In vitro and in vivo evidence for the formation of methyl radical from procarbazine: a spin-trapping study

An innovative GC-MS, NMR and ESR combined, gas-phase investigation during chemical vapor deposition of silicon oxynitrides films from tris(dimethylsilyl)amine

Tris(dimethylsilyl)amine (TDMSA) is used in the presence of O2 and NH3 for the atmospheric pressure chemical vapor deposition (CVD) of conformal, corrosion barrier silicon oxynitride (SiOxNy) films at moderate temperature. Plausible decomposition pathways taking place during the process, as well as resulting gas-phase by-products, are investigated by an innovative methodology, coupling solid-state films characteristics with gas phase analysis. Liquid NMR, gas chromatography coupled with mass spectrometry (GC-MS) and electron spin resonance (ESR) allow probing stable compounds and radical intermediate species in the gas phase. At least fifteen by-products are identified, including silanols, siloxanes, disilazanes, silanamines, and mixed siloxane-silanamine molecules, in addition to more usual compounds such as water. The radical dimethylsilane, Me2HSi˙, is noted across all experiments, hinting at the decomposition of the TDMSA precursor. Deposition of SiOxNy films occurs even in the absence of NH3, demonstrating the judicious choice of the silanamine TDMSA as a dual source of nitrogen and silicon. Additionally, the presence of Si-H bonds in the precursor structure allows formation of SiOxNy films at temperatures lower than those required by other conventional silazane/silanamine precursors. Addition of NH3 in the inlet gas supply results in lower carbon impurities in the films. The identified by-products and formulated decomposition and gas-phase reactions provide stimulating insight and understanding of the deposition mechanism of SiOxNy films by CVD, offering possibilities for the investigation of representative chemical models and process simulation.

Free radical-mediated cytosine C-5 methylation triggers epigenetic changes during carcinogenesis

The methylation of the C-5 position of deoxycytidine (dC) in the promoter regions of tumor suppressor genes is often observed in cancer cells. We found that various environmental agents, as well as endogenous compounds such as methionine sulfoxide (MetO), generate methyl radicals and modify dC to form 5-methyl-dC in DNA in vitro. We confirmed that both DNA methylation and cancer incidence in the liver were increased by the administration of MetO to oxidatively stressed mice. In this review, we summarize previous reports on methyl radical generation in vitro and in vivo and DNA modifications by methyl radicals, including our discoveries, as well as our recent experimental evidence suggesting that free radical-mediated dC methylation triggers epigenetic changes.

Quercetin, a Flavonoid Antioxidant, Ameliorated Procarbazine-Induced Oxidative Damage to Murine Tissues

Procarbazine (PCZ) (indicated in Hodgkin's disease), is an alkylating agent known to generate free radicals in vivo, while Quercetin (QCT) is a flavonoid antioxidant with proven free radical scavenging capacity. This study investigated the protective effects of QCT on PCZ-induced oxidative damage in the rat. Male Wistar rats (160-180 g) were randomized into five groups (n = 5/group): I (control), II PCZ-treated (2 mg/kg body weight (bw) for seven days); III pre-treated with QCT (20 mg/kg bw) for seven days, followed by PCZ for seven days; IV co-treated with PCZ and QCT for seven days and V administered QCT alone for seven days. PCZ caused a significant increase in plasma total bilirubin, urea, and creatinine when compared with control (P < 0.05). Similarly, plasma activities of alkaline phosphatase (ALP), aspartate aminotransferase (AST), alanine aminotransferase (ALT), and γ-glutamyl transferase (γ-GT) were significantly increased in the PCZ-treated group relative to control. Furthermore, PCZ caused a significant decrease in the activities of hepatic superoxide dismutase (SOD), catalase (CAT) and glutathione-S-transferase (GST) as well as levels of ascorbic acid (AA) and glutathione (GSH). This was followed by a significant increase in hepatic malondialdehyde (MDA) content. However, QCT pre-treatment and co-treatment ameliorated the PCZ-induced changes in plasma levels of urea, creatinine, and bilirubin as well as the activities of ALP, AST, ALT, and GGT. QCT also ameliorated hepatic AA and GSH levels and the activities of SOD, CAT, and GST. This all suggests that QCT protected against PCZ-induced oxidative damage in rats.

