Reaction rate coefficients of hydroxyl radical-induced DNA single- and α-type double-strand breaks

With a model system of pBR322 plasmid DNA solution in vitro, the dose effects of radiation- induced single- and double-strand breaks (SSB and DSB) were measured and DSB was distinguished into α- and β-types. Under the condition of low scavenging capacity existing in the irradiated DNA solution, SSB and αDSB were mainly induced by hydroxyl radicals (^·OH). Moreover, a certain relationship was obtained between the SSB and αDSB yields and the DNA concentration. It was found that when the DNA solution was irradiated in the presence of 2.5 mmol dm^–3 mannitol, the reciprocals of G(SSB) and G(αDSB), respectively, were linearly related to the reciprocal of the DNA concentration, i.e. the competition reactions of DNA and mannitol for ^·OH radicals can be described by second-order kinetics. The rate coefficients and the efficiencies of the ^·OH radical inducing SSB were deduced. Also, the reaction rate coefficients and the efficiencies for the induction of αDSB from SSB by the ^·OH radical transfer mechanism, were first derived from the competition kinetics. Received: 27 October 1999 / Accepted: 15 March 2000     

Reassignment of organic peroxyl radical adducts.

The study of the important role of peroxyl radicals in biological systems is limited by their difficult detection with direct electron spin resonance (ESR). Many ESR spectra were assigned to 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/peroxyl radical adducts based only on the close similarity of their ESR spectra to that of DMPO/superoxide radical adduct in conjunction with their insensitivity to superoxide dismutase, which distinguishes the radical adduct from DMPO/superoxide radical adduct. Later, the spin-trapping literature reported that DMPO/peroxyl radical adducts have virtually the same hyperfine coupling constants as synthesized alkoxyl radical adducts, raising the issue of the correct assignment of peroxyl radical adducts. However, using 17O-isotope labelling, the methylperoxyl and methoxyl radical adducts should be distinguishable. We have reinvestigated the spin trapping of the methylperoxyl radical. The methylperoxyl radical was generated in aerobic solution with 17O-molecular oxygen either in a Fenton system with dimethylsulfoxide or in a chloroperoxidase system with tert-butyl hydroperoxide. Two different spin traps, DMPO and 2,2,4-trimethyl-2H-imidazole-1-oxide (TMIO), were used to trap methylperoxyl radical. 17O-labelled methanol was used to synthesize methoxyl radical adducts by nucleophylic addition. It was shown that the 17O hyperfine coupling constants of radical adducts formed in methylperoxyl radical-generating systems are identical to that of the methoxyl radical adduct. Therefore, methylperoxyl radical-producing systems form detectable methoxyl radical adduct, but not detectable methylperoxyl radical adducts at room temperature. One of the possible mechanisms is the decomposition of peroxyl radical adduct with the formation of secondary alkoxyl radical adduct. These results allow us to reinterpret previously published data reporting detection of peroxyl radical adducts. We suggest that detection of 17O-alkoxyl radical adduct from 17O-labelled molecular oxygen can be used as indirect evidence for peroxyl radical generation.     

Effect of lecithin on the inhibitory efficiency of α-tocopherol during methyl oleate oxidation

It has been established that the effect of lecithin on the oxidation of methyl oleate depends on the rate of the free radical initiation: lecithin does not affect the reaction rate at high rates of radical initiation (the initiated oxidation) and accelerates the oxidation at low rates (autooxidation), causing the reduction of the induction period. The inhibitory action of mixtures of lecithin and the natural antioxidant α-tocopherol depends on the concentrations of both α-tocopherol and lecithin at different rates of oxidation. The inhibitory efficiency of mixtures with low concentrations of α-tocopherol is close to additive. Increasing the initial concentration of α-tocopherol in mixtures causes a gain in the synergistic effect. The synergistic effect depends extremally on lecithin concentration at high concentrations of α-tocopherol. The mechanism of the synergistic effect for the mixtures of α-tocopherol and lecithin is discussed.     

