Oxidation of spermine by an amine oxidase from lentil seedlings

Spermine is a substrate of lentil (Lens culinaris) seedling amine oxidase and the oxidation products are reversible inactivators of the enzyme. The spermine is oxidized at the terminal amino groups to a dialdehyde: 2 moles of hydrogen peroxide and 2 moles of ammonia per mole of spermine are formed. The pH optimum of the enzyme with spermine is 7.9 in TI-HCI buffer; the K_m value is 4.4·10⁻⁴ molar, similar to that found with other substrates (putrescine and spermidine).     

An essential role of active site arginine residue in iodide binding and histidine residue in electron transfer for iodide oxidation by horseradish peroxidase

The objective of the present study is to delineate the role of active site arginine and histidine residues of horseradish peroxidase (HRP) in controlling iodide oxidation using chemical modification technique. The arginine specific reagent, phenylglyoxal (PGO) irreversibly blocks iodide oxidation following pseudofirst order kinetics with second order rate constant of 25.12 min^-1 M^-1. Radiolabelled PGO incorporation studies indicate an essential role of a single arginine residue in enzyme inactivation. The enzyme can be protected both by iodide and an aromatic donor such as guaiacol. Moreover, guaiacol-protected enzyme can oxidise iodide and iodide-protected enzyme can oxidise guaiacol suggesting the regulatory role of the same active site arginine residue in both iodide and guaiacol binding. The protection constant (K_p) for iodide and guaiacol are 500 and 10 M respectively indicating higher affinity of guaiacol than iodide at this site. Donor binding studies indicate that guaiacol competitively inhibits iodide binding suggesting their interaction at the same binding site. Arginine-modified enzyme shows significant loss of iodide binding as shown by increased K_d value to 571 mM from the native enzyme (K_d = 150 mM). Although arginine-modified enzyme reacts with H₂O₂ to form compound II presumably at a slow rate, the latter is not reduced by iodide presumably due to low affinity binding.The role of the active site histidine residue in iodide oxidation was also studied after disubstitution reaction of the histidine imidazole nitrogens with diethylpyrocarbonate (DEPC), a histidine specific reagent. DEPC blocks iodide oxidation following pseudofirst order kinetics with second order rate constant of 0.66 min^-1 M^-1. Both the nitrogens (, ) of histidine imidazole were modified as evidenced by the characteristic peak at 222 nm. The enzyme is not protected by iodide suggesting that imidazolium ion is not involved in iodide binding. Moreover, DEPC-modified enzyme binds iodide similar to the native enzyme. However, the modified enzyme does not form compound II but forms compound I only with higher concentration of H₂O₂ suggesting the catalytic role of this histidine in the formation and autoreduction of compound I. Interestingly, compound I thus formed is not reduced by iodide indicating block of electron transport from the donor to the compound I. We suggest that an active site arginine residue regulates iodide binding while the histidine residue controls the electron transfer to the heme ferryl group during oxidation.     

Sarcoplasmic reticulum. XVI. The permeability of phosphatidyl choline vesicles for calcium

The Ca permeability of phosphatidyl choline vesicles of diverse fatty acid composition was measured. The rate of ⁴⁵Ca release from liposomes equilibrated with 1 mm⁴⁵CaCl₂ was found to be about 8 × 10⁻¹⁸ moles of Ca/cm²/sec for egg lecithin and about 5.3 × 10⁻¹⁷ moles of Ca/cm²/sec for dioleyllecithin at 30 °. Incorporation of cholesterol into dioleyllecithin micelles reduced the rate of Ca release. The Ca permeability of the phosphatidyl choline micelles was insensitive to changes in the pH, calcium or sodium concentration of the medium but increased with increasing temperature. The effect of temperature was most marked with dioleyl lecithin dispersions, but was clearly apparent with dipalmitoyl, plant, bovine, and egg lecithins as well. The activation energy of Ca release fell in the range of 4.2–9.6 kcal/mole. Macrocyclic antibiotics (valinomycin, tyrocidin, and gramicidin) at relatively high concentration increased the rate of Ca release similarly to their effects on fragmented sarcoplasmic reticulum membranes.     