Post-trapping derivatization of radical-derived EPR-silent adducts: application to free radical detection by HPLC/UV in chemical, biochemical, and biological systems and comparison with EPR spectroscopy

Free radicals are conventionally detected by electron paramagnetic resonance (EPR) spectroscopy after being trapped as spin adducts. Albeit this technique has demonstrated utmost efficacy in studying free radicals, its application to biological settings is intrinsically hampered by the inevitable bioreduction of radical-derived paramagnetic adducts. Herein, we describe a reliable technique to detect and quantify free radical metabolites, wherein reduced alkyl- and phenyl-5,5-dimethyl-1-pyrroline N-oxide (DMPO) adducts are converted into ultrastable N-naphthoate esters. To mimic the ubiquitous in vivo microenvironment, bioreductants, exogenous thiols, and sodium borohydride were studied. Nitroxyl reduction was confirmed using EPR and triphenyltetrazolium chloride. The formation of the N-naphthoyloxy derivatives was established by liquid chromatography/mass spectrometry (LC/MS). The derivatives were chromatographed using a binary eluent. HPLC and internal standards were synthesized using Grignard addition. The labeled DMPO adduct is (1) fluorescent, (2) stable as opposed to nitroxyl radical adducts, (3) biologically relevant, and (4) excellently chromatographed. Applications encompassed chemical, biochemical, and biological model systems generating C-centered radicals. Different levels of phenyl radicals produced in situ from whole blood were successfully determined. The method is readily applicable to the detection of hydroxyl radical. Analogously, DMPO, the spin trap, could be detected with extreme sensitivity suitable for in vivo applications. The developed method proved to be a viable alternative to EPR, where for the first time the reductive loss of paramagnetic signals of DMPO-trapped free radicals is transformed into fluorescence emission. We believe the proposed methodology could represent a valuable tool to probe free radical metabolites in vivo using DMPO, the least toxic spin trap.

DNA methylation at the C-5 position of cytosine by methyl radicals: a possible role for epigenetic change during carcinogenesis by environmental agents

During carcinogenesis, methylation of the C-5 position of cytosines in the promoter region of tumor suppressor genes is often observed. Enzymatic DNA methylation is a widely accepted mechanism for this phenomenon. It is interesting to propose a free radical mechanism for 5-methyldeoxycytidine (m(5)dC) production, because the C-5 position of cytosine is an active site for free radical reactions. When deoxycytidine (dC) and cumene hydroperoxide (CuOOH), a tumor promoter and a methyl radical producer, were reacted in the presence of ferrous ion at pH 7.4, the formation of m(5)dC was observed. The same reaction also proceeded with t-butyl hydroperoxide (BuOOH). The formation of m(5)dC was also observed in DNA by the CuOOH treatment. This is the first report of chemical DNA methylation at cytosine C-5 by environmental tumor promoters. We propose here that this reaction is one of the important mechanisms of de novo DNA methylation during carcinogenesis, because methyl radicals are produced by the biotransformation of various endogenous and exogenous compounds.

The effect of enzyme-inducing antiseizure drugs on the pharmacokinetics and tolerability of procarbazine hydrochloride

PURPOSE

Procarbazine hydrochloride (PCB) is one of the few anticancer drugs with activity against high-grade gliomas. This study was conducted to determine if the maximum tolerated dose and pharmacokinetics of PCB are affected by the concurrent use of enzyme-inducing antiseizure drugs (EIASD).