Shortage of Lipid-radical Cycles in Membranes as a Possible Prime Cause of Energetic Failure in Aging and Alzheimer Disease

Polyunsaturated fatty acids (PUFA) and α-tocopherol (α-TOH) are the most oxygen-sensitive constituents of cells. α-TOH is a member of the vitamin E family that is considered the most important lipophilic antioxidant in cell membranes. Its importance is emphasized by the involvement of oxidative stress in injury to the central nervous system and neurodegenerative diseases. Currently, α-TOH transfer protein (TTP), is believed to play a significant role in maintaining the vitamin status but the presence of α-TOH in membranes is required but not sufficient to protect the membranes against lipid hydroperoxides (LOOH) formation. The lipid-radical theory presented in this review considers the role of two membrane factors—α-tocopherol and cytochrome b5; these factors secure the functioning of lipid-radical cycles and the participation of lipid-radical reactions in the key membrane processes. The prominent intermembrane reaction realized via a protein–lipid interaction, during which electron transport from cytochrome b5—located in the outer membrane—to peroxyl radical (LOO^·)—located in inner membrane—causes reduction of the peroxyl radical: cyt.b5red + LOO^· → cyt.b5ox + LOO⁻. This secures an interaction of α-TOH with other intermediate, LOO⁻ excepting the LOOH formation. The discussion will be focused on the consequences of ineffective electron transfer to LOO^· and excessive oxidative pathway of metabolism of the PUFA (LOO^· → LOOH). Assuming the operation of cytochrome b5/α-tocopherol-controlled lipid-radical cycles and considering the role of the cycles in membrane bioenergetics we arrive at a model for effective function of adenine nucleotide translocator and ATP synthesis in mitochondria. This paper summarizes our experimental evidence that the oxidative and non-oxidative pathways of metabolism of PUFA via their respective intermediates occur in the cells. While this fact is not widely appreciated it may be relevant to elucidation of new mechanisms of neurodegenerative diseases.     

Molecular characterization of soluble factors from human menstrual effluent that induce epithelial to mesenchymal transitions in mesothelial cells

We have studied menstrual effluent in order to identify soluble menstrual factors that induce epithelial to mesenchymal transitions (EMT) in mesothelial cells. A variety of molecules, such as nitric oxide and its reaction products, proteases (i.e. matrix metalloproteinases, plasmin) and proteins and/or peptides (i.e. growth factors: b-fibroblast growth factor, epidermal growth factor, hepatocyte growth factor, transforming growth factor-β; cytokines: interleukin 1β, tumour necrosis factor-α [TNF-α]) may be involved in this process. We have demonstrated that TNF-α is involved in EMT, whereas the other molecules are not. Biochemical analysis has shown that the inducing menstrual factors are heat-labile molecules, are uncharged at neutral pH, have a molecular weight between 50–70 kDa (or are bound in complexes of that size) and are eluted in the albumin fraction during gel filtration chromatography. Further analysis of this fraction by using proteomics and mass spectrometry has led to the identification of α-enolase and haemoglobin whose inhibition partially prevents EMT. When antibodies against TNF-α, α-enolase and haemoglobin are combined, EMT is almost completely inhibited. Thus, the candidates for soluble menstrual factors that induce mesothelial EMT are TNF-α, α-enolase and haemoglobin.     

Vitamin E: action, metabolism and perspectives

Natural vitamin E includes four tocopherols and four tocotrienols. RRR-α-tocopherol is the most abundant form in nature and has the highest biological activity. Although vitamin E is the main lipid-soluble antioxidant in the body, not all its properties can be assigned to this action. As antioxidant, vitamin E acts in cell membranes where prevents the propagation of free radical reactions, although it has been also shown to have pro-oxidant activity. Non-radical oxidation products are formed by the reaction between α-tocopheryl radical and other free radicals, which are conjugated to glucuronic acid and excreted through the bile or urine. Vitamin E is transported in plasma lipoproteins. After its intestinal absorption vitamin E is packaged into chylomicrons, which along the lymphatic pathway are secreted into the systemic circulation. By the action of lipoprotein lipase (LPL), part of the tocopherols transported in chylomicrons are taken up by extrahepatic tissues, and the remnant chylomicrons transport the remaining tocopherols to the liver. Here, by the action of the “α-tocopherol transfer protein”, a major proportion of α-tocopherol is incorporated into nascent very low density lipoproteins (VLDL), whereas the excess of α-tocopherol plus the other forms of vitamin E are excreted in bile. Once secreted into the circulation, VLDL are converted into IDL and LDL by the action of LPL, and the excess of surface components, including α-tocopherol, are transferred to HDL. Besides the LPL action, the delivery of α-tocopherol to tissues takes place by the uptake of lipoproteins by different tissues throughout their corresponding receptors. Although we have already a substantial information on the action, effects and metabolism of vitamin E, there are still several questions open. The most intriguing is its interaction with other antioxidants that may explain how foods containing small amounts of vitamin E provide greater benefits than larger doses of vitamin E alone.     

