Free radical formation in isolated murine keratinocytes treated with organic peroxides and its modulation by antioxidants

Tert-Butyl Hydroperoxide Stimulated Apoptosis Independent of Prostaglandin E2 and IL-6 in the HTR-8/SVneo Human Placental Cell Line

Significant gaps exist in our knowledge of how cellular redox status, sometimes referred to as oxidative stress, impacts placental trophoblasts. The present study used tert-butyl hydroperoxide (TBHP) as a known generator of reactive oxygen species (ROS) in the extravillous trophoblast cell line HTR-8/SVneo to examine the role of cellular redox disruption of prostaglandin E₂ (PGE₂) and the cytokine IL-6 in cell death. Cells were exposed to 0, 12.5, 25, or 50 μM TBHP for 4, 8, and 24 h to ascertain effects on cell viability, caspase 3/7 activity, PGE₂ release, PTGS2 mRNA expression, and IL-6 release. Experiments with inhibitors included the cyclooxygenase inhibitor indomethacin, mitogen-activated protein kinase inhibitors (PD169316, U0126, or SP600125), or treatments to counter expected consequences of TBHP-stimulated generation of ROS (deferoxamine [DFO], butylated hydroxyanisole [BHA], and N,N'-diphenyl-1,4-phenylenediamine [DPPD]) using 24-h exposure to 50 μM TBHP. Cell viability, measured by ATP content, decreased 24% relative to controls with a 24-h exposure to 50 μM TBHP, but not at lower TBHP concentrations nor at earlier time points. Exposure to 50 μM TBHP increased caspase 3/7 activity, an indicator of apoptosis, after 8 and 24 h. Antioxidant treatment markedly reduced TBHP-stimulated caspase 3/7 activity, PGE₂ release, and IL-6 release. TBHP-stimulated IL-6 release was blocked by PD169316 but unaltered by indomethacin. These data suggest that TBHP-stimulated IL-6 release and caspase 3/7 activation were independent of PGE₂ yet were interrupted by treatments with known antioxidant properties, providing new insight into relationships between PGE₂, IL-6, and apoptosis under conditions of chemically induced cellular oxidation.

Formation of methyl radicals derived from cumene hydroperoxide in reconstructed human epidermis: an EPR spin trapping confirmation by using 13C-substitution

Dermal exposure to cumene hydroperoxide (CumOOH) during manufacturing processes is a toxicological issue for the industry. Its genotoxicity, mutagenic action, ability to promote skin tumour, capacity to induce epidermal hyperplasia, and aptitude to induce allergic and irritant skin contact dermatitis are well known. These toxic effects appear to be mediated through the activation to free radical species such as hydroxyl, alkoxyl, and alkyl radicals characterised basically by electron paramagnetic resonance (EPR) and spin-trapping (ST) techniques. To be a skin sensitiser CumOOH needs to covalently bind to skin proteins in the epidermis to form the antigenic entity triggering the immunotoxic reaction. Cleavage of the O-O bond allows formation of unstable CumO^•/CumOO^• radicals rearranging to longer half-life specific carbon-centred radicals R^• proposed to be at the origin of the antigen formation. Nevertheless, it is not still clear which R^• is precisely formed in the epidermis and thus involved in the sensitisation process. The aim of this work was to elucidate in conditions closer to real-life sensitisation which specific R^• are formed in a 3D reconstructed human epidermis (RHE) model by using ¹³C-substituted CumOOH at carbon positions precursors of potentially reactive radicals and EPR-ST. We demonstrated that most probably methyl radicals derived from β-scission of CumO^• radicals occur in RHE through a one-electron reductive pathway suggesting that these could be involved in the antigen formation inducing skin sensitisation. We also describe a coupling between nitroxide radicals and β position ¹³C atoms that could be of an added value to the very few examples existing for the coupling of radicals with ¹³C atoms.

Use of a cocktail of spin traps for fingerprinting large range of free radicals in biological systems

It is well established that the formation of radical species centered on various atoms is involved in the mechanism leading to the development of several diseases or to the appearance of deleterious effects of toxic molecules. The detection of free radical is possible using Electron Paramagnetic Resonance (EPR) spectroscopy and the spin trapping technique. The classical EPR spin-trapping technique can be considered as a "hypothesis-driven" approach because it requires an a priori assumption regarding the nature of the free radical in order to select the most appropriate spin-trap. We here describe a "data-driven" approach using EPR and a cocktail of spin-traps. The rationale for using this cocktail was that it would cover a wide range of biologically relevant free radicals and have a large range of hydrophilicity and lipophilicity in order to trap free radicals produced in different cellular compartments. As a proof-of-concept, we validated the ability of the system to measure a large variety of free radicals (O-, N-, C-, or S- centered) in well characterized conditions, and we illustrated the ability of the technique to unambiguously detect free radical production in cells exposed to chemicals known to be radical-mediated toxic agents.

