxide dismutase in the rat cochlea.

There has been recent increasing interest in the involvement of superoxide radicals (O2-) and their scavenging enzymes, the superoxide dismutases, in the patho-physiology of certain diseases. Since mitochondria are significant intracellular sources of O2- and important targets of oxidant injury, determining the intracochlear localization of mitochondrial O2- scavenging enzyme may provide important insight into the pathogenesis of injury due to cochlear oxidants. In order to locate the mitochondrial O2- scavenging enzyme, manganese superoxide dismutase (MnSOD), the authors used a modified immunoglobulin peroxidase bridge sequence method to detect MnSOD in paraffin-embedded, formalin-fixed rat cochleas. Site-specific immunolocalization of MnSOD could be demonstrated in the cochlear labyrinth, suggesting that the generation of intracochlear O2- was possibly implicated in the metabolically active sites and sites rich in vascularity. This study also provided a useful probe for detecting MnSOD immunohistochemically from ethylenediamine tetra-acetic acid-treated materials without requiring an antigen retrieval procedure.

Homolytic carbon to phosphorus bond scission of some phosphonates catalyzed by bacterial carbon-phosphorus lyase

The production of volatile degradation products of phosphonates was monitored to investigate the mechanism involved in the biodegradation of propylphosphonic acid and phenylphosphonic acid byRhizobium sp MMM101a. The biodegradation of propylphosphonic acid gave rise to the production, in decreasing order, of propane, methane, ethane, 1-butene, propene, isobutene, butane and ethene. The formation of these degradation products was strongly reduced by adding catalase to the growing cultures indicating the involvement of peroxides in the biodegradation mechanism. OH0 radical scavengers did not reduce the rate of biodegradation, and therefore these radicals appear not to be involved. Addition of ascorbate, a known hydroxylating agent in biological systems, increased the amount of biodegradation products. The involvement of iron in the degradation was indicated and was optimal at a concentration of 950 µM. This suggests the involvement of a metalloenzyme involving iron and peroxide. The decomposition of phenylphosphonic acid yielded benzene and biphenyl. No phenol could be detected, again suggesting that OH0 radicals were not involved in the biodegradation. The presence of deuterated benzene did not result in the occurrence of biphenyl consisting of one nondeuterated and a deuterated ring, which is chemically more likely. It therefore appears that the degradation of the phosphonates occurs on a multicentered enzyme. The diversity of the products generated by this bacterium from phosphonates, many of them due to rearrangement of the carbon moiety of the substrate molecule, suggests an overall involvement of superoxide radicals in the homolytic carbon to phosphorus bond scission.

Involvement of Superoxide Radical in Signal Transduction Regulating Stomatal Movements

Stomatal movements are regulated by alternate phase transition in the lipophilic domains of membrane initiated by blue light through activation of plasmalemma redox system. ABA blocks the blue light activated redox reactions leading to the accumulation of superoxide (O·−2) radical inducing melting of the membrane and increased fluidity. Phenolic compounds which reverse the ABA effects reduce O·−2 radical and thereby reverse phase transition in lipophilic domains of membrane. Stomatal movements thus appear to be regulated by blue light activated redox reactions involving formation of O·−2 radical and its reduction by the reducing equivalents available in the GCPs.

Albumin-binding superoxide dismutase with a prolonged half-life reduces reperfusion brain injury

To test the possible involvement of superoxide radicals in the pathogenesis of reperfusion injury, we synthesized a superoxide dismutase (SOD) derivative [poly(styrene-co-maleic acid) butyl ester (SM) covalently linked to SOD] that circulates bound to albumin, has a prolonged in vivo half-life, and accumulates in pH-decreased tissues. The protective effects of SM-SOD on regional cerebral blood flow, intracranial pressure, cardiac index, vascular permeability, and neurological outcome were investigated using a model of global brain ischemia in dogs. Intra-arterial injection of SM-SOD (10 mg/kg) just before reperfusion increased reactive hyperemia (SM-SOD, 160 +/- 36 ml.100 g-1.min-1, means +/- SD, n = 6; control, 100 +/- 34 ml.100 g-1.min-1, n = 6, P = 0.015), ameliorated delayed hypoperfusion (7 h after ischemia: SM-SOD, 40 +/- 14 ml.100 g-1.min-1; control 17 +/- 6 ml.100 g-1.min-1, P = 0.003), vascular permeability, and neurological outcome without affecting the cardiac index. These results indicate that superoxide radicals and/or their metabolite(s) might play a critical role in the pathogenesis of reperfusion injury in the brain.