EXPERIMENTAL DESIGN

Adults with recurrent high-grade glioma were divided into cohorts who were (+) and were not (-) taking EIASDs. PCB was given orally for 5 consecutive days each month. Six patients were evaluated at each dose level beginning with 200 mg/m2/d and escalated using the modified continual reassessment method. Toxicity and response were assessed. Pharmacokinetic studies were done with a new electrospray ionization mass spectrometry assay.

RESULTS

Forty-nine patients were evaluated. The maximum tolerated dose was 393 mg/m2/d for the +EIASD group and the highest dose evaluated in -EIASD patients was 334 mg/m2/d. Myelosuppression was the primary dose-limiting toxicity. Significant hepatic dysfunction occurred in three patients in the +EIASD cohort. Four partial responses (8%) and no complete responses were observed. PCB exhibited linear pharmacokinetics with no significant differences between the two cohorts. A marked increase in peak PCB levels was noted on day 5 relative to day 1, which was not attributable to drug accumulation.

CONCLUSIONS

This study suggests that (a) EIASD use does not significantly affect the pharmacokinetics of PCB; (b) changes in the peak plasma concentration of PCB, consistent with decreased apparent oral clearance due to autoinhibition of hepatic metabolism, occur with daily dosing; and (c) severe hepatic dysfunction may accompany this administration schedule.

MGMT: key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against alkylating agents that generate, among other lesions, O(6)-alkylguanine in DNA (collectively termed O(6)-alkylating agents [O(6)AA]). The defense is highly important, since O(6)AA are common environmental carcinogens, are formed endogenously during normal cellular metabolism and possibly inflammation, and are being used in cancer therapy. O(6)AA induced DNA damage is subject to repair, which is executed by MGMT, AlkB homologous proteins (ABH) and base excision repair (BER). Although this review focuses on MGMT, the mechanism of repair by ABH and BER will also be discussed. Experimental systems, in which MGMT has been modulated, revealed that O(6)-methylguanine (O(6)MeG) and O(6)-chloroethylguanine are major mutagenic, carcinogenic, recombinogenic, clastogenic and killing lesions. O(6)MeG-induced clastogenicity and cell death require MutS alpha-dependent mismatch repair (MMR), whereas O(6)-chloroethylguanine-induced killing occurs independently of MMR. Extensive DNA replication is required for O(6)MeG to provoke cytotoxicity. In MGMT depleted cells, O(6)MeG induces apoptosis almost exclusively, barely any necrosis, which is presumably due to the remarkable ability of secondarily formed DNA double-strand breaks (DSBs) to trigger apoptosis via ATM/ATR, Chk1, Chk2, p53 and p73. Depending on the cellular background, O(6)MeG activates both the death receptor and the mitochondrial apoptotic pathway. The inter-individual expression of MGMT in human lymphocytes is highly variable. Given the key role of MGMT in cellular defense, determination of MGMT activity could be useful for assessing a patient's drug sensitivity. MGMT is expressed at highly variable amounts in human tumors. In gliomas, a correlation was found between MGMT activity, MGMT promoter methylation and response to O(6)AA. Although the human MGMT gene is inducible by glucocorticoids and genotoxins such as radiation and alkylating agents, the role of this induction in the protection against carcinogens and the development of chemotherapeutic alkylating drug resistance are still unclear. Modulation of MGMT expression in tumors and normal tissue is currently being investigated as a possible strategy for improving cancer therapy.

Non-irradiation-derived reactive oxygen species (ROS) and cancer: therapeutic implications