Dermal Exposure to Cumene Hydroperoxide: Assessing Its Toxic Relevance and Oxidant Potential

Cumene hydroperoxide (CHP) is a high production volume chemical that is used to generate phenol and acetone. Dermal exposure to CHP was hypothesized to result in systemic tissue toxicity, production of free radicals, and consequent decrease in plasma antioxidant levels. To evaluate the hypothesis and characterize the toxicity of CHP, male and female B6C3F1/N mice and F344/N rats were exposed to varying doses of CHP applied topically for 14 or 90 days. No significant changes in survival or body weight of mice and rats were observed following 14 days of exposure. However, 90 days of CHP exposure at the high dose (12 mg/kg) triggered a significant decrease (-15%) in the body weight of the male rat group only. Irritation of the skin was observed at the site of application and was characterized by inflammation and epidermal hyperplasia. In treated animals, histology of liver tissue, free radical generation, and antioxidant levels in blood plasma were not significantly changed as compared to the corresponding controls. Consistent with the lack of systemic damage, no increase in micronucleated erythrocytes was seen in peripheral blood. In conclusion, topical CHP application caused skin damage only at the application site and did not cause systemic tissue impairment.

The organic air pollutant cumene hydroperoxide interferes with NO antioxidant role in rehydrating lichen

Organic pollutants effects on lichens have not been addressed. Rehydration is critical for lichens, a burst of free radicals involving NO occurs. Repeated dehydrations with organic pollutants could increase oxidative damage. Our aim is to learn the effects of cumene hydroperoxide (CP) during lichen rehydration using Ramalina farinacea (L.) Ach., its photobiont Trebouxia spp. and Asterochloris erici. Confocal imaging shows intracellular ROS and NO production within myco and phycobionts, being the chloroplast the main source of free radicals. CP increases ROS, NO and lipid peroxidation and reduces chlorophyll autofluorescence, although photosynthesis remains unaffected. Concomitant NO inhibition provokes a generalized increase of ROS and a decrease in photosynthesis. Our results suggest that CP induces a compensatory hormetic response in Ramalina farinacea that could reduce the lichen's antioxidant resources after repeated desiccation-rehydration cycles. NO is important in the protection from CP.

Pro/antioxidant status and AP-1 transcription factor in murine skin following topical exposure to cumene hydroperoxide

Organic peroxides, widely used in the chemical and pharmaceutical industries, can act as skin tumor promoters and cause epidermal hyperplasia. They are also known to trigger free radical generation. The present study evaluated the effect of cumene hydroperoxide (Cum-OOH) on the induction of activator protein-1 (AP-1), which is linked to the expression of genes regulating cell proliferation, growth and transformation. Previously, we reported that topical exposure to Cum-OOH caused formation of free radicals and oxidative stress in the skin of vitamin E-deficient mice. The present study used JB6 P+ mouse epidermal cells and AP-1-luciferase reporter transgenic mice to identify whether exposure to Cum-OOH caused activation of AP-1, oxidative stress, depletion of antioxidants and tumor formation during two-stage carcinogenesis. In vitro studies found that exposure to Cum-OOH reduced the level of glutathione (GSH) in mouse epidermal cells (JB6 P+) and caused the induction of AP-1. Mice primed with dimethyl-benz[a]anthracene (DMBA) were topically exposed to Cum-OOH (82.6 micromol) or the positive control, 12-O-tetradecanoylphorbol-13-acetate (TPA, 17 nmol), twice weekly for 29 weeks. Activation of AP-1 in skin was detected as early as 2 weeks following Cum-OOH or TPA exposure. No AP-1 expression was found 19 weeks after initiation. Papilloma formation was observed in both the DMBA-TPA- and DMBA-Cum-OOH-exposed animals, whereas skin carcinomas were found only in the DMBA-Cum-OOH-treated mice. A greater accumulation of peroxidative products (thiobarbituric acid-reactive substances), inflammation and decreased levels of GSH and total antioxidant reserves were also observed in the skin of DMBA-Cum-OOH-exposed mice. These results suggest that Cum-OOH-induced carcinogenesis is accompanied by increased AP-1 activation and changes in antioxidant status.