Electron transport systems of Candida utilis. Purification and properties of the respiratory chain-linked external NADH dehydrogenase+

The respiratory chain-linked external NADH dehydrogenase has been isolated from Candida utilis in highly purified form. The enzyme is soluble and has a molecular weight of approx. 1.5 · 10⁶. The enzyme contains two moles of FMN per mole of enzyme and is composed of two large subunits of mol. wt. 270 000 and eight smaller subunits of mol. wt. 135 000. Iron and copper are present in the preparations, but appear to be contaminants. The enzyme catalyzes the oxidation of NADH and NADPH at nearly equal rates and reacts readily with 2,6-dichlorophenolindophenol, CoQ₆ and CoQ₁ derivatives as acceptors. Rotenone (10^?5 M) and seconal (10^?3 M) do not inhibit enzymatic activity.     

Superoxide Production by a Manganese-Oxidizing Bacterium Facilitates Iodide Oxidation

The release of radioactive iodine (i.e., iodine-129 and iodine-131) from nuclear reprocessing facilities is a potential threat to human health. The fate and transport of iodine are determined primarily by its redox status, but processes that affect iodine oxidation states in the environment are poorly characterized. Given the difficulty in removing electrons from iodide (I⁻), naturally occurring iodide oxidation processes require strong oxidants, such as Mn oxides or microbial enzymes. In this study, we examine iodide oxidation by a marine bacterium, Roseobacter sp. AzwK-3b, which promotes Mn(II) oxidation by catalyzing the production of extracellular superoxide (O₂⁻). In the absence of Mn²⁺, Roseobacter sp. AzwK-3b cultures oxidized ∼90% of the provided iodide (10 μM) within 6 days, whereas in the presence of Mn(II), iodide oxidation occurred only after Mn(IV) formation ceased. Iodide oxidation was not observed during incubations in spent medium or with whole cells under anaerobic conditions or following heat treatment (boiling). Furthermore, iodide oxidation was significantly inhibited in the presence of superoxide dismutase and diphenylene iodonium (a general inhibitor of NADH oxidoreductases). In contrast, the addition of exogenous NADH enhanced iodide oxidation. Taken together, the results indicate that iodide oxidation was mediated primarily by extracellular superoxide generated by Roseobacter sp. AzwK-3b and not by the Mn oxides formed by this organism. Considering that extracellular superoxide formation is a widespread phenomenon among marine and terrestrial bacteria, this could represent an important pathway for iodide oxidation in some environments.     

A model for the effects of primary substrates on the kinetics of reductive dehalogenation

A kinetic model that describes substrate interactions during reductive dehalogenation reactions is developed. This model describes how the concentrations of primary electron-donor and -acceptor substrates affect the rates of reductive dehalogenation reactions. A basic model, which considers only exogenous electron-donor and -acceptor substrates, illustrates the fundamental interactions that affect reductive dehalogenation reaction kinetics. Because this basic model cannot accurately describe important phenomena, such as reductive dehalogenation that occurs in the absence of exogenous electron donors, it is expanded to include an endogenous electron donor and additional electron acceptor reactions. This general model more accurately reflects the behavior that has been observed for reductive dehalogenation reactions. Under most conditions, primary electron-donor substrates stimulate the reductive dehalogenation rate, while primary electron acceptors reduce the reaction rate. The effects of primary substrates are incorporated into the kinetic parameters for a Monod-like rate expression. The apparent maximum rate of reductive dehalogenation (q _{m, ap} ) and the apparent half-saturation concentration (K _ap ) increase as the electron donor concentration increases. The electron-acceptor concentration does not affect q _{m, ap} , but K _ap is directly proportional to its concentration.Definitions for model parameters RX halogenated aliphatic substrate - E-M ⁿ reduced dehalogenase - E-M ⁿ⁺² oxidized dehalogenase - [E-M ⁿ ] steady-state concentration of the reduced dehalogenase (moles of reduced dehalogenase per unit volume) - [E-M ⁿ⁺²] steady-state concentration of the oxidized dehalogenase (moles of reduced dehalogenase per unit volume) - DH₂ primary exogenous electron-donor substrate - A primary exogenous electron-acceptor substrate - A2 second primary exogenous electron-acceptor substrate - X biomass concentration (biomass per unit volume) - f fraction of biomass that is comprised of the dehalogenase (moles of dehalogenase per unit biomass) - stoichiometric coefficient for the reductive dehalogenation reaction (moles of dehalogenase oxidized per mole of halogenated substrate reduced) - stoichiometric coefficient for oxidation of the primary electron donor (moles of dehalogenase reduced per mole of donor oxidized) - stoichiometric coefficient for oxidation of the endogenous electron donor (moles of dehalogenase reduced per unit biomass oxidized) - stoichiometric coefficient for reduction of the primary electron acceptor (moles of dehalogenase oxidized per mole of acceptor reduced) - stoichiometric coefficient for reduction of the second electron acceptor (moles of dehalogenase oxidized per mole of acceptor reduced) - r _RX rate of the reductive dehalogenation reaction (moles of halogenated substrate reduced per unit volume per unit time) - r _d1 rate of oxidation of the primary exogenous electron donor (moles of donor oxidized per unit volume per unit time) - r _d2 rate of oxidation of the endogenous electron donor (biomass oxidized per unit volume per unit time) - r _a1 rate of reduction of the primary exogenous electron acceptor (moles of acceptor reduced per unit volume per unit time) - r _a2 rate of reduction of the second primary electron acceptor (moles of acceptor reduced per unit volume per unit time) - k _RX mixed second-order rate coefficient for the reductive dehalogenation reaction (volume per mole dehalogenase per unit time) - k _d1 mixed-second-order rate coefficient for oxidation of the primary electron donor (volume per mole dehalogenase per unit time) - k _d2 mixed-second-order rate coefficient for oxidation of the endogenous electron donor (volume per mole dehalogenase per unit time) - b first-order biomass decay coefficient (biomass oxidized per unit biomass per unit time) - k _a1 mixed-second-order rate coefficient for reduction of the primary electron acceptor (volume per mole dehalogenase per unit time) - k _a2 mixed-second-order rate coefficient for reduction of the second primary electron acceptor (volume per mole dehalogenase per unit time) - q _m,ap apparent maximum specific rate of reductive dehalogenation (moles of RX per unit biomass per unit time) - K _ap apparent half-saturation concentration for the halogenated aliphatic substrate (moles of RX per unit volume) - k _ap apparent pseudo-first-order rate coefficient for reductive dehalogenation (volume per unit biomass per unit time)     

