Epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene via a hydroperoxide-dependent mechanism catalyzed by lipoxygenases

5-Lipoxygenase Contributes to Benzo[a]pyrene-Induced Cytotoxicity and DNA Damage in Human Bronchial Epithelial Cells

Metabolic activation of indirect-acting carcinogens in target organs is a recognized mechanism of carcinogenesis. This study aimed to determine the role of benzo[a]pyrene (BaP) metabolism enzymes lipoxygenase (LOX), cytochrome P4501A1 (CYP1A1), and prostaglandin synthetase (PGS) in the cytotoxicity and DNA damage induced by BaP in the human tracheobronchial epithelial cells (HBECs) using RNA interference strategy and metabolic enzyme inhibitors. Our results showed that in three epithelial cell lines (HBE, HTR-8/SVneo, and HaCat), BaP significantly upregulated 5-LOX protein expression. 15-LOX-2 expression also increased with increasing BaP concentration, but the change was less pronounced than that of 5-LOX. BaP caused significant cytotoxicity, DNA strand breaks, and 8-hydroxy-2'-deoxyguanosine formation in HBE, which was inhibited by 5-LOXshRNA, a specific inhibitor of 5-LOX (AA861), the CYP1A1 inhibitor α-naphthoflavone, and the PGS inhibitor naproxen. The protective effects of 5-LOXshRNA were stronger than AA861, naproxen and α-naphthoflavone. We conclude that BaP may be activated more by 5-LOX than by CYP1A1 and PGS to produce cytotoxicity and DNA damage in HBE.

Garlic in health and disease

The present article reviews the historical and popular uses of garlic, its antioxidant, haematological, antimicrobial, hepatoprotective and antineoplastic properties and its potential toxicity (from sulfoxide). Garlic has been suggested to affect several cardiovascular risk factors. It has also been shown that garlic and its organic allyl sulfur components are effective inhibitors of the cancer process. Since garlic and its constituents can suppress carcinogen formation, bioactivation and tumour proliferation, it is imperative that biomarkers be established to identify which individuals might benefit most. Garlic powder, aged garlic and garlic oil have demonstrated antiplatelet and anticoagulant effects by interfering with cyclo-oxygenase-mediated thromboxane synthesis. Garlic has also been found to have synergistic effects against Helicobacter pylori with a proton pump inhibitor. The active compound allicin may affect atherosclerosis not only by acting as an antioxidant, but also by other mechanisms, such as lipoprotein modification and inhibition of LDL uptake and degradation by macrophages. Freshly prepared garlic homogenate protects against isoniazid+rifampicin-induced liver injury in experimental animal models. Several mechanisms are likely to account for this protection.

Bioavailability and risk assessment of orally ingested polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a family of toxicants that are ubiquitous in the environment. These contaminants generate considerable interest, because some of them are highly carcinogenic in laboratory animals and have been implicated in breast, lung, and colon cancers in humans. These chemicals commonly enter the human body through inhalation of cigarette smoke or consumption of contaminated food. Of these two pathways, dietary intake of PAHs constitutes a major source of exposure in humans. Although many reviews and books on PAHs have been published, factors affecting the accumulation of PAHs in the diet, their absorption following ingestion, and strategies to assess risk from exposure to these hydrocarbons following ingestion have received much less attention. This review, therefore, focuses on concentrations of PAHs in widely consumed dietary ingredients along with gastrointestinal absorption rates in humans. Metabolism and bioavailability of PAHs in animal models and the processes, which influence the disposition of these chemicals, are discussed. The utilitarian value of structure and metabolism in predicting PAH toxicity and carcinogenesis is also emphasized. Finally, based on intake, disposition, and tumorigenesis data, the exposure risk to PAHs from diet, and contaminated soil is presented. This information is expected to provide a framework for refinements in risk assessment of PAHs from a multimedia exposure perspective.

