Reductive role of glutathione in the redox cycling of oxidizable drugs

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

Myeloperoxidase-catalyzed redox-cycling of phenol promotes lipid peroxidation and thiol oxidation in HL-60 cells

Various types of cancer occur in peroxidase-rich target tissues of animals exposed to aryl alcohols and amines. Unlike biotransformation by cytochrome P450 enzymes, peroxidases activate most substrates by one-electron oxidation via radical intermediates. This work analyzed the peroxidase-dependent formation of phenoxyl radicals in HL-60 cells and its contribution to cytotoxicity and genotoxicity. The results showed that myeloperoxidase-catalyzed redox cycling of phenol in HL-60 cells led to intracellular formation of glutathionyl radicals detected as GS-DMPO nitrone. Formation of thiyl radicals was accompanied by rapid oxidation of glutathione and protein-thiols. Analysis of protein sulfhydryls by SDS-PAGE revealed a significant oxidation of protein SH-groups in HL-60 cells incubated in the presence of phenol/H2O2 that was inhibited by cyanide and azide. Additionally, cyanide- and azide-sensitive generation of EPR-detectable ascorbate radicals was observed during incubation of HL-60 cell homogenates in the presence of ascorbate and H2O2. Oxidation of thiols required addition of H2O2 and was inhibited by pretreatment of cells with the inhibitor of heme synthesis, succinylacetone. Radical-driven oxidation of thiols was accompanied by a trend toward increased content of 8-oxo-7,8-dihydro-2'-deoxyguanosine in the DNA of HL-60 cells. Membrane phospholipids were also sensitive to radical-driven oxidation as evidenced by a sensitive fluorescence HPLC-assay based on metabolic labeling of phospholipids with oxidation-sensitive cis-parinaric acid. Phenol enhanced H2O2-dependent oxidation of all classes of phospholipids including cardiolipin, but did not oxidize parinaric acid-labeled lipids without addition of H2O2. Induction of a significant hypodiploid cell population, an indication of apoptosis, was detected after exposure to H2O2 and was slightly but consistently and significantly higher after exposure to H2O2/phenol. The clonogenicity of HL-60 cells decreased to the same extent after exposure to H2O2 or H2O2/phenol. Treatment of HL-60 cells with either H2O2 or H2O2/phenol at concentrations adequate for lipid peroxidation did not cause a detectable increase in chromosomal breaks. Detection of thiyl radicals as well as rapid oxidation of thiols and phospholipids in viable HL-60 cells provide strong evidence for redox cycling of phenol in this bone marrow-derived cell line.

Activation of caspase-3-like protease by digitonin-treated lysosomes

Apoptosis, a naturally occurring programmed cell death or cell 'suicide', has been paid much attention as one of the critical mechanisms for morphogenesis and tissue remodeling. Activation of cysteine aspartases (caspases) is one of the critical steps leading to apoptosis. Although a mitochondria-mediated pathway has been postulated to be one of the activation mechanism of caspase-3, another subcellular compartment might be involved in the activation of the enzyme. The present study shows that the supernatant fraction of digitonin-treated lysosomes strongly activates Ac-DEVD-CHO inhibitable caspase-3-like protease. Activation of caspase-3-like protease by digitonin-treated lysosomal fractions was specifically suppressed by leupeptin and E-64, inhibitors of cysteine protease. These results indicate that leakage of lysosomal cysteine protease(s) into the cytosolic compartment might be involved in the activation of caspase-3-like protease.

Antibacterial effect of 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1, 4-naphthoquinone-4-imine on Staphylococcus aureus

The mechanism by which a new naphthoquinone derivative, the 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1, 4-naphthoquinone-4-imine (INQI-E) has antibacterial effect against Staphylococcus aureus was studied. The interaction of INQI-E with the bacteria was followed by absorption spectroscopy at 323 and 490 nm. The absorption band of INQI-E at 490 nm undergoes a hypochromic shift with a decrease of intensity. This effect was found to be reversible by oxygenation during the first hours of incubation. The participation of an oxidation-reduction process related to the respiratory chain was demonstrated by oxygen consumption. An increase in O2 uptake and inhibition of S. aureus growth was observed. Experiments with three inhibitors of the respiratory chain demonstrated that the pathway induced by INQI-E was antimycin-resistant and KCN- and salicylhydroxamic acid (SHAM)-sensitive, which suggests that INQI-E is capable of diverting the normal electron flow to an alternate superoxide-producing route. On the other hand, experiments with Tiron, a specific scavenger of superoxide, hindered the effect of INQI-E against S. aureus, indicating that the inhibitory growth effect of this quinone-imine is mainly due to the production of the cytotoxic superoxide radical.