Induction of tumor necrosis factor-alpha by UVB: a role for reactive oxygen intermediates and eicosanoids

UVB irradiation induces nuclear factor-kappaB (NF-kappaB) activation, tumor necrosis factor-alpha (TNF-alpha) expression and reactive oxygen intermediates (ROI) in keratinocytes. We investigated whether ROI play a role in UVB-induced TNF-alpha mRNA expression. The antioxidants N-acetyl cysteine, NAC, epigallocathin gallate, EGCG, butylated hydroxyanisole (BHA) and vitamin C could reduce UVB-induced TNF-alpha mRNA levels to various degrees; vitamin E (alpha-tocopherol) had no effect. BHA was the most potent inhibitor. The oxidant tertiary butylated hydroperoxide could effectively induce TNF-alpha mRNA expression. Nordihydroguaiaretic acid (NDGA) and MK-886, inhibitors of lipoxygenase (LOX), and indometacin and quinacrine, inhibitors of cyclooxygenase (COX) and phospholipase A2, respectively, could also reduce UVB-induced TNF-alpha mRNA expression. Inhibition by NDGA was in concordance with the results for BHA. NDGA, indometacin, quinacrine and BHA could also effectively inhibit the inhibitor of NF-kappaB degradation, thereby maintaining NF-kappaB inactivity. In conclusion, we show that ROI are implicated in the induction of TNF-alpha mRNA by UVB and that not all antioxidants are equally effective inhibitors. COX products and more importantly LOX products, which themselves are products of an oxidative metabolism, are the main ROI implicated in this induction of TNF-alpha expression by UVB probably via activation of NF-kappaB.

Final report on the safety assessment of BHT(1)

BHT is the recognized name in the cosmetics industry for butylated hydroxytoluene. BHT is used in a wide range of cosmetic formulations as an antioxidant at concentrations from 0.0002% to 0.5%. BHT does penetrate the skin, but the relatively low amount absorbed remains primarily in the skin. Oral studies demonstrate that BHT is metabolized. The major metabolites appear as the carboxylic acid of BHT and its glucuronide in urine. At acute doses of 0.5 to 1.0 g/kg, some renal and hepatic damage was seen in male rats. Short-term repeated exposure to comparable doses produced hepatic toxic effects in male and female rats. Subchronic feeding and intraperitoneal studies in rats with BHT at lower doses produced increased liver weight, and decreased activity of several hepatic enzymes. In addition to liver and kidney effects, BHT applied to the skin was associated with toxic effects in lung tissue. BHT was not a reproductive or developmental toxin in animals. BHT has been found to enhance and to inhibit the humoral immune response in animals. BHT itself was not generally considered genotoxic, although it did modify the genotoxicity of other agents. BHT has been associated with hepatocellular and pulmonary adenomas in animals, but was not considered carcinogenic and actually was associated with a decreased incidence of neoplasms. BHT has been shown to have tumor promotion effects, to be anticarcinogenic, and to have no effect on other carcinogenic agents, depending on the target organ, exposure parameters, the carcinogen, and the animal tested. Various mechanism studies suggested that BHT toxicity is related to an electrophillic metabolite. In a predictive clinical test, 100% BHT was a mild irritant and a moderate sensitizer. In provocative skin tests, BHT (in the 1% to 2% concentration range) produced positive reactions in a small number of patients. Clinical testing did not find any depigmentation associated with dermal exposure to BHT, although a few case reports of depigmentation were found. The Cosmetic Ingredient Review Expert Panel recognized that oral exposure to BHT was associated with toxic effects in some studies and was negative in others. BHT applied to the skin, however, appears to remain in the skin or pass through only slowly and does not produce systemic exposures to BHT or its metabolites seen with oral exposures. Although there were only limited studies that evaluated the effect of BHT on the skin, the available studies, along with the case literature, demonstrate no significant irritation, sensitization, or photosensitization. Recognizing the low concentration at which this ingredient is currently used in cosmetic formulations, it was concluded that BHT is safe as used in cosmetic formulations.