Induction of lipid peroxidation in biomembranes by dietary oil components

Prooxidant formation and resulting lipid peroxidation are supposed to be involved in the pathogenesis of various diseases including cancer. Cancer risk is possibly influenced by the composition of diet with high intake of fat and red meat being harmful and high consumption of fruits and vegetables being protective. Since dietary oils may contain potential prooxidants, the aim of the present study was to prove (i) whether oxidative stress in biomembranes may be induced by dietary oils and if, (ii) which impact it has on the viability and proliferation of cultured colon (carcinoma) cells. Lipid hydroperoxide content in dietary oils increased after heating. Linoleic acid hydroperoxide (LOOH) and/or oils with different hydroperoxide contents induced lipid peroxidation in liposomes, erythrocyte ghosts and colon cells. Upon incubation with liposomes, both LOOH and heated oil induced lipid peroxidation only in the presence of iron and ascorbate. LOOH was sufficient to start lipid peroxidation of erythrocyte ghosts. LOOH incorporates into the lipid bilayer decreasing membrane fluidity and initiating lipid peroxidation in the lipid phase. When cultured cells (IEC18 intestinal epithelial cells, SW480 and HT29/HI1 colon carcinoma cells) were exposed to LOOH, they responded by cell death both via apoptosis and necrosis. Cells with higher degree of membrane unsaturation were more susceptible and antioxidants (vitamin E and selenite) were protective indicating the involvement of oxidative stress. Thus, peroxidation of biomembranes can be initiated by lipid hydroperoxides from heated oils. Dietary consumption of heated oils may lead to oxidative damage and to cell death in the colon. This may contribute to the enhanced risk of colon cancer due to regenerative cell proliferation.

Mechanisms by which garlic and allyl sulfur compounds suppress carcinogen bioactivation. Garlic and carcinogenesis

Overall, a host of studies provides compelling evidence that garlic and its organic allyl sulfur components are effective inhibitors of the cancer process. These studies reveal that the benefits of garlic are not limited to a specific species, to a particular tissue, or to a specific carcinogen. Several mechanisms are likely to account for this protection. Notable among these is a depression in nitrosamine formation and a reduction in carcinogen bioactivation. The benefits provided by garlic must be viewed as part of the entire diet, since several dietary constituents can influence the degree of protection. More than one compound is responsible for the anticancer properties associated with garlic. Future research should focus on how genetic variability and daily environmental factors influence the anticancer benefits attributed to garlic and its allyl sulfur components.

A historical perspective on garlic and cancer

Epidemiological and laboratory studies provide insight into the anticarcinogenic potential of garlic and its constituent compounds. Both water- and lipid-soluble allyl sulfur compounds are effective in blocking a myriad of chemically induced tumors. Part of the protection from these compounds probably relates to a block in nitrosamine formation and metabolism. However, blockage in the initiation and promotion phases of the carcinogenicity of various compounds, including polycyclic hydrocarbons, provide evidence that garlic and its constituents can alter several phase I and II enzymes. Their ability to block experimentally induced tumors in a variety of sites including skin, mammary and colon, suggests a general mechanism of action. Changes in DNA repair and in immunocompetence may also account for some of this protection. Some, but not all, allyl sulfur compounds can also effectively retard tumor proliferation and induce apoptosis. Changes in cellular thiol and phosphorylation stains may account for some of these antitumorigenic properties. The anticarcinogenic potential of garlic can be influenced by several dietary components including specific fatty acids, selenium, and vitamin A. Since garlic and its constituents can suppress carcinogen formation, carcinogen bioactivation, and tumor proliferation it is imperative that biomarkers be established to identify which individuals might benefit most and what intakes can occur with ill consequences.