Cellular injury induced by oxidative stress is mediated through lysosomal damage

Cultured primary hepatocytes pretreated (protected) with the iron chelator deferoxamine or the antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD) were resistant to the toxicity of 5 microM naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) during a 180-min exposure. Hepatocytes exposed to naphthazarin without any protection were abruptly depleted of intracellular reduced glutathione, and the level of cytosolic Ca2+ was rapidly increased. This was followed by lipid peroxidation, measured as accumulation of malondialdehyde (MDA) and 4-hydroxyalkenals (4-HNA) intra- and extracellularly; decrease in ATP levels; destabilization of lysosomes; and finally cell death. The stability of the lysosomal membranes was evaluated by determining retention of the lysosomotropic weak base acridine orange (AO). Naphthazarin exposure caused leakage of protons from the acidic compartment, as indicated by relocalization of AO to the cytosol. Protection of the cell cultures with deferoxamine or DPPD prevented destabilization of lysosomes and cell killing. It also reduced the loss of ATP but did not prevent the depletion of glutathione or the increase in Ca2+. In cells subjected to naphthazarin exposure, DPPD protection also completely inhibited lipid peroxidation, whereas deferoxamine pretreatment only slightly reduced the intracellular accumulation of MDA and 4-HNA but completely prevented cell rupture and the leakage of these lipid peroxidation products to the medium that took place in large amounts from unprotected cells exposed to naphthazarin. Deferoxamine is taken up by endocytosis and is thus transported to the acidic vacuolar apparatus, whereas the lipophilic DPPD is rapidly distributed throughout the cells. Inhibiting endocytosis during deferoxamine pretreatment, by incubating at +4 degrees C or by preexposure to a mixture of the endocytosis-inhibitors cytochalasin B and monensin, abolished the protective effect of deferoxamine. The findings suggest that naphthazarin-induced cell killing is not caused directly by either thiol oxidation or an increase in cytosolic free Ca2+, but rather is preceded by lysosomal destabilization, which may be prevented either by inhibition of cellular peroxidation in general or by prevention of iron-catalyzed oxidative reactions, and involves peroxidation of cellular membranes, energy depletion, and leakage of lysosomal content. DPPD would protect against cell killing by preventing lipid peroxidation of cellular membranes in general, whereas deferoxamine seems to allow a limited general cellular peroxidation but specifically prevents peroxidation and fragmentation of lysosomal membranes by chelating intralysosomal iron and, consequently, leakage of destructive lysosomal contents with ensuing cell rupture and death. Thus, a certain degree of cellular peroxidation does not appear to be lethal as long as lysosomal membranes are protected, placing lysosomes into a category of cellular loci minora resistentia.

Radiation chemistry applied to drug design

Radiation chemistry can contribute to drug design by quantifying redox properties of drugs (useful parameters in quantitative structure-activity relationships), and where free radicals are suspected intermediates in drug action, radiation can be used to generate these putative species and help characterize relevant reactions. Steady radiolysis produces radicals at a readily-varied but quantified rate; pulse radiolysis with fast spectrophotometric and/or conductimetric detection enables the kinetic properties of radicals to be monitored directly. Using these methods, radical intermediates from drugs with specific cytotoxicity towards hypoxic cells have been shown to react rapidly with oxygen, a reaction probably responsible for the therapeutic differential. Radical oxidants from activated neutrophils include superoxide and hydroxyl radicals, and radiation-chemical methods have an important role to play in rational drug design to exploit such oxidative chemistry. Antioxidants can also be evaluated quantitatively by radiolysis methods; the conjugation reactions of thiyl radicals with thiolate and oxygen are now recognised to be major contributions of pulse radiolysis to thiol biochemistry.