Percutaneous permeation and skin irritation of JP-8+100 jet fuel in a porcine model

JP-8 is the major jet fuel used by US Air Force. JP-8+100 is a new jet fuel recently introduced by the US Air Force, which contains JP-8 plus three performance additives [butylated hydroxytoluene (BHT), metal deactivator (MDA) and 8Q405]. The purpose of the present study was to investigate the percutaneous permeation of JP-8+100 across pig ear skin in vitro and to study the effect of JP-8+100 exposure on the skin barrier function, moisture content and irritation in Yucatan minipigs. The influence of performance additives on the permeation of JP-8 was studied by adding each additive individually to JP-8. The percutaneous permeation and skin irritation data obtained with JP-8+100 were compared with that of JP-8. JP-8+100 spiked with 5.0 microCi of radiolabeled [14C]tridecane, nonane, naphthalene or toluene (selected components of JP-8+100) was used for the in vitro percutaneous permeation studies. For skin irritation studies, 250 microl of JP-8+100 was placed in a Hill top chamber and affixed over the marked treatment area for 24 h. The components of JP-8+100 such as tridecane, nonane, naphthalene and toluene permeated readily through pig ear skin without any apparent lag time. Compared to JP-8, the permeation of tridecane, toluene and nonane from JP-8+100 was significantly lower (P<0.05). However, the permeation of naphthalene from JP-8+100 was significantly higher than from JP-8. When BHT was added to JP-8, the permeation of all four chemicals were significantly decreased (P<0.05). Though the addition of 8Q405 to JP-8 decreased the permeation of all four chemicals, the values were not significantly different (P>0.05) from that of JP-8. Addition of MDA did not show any significant change in the permeation of the selected chemicals from JP-8. Application of JP-8+100 increased the transepidermal water loss (TEWL) about three times compared to the baseline level. The skin moisture content decreased consistently after the application of JP-8+100, though it was not significantly different (P>0.05) from the baseline level. JP-8+100 caused a moderate erythema (score: 1.60) and a moderate to severe edema (score: 2.60). These results suggest that JP-8+100 produces significant changes in the barrier function of the skin and a local irritant effect upon occlusive dermal exposure. However there was no significant difference in the skin irritation data observed from JP-8 and JP-8+100.

Redox-modulated pathways in inflammatory skin diseases

Inflammatory skin diseases account for a large proportion of all skin disorders and constitute a major health problem worldwide. Contact dermatitis, atopic dermatitis, and psoriasis represent the most prevalent inflammatory skin disorders and share a common efferent T-lymphocyte mediated response. Oxidative stress and inflammation have recently been linked to cutaneous damage in T-lymphocyte mediated skin diseases, particularly in contact dermatitis. Insights into the pathophysiology responsible for contact dermatitis can be used to better understand the mechanism of other T-lymphocyte mediated inflammatory skin diseases, and may help to develop novel therapeutic approaches. This review focuses on redox sensitive events in the inflammatory scenario of contact dermatitis, which comprise for example, several kinases, transcription factors, cytokines, adhesion molecules, dendritic cell surface markers, the T-lymphocyte receptor, and the cutaneous lymphocyte-associated antigen (CLA). In vitro and animal studies clearly point to a central role of several distinct but interconnected redox-sensitive pathways in the pathogenesis of contact dermatitis. However, clinical evidence that modulation of the skin's redox state can be used therapeutically to modulate the inflammatory response in contact dermatitis is presently not convincing. The rational for this discrepancy seems to be multi-faceted and complex and will be discussed.

A biologically based pharmacodynamic model for lipid peroxidation stimulated by trichloroethylene in vitro

It is often necessary for chemical risk assessment to determine a quantitative relationship between the internal dose of a chemical and its biological effect. The tool best suited for this purpose is a biologically based pharmacodynamic (BBPD) model. Such a BBPD model was developed previously (10) to simulate chemically induced lipid peroxidation, and it was experimentally calibrated in precision-cut mouse liver slices in vitro. The BBPD model simulated formation of lipid hydroperoxides and thiobarbituric acid reactive substances (TBARS) over time and was originally calibrated with different concentrations of tert-butyl hydroperoxide and bromotrichloromethane. The objective of the present work is to refine this BBPD model so it can describe the kinetics and the dose response of lipid peroxidation induced by a weakly pro-oxidant chemical, trichloroethylene (TCE). The chemical-dependent model parameters were optimized to reflect the chemistry of TCE. Two basic algorithms, linear and square root, for the description of stoichiometric free radical production from TCE were tested. Predictions with the square root algorithm fit the experimental data employing TBARS as an end point better than those by the linear algorithm. The calibrated BBPD model will be used to support our future mathematical description of TCE pharmacodynamics in vivo.