Spin trapping of lipid radicals with DEPMPO-derived spin traps: detection of superoxide, alkyl and alkoxyl radicals in aqueous and lipid phase

The spin trap 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) forms a superoxide adduct with a half-life of almost 15 min. DEPMPO is very hydrophilic and its use for the detection of radicals in the lipid phase (lipid-derived radicals and superoxide generated in the lipid phase) is therefore limited due to its very low concentration in the lipid phase. For the detection of lipid-derived radicals, three derivatives of DEPMPO with increasing degree of lipid solubility have been investigated: 5-(di-n-propoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DPPMPO), 5-(di-n-butoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DBPMPO), and 5-(bis-(2-ethylhexyloxy)phosphoryl)-5-methyl-1-pyrroline N-oxide (DEHPMPO). As compared with the spin trap DMPO, the half-lives of the respective superoxide adducts were clearly higher in aqueous solutions of the spin traps, which facilitates qualitative ESR measurements. The stability of the superoxide spin adducts formed with the various lipophilic spin traps in aqueous buffer were similar to those observed with DEPMPO (half-life: 7-11 min.). In model experiments using Fe(3+)-catalyzed nucleophilic addition of methanol or tert-butanol to the respective spin trap the respective alkoxyl radical adducts were formed in aqueous solution as transient species in the presence of high concentrations of the alcohol. Upon dilution with water the alkoxyl group was substituted by water, giving the respective hydroxyl adduct of the spin trap. Care must therefore be taken when Fenton-type reactions are used for the generation of radicals such as the use of Fe(2+) complexes with phosphate or DTPA or inactivation of iron by addition of "Desferal" (Novarti's Pharma GmbH, Vienna, Austria) after a short incubation time. Addition of Fe(2+) under anaerobic conditions to an aqueous suspension of linoleic acid hydroperoxide and the spin trap resulted in the detection of three different species: a carbon-centered radical adduct, an acyl radical adduct, and the hydroxyl adduct. In the presence of oxygen a different species was observed with DEPMPO, DPPMPO, and DBPMPO, which was only slightly suppressed upon the addition of SOD, possibly the respective spin adduct of either the alkylperoxyl radical or, in analogy to DMPO, a secondary alkoxyl radical.

Roles of cyclooxygenase and lipoxygenases in ochratoxin A genotoxicity in human epithelial lung cells

The roles of constitutive prostaglandin-H-synthetase (PGHS) and lipoxygenases in ochratoxin A (OTA) genotoxicity, as reflected by DNA adduct formation, have been investigated in vitro: (1) in culture of human epithelial cells and (2) by incubation in presence of pig seminal vesicle microsomes. Indomethacin (0.1 μM), which inhibits PGHS and significantly increases leukotriene C(4) production by enhancement of lipoxygenases, enhanced formation of OTA-DNA adducts tenfold. At highest dose of 10 μM, indomethacin inhibited all pathways (PGHS and lipoxygenases) and thus prevented OTA-DNA adduct formation. Nordihydroguaiaretic acid, which inhibits lipoxygenases, suppressed OTA-DNA adduct formation. The OTA metabolites formed were analysed by HPLC. OTα, 4[R]- and 4[S]-hydroxy-OTA and a unidentified derivative were formed in control cells. After pre-incubation with indomethacin (0.1 μM), further unidentified metabolites were obtained. They were similar to those obtained in presence of pig seminal vesicle microsomes. These data demonstrate that OTA is biotransformed into genotoxic metabolites via a lipoxygenase, whereas PGHS decreases OTA genotoxicity.

Metabolism and activation of cyclopenta polycyclic aromatic hydrocarbons in liver tissue from rats and humans