Glutathionyl- and hydroxyl radical formation coupled to the redox transitions of 1,4-naphthoquinone bioreductive alkylating agents during glutathione two-electron reductive addition

The kinetic parameters of the redox transitions subsequent to the two-electron transfer implied in the glutathione (GSH) reductive addition to 2- and 6-hydroxymethyl-1,4-naphthoquinone bioalkylating agents were examined in terms of autoxidation, GSH consumption in the arylation reaction, oxidation of the thiol to glutathione disulfide (GSSG), and free radical formation detected by the spin-trapping electron spin resonance method. The position of the hydroxymethyl substituent in either the benzenoid or the quinonoid ring differentially influenced the initial rates of hydroquinone autoxidation as well as thiol oxidation. Thus, GSSG- and hydrogen peroxide formation during the GSH reductive addition to 6-hydroxymethyl-1,4-naphthoquinone proceeded at rates substantially higher than those observed with the 2-hydroxymethyl derivative. The distribution and concentration of molecular end products, however, was the same for both quinones, regardless of the position of the hydroxymethyl substituent. The [O2]consumed/[GSSG]formed ratio was above unity in both cases, thus indicating the occurrence of autoxidation reactions other than those involved during GSSG formation. EPR studies using the spin probe 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) suggested that the oxidation of GSH coupled to the above redox transitions involved the formation of radicals of differing structure, such as hydroxyl and thiyl radicals. These were identified as the corresponding DMPO adducts. The detection of either DMPO adduct depended on the concentration of GSH in the reaction mixture: the hydroxyl radical adduct of DMPO prevailed at low GSH concentrations, whereas the thiyl radical adduct of DMPO prevailed at high GSH concentrations. The production of the former adduct was sensitive to catalase, whereas that of the latter was sensitive to superoxide dismutase as well as to catalase. The relevance of free radical formation coupled to thiol oxidation is discussed in terms of the thermodynamic and kinetic properties of the reactions involved as well as in terms of potential implications in quinone cytotoxicity.

Reduction of daunorubicin in the presence of sulfur-containing peptides

Daunorubicin, an anthracycline antitumor antibiotic, was reduced in the presence of reduced (GSH) or oxidized (GSSG) glutathione to evaluate the possibilities of detoxification or of potentiation of the drug by these compounds. The reductants were .COO- free radicals produced by gamma radiolysis. In both cases, the final product is 7-deoxydaunomycinone, i.e., the same as without glutathione. The reduction yield is also the same as without GSH or GSSG (0.23 mumol.J-1). No glutathione depletion was observed. Limits for the rate constants of some possible nonenzymatic detoxification reactions are given. To evaluate the possible interactions of daunorubicin with sulfur-containing proteins, the reduction of this drug by .COO- free radicals was also studied in the presence of a polypeptide containing two disulfide bridges, aponeocarzinostatine. The final product is also 7-deoxydaunomycinone. The yields of reduction of the drug and of a protein disulfide bridge are, respectively, 0.23 mumol.J-1 and less than or equal to 6 nmol.J-1. These values indicate that disulfide radical anions of the protein can reduce the drug, giving back the disulfide bridge, but that the drug transients neither oxidize nor reduce the protein.

Bioreductive activation of quinones: redox properties and thiol reactivity

Redox properties and thiol reactivity are central to the therapeutic and toxicological properties of quinones. The use of other physicochemical parameters to establish predictive relationships for redox properties of quinones is discussed, and attention drawn to situations where such relationships may be unreliable. The rates of reaction of semiquinone radicals with oxygen, including those of chemotherapeutic agents such as mitomycin and the anthracyclines, can be predicted with reasonable confidence from the redox properties. The reactions of quinones with thiols such as glutathione produces reduced quinones and radicals, but the reactions are complex and all the features are not well understood.