Generation of transgenic animals expressing the α and β chains of the autoreactive T-cell receptor

Transgenic animal analysis has become a key approach used to study the gene functions and to model various human diseases, including autoimmune disorders. Such disorders are caused by the activation of T-cell clones whose T-cell receptors (TCRs) have a high affinity for syngeneic MHC molecules. The genes coding for the α and β chains of the autoreactive TCR were cloned from hybridoma 7, which was specific for syngeneic Ab MHC class II molecules. Amplified DNA fragments containing rearranged genomic DNA of the α and β chains of hybridoma 7 were cloned into special cassette vectors that contained the natural promoter and enhancer elements ensuring direct expression of the α- and β-chain genes in T cells of transgenic animals. The animals obtained with the vectors expressed the α or β chain on the majority of peripheral Tcells. The animals are suitable for studying the features of the intrathymic selection and maturation of T cells and provide an experimental model for developing new approaches to therapy of autoimmune diseases.     

Role of ��-tocopherol in cellular signaling: ��-tocopherol inhibits stress-induced mitogen-activated protein kinase activation

Tocopherols belong to the plant-derived poly phenolic compounds known for antioxidant functions in plants and animals. Activation of mitogen-activated protein kinases (MAPK) is a common reaction of plant cells in defense-related signal transduction pathways. We report a novel non-antioxidant function of α-tocopherol in higher plants linking the physiological role of tocopherol with stress signalling pathways. Pre-incubation of a low concentration of 50 μM α-tocopherol negatively interferes with MAPK activation in elicitor-treated tobacco BY2 suspension culture cells and wounded tobacco leaves, whereas pre-incubated BY2 cells with α-tocopherol phosphate did not show the inhibitory effect on stimuli-induced MAPK activation. The decreased MAPK activity was neither due to a direct inhibitory effect of α-tocopherol nor due to the induction of an inhibitory or inactivating activity directly affecting MAPK activity. The data support that the target of α-tocopherol negatively regulates an upstream component of the signaling pathways that leads to stress dependent MAPK activation.     

Spectral power of θ and α-EEG bands in preschool children with different levels of visual set plasticity

The spectral powers (SPs) of the θ and α EEG rhythms were studied at different stages of visual set in children from five to seven years of age. Children with a plastic set had a greater α band SP than those with a rigid set. At the set formation stage, children with a rigid set displayed an increase in the SP of the θ band, which is a manifestation of an “immature” activation reaction. Children with a plastic set exhibited an increase in the α band SP in the occipital lobes of both hemispheres. The dynamics of the SP of the EEG rhythms differed in children younger and older than six years. At the set formation stage, children from five to six years of age exhibited the above-mentioned “immature” activation reaction, while those from six to seven years of age showed an increase in the α band SP in the occipital lobes. At the set actualization stage, an increase in the α band SP was observed in the right caudal areas, while the set extinction stage was marked by a bilateral depression of the α band in the frontal lobes. The importance of the involvement of specific cortical areas in visual perception and the formation of a plastic set is discussed.     

The immunologically active site of prothymosin α is located at the carboxy-terminus of the polypeptide. Evaluation of its in vitro effects in cancer patients

Prothymosin α (proTα) is a 109 amino acid long polypeptide presenting distinct immunoenhancing activity in vitro and in vivo. Recent reports suggest that in apoptotic cells, proTα is cleaved by caspases at its carboxy(C)-terminus generating potentially bioactive fragments. In this study, we identified the peptide segment of proTα presenting maximum immunomodulatory activity. Calf thymus proTα was trypsinised, and the five fragments produced (spanning residues 1–14, 21–30, 31–87, 89–102 and 103–109) were tested for their ability to stimulate healthy donor- and cancer patient-derived peripheral blood mononuclear cell (PBMC) proliferation in autologous mixed lymphocyte reaction (AMLR), natural killer and lymphokine-activated killer cell activity, intracellular production of perforin, upregulation of adhesion molecules and CD25 expression. ProTα(89–102) and proTα(103–109) significantly fortified healthy donor-lymphocytes’ immune responses to levels comparable to those induced by intact proTα. These effects were more pronounced in cancer patients, where peptides proTα(89–102) and proTα(103–109) partly, however significantly, restored the depressed AMLR and cytolytic ability of PBMC, by simulating the biological activity exerted by intact proTα. ProTα(1–14), proTα(21–30) and proTα(31–87) marginally upregulated lymphocyte activation. This is the first report showing that proTα’s immunomodulating activity can be substituted by its C-terminal peptide(s). Whether generation and externalization of such immunoactive proTα fragments occurs in vivo, needs further investigation. However, if these peptides can trigger immune responses, they may eventually be used therapeutically to improve some PBMC functions of cancer patients.     