The metabolism of radiolabelled benz(j)aceanthrylene (B(j)A) was studied by high performance liquid chromatography (HPLC) using suspensions of hepatocytes and liver microsomes from control- or Aroclor 1254 (PCB)-treated rats, or with human liver microsomes (five different donors) as activation systems. The major metabolites formed in hepatocytes were sulfate conjugates, indicating that sulfation is an important detoxication pathway for B(j)A. In incubations with B(j)A and rat or human liver microsomes, the major metabolite formed was B(j)A-1,2-diol. Studies with rat liver microsomes using antibodies (Ab) towards either P4501A1, 1A2 or 3A2, resulted in approximately 30% reduction in covalent binding with all Ab-using microsomes from control animals, whereas with microsomes from PCB-treated animals an 85% reduction was observed using Ab towards P4501A2, and only minor reductions were observed with 1A1 or 3A2. When compared to B(j)A and benzo(a)pyrene (B(a)P), benz(1)aceanthrylene (B(l)A) caused higher numbers of revertants in the Salmonella assay when plated with rat liver microsomes from control animals or human liver microsomes. The total DNA adduct levels in hepatocytes from control animals after 2 h exposure to 30 micrograms/ml (120 microM) B(j)A or B(l)A, as measured by the 32P-postlabelling technique, were 3.8 +/- 1.5 and 10.1 +/- 5.8 fmol/microgram DNA, respectively. PCB-treatment decreased the total level of B(j)A adducts slightly (1.8 +/- 0.5 fmol/microgram DNA), whereas in contrast the level of B(1)A adducts was increased (24.5 +/- 20.1 fmol/microgram DNA). The major DNA adduct formed in control hepatocytes exposed to B(j)A co-chromatographed with B(j)A-1,2-oxide, which also appeared to be the major adduct formed when rat or human liver microsomes were co-incubated with calf thymus DNA. The total DNA adduct levels in the modified calf thymus DNA after 30 min exposure to 30 micrograms/ml B(j)A, B(l)A or B(a)P using rat liver microsomes form control animals, were 3.6, 66.3 and 1.4 fmol/microgram DNA, respectively. These levels increased to 22.7, 93.3 and 7.4 fmol/microgram DNA, respectively, using microsomes from PCB-treated animals. With human liver microsomes, the total DNA adduct levels after exposure to B(j)A, B(l)A or B(a)P, ranged between 0.4-1.0, 0.3-4.3, and 0.1-0.3 fmol/microgram DNA, respectively. Overall, the present data supports the notion that oxidation at the cyclopenta-ring is an important activation pathway for B(j)A, and indicate that the activation mechanism for B(j)A is similar in rat and human liver tissue.

Detection of free radicals produced from the reaction of cytochrome P-450 with linoleic acid hydroperoxide

The ESR spin-trapping technique was employed to investigate the reaction of rabbit cytochrome P-450 1A2 (P450) with linoleic acid hydroperoxide. This system was compared with chemical systems where FeSO4 or FeCl3 was used in place of P450. The spin trap 5, 5'-dimethyl-1-pyrroline N-oxide (DMPO) was employed to detect and identify radical species. The DMPO adducts of hydroxyl, O2-., peroxyl, methyl and acyl radicals were detected in the P450 system. The reaction did not require NADPH-cytochrome P-450 reductase or NADPH. The same DMPO-radical adducts were detected in the FeSO4 system. Only DMPO-.OH radical adduct and carbon-centred radical adducts were detected in the FeCl3 system. Peroxyl radical production was completely O2-dependent. We propose that polyunsaturated fatty acids are initially reduced to form alkoxyl radicals, which then undergo intramolecular rearrangement to form epoxyalkyl radicals. Each epoxyalkyl radical reacts with O2, forming a peroxyl radical. Subsequent unimolecular decomposition of this peroxyl radical eliminates O2-. radical.

Dioxygenase and co-oxidase activities of rat hepatic cytosolic lipoxygenase

The role of rat liver cytosolic lipoxygenase in the metabolism of benzidine was studied using linoleic acid as a cosubstrate. Under optimum assay conditions, cytosolic dioxygenase activity in the presence of 3.5 mM linoleic acid at pH 7.2 was 74.07 +/- 1.43 nmoles/min/mg protein. Benzidine was oxidized at the rate of 3.18 +/- 0.13 nmoles/min/mg cytosolic protein to benzidine diimine at pH 7.2 in the presence of 3.65 mM linoleic acid. Both dioxygenase and co-oxidase reactions were inhibited by nordihydroguaiaretic acid in a concentration-dependent manner. Partially purified preparations of rat liver lipoxygenase, free of hemoglobin, exhibited a dioxygenase activity of 223.1 +/- 65.9 nmoles/min/mg protein and co-oxidase activity of 6.1 +/- 0.5 nmoles/min/mg protein toward benzidine. These results suggest that hepatic lipoxygenase may play an important role in the metabolism of this hepatocarcinogen.