Effect of glutathione on the redox transitions of naphthohydroquinone derivatives formed during DT-diaphorase catalysis

The oxidation of GSH coupled to the redox transitions of 1,4-naphthoquinone derivatives during DT-diaphorase catalysis was examined. The quinones studied included 1,4-naphthoquinone and its dimethoxy- and hydroxy derivatives and were selected according to their different ability to undergo nucleophilic addition with GSH and the dual effect of superoxide dismutase on hydroquinone autoxidation. GSH was oxidized to GSSG during the redox transitions of the above quinones, regardless of their substitution pattern. This effect was accompanied by an increase of total O2 consumption, indicating the ability of GSH to support quinone redox cycling. The values for the relationship [O2]consumed/[GSSG]formed were, with every quinone examined, above unity, thus pointing to the occurrence of autoxidation reactions other than those involved during GSSG formation. These results are discussed in terms of the functional group chemistry of the quinones and the thermodynamic properties of the reactions involved in the reduction of the semi- to the hydro-quinone by GSH.

Study of the redox properties of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) and its glutathionyl conjugate in biological reactions: one- and two-electron enzymatic reduction

Naphthazarin (5,8-dihydroxy-1,4-naphthoquinone), the basic unit of several tetracyclic antitumor antibiotics, and its glutathione conjugate were reduced by the one- and two-electron transfer flavoproteins NADPH-cytochrome P450 reductase and DT-diaphorase to their semi- and hydroquinone forms, respectively. Kinetic studies performed on purified DT-diaphorase showed the following results: KNADPHm = 68 microM, KQuinonem = 0.92 microM, and Vmax 1300 nmol X min-1 X microgram enzyme-1. Similar studies performed on purified NADPH-cytochrome P450 reductase indicated a lower KNADPHm (10.5 microM) and higher KQuinonem (2.3 microM). The Vmax values were 20-fold lower (46 nmol X min-1 X micrograms enzyme-1) than those observed with DT-diaphorase. DT-diaphorase reduced the naphthazarin-glutathione conjugate with an efficiency 5-fold lower than that observed with the parent quinone. The nucleophilic addition of GSH to naphthazarin proceeded with GSH consumption at rates slower than those observed with 1,4-naphthoquinone and its monohydroxy derivative, 5-hydroxy-1,4-naphthoquinone. The initial rate of GSH consumption during these reactions did not vary whether the assay was carried out under anaerobic or aerobic conditions. Autoxidation accompanied the DT-diaphorase and NADPH-cytochrome P450 reductase catalysis of naphthazarin and its glutathionyl adduct as well as the 1,4-reductive addition of GSH to naphthazarin. Superoxide dismutase at catalytic concentrations (nM range) enhanced slightly (1.1- to 1.6-fold) the autoxidation following the enzymatic catalysis of naphthazarin. Autoxidation during the GSH reductive addition to 1,4-naphthoquinones decreased with increasing number of -OH substituents, 1,4-naphthoquinone greater than 5-hydroxy-1,4-naphthoquinone greater than 5,8-dihydroxy-1,4-naphthoquinone, thus revealing that the contribution of redox transitions other than autoxidation, e.g., cross-oxidation, to the decay of the primary product of nucleophilic addition increases with increasing number of -OH substituents. Superoxide dismutase enhanced substantially the autoxidation of glutathionyl-naphthohydroquinone adducts, thereby affecting only slightly the total GSH consumed and GSSG formed during the reaction. The present results are discussed in terms of the relative contribution of one- and two-electron transfer flavoproteins to the bioreductive activation of naphthazarin and its glutathionyl conjugate as well as the importance of autoxidation reactions in the mechanism(s) of quinone cytotoxicity.