Pathogenesis of cytomegalovirus-associated pneumonitis in ICR mice: possible involvement of superoxide radicals.

SummaryWe have studied the pathogenesis of murine cytomegalovirus (MCMV) pneumonitis in immunocompetent ICR mice and in mice treated with cyclophosphamide (CP). Intranasal infection of immunocompetent mice with MCMV resulted in transient and self-limited pulmonary lesions. When mice were given 200 mg/kg of CP one day before virus infection, transient splenic atrophy and subsequent splenic hypertrophy were induced, and the lesions in the lung were markedly augmented in their number and size although there was no significant enhancement of the virus growth. The augmentation coincided with the period of splenic hypertrophy. A marked increase in the number of pulmonary lesions was also induced in mice given 100 mg/kg of CP every 4 days following the initial dose of 200 mg/kg. In these mice, however, continuous splenic atrophy and augmented replication of MCMV in the lung were observed. When the activity of xanthine oxidase (XO) in lung tissue homogenates was measured, the activity was found to significantly increase after intranasal infection with MCMV irrespective of CP administration and there was a good correlation between the elevation of XO activity and the degree of pathological changes in the lung. In addition, we found that the administration of allopurinol, a specific inhibitor of XO and superoxide dismutase, a superoxide radical scavenger, reduced the number of the pulmonary lesions. These results suggest that superoxide radicals are involved in the pathogenesis of MCMV-associated pneumonitis in ICR mice.

Involvement of superoxide radicals in the mouse two-cell block.

The effect of oxygen toxicity on the development of mammalian embryos was assessed by the use of superoxide dismutase (SOD), a potent scavenger of superoxide radicals. Mouse pronuclear embryos recovered 17 h after human chorionic gonadotropin (hCG) were cultured in medium BWW at 37 degrees C under an atmosphere of 5% CO2 in air. Culture of mouse pronuclear embryos in the presence of Cu.Zn-SOD (500 micrograms/ml) significantly increased the blastulation rate (44.6%) when compared with the control culture system (4.2%). Essentially the same effects were observed in SOD containing either Mn or Fe in the catalytic center. Heat treatment of the SOD preparation, and the addition of anti-SOD antibodies to the culture medium, significantly reduced the attenuation of the two-cell block by SOD, indicating that this effect is SOD dependent. SOD activity was detected in rabbit oviduct fluid (3.675 +/- 3.084 mIU/mg protein) by electron spin resonance. These results suggest that active oxygen is involved in the two-cell block phenomenon in mouse embryos exposed to air and that SOD in the oviduct may play an important role in the protection of embryos from superoxide radicals.

Inhibition of carrageenin-induced paw edema by a superoxide dismutase derivative that circulates bound to albumin

Although the possible involvement of superoxide radical and its metabolite(s) in the pathogenesis of various types of edema have been suggested, direct evidence supporting this concept is lacking. Since intravenously administered Cu 2+Zn 2+-type superoxide dismutase (SOD) rapidly disappeared from the circulation with a half-life of 4 min, the enzyme could not be used to test whether superoxide radicals played a critical role in the modulation of vascular permeability. We previously synthesized a SOD derivative (SM-SOD) by linking poly(styrene co-maleic acid butyl ester) (SM) to the enzyme (Ogino, T., Inoue, M., Ando, Y., Awai, M., Maeda, H. and Morino Y. (1988) Int. J. Pept. Protein Res. 32, 1583–1588); SM-SOD circulates bound to albumin with a half-life of 6 h. To test whether superoxide radicals play an important role in the regulation of vascular permeability, the effect of SM-SOD on experimental paw edema was studied in the rat. Subcutaneous injections of carrageenin to the paw rapidly induced local edema by increasing vascular permeability. Intravenous administration of SM-SOD markedly inhibited the carrageenin-induced increase in vascular permeability and suppressed the development of paw edema. In contrast, the same dose of SOD showed no such inhibitory effect. These results suggested that superoxide radical and/or its metabolite(s) might play a critical role in the pathogenesis of carrageenin-induced vasogenic edema.

The effects of free radical scavengers on arachidonic acid metabolism by ovine placental microsomes

1. The effect of free radical (superoxide and hydroxyl) and hydrogen peroxide scavengers on the formation of prostaglandin E 2 by ovine placental microsomes was investigated. 2. Of the free radical scavengers tested only Mn Desferal, a low molecular weight (712 Da) superoxide scavenger, at concentrations of 0.5-5 mmol/1 significantly ( P < 0.001) stimulated PGE 2 formation by ovine placental microsomes. 3. This observation is consistent with the involvement of superoxide radicals in the inactivation of prostaglandin G/H synthase (PGHS). 4. The results obtained further suggest that if superoxide radicals are important regulators of PGHS activity in the ovine placenta, they are not accessible to large molecular weight free radical scavengers.