Owing to their chemical reactivity, radicals have cytocidal properties. Destruction of cells by irradiation-induced radical formation is one of the most frequent interventions in cancer therapy. An alternative to irradiation-induced radical formation is in principle drug-induced formation of radicals, and the formation of toxic metabolites by enzyme catalysed reactions. Although these developments are currently still in their infancy, they nevertheless deserve consideration. There are now numerous examples known of conventional anti-cancer drugs that may at least in part exert cytotoxicity by induction of radical formation. Some drugs, such as arsenic trioxide and 2-methoxy-estradiol, were shown to induce programmed cell death due to radical formation. Enzyme-catalysed radical formation has the advantage that cytotoxic products are produced continuously over an extended period of time in the vicinity of tumour cells. Up to now the enzymatic formation of toxic metabolites has nearly exclusively been investigated using bovine serum amine oxidase (BSAO), and spermine as substrate. The metabolites of this reaction, hydrogen peroxide and aldehydes are cytotoxic. The combination of BSAO and spermine is not only able to prevent tumour cell growth, but prevents also tumour growth, particularly well if the enzyme has been conjugated with a biocompatible gel. Since the tumour cells release substrates of BSAO, the administration of spermine is not required. Combination with cytotoxic drugs, and elevation of temperature improves the cytocidal effect of spermine metabolites. The fact that multidrug resistant cells are more sensitive to spermine metabolites than their wild type counterparts makes this new approach especially attractive, since the development of multidrug resistance is one of the major problems of conventional cancer therapy.

Cytochrome P450 pharmacogenetics and cancer

The cytochromes P450 (CYPs) are key enzymes in cancer formation and cancer treatment. They mediate the metabolic activation of numerous precarcinogens and participate in the inactivation and activation of anticancer drugs. Since all CYPs that metabolize xenobiotics are polymorphic, much emphasis has been put on the investigation of a relationship between the distribution of specific variant CYP alleles and risk for different types of cancer, but a consistent view does not yet exist. This is to a great extent explained by the fact that the CYPs involved in activation of precarcinogens are in general not functionally polymorphic. This is in contrast to CYPs that are active in drug biotransformation where large interindividual differences in the capacity to metabolize therapeutic drugs are seen as a consequence of polymorphic alleles with altered function. This includes also some anticancer drugs like tamoxifen and cyclophosphamide metabolized by CYP2D6, CYP2C19 and CYP2B6. Some P450 forms are also selectively expressed in tumours, and this could provide a mechanism for drug resistance, but also future therapies using these enzymes as drug targets can be envisioned. This review gives an up-to-date description of our current knowledge in these areas.

In vitro methods in human drug biotransformation research: implications for cancer chemotherapy

Anticancer drugs have a complex pharmacological and toxicological profile with a narrow therapeutic index. It is therefore critical to understand the factors that contribute to the marked intersubject variability in the pharmacokinetics and pharmacodynamics often observed with anticancer compounds. Since hepatic and extra-hepatic drug metabolism represents a major drug disposition pathway, extensive efforts are made to thoroughly investigate metabolism of anticancer compounds during the pre-clinical and clinical development phases as well as to address issues encountered during the clinical use of an approved drug. In recent years there has been a significant paradigm shift in pre-clinical/non-clinical drug metabolism studies. Most importantly, this has included a reduced reliance on animal models and increased use of human tissues (i.e. human liver microsomes and other cellular fractions, primary culture of human hepatocytes, cDNA expressed human-specific enzymes and cell-based reporter assays). Typically, experiments are performed using these tools to identify the phase I and/or phase II enzymes involved in metabolism of the drug/investigational agent and for metabolic fingerprinting. Additionally, issues pertaining to the rate, extent and mechanism(s) of the inhibition or induction of the metabolic pathways are also investigated. These studies provide important clues about various aspects of the disposition of a therapeutic agent including first-pass metabolism, elimination half-life, overall bioavailability and the potential for drug-drug interactions. The methodologies used for in vitro assessment of drug metabolism and their applications to drug development and clinical therapeutics with special emphasis on anticancer drugs are reviewed in this manuscript.