On the reaction of sulfenyl iodide derivatives with azide

Based on the suggested mechanism of the Raschig catalytic iodine-azide reaction the use of azide for the azotometric estimation of sulfenyl iodide groups is proposed. In the Raschig reaction reduction of iodine to iodide and oxidation of azide to elementary nitrogen is specifically catalyzed by bivalent sulfur compounds; the reaction is usually formulated to proceed via hypothetical sulfenyl iodide derivatives. This has been explored with the use of available, relatively stable sulfenyl iodide derivatives. The -SI group oxidizes azide to nitrogen stoichiometrically: 1 mole of a sulfenyl iodide consumes 2 moles of sodium azide and yields 3 moles of elementary nitrogen. The specificity and limitations of the method are discussed.     

Binding of a Phosphorylated Inhibitor to Ribulose Bisphosphate Carboxylase/Oxygenase during the Night

The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase was measured at various times during the purification of the enzyme from leaves of Nicotiana tabacum which were collected either 1 hour before the start of the photoperiod (predawn) or in the middle of the photoperiod (midday). The activity of the enzyme in extracts of the predawn leaves (0.8 units/mg enzyme) was consistently about 2-fold lower than that measured in extracts of midday leaves (1.7 units/mg enzyme). The activity of the predawn enzyme was increased to that of the midday enzyme following removal of CO₂ and Mg²⁺ (deactivation), (NH₄)₂SO₄ precipitation, or incubation in SO₄²⁻ (18 millimolar required for one-half maximal increase). Following purification to >95% homogeneity, the predawn enzyme was found to have ~0.5 moles of bound organic phosphate per mole of enzyme active sites, while the midday enzyme had only ~0.08 moles of bound organic phosphate per mole of enzyme active sites. Deactivation of the predawn enzyme or treatment with 0.2 molar SO₄²⁻ resulted in the removal of most of the bound organic phosphate. These findings support the hypothesis that following the night period about 50% of the enzyme is catalytically inactive because of the tight-binding of a small molecular weight, phosphorylated inhibitor at the active site.     