Effects of boron doping on structural,electronic, elastic,and optical properties of energetic crystal 2,6-diamino-3,5-dinitropyrazine-1-oxide: a theoretical study using the first principles calculation and Hirshfeld surface analysis

The activation or functionalization of the saturated C-H is an extremely active field at present. We have explored the triplet state thioxanthone in reactivity of the hydrogen transfer reaction between donors and acceptors. In our works, two donors with quasi-inert sp3 C-H of skipped diene (3,6-nonadiene) and cyclic acetals (benzodioxole) reacted with type II photoinitiators (triplet state of thioxanthone series, TXs) as acceptors are investigated. The excited energies of TXs were obtained by time-dependent density functional theory (TD-DFT). TXs show obvious photosensibility based on their low reorganization energies (< 60 kcal mol−1). The isoentropy reactions had linear geometries of transition state (TS). The distortion/interaction model was used to probe the existence of interaction between acceptors and donors in saddle point. The distortion energy and activation barrier of benzodioxole are much higher than those of the corresponding 3,6-nonadiene. The lower bond dissociation energy noticeably affect the transition state. The reaction of triplet state of TXs with skipped dienes were found to have an anomalous low tunneling factors by using Wigner correction and early transition state by using the bond-energy–bond-order method. The triplet state of TXs photoinitiator can induced the hydrogen abstraction from saturated cyclic acetals and the skipped alkadienes. The hydrogen abstraction experiment are confirmed by UV and real-time FTIR.     

Assessment of the effect of candidate anti-inflammatory treatments on the interaction between meningococci and inflammatory cellsin vitro in a whole blood model

A wide range of immunomodulating agents are now available which may be of benefit in reducing inflammatory cell activation in meningococcal sepsis. In order to facilitate selection of candidate anti-inflammatory agents for clinical trials, we have used an in vitro whole blood model to evaluate the effects on meningococcal induced neutrophil and monocyte activation, of dexamethasone, prostacyclin, pentoxifylline and a human IgM anti-lipid A monoclonal antibody (HA-1A). Known concentrations of heat and penicillin killed meningococci were added to whole blood and the time course of cellular activation was determined. Using elastase-α ₁-antitrypsin (elastase-α ₁-AT) and TNFα production as markers of neutrophil and monocyte activation respectively, plasma levels of elastase-α ₁-AT and TNFα were found to increase in a dose-dependant manner. Elastase-α ₁-AT was detected early, with most release occurring between 15–30 min whereas TNFα was detected later, between 120–180 min. Dexamethasone, prostacyclin and pentoxifylline caused a dose dependant inhibition of TNFα release but had no effect on elastase release. HA-1A had no effect on either TNFα or elastase release. This model may be useful in determining the sequence of inflammatory cell activation and in selecting candidate anti-inflammatory agents for evaluation in clinical trials.     

Activation of Bax by joint action of tBid and mitochondrial outer membrane: Monte Carlo simulations

The mitochondrial pathway of apoptosis proceeds when molecules, such as cytochrome c, sequestered between the outer and inner mitochondrial membranes are released to the cytosol by mitochondrial outer membrane (MOM) permeabilization. Bax, a member of the Bcl-2 protein family, plays a pivotal role in mitochondrion-mediated apoptosis. In response to apoptotic stimuli, Bax integrates into the MOM, where it mediates the release of cytochrome c from the intermembrane space into the cytosol, leading to caspase activation and cell death. The pro-death action of Bax is regulated by interactions with both other prosurvival proteins, such as tBid, and the MOM, but the exact mechanisms remain largely unclear. Here, the mechanisms of integration of Bax into a model membrane mimicking the MOM were studied by Monte Carlo simulations preceded by a computer prediction of the docking of tBid with Bax. A novel model of Bax activation by tBid was predicted by the simulations. In this model, tBid binds to Bax at an interaction site formed by Bax helices α1, α2, α3 and α5 leading, due to interaction of the positively charged N-terminal fragment of tBid with anionic lipid headgroups, to Bax reorientation such that a hydrogen-bonded pair of residues, Asp98 and Ser184, is brought into close proximity with negatively charged lipid headgroups. The interaction with these headgroups destabilizes the hydrogen bond which results in the release of helix α9 from the Bax-binding groove, its insertion into the membrane, followed by insertion into the membrane of the α5–α6 helical hairpin. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Neuroprotective mechanisms of puerarin in middle cerebral artery occlusion-induced brain infarction in rats