Evaluation of skin sensitization potential of jet fuels by murine local lymph node assay

Jet A and JP-8 are the major jet fuels used in civilian and military (US Air Force) flights, respectively. JP-8+100 is a new jet fuel recently introduced by the US Air Force. Besides lung exposure, skin is the potential route of exposure to jet fuels. The purpose of the present study was to investigate the skin sensitization potential of jet fuels (Jet A, JP-8 and JP-8+100) using murine Local lymph node assay (LLNA). Female CBA/Ca mice (8-12-weeks-old) were used in the study. Dinitrochlorobenzene (DNCB, 0.25% w/v) and paraaminobenzoic acid (PABA, 2.5% w/v) were used as positive and negative control, respectively and acetone: olive oil (4:1, AOO) was used as the vehicle (control). All three jet fuels caused a proliferative activity significantly greater than the control (P<0.01). Our results demonstrate that JP-8 is a weak skin sensitizer [stimulation index (SI)=3.17]. The SI of Jet A and JP-8+100 were 2.44 and 2.38, respectively, hence are not considered as skin sensitizers. Interestingly, the SI of JP-8 with butylated hydroxytoluene (BHT) was consistently lower than JP-8, though the difference was not statistically significant (P>0.05). BHT, which is an antioxidant additive of JP-8+100, reduced the skin sensitization potential of JP-8. Furthermore, the lower SI of JP-8+100 could be partially attributed to the presence of BHT. The findings reported here suggest that care should be taken to minimize dermal exposure to jet fuels especially JP-8 to avoid skin sensitization.

trans-resveratrol protects embryonic mesencephalic cells from tert-butyl hydroperoxide: electron paramagnetic resonance spin trapping evidence for a radical scavenging mechanism

In recent years, the antioxidant and other pharmacological properties of resveratrol, a natural product present in grapes and wine, have attracted considerable interest from the biomedical research community. In an examination of the potential neuroprotective properties of the compound, we have investigated the ability of resveratrol to protect rat embryonic mesencephalic tissue, rich in dopaminergic neurones, from the prooxidant tert-butyl hydroperoxide. Using the electron paramagnetic resonance (EPR) spin-trapping technique, the main radicals detected in cell suspensions were the tert-butoxyl radical and the methyl radical, indicating the one-electron reduction of the peroxide followed by a beta-scission reaction. The appearance of EPR signals from the trapped radicals preceded the onset of cytotoxicity, which was almost exclusively necrotic in nature. The inclusion of resveratrol in incubations resulted in the marked protection of cells from tert-butyl hydroperoxide. In parallel spin-trapping experiments, we were able to demonstrate the scavenging of radicals by resveratrol, which involved direct competition between resveratrol and the spin trap for reaction with the radicals. To our knowledge, this is the first example in which cytoprotection by resveratrol has been demonstrated by EPR spin-trapping competition kinetics to be due to its scavenging of the radicals responsible for the toxicity of a prooxidant.

Skin roughness and wrinkle formation induced by repeated application of squalene-monohydroperoxide to the hairless mouse

The present study examines the cumulative effects of sub-erythema application of squalene-monohydroperoxide (Sq-OOH), the initial products of UV-peroxidated squalene, to the skin of hairless mice. Sq-OOH was isolated by the methanol extraction and preparative HPLC method. Repeated topical application of 10 mM Sq-OOH to hairless mice for 3 weeks induced definite skin roughness and crinkle formation. 3-D surface parameter analysis revealed changes in all roughness parameters (number of furrows and crests, distance between a furrow and next crest, and irregularity) of the group treated with more than 3 mM Sq-OOH compared to the control group. These skin surface changes were not induced by squalene, squalene-monohydroxide (Sq-OH) or organic hydroperoxides such as tert-butyl hydroperoxide and cumene-hydroperoxide at 10 mM. Similarly, such changes were not induced by primary irritants, such as sodium lauryl sulfate and n-tetradecane under the same experimental conditions. Skin conductance decreased, following application of 10 mM Sq-OOH. Histological observation revealed that application of 10 mM Sq-OOH induced slight hyperkeratosis, moderate epidermal thickening and slight hyperplasia of sebaceous glands.