32P-labeling of bovine tryptophanyl-tRNA synthetase with [32P]pyrophosphate

The covalent derivative of the tryptophanyl-tRNA synthetase obtained under the action of³²PP_i contains one mole of the covalently bound pyrophosphate (or 2 moles of orthophosphate) per mole of dimeric enzyme. Dephosphorylation with alkaline phosphatase causes practically no changes of enzymatic activity although the enzyme looses its ability to bind PP_i.Enzymes tryptophanyl-tRNA synthetase (EC 6.1.1.2), alkaline phosphatase (EC 3.1.3.1), inorganic pyrophosphatase (EC 3.6.1.1)     

Mechanism of horseradish peroxidase-catalyzed conversion of iodine to iodide in the presence of EDTA and H2O2

EDTA not only blocks the horseradish peroxidase (HRP)-catalyzed iodide oxidation to I-3 but also causes an enzymatic conversion of oxidized iodine species to iodide (Banerjee, R. K., De, S. K., Bose, A. K., and Datta, A. G. (1986) J. Biol. Chem. 261, 10592-10597). The EDTA effect on both of these reactions can be withdrawn with a higher concentration of iodide and not with H2O2. Spectral studies indicate a possible interaction of EDTA with HRP as evidenced by the formation of modified compound 1 with H2O2 at 416 nm instead of 412 nm in the absence of EDTA. EDTA causes a hypochromic effect on HRP at 402 nm which undergoes the bathochromic red shift to 416 nm by H2O2. The addition of iodide to the 416 nm complex causes the reappearance of the Soret band of HRP at 402 nm. Among various EDTA analogues tested, N-N-N'-N'-tetramethylethylenediamine (TEMED) is 80% as effective as EDTA in the conversion of I-3 to iodide and produces a spectral shift of HRP similar to EDTA. Interaction of EDTA with HRP is further indicated by the hyperchromic effect of HRP and H2O2 on the absorption of EDTA at 212 nm. The addition of oxidized iodine species produces a new peak at 230 nm due to formation of iodide. EDTA at a higher concentration can effectively displace radioiodide specifically bound to HRP indicating its interaction at the iodide-binding site. The enzyme, after radioiodide displacement with EDTA, shows a characteristic absorption maximum at 416 nm on the addition of H2O2, indicating that EDTA is bound with the enzyme. Both positive and negative circular dichroism spectra of HRP and the HRP.H2O2 complex, characteristic of heme absorption, are altered by EDTA, suggesting an EDTA-induced conformational change at or near the heme region. This is associated with a change of affinity of heme toward H2O2 and azide. It is postulated that EDTA interacts at the iodide-binding site of the HRP inducing a new conformation that blocks iodide oxidation but is suitable to convert iodine to iodide by a redox reaction with H2O2.     

Incorporation of 14C-Tyrosine into Soluble Ribonucleic Acid from Baker’s Yeast

The incorporation of ¹⁴C-tyrosine into S-RNA catalyzed by a partially purified tyrosine activating enzyme from baker’s yeast was observed. The maximum incorporation was shown in the presence of 5 μmoles of ATP, 10 μmoles of MgCl₂ and 10~100 μmoles of KCl in the reaction mixture of total volume of 1ml, at pH 7.8 when 1.2 mg of S-RNA, 0.1 μmole of ¹⁴C-tyrosine and 400 μg of enzyme protein were used. Beyond the concentration of ATP, MgCl₂ and KCl described above, the tendency of inhibition was observed. The incorporation was strongly inhibited by pCMB and reactivated by cysteine. Manganese and calcium ions were effective as substitutes for magnesium. S-RNA used was prepared from whole baker’s yeast cell with phenol, but S-RNA obtained from the supernatant of the ground yeast had lost its incorporating activity.     

Occlusion of Rb+ by detergent-solubilized (Na++K+)-ATPase from shark salt glands

Occlusion of Rb⁺ by C₁₂E₈-solubilized (Na⁺+K⁺)-ATPase from shark salt glands has been measured. The rate of de-occlusion at room temperature is about 1 s⁻¹, which is the same as for the membrane-bound enzyme. The amount of Rb⁺ occluded is 3 moles Rb⁺ per mole membrane-bound shark enzyme, whereas only about 2 moles Rb⁺ are occluded by the C₁₂E₈-solubilized enzyme..     