A probe for intracellular concentrations of drugs: delayed fluorescence from acridine orange

The aim of this work is to develop fluorescent probes that will indicate effective concentrations of therapeutic agents, or endogenous protectors, at important cellular sites. Acridine orange associates with nucleic acids and emits a 'delayed' fluorescence signal. This signal is quenched by oxidants such as oxygen, nitroaryl radiosensitizers, adriamycin and mitomycin-c, and reductants such as thiols, ascorbate and other radioprotectors. The quenching of the acridine orange delayed fluorescence reflects the effective concentration of these therapeutically-important oxidants and reductants near DNA. The relative concentration of basic radiosensitizers such as pimonidazole (Ro 03-8799) near the DNA is greater than that of misonidazole. Thiols quench the delayed fluorescence signal according to the degree of ionization of the thiol function; this may model the reactivity of thiols with guanine radical sites in DNA. Ascorbate and aminopyrine do not quench the delayed fluorescence from cells stained with acridine orange as these compounds are taken up by cells very inefficiently.

Weiss lecture. Radiation chemistry: basic, strategic or tactical science?

The work of Weiss in the 1930s, particularly with Haber, has only recently been recognized to have implications in biology and medicine. Similarly, research in radiation chemistry and the application of the pulse radiolysis technique, for example, have implications far beyond traditional radiation chemistry. Some examples of such research are discussed against a background of categorization into 'basic', 'strategic' or 'tactical' science, rather than 'pure' and 'applied'. Examples discussed include redox properties of free radicals, which are now on a firm quantitative basis, and the identification and characterization of nitro radicals as intermediates in drug metabolism. Radical reactions often take place in multicomponent systems, and the techniques of radiation chemistry can be used to probe, for example, events occurring at interfaces in micelles. Industrial processes involving radiation are attracting investment, particularly in Japan. Radiation chemistry deserves further support to exploit its full potential, but much research using radiation chemical techniques often appears, probably more appropriately, under different labels.

Properties of the radicals formed by one-electron oxidation of acetaminophen--a pulse radiolysis study

The semi-iminoquinone radical of acetaminophen, which has previously been proposed as a possible hepatotoxic intermediate in the cytochrome P-450 catalysed oxidation of acetaminophen, has been generated and studied by pulse radiolysis. In the absence of other reactive solutes, the radical decays rapidly by second order kinetics with a rate constant (2k2) of (2.2 +/- 0.4) x 10(9) M-1 sec-1. In alkaline solutions the radical deprotonates with a pK of 11.1 +/- 0.1 to form a radical-anion, as confirmed by the effect of ionic strength on the rate of radical decay. The acetaminophen radical-anion reacts with resorcinol at high pH values, leading to the formation of a transient equilibrium from which the one-electron reduction potential of the semi-iminoquinone radical of acetaminophen is estimated to be +0.707 +/- 0.01 V at pH 7. This value predicts that acetaminophen should be oxidised by thiyl radicals. This was confirmed by pulse radiolysis experiments for reaction of the cysteinyl radical, for which rate constants of 7 x 10(6) M-1 sec-1 at pH 7 and 2.7 x 10(8) M-1 sec-1 at pH 11.3 were obtained. The reaction of O2 with the acetaminophen semi-iminoquinone radical could not be detected by pulse radiolysis, and alternative mechanisms for superoxide radical formation are discussed.

Role of nonprotein thiols in enzymatic reduction of 2-nitroimidazoles

The role of nonprotein thiols (NPSH) in the enzymatic reduction of the nitro function in 2-nitroimidazoles (2-NI) has been investigated. The addition of NPSH has been shown previously to protect cells from the hypoxic cytotoxicity of 2-NI, whereas depletion of NPSH enhances the hypoxic cytotoxicity. In this report, we have investigated the effects of thiol depleting agents, N-ethylmaleimide (NEM) and diethyl maleate (DEM), on the enzymatic reduction of the nitro group. Cytosolic and microsomal fractions of rat hepatic tissue and xanthine oxidase were employed as sources of nitro reductases. Addition of NPSH caused an enhancement in the reduction of the nitro group of 2-NI; cysteine was significantly more effective than glutathione (GSH) in stimulating the enzymatic reduction. The reduction of the nitro function was decreased markedly in the presence of NEM or DEM. Addition of cysteine or GSH reversed the inhibition with NEM. Both NEM and DEM also attenuated the enhancement of reduction observed after the addition of NPSH. These results suggest that the addition of NPSH facilities the reduction of the nitro function to the reduced intermediates that may be inactivated by an excess of NPSH, whereas the depletion of NPSH allows the accumulation of the toxic nitro radicals causing increased cytotoxicity.