Puerarin, a major isoflavonoid derived from the Chinese medical herb Radix puerariae (kudzu root), has been reported to be useful in the treatment of various cardiovascular diseases. In the present study, we examined the detailed mechanisms underlying the inhibitory effects of puerarin on inflammatory and apoptotic responses induced by middle cerebral artery occlusion (MCAO) in rats. Treatment of puerarin (25 and 50 mg/kg; intraperitoneally) 10 min before MCAO dose-dependently attenuated focal cerebral ischemia in rats. Administration of puerarin at 50 mg/kg, showed marked reduction in infarct size compared with that of control rats. MCAO-induced focal cerebral ischemia was associated with increases in hypoxia-inducible factor-1α (HIF-1α), inducible nitric oxide synthase (iNOS), and active caspase-3 protein expressions as well as the mRNA expression of tumor necrosis factor-α (TNF-α) in ischemic regions. These expressions were markedly inhibited by the treatment of puerarin (50 mg/kg). In addition, puerarin (10~50 μM) concentration-dependently inhibited respiratory bursts in human neutrophils stimulated by formyl-Met-Leu-Phe. On the other hand, puerarin (20~500 μM) did not significantly inhibit the thiobarbituric acid-reactive substance reaction in rat brain homogenates. An electron spin resonance (ESR) method was conducted on the scavenging activity of puerarin on the free radicals formed. Puerarin (200 and 500 μM) did not reduce the ESR signal intensity of hydroxyl radical formation. In conclusion, we demonstrate that puerarin is a potent neuroprotective agent on MCAO-induced focal cerebral ischemia in vivo. This effect may be mediated, at least in part, by the inhibition of both HIF-1α and TNF-α activation, followed by the inhibition of inflammatory responses (i.e., iNOS expression), apoptosis formation (active caspase-3), and neutrophil activation, resulting in a reduction in the infarct volume in ischemia-reperfusion brain injury. Thus, puerarin treatment may represent a novel approach to lowering the risk of or improving function in ischemia-reperfusion brain injury-related disorders.     

Mechanical Properties of Mammalian and Fish Gelatins as a Function of the Contents of α-Chain, β-Chain,and Low and High Molecular Weight Fractions

Small-strain oscillatory measurements and size-exclusion chromatography coupled to multiangle laser light scattering were used to study the mechanical properties and the molecular weight distribution, respectively, of acid porcine skin gelatins (type A), lime bovine bone gelatins (type B), and cold water fish gelatins, while principal component analysis (PCA) and partial least squares regression were used to relate the mechanical properties with the molecular weight distribution. The present study suggests a linear relationship between the mechanical properties and the fractions of low molecular weight (LMW) molecules, α-chains, β-chains, and high molecular weight (HMW) molecules. The Bloom value for mammalian gelatin was positively correlated with the fractions of α-chains, β-chains, and HMW molecules and negatively correlated with the fraction of LMW molecules. The dynamic storage modulus for cold water fish gelatin was positively correlated with the fractions of β-chains and HMW molecules and negatively correlated with the fractions of LMW molecules and α-chains.     

Quantum chemical calculations of electron transfer from a model ascorbic acid free radical to ketoaldehydes

Ab-initio SCF calculations have been performed on the anion radical derived from α-hydroxytetronic acid as a model for the ascorbic anion radical, and its interaction with glyoxal. The results show that the anion radical can act as an effective electrondonor to glyoxal, and provide an explanation for the enhancement of charge transfer in amine/dicarbonyl systems by ascorbate.     