Interactions Between Antiretrovirals and Antineoplastic Drug Therapy

Despite the established impact of highly active antiretroviral therapy (HAART) in reducing HIV-related morbidity and mortality, malignancy remains an important cause of death. Patients who receive the combination of cancer chemotherapy and HAART may achieve better response rates and higher rates of survival than patients who receive antineoplastic therapy alone. However, the likelihood of drug interactions with combined therapy is high, since protease inhibitors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are substrates and potent inhibitors or inducers of the cytochrome P450 (CYP) system. Since many antineoplastic drugs are also metabolised by the CYP system, coadministration with HAART could result in either drug accumulation and possible toxicity, or decreased efficacy of one or both classes of drugs. Although formal, prospective pharmacokinetic interaction studies are not available in most instances, it is possible to infer the nature of drug interactions based on the metabolic fates of these agents. Paclitaxel and docetaxel are both metabolised by the CYP system, although differences exist in the nature of the isoenzymes involved. Case reports describing adverse consequences of concomitant taxane-antiretroviral therapy exist. Although other confounding factors may have been present, these cases serve as reminders of the vigilant monitoring necessary when taxanes and HAART are coadministered. Similarly, vinca alkaloids are substrates of CYP3A4 and are, thus, vulnerable to PI- or NNRTI-mediated changes in their pharmacokinetics. Interactions with the alkylating agents cyclophosphamide and ifosfamide are complicated as a result of the involvement of the CYP3A4 and CYP2B6 isoenzymes in both the metabolic activation of these drugs and the generation of potentially neurotoxic metabolites. Existing data regarding the metabolic fate of the anthracyclines doxorubicin and daunorubicin suggest that clinically detrimental interactions would not be expected with coadministered HAART. Commonly used endocrine therapies are largely substrates of the CYP system and may, therefore, be amenable to modulation by concomitant HAART. In addition, tamoxifen itself has been associated with reduced concentrations of both anastrozole and letrozole, raising the concern that similar inducing properties may adversely affect the outcome of PI- or NNRTI-based therapy. Similarly, dexamethasone is both a substrate and concentration-dependent inducer of CYP3A4; enhanced corticosteroid pharmacodynamics may result with CYP3A4 inhibitors, while the efficacy of concomitant HAART may be compromised with prolonged dexamethasone coadministration. Since PIs and NNRTIs may also induce or inhibit the expression of P-glycoprotein, the potential for additional interactions to arise via modulation of this transporter also exists. Further research delineating the combined safety and pharmacokinetics of antiretrovirals and antineoplastic therapy is necessary.

Identification of novel enzyme-prodrug combinations for use in cytochrome P450-based gene therapy for cancer

Gene-directed enzyme prodrug therapy can be used to increase the therapeutic activity of anti-cancer prodrugs that undergo liver cytochrome P450 (CYP)-catalyzed prodrug to active drug conversion. The present report describes a cell-culture-based assay to identify CYP gene-CYP prodrug combinations that generate bystander cytotoxic metabolites and that may potentially be useful for CYP-based gene therapy for cancer. A panel of rat liver microsomes, comprising distinct subsets of drug-inducible hepatic CYPs, was evaluated for prodrug activation in a four-day 9L gliosarcoma cell growth inhibition assay. A strong NADPH- and liver microsome-dependent increase in 9L cytotoxicity was observed for the CYP prodrugs cyclophosphamide, ifosfamide, and methoxymorpholinyl doxorubicin (MMDX) but not with three other CYP prodrugs, procarbazine, dacarbazine, and tamoxifen. MMDX activation was potentiated approximately 250-fold by liver microsomes from dexamethasone-induced rats (IC(50) (MMDX) approximately 0.1nM), suggesting that dexamethasone-inducible CYP3A enzymes contribute to activation of this novel anthracycline anti-tumor agent. This CYP3A dependence was verified in studies using liver microsomes from uninduced male and female rats and by using the CYP3A-selective inhibitors troleandomycin and ketoconazole. These findings highlight the advantages of using cell culture assays to identify novel CYP prodrug-CYP gene combinations that are characterized by production of cell-permeable, cytotoxic metabolites and that may potentially be incorporated into CYP-based gene therapies for cancer treatment.