The role of myeloperoxidase in the oxidation of biologically active polyhydroxyphenols (substituted catechols)

The reaction of myeloperoxidase with biologically active polyhydroxyphenols (substituted catechols): catecholamine, norepinephrine and 2,4,5-trihydroxyphenylalanine [Phe(OH)3] were investigated by using the ESR spin-stabilization technique and rapid-scan spectrophometry in the millisecond time scale. The results presented here indicate that dihydroxyphenols and trihydroxyphenols are substrates in the myeloperoxidase reaction. The data of ESR and rapid-scan optical investigation of the myleoperoxidase reaction with the dihydroxyphenols catecholamine and norepinephrine clearly indicate a normal peroxidase-type pathway of catecholamine degradation. The first evidence of o-semiquinone radical formation as a product of the enzymatic oxidation of catecholamine by myeloperoxidase is reported. The results obtained by rapid-scan spectrophotometric investigation of enzyme intermediate formation and decay showed qualitative agreement with the spin-stabilization studies. The first results on the reaction of myeloperoxidase with the trihydroxyphenol Phe(OH)3 presented here, indicate that it plays a role as an electron donor for myeloperoxidase I, but we were unable to obtain evidence that a normal peroxidase cycle is occurring. The inhibitory effect of superoxide dismutase on product formation was evident and indicate the involvement of superoxide radicals in the process. Attention is drawn to the biochemical and toxicological implications of these and other related studies of substituted catechol peroxidation by mammalian peroxidases.

2,3,7,8 Tetrachlorodibenzo- P-dioxin induces oxygen activation associated with cell respiration

We have investigated the influence of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) on bioenergetic functions of isolated heart-mitochondria. Electron transfer and energy conservation activities were found to be decreased in the presence of very amounts of the polychlorinated biphenyl compound (1.5 nmol/mg mitochondrial protein). The effect was greatest when substrates for complex I were used. In this case coupling of oxidative phosphorylation to respiration was drastically diminished, essentially at the expense of state 3 respiration, and P/O values were found around 2 instead of 3. Succinate-related energy conservation remained practically unaffected in the presence of TCDD, suggesting an interference of the toxic compound at coupling site I. SOD plus catalase found to protect energy-linked respiration from the effect of dioxin indicating the involvement of superoxide radicals and H 2O 2 in the development of the observed phenomena. The present contribution provides experimental evidence on the formation of these oxygen species in the presence of TCDD. Furthermore, the site of action of TCDD is demonstrated and discussedd in relation to the oxygen radical formation observed.

Role of oxyradicals in the inactivation of catalase by ozone

The antioxidant enzymes, catalase and superoxide dismutase, are inactivated upon exposure to ozone. In this study, the mechanism of this inactivation was examined using catalase as a model system. The data show that the inactivation of catalase is dependent on ozone concentration, time of exposure, and pH. Loss of catalase activity is accompanied with loss of the heme spectra. Tiron, desferal-Mn, trolox-c, and pyruvate protect the enzyme against ozone inactivation. SOD is less effective due to its inactivation by ozone. On the other hand, alcohols do not provide significant protection. The data suggest the possible involvement of superoxide radicals in the inactivation of catalase by ozone.

Involvement of Superoxide Radical in Extracellular Ferric Reduction by Iron-Deficient Bean Roots

The recent proposal of Tipton and Thowsen (Plant Physiol 79: 432-435) that iron-deficient plants reduce ferric chelates in cell walls by a system dependent on the leakage of malate from root cells was tested. Results are presented showing that this mechanism could not be responsible for the high rates of ferric reduction shown by roots of iron-deficient bean (Phaseolus vulgaris L. var Prélude) plants. The role of O(2) in the reduction of ferric chelates by roots of iron-deficient bean plants was also tested. The rate of Fe(III) reduction was the same in the presence and in the absence of O(2). However, in the presence of O(2) the reaction was partially inhibited by superoxide dismutase (SOD), which indicates a role for the superoxide radical, O(2) ([unk]), as a facultative intermediate electron carrier. The inhibition by SOD increased with substrate pH and with decrease in concentration of the ferrous scavenger bathophenanthroline-disulfonate. The results are consistent with a mechanism for transmembrane electron transport in which a flavin or quinone is the final electron carrier in the plasma membrane. The results are discussed in relation to the ecological importance that O(2) ([unk]) may have in the acquisition of ferric iron by dicotyledonous plants.