Both α and β chain polymorphisms determine the specificity of the disease-associated HLA-DQ2 molecules,with β chain residues being most influential

 We compared the peptide binding specificity of three HLA-DQ molecules; HLA-DQ(α1^*0501, β1^*0201), HLA-DQ(α1^*0201, β1^*0202), and HLA-DQ(α1^*0501, β1^*0301). The first of these molecules confers susceptibility to celiac disease and insulin-dependent diabetes mellitus, while the two latter molecules, which share either the α chain or the nearly identical β chain with HLA-DQ(α1^*0501, β1^*0201), do not predispose to these disorders. The binding of peptides was detected in biochemical binding assays as inhibition of binding of radiolabeled indicator peptides to affinity-purified HLA-DQ molecules. Binding experiments with several peptides demonstrated a clear difference in peptide binding specificity between the three HLA-DQ molecules. Further, single amino acid substitution analyses indicated that the HLA-DQ molecules have different peptide binding motifs. The experimental data were corroborated by computer modelling analysis. Our data suggest that the three HLA-DQ molecules prefer large hydrophobic residues in P1 of peptides with subtle differences in side-chain preferences. HLA-DQ(α1^*0501, β1^*0201) and HLA-DQ(α1^*0201, β1^*0202) both prefer large hydrophobic residues in P9, whereas HLA-DQ(α1^*0501, β1^*0301) prefers much smaller residues in this position. HLA-DQ(α1^*0501, β1^*0201) and HLA-DQ(α1^*0201, β1^*0202), in contrast to HLA-DQ(α1^*0501, β1^*0301), prefer negatively charged residues in P4 and P7. A less prominent P6 pocket also appears to differ between the three HLA-DQ molecules. Our results indicate that polymorphic residues of both the α and the β chain determine the peptide binding specificity of HLA-DQ(α1^*0501, β1^*0201), but that the β chain polymorphisms appears to play the most important role. The information on peptide residues which are advantageous and deleterious for binding to these HLA-DQ molecules may make possible the prediction of characteristic features of peptide that bind to HLA-DQ(α1^*0501, β1^*0201) and precipitate celiac disease. Received: 2 July 1996 / Revised: 7 August 1995     

Up-regulation of HIF-1α is associated with neuroprotective effects of agmatine against rotenone-induced toxicity in differentiated SH-SY5Y cells

Agmatine, a metabolite generated by arginine decarboxylation, has been reported as neuromodulator and neuroactive substance. Several findings suggest that agmatine displays neuroprotective effects in several models of neurodegenerative disorders, such as Parkinson’s disease (PD). It has been hypothesized that biogenic amines may be involved in neuroprotection by scavenging oxygen radicals, thus preventing the generation of oxidative stress. Mitochondrial dysfunction, that leads to a reduction of oxygen consumption, followed by activation of prolyl hydroxylase and decrease of hypoxia-inducible factor 1 alpha (HIF-1α) levels, has been demonstrated to play a role in PD pathogenesis. Using rotenone-treated differentiated SH-SY5Y cells as the in vitro PD model, we here investigated the molecular mechanisms underlying agmatine neuroprotective effects. Our results showed that the preliminary addition of agmatine induces HIF-1α activation, and prevents the rotenone-induced production of free radical species, and the activation of apoptotic pathways by inhibiting mitochondrial membrane potential decrease and caspase 3 as well as cytochrome c increase. Notably, these effects are mediated by HIF-1α, as indicated by experiments using a HIF-1α inhibitor. The present findings suggest that the treatment with agmatine is able to counteract the neuronal cell injury evoked by mitochondrial toxins.     

Reaction profile of the last step in cytochrome P-450 catalysis revealed by studies of model complexes

 A series of oxoiron(IV) porphyrin cation radical complexes was investigated as compound I analogs of cytochrome P-450. Both the spectroscopic features and the reactivities of the complexes in oxygen atom transfer to olefins were examined as a function of only one variable, the axial ligand trans to the oxoiron(IV) bond. The results disclosed two important kinetic steps – electron transfer from olefin to oxoiron(IV) and intramolecular electron transfer from metal to porphyrin radical – which are affected differently by the axial ligands. The large kinetic barrier of the latter step in the reaction of olefins with the perchlorato-bound oxoiron(IV) porphyrin cation radical complex enabled the trapping of a reaction intermediate in which the metal, but not the porphyrin radical, is reduced. The first electron transfer step is probably followed by σ-bond formation, which readily accounts for formation of isomerized organic products at low temperatures. It is finally postulated that part of the enhanced oxygenation activities of cytochrome P-450 monooxygenases and chloroperoxidases is due to a lowering of the energy barrier for the second electron transfer step via participation of their redox-active cysteinate ligand. Received: 16 January 1997 / Accepted: 24 May 1997