Poly-ADP ribose polymerase activates nuclear proteasome to degrade oxidatively damaged histones

The 20S proteasome has been shown to be largely responsible for the degradation of oxidatively modified proteins in the cytoplasm. Nuclear proteins are also subject to oxidation, and the nucleus of mammalian cells contains proteasome. In human beings, tumor cells frequently are subjected to oxidation as a consequence of antitumor chemotherapy, and K562 human myelogenous leukemia cells have a higher nuclear proteasome activity than do nonmalignant cells. Adaptation to oxidative stress appears to be one element in the development of long-term resistance to many chemotherapeutic drugs and the mechanisms of inducible tumor resistance to oxidation are of obvious importance. After hydrogen peroxide treatment of K562 cells, degradation of the model proteasome peptide substrate suc-LLVY-MCA and degradation of oxidized histones in nuclei increases significantly within minutes. Both increased proteolytic susceptibility of the histone substrates (caused by modification by oxidation) and activation of the proteasome enzyme complex occur independently during oxidative stress. This rapid up-regulation of 20S proteasome activity is accompanied by, and depends on, poly-ADP ribosylation of the proteasome, as shown by inhibitor experiments, 14C-ADP ribose incorporation assays, immunoblotting, in vitro reconstitution experiments, and immunoprecipitation of (activated) proteasome with anti-poly-ADP ribose polymerase antibodies. The poly-ADP ribosylation-mediated activated nuclear 20S proteasome is able to remove oxidatively damaged histones more efficiently and therefore is proposed as an oxidant-stimulatable defense or repair system of the nucleus in K562 leukemia cells.

Activation of alkylhydrazines to free radical intermediates by ethanol-inducible cytochrome P-4502E1 (CYP2E1)

Electron spin resonance (EPR) spectroscopy analysis using the spin trapping agent 4-pyridyl-1-oxide-t-butyl nitrone (4-POBN) was used to measure the formation of free radical intermediates during NADPH-dependent oxidation of 1-methyl-, 1-ethyl-, and 1-isopropylhydrazine in rat liver microsomes and in reconstituted enzyme systems. The experiments in microsomes revealed that the specific activation of the hydrazines, as measured by the EPR signal intensities, was about two-fold higher, when expressed per nmol of P-450, in microsomes from rats treated with ethanol (EtOH) as compared to membranes isolated from either phenobarbital (PB)-, beta-naphthoflavone (beta-NF)-treated or control rats. Furthermore, kinetic experiments revealed that EtOH-microsomes had an apparent affinity for 1-ethylhydrazine about one order of magnitude higher than PB-microsomes. In reconstituted vesicular systems composed of phospholipids, NADPH cytochrome P-450 reductase and P-450, the intensities of EPR signals produced by the formation of the methyl-, ethyl- and isopropyl-free radicals, were 3- to 5-fold more intense in membrane vesicles containing ethanol-inducible CYP2E1 than phenobarbital-inducible CYP2B1. By contrast, CYP1A2, CYP2B4 and CYP2C4 were inefficient catalysts of radical formation. Desferrioxamine, catalase and superoxide dismutase did not influence the extent of ethyl radicals formed in EtOH-microsomes, indicating that hydroxyl radicals are not involved in the CYP2E1-dependent activation of 1-ethylhydrazine. Addition of cytochrome b5, an efficient donor of the second electron to P-450 and hence an inhibitor of the formation of the oxy-cytochrome P-450 complex, increased to be consistent with the results, did not influence the amount of ethyl radicals trapped. In liver microsomes from untreated rats selective substrates of CYP2E1, such as diethyl-dithiocarbamate and p-nitrophenol, as well as anti-CYP2E1-IgG, inhibited the free radical formation from 1-ethylhydrazine by about 60%. The anti-CYP2E1 IgG used significantly inhibited ethyl radical production also in human liver microsomes incubated with 1-ethylhydrazine and 4-POBN. Taken together, these results indicate that CYP2E1, as compared to other rat liver cytochromes P-450, is an efficient catalyst of transformation of alkylhydrazines to free radical intermediates, a finding that might be of importance in the development of the toxicity of these compounds.