Polymerization of tubulin in the presence of colchicine or podophyllotoxin. Formation of a ribbon structure induced by guanylyl-5'-methylene diphosphonate

GTP-dependent in vitro polymerization of rat brain microtubular protein is inhibited to 50% by substoichiometric concentrations of the antimitotic drugs colchicine (0.12 mol/mol of tubulin) and podophyllotoxin (0.14 mol/mol of tubulin). Substitution of pp(CH₂)pG² for GTP, however, results in an extensive microtubular protein polymerization at such concentrations. In the presence of pp(CH₂)pG, suprastoichiometric concentrations of podophyllotoxin (19 mol/mol of tubulin) are required to inhibit the polymerization process by 50%. Colchicine is very ineffective since 3 × 10⁵ moles/mole of tubulin are required to give a 50% inhibition. Electron microscopical analysis shows that the polymers formed by microtubular protein in the presence of suprastoichiometric concentrations of drugs are not the normal short microtubules typical of pp(CH₂)pG-driven polymerization, but are ribbons with three or four protofilaments. The colchicine content of the harvested ribbons has been measured directly and found to be approximately 0.8 moles colchicine/mole of tubulin. Treatment of microtubular protein with substoichiometric concentrations of drugs results in an increase in the number of protofilaments forming the ribbons. Many of the ribbons can close into morphologically normal microtubules when microtubular protein is treated with only 0.05 moles of either colchicine or podophyllotoxin per mole of tubulin.     

Uptake of iodide in the marine haptophyte Isochrysis sp. (T.ISO) driven by iodide oxidation

Uptake of iodide was studied in the marine microalga Isochrysis sp. (isol. Haines, T.ISO) during short‐term incubations with radioactive iodide (¹²⁵I^?). Typical inhibitors of the sodium/iodide symporter (NIS) did not inhibit iodide uptake, suggesting that iodide is not taken up through this transport protein, as is the case in most vertebrate animals. Oxidation of iodide was found to be an essential step for its uptake by T.ISO and it seemed likely that hypoiodous acid (HOI) was the form of iodine taken up. Uptake of iodide was inhibited by the addition of thiourea and of other reducing agents, like L‐ascorbic acid, L‐glutathione and L‐cysteine and increased after the addition of oxidized forms of the transition metals Fe and Mn. The simultaneous addition of both hydrogen peroxide (H₂O₂) and a known iodide‐oxidizing myeloperoxidase (MPO) significantly increased iodine uptake, but the addition of H₂O₂ or MPO separately, had no effect on uptake. This confirms the observation that iodide is oxidized prior to uptake, but it puts into doubt the involvement of H₂O₂ excretion and membrane‐bound or extracellular haloperoxidase activity of T.ISO. The increase of iodide uptake by T.ISO upon Fe(III) addition suggests the nonenzymatic oxidation of iodide by Fe(III) in a redox reaction and subsequent influx of HOI. This is the first report on the mechanism of iodide uptake in a marine microalga.     

Isolation of Iodide-Oxidizing Bacteria from Iodide-Rich Natural Gas Brines and Seawaters

Iodide-oxidizing bacteria (IOB), which oxidize iodide (I⁻) to molecular iodine (I₂), were isolated from iodide-rich (63 μM to 1.2 mM) natural gas brine waters collected from several locations. Agar media containing iodide and starch were prepared, and brine waters were spread directly on the media. The IOB, which appeared as purple colonies, were obtained from 28 of the 44 brine waters. The population sizes of IOB in the brines were 10² to 10⁵ colony-forming units (CFU) mL⁻¹. However, IOB were not detected in natural seawaters and terrestrial soils (fewer than 10 CFU mL⁻¹ and 10² CFU g wet weight of soils⁻¹, respectively). Interestingly, after the enrichment with 1 mM iodide, IOB were found in 6 of the 8 seawaters with population sizes of 10³ to 10⁵ CFU mL⁻¹. 16S rDNA sequencing and phylogenetic analyses showed that the IOB strains are divided into two groups within the α-subclass of the Proteobacteria. One of the groups was phylogenetically most closely related to Roseovarius tolerans with sequence similarities between 94% and 98%. The other group was most closely related to Rhodothalassium salexigens, although the sequence similarities were relatively low (89% to 91%). The iodide-oxidizing reaction by IOB was mediated by an extracellular enzyme protein that requires oxygen. Radiotracer experiments showed that IOB produce not only I₂ but also volatile organic iodine, which were identified as diiodomethane (CH₂I₂) and chloroiodomethane (CH₂ClI). These results indicate that at least two types of IOB are distributed in the environment, and that they are preferentially isolated in environments in which iodide levels are very high. It is possible that IOB oxidize iodide in the natural environment, and they could significantly contribute to the biogeochemical cycling of iodine.     