Menadione causes increases in the level of glutathione and in the activity of gamma-glutamylcysteine synthetase in cultured Chinese hamster V79 cells

Incubation of cultured Chinese hamster V79 cells with menadione (2-methyl-1,4-naphthoquinone), a generator of superoxide anion radicals, caused a rapid increase in the level of glutathione disulfide (GSSG) and a decrease in the level of glutathione (GSH), which followed a 1.5- to 2-fold increase in the level of GSH during post-treatment incubation. Menadione also caused a concentration- and time-dependent increase in the activity of gamma-glutamylcysteine synthetase (gamma-GCS), a rate-limiting enzyme in the synthesis of GSH. These results suggested that the increase in level of GSH after treatment with menadione was due to the increase in the activity of gamma-GCS. Dicoumarol, an inhibitor of DT-diaphorase, did not influence the increase in the activity of gamma-GCS caused by menadione but it did enhance the cytotoxicity and the increase in the level GSSG caused by menadione. This result suggested that neither the DT-diaphorase-mediated metabolism of menadione nor the increase in level of GSSG caused by menadione was associated with the increase in the activity of gamma-GCS. Chelators of divalent iron and copper (I), and cycloheximide did not influence the increase in the activity of gamma-GCS caused by menadione. Thus, it appeared that reactive oxygen radicals, generated from hydrogen peroxide by an iron- or copper-catalyzed Fenton reaction, were not responsible for the increase in the activity of gamma-GCS and that the increase was not an inducible phenomenon.

Hydrogen peroxide increases the activity of gamma-glutamylcysteine synthetase in cultured Chinese hamster V79 cells

Hydrogen peroxide (H2O2) caused a rapid and a concentration-dependent increase in the activity of gamma-glutamylcysteine synthetase (gamma-GCS) in cultured Chinese hamster V79 cells. The increase in the activity was transient and declined rapidly during post-treatment incubation. Inhibition of protein synthesis by cycloheximide, chelation of divalent iron by o-phenanthroline, and scavenging of free radicals by butyl-4-hydroxyanisole failed to suppress the increase in activity of gamma-GCS caused by H2O2. However, catalase completely inhibited the increase in the activity of the enzyme. H2O2 did not change the level of total glutathione (GSH + GSSG) but it oxidized GSH. The increase in levels of GSSG caused by H2O2 was enhanced by o-phenanthroline. These results suggest that the increase in activity of gamma-GCS caused by H2O2 is not an inducible phenomenon, nor it is attributable to the action of free radicals generated by an iron-catalyzed Fenton reaction. Furthermore, the changes in levels of GSH and GSSG caused by H2O2 appear not to be responsible for the increase in activity of gamma-GCS caused by the hydroperoxide. However, chemical reduction of the enzyme, the activity of which had been increased by H2O2, resulted in a decrease in tha activity, suggesting the involvement of oxidation of the enzyme in the increased activity of gamma-GCS caused by H2O2. The results also suggest that the activity of gamma-GCS in cultured V79 cells can be regulated by the cellular oxidation-reduction state.

Mitochondrial metabolism of a hydroperoxide to free radicals in human endothelial cells: an electron spin resonance spin-trapping investigation

Oxidative damage to the vascular endothelium may be an important event in the promotion of atherosclerosis. Several lines of evidence suggest that lipid hydroperoxides may be responsible for the induction of such damage. Hydroperoxides cause loss of endothelial cell integrity, increase the permeability of the endothelium to macromolecules, and compromise its ability to control vascular tone via the secretion of vasoactive molecules in response to receptor stimulation. The molecular mechanisms responsible for these effects are, however, poorly understood. In this paper, we describe an e.s.r. spin-trapping investigation into the metabolism of the model hydroperoxide compound tert-butylhydroperoxide to reactive free radicals in intact human endothelial cells. The hydroperoxide is shown to undergo a single electron reduction to form free radicals. Experiments with metabolic poisons indicate that the mitochondrial electron-transport chain is the source of electrons for this reduction. The metal-ion-chelating agent desferrioxamine was found to prevent cell killing by tert-butylhydroperoxide, but did not affect free radical formation, suggesting that free metal ions may serve to promote free-radical chain reactions involved in cell killing following the initial conversion of the hydroperoxide to free radicals by mitochondria. These processes may well be responsible for many of the reported effects of hydroperoxides on endothelial cell integrity and function.