Profile of procarbazine-induced embryotoxicity in an embryo hepatocyte co-culture system and after in utero glutathione depletion

Procarbazine (PCZ) is an antineoplastic agent useful in the treatment of Hodgkin's disease, brain tumors, and chronic leukemia. PCZ is dysmorphogenic to developing embryos exposed in vivo or cultured in the serum of PCZ-treated rats. However, embryos directly cultured with PCZ (up to 400 micrograms/ml) or PCZ plus S-9 liver fractions are unaffected. Since intact liver cells provide several advantages over hepatic subcellular fractions for in vitro bioactivation, we exposed rat embryos to PCZ in an embryo/hepatocyte co-culture system. Gestation day (GD) 9.5 rat embryos exposed to 0, 200, 300, or 400 micrograms PCZ/ml in the presence of untreated or phenobarbital induced male rat hepatocytes failed to display toxicity. However, in a companion study GD 9.5 rat embryos cultured in the serum from PCZ-treated rats exhibited developmental deficiencies. Studies have shown that the formation of toxic metabolites can result from glutathione (GSH) conjugation of toxicants in the liver. Therefore, in a second set of experiments, rat embryos were cultured in serum from rats pretreated with two GSH depleters (phorone and buthionine sulfoximine) and subsequently dosed with PCZ. Effects on development were enhanced when embryos were cultured in the serum from PCZ-treated/GSH depleted rats. These data indicate that PCZ requires in vivo activation to be dysmorphogenic and further suggest that the metabolite(s) responsible for procarbazine embryo-toxicity are formed readily under conditions of low GSH levels. This argues against a glutathione conjugate as the ultimate toxicant.

Free radical activation of acetaldehyde and its role in protein alkylation

The formation of carbon centered free radicals, identified as methylcarbonyl species, was observed using ESR spectroscopy and the spin trapping agent 4-pyridyl-1-oxide-N-t-butyl nitrone (4-POBN) during the oxidation of acetaldehyde by xanthine oxidase. The reaction was dependent upon the presence of OH. radicals and was inhibited by the addition of superoxide dismutase, catalase or OH. radical scavengers. The generation of methylcarbonyl radicals was associated with a doubling of stable acetaldehyde adducts with serum albumin, and 4-POBN or superoxide dismutase and catalase, completely blocked this effect. Thus, methylcarbonyl radicals contributed to acetaldehyde-mediated protein alkylation which is involved in causing toxic as well as immunological reactions ascribed to acetaldehyde.

Alkylation and cleavage of DNA by carbon-centered radical metabolites

Although carbon-centered radicals are formed during the metabolism of several genotoxic compounds, they have received little attention as DNA damaging agents. Carbon-centered radicals, however, can both cleave the DNA backbone and alkylate DNA bases, as has been demonstrated to occur in chemical and biochemical systems. Also, in vivo DNA alkylation by methyl radicals has been evidenced by isolation of C8-methylguanine in hydrolysates of DNA from rats administered 1,2-dimethylhydrazine. While most of the studies related to DNA damage by free radicals have been focused on oxyradicals, further studies on DNA alterations promoted by carbon-centered radicals may be necessary to elucidate the mechanisms of action of chemical mutagens and carcinogens.

Possible role of free radical intermediates in hepatotoxicity of hydrazine derivatives

Hydrazine derivatives constitute a wide group of compounds and have found application in industry, agriculture, and (as therapeutical agents) medicine. In spite of their widely spread use, several hydrazine derivatives are known to exert hepatotoxic effects and are carcinogenic. Free radical species are produced during the hepatic biotransformation of alkylhydrazines by both rat and humans liver microsomes. Cytochrome P-450 dependent monoxygenase system is responsible for the production of these reactive species and specific cytochrome P-450 isoenzymes appear to catalyze the formation of free radical intermediates. Free radicals generated during the metabolism of alkylhydrazines are capable of inducing oxidative stress in isolated hepatocytes and might contribute to the development of cell injury.