Radical formation during the peroxidase catalyzed metabolism of carcinogens and xenobiotics: the reactivity of these radicals with GSH, DNA, and unsaturated lipid

Radicals generated by the peroxidase catalyzed oxidation of a wide variety of substrates oxidize GSH, NADH, or arachidonate with accompanying oxygen activation. Substrates studied include carcinogens, drugs, or xenobiotics. The effectiveness of the various radicals is partly related to their one-electron oxidation potential. High redox potential radicals were particularly effective at oxidizing these biomolecules. Low redox potential radicals did not react with GSH, NADH, or arachidonate, but can directly activate oxygen to form hydroxyl radicals or undergo scission to carbon radicals. The hydroxyl and carbon radicals have a high redox potential and readily oxidize biomolecules. DNA strand breakage also occurs with some high redox potential radicals, but DNA did not react with low redox potential radicals. The extensive binding of xenobiotics to DNA in the peroxidase system was attributed to noncovalent binding by polymeric products or covalent binding by the two electron oxidation product (formed by radical dismutation or oxidation). The latter can cause alkali labile DNA strand breaks. GSH conjugate formation was also attributed to the two electron oxidation product. Radicals have been trapped in intact cells and oxygen activation or lipid peroxidation has been demonstrated but it is still not clear whether the associated GSH oxidation, DNA strand breakage and cytotoxicity is the result of direct action by radicals. Indirect enzymic mechanisms for free radical mediated DNA strand breakage and cytotoxicity are discussed.

Control of the generation and reactions of free radicals in biological systems by kinetic and thermodynamic factors

Quantifiable redox properties are useful predictors of substrate reactivity in enzyme-catalysed redox reactions of e.g. nitroreductases or peroxidases. Redox properties may also control the rates of electron-transfer reactions between radical products of reduction and oxidation, and endogenous oxidants and reductants respectively. However, in numerous instances prototropic properties of substrate or radical may have profound kinetic consequences, protonation of radicals frequently slowing down electron-transfer reactions. Further, reactions which are thermodynamically extremely unfavourable may still proceed if radical products are removed from the pre-equilibrium efficiently. Thus kinetic considerations often outweigh the purely thermodynamic viewpoint.

Oxidation of glutathione and reduced pyridine nucleotides by the myotoxic and mutagenic aromatic amine, 1,2,4-triaminobenzene

The myotoxic and mutagenic aromatic amine, 1,2,4-triaminobenzene, has been shown to oxidise glutathione and reduced pyridine nucleotides in a cyclic reaction which generates both superoxide radical and hydrogen peroxide. It is suggested that the process is initiated by the triaminobenzene radical, formed by autoxidation of the amine. This, by mediating the one-electron oxidation of GSH and NAD(P)H, is able to establish a radical chain-reaction leading to the formation of GSSG and NAD(P)+. This reaction may be of significance in the pathogenesis of the toxic effects of 1,2,4-triaminobenzene, not only by forming reactive free-radicals but also by compromising cellular defences against oxidative attack.

Peroxidase activation of 1-naphthol to naphthoxy or naphthoxy-derived radicals and their reaction with glutathione

1-Naphthol was metabolised by horseradish peroxidase (HRP) in a H2O2-dependent reaction to methanol-soluble and covalently bound products. Spectrophotometric and electron spin resonance (ESR) studies established that HRP catalysed the one electron oxidation of 1-naphthol to naphthoxy or a naphthoxy-derived radical. Inclusion of glutathione (GSH) in the reaction caused a dose-dependent inhibition of covalent binding and an increase in the amount of unmetabolised 1-naphthol present at the end of the incubation. gamma-Radiolysis studies suggest that this is due to the reduction of naphthoxy radicals by GSH yielding 1-naphthol and GS.. In agreement with this, HRP-catalysed-oxidation of 1-naphthol in the presence of GSH, was found to stimulate oxidised glutathione (GSSG) formation.