Effets de l'iode sur la croissance des frondes d'Asparagopsis armata (Rhodophycées,Bonnemaisoniales) obtenues en culture à partir de rameaux à harpons

Abstract

Effects of iodine on the growth of the « fronds » of Asparagopsis armata (Rhodophyceae, Bonnemaisoniales) in culture from spear bearing branches. – By adding doses of 5 μ moles per litre of iodide or iodate to a medium changed every six days, maximum growth for the « fronds » of Asparagopsis armata is obtained. Initial doses of iodide and iodate higher than 15 μ mole per litre inhibit the growth of this alga.     

Fatty acid synthetase complex: Selective inactivation by phenylmethylsulphonyl fluoride

Reaction of pigeon and rat liver fatty acid synthetases with phenylmethylsulphonyl fluoride at pH 7.0 results in rapid and complete loss of activity for fatty acid synthesis. Acetyl and malonyl transacylation, two reductions, dehydration and condensation-CO₂ exchange reactions are not appreciably altered in the modified enzyme. However, the deacylation of palmityl CoA is completely inhibited. Complete inactivation results in the incorporation of about 1.9 moles of ¹⁴C-phenylmethylsulphonyl groups/mole of the enzyme complex. These results suggest that either two moles of a fatty acyl deacylase or two deacylases with different fatty acyl chain length specificities may be functional in the enzyme complex.     

Activation of indole-3-acetic acid oxidase from horseradish and Prunus by phenols and H2O2

The enzyme-catalysed oxidation of indole-3-acetic acid (IAA) was sytematically investigated with respect to enzyme source and cofactor influence using differential spectrophotometry and oxygen uptake measurement. Commercially-available horseradish peroxidase (HRP) and a peroxidase preparation from Prunus phloem showed identical catalytic properties in degrading IAA. There was no lag phase of IAA oxidation with any of the reaction mixtures tested. Monophenols exhibited a much stronger stimulatory effect than inorganic cofactors, but during the incubation of IAA the phenols were also gradually oxidised. Hydrogen peroxide (H₂O₂) in combination with monophenols accelerated peroxidation of the monophenol and IAA oxidation simutaneously. Since photometric determination of IAA was affected by oxidation products of dichlorophenol or phenol contamination of the enzyme preparation used, the standard IAA absorption measurements appear to be susceptible to methodological errors. Under certain incubation conditions a catalase-like activity of HRP during the course of IAA oxidation was noted and substrate inhibition was observed above 1.5 × 10^\s-4 M IAA. Some concepts concerning the mode of activation of the enzyme-catalysed IAA oxidation are deduced from the experimental results.     

Chemiluminescent assay of β-D-galactosidase based on indole luminescence

We developed a novel chemiluminescent assay of β-D -galactosidase (β-gal) based on the chemiluminescence of indole. 5-Bromo-4-chloro-3-indolyl-β-D -galactopyranoside (X-gal) was used as a substrate for β-gal and also as a light emitter. X-gal was hydrolysed by β-gal to liberate free indoxyl, followed by oxidation to indigo dye, and simultaneously produces hydrogen peroxide (H₂O₂). H₂O₂ reacts with the residual X-gal in the presence of horseradish peroxidase (HRP) to emit light. The measurable range of β-gal obtained by this method was 6 × 10⁻¹⁴ mol/L to 6 × 10⁻¹¹ mol/L; the detection limit was 3 amol/assay. This chemiluminescent assay could be applied to an enzyme immunoassay of thyroxine using β-gal as the enzyme label. © 1998 John Wiley & Sons, Ltd.     

Reduction of [(FeTPP)2O]+SbF6− by iodide ion in dichloromethane

The oxidation in dichloromethane of the iodide ion of cetyltrimethyl ammonium iodide to iodine by the compound consisting of the hexafluoroantimonate anion and the dimeric iron tetraphenylporphyrin (bridged by oxygen) cation, [Fe(TPP)₂O]⁺SbF₆⁻, has been studied spectrophotometrically using the stopped-flow method. At 273 K, d[Fe(TPP)₂O]/dt is equal to 8 × 10⁴[[Fe(TPP)₂O]⁺]-[1⁻] mol dm⁻³ s⁻¹; from this and other measurements E_a is estimated to be 13 kJ mol⁻¹. These results will be compared with other relevant kinetic data, and a possible reaction mechanism will be considered.