Bioactivation of benzo(a)pyrene-7,8-dihydrodiol catalyzed by lipoxygenase purified from human term placenta and conceptal tissues

Bioactivation of 14C-benzo(a)pyrene-7,8-dihydrodiol catalyzed by lipoxygenase purified from human term placenta of nonsmoking women and intrauterine conceptal tissues (at 4 weeks of gestation) was investigated. Incubation of 14C-benzo(a)pyrene-7,8-dihydrodiol with 3 mM linoleic acid in the presence of lipoxygenase purified from either human term placenta or intrauterine conceptal tissues resulted in co-oxidation generating several soluble and protein-bound metabolites of benzo(a)pyrene-7,8-dihydrodiol. The co-oxidation was inhibited significantly by the specific lipoxygenase inhibitor, nordihydroguaiaretic acid. Substitution for the active enzyme in the reaction mixture with heat denatured enzyme resulted in almost complete abolition of benzo(a)pyrene-7,8-dihydrodiol co-oxidation. These results suggest that lipoxygenase in the placentas and intrauterine conceptal tissues is capable of metabolizing benzo(a)pyrene-7,8-dihydrodiol to several reactive metabolites and may represent one of the major xenobiotic metabolizing pathways of bioactivating chemicals in the intrauterine compartment.

Hepatocarcinogenic potency of mixed and pure enantiomers of trans-7,8-dihydrobenzo[a]pyrene-7,8-diol in trout

The hepatocarcinogenic potency of pure and racemic trans-7,8-dihydrobenzo[a]pyrene-7,8-diol was investigated in embryos and sac-fry rainbow trout. Embryos microinjected with (+/-)-trans-7,8-dihydrobenzo[a]pyrene-7,8-diol ((+/-) BP-7,8-DHD) developed liver tumors 9 months after hatching. However, this exposure protocol resulted in high mortalities. Microinjection of newly hatched sac-fry with 0.01-1.0 microgram of (+/-) BP-7,8-DHD resulted in a dose-dependent production of liver tumors (0-13%) similar to the results with embryos but without the problem of high mortalities. Co-injection of sac-fry with (+/-) BP-7,8-DHD and either beta-naphthoflavone or carbon tetrachloride significantly enhanced the tumor response (approx. 3-fold). The relative carcinogenic potencies of the pure (+) and (-) enantiomers of BP-7,8-DHD were evaluated by microinjection into sac-fry at doses of 0.5-5.0 micrograms. The results demonstrated that the (-) enantiomer was 4-18 times more potent than the (+). Mixed carcinomas were the most prevalent liver tumors observed. These results demonstrate that trout embryos and sac-fry are both responsive to hepatocarcinogenesis initiation by injection with BP-7,8-DHD. The marked enhancement seen with co-injection of sac-fry with beta-naphthoflavone or carbon tetrachloride suggests that both cytochrome P-450-dependent and lipid peroxidation-dependent pathways could be involved in bioactivation of this compound, presumably through epoxidation at the 9,10-position. As is the case with mammals, the (-) enantiomer of BP-7,8-DHD is a more potent carcinogen than the (+) enantiomer.

Evidence against the 1:2:2:1 quartet DMPO spectrum as the radical adduct of the lipid alkoxyl radical

It was reported that the electron paramagnetic resonance (EPR) spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/lipid alkoxyl radical exhibited a quartet with 1:2:2:1 relative intensity that is identical to that of DMPO/hydroxyl radical (K. M. Schaich and D. C. Borg, 1990, Free Radicals Res. Commun. 9, 267-278). We repeated these EPR experiments using HPLC separation of radical adducts and isotope substitution. We found that the HPLC/EPR chromatogram of the radical adduct with a 1:2:2:1 quartet obtained by the reduction of methyl linoleate hydroperoxide (MLOOH) with Fe2+ exhibited identical retention time to that of the DMPO/OH radical adduct obtained from the Fenton reaction in two different solvent systems. Upon performing the same reaction in 17O-enriched water, the 17O-hyperfine coupling constants due to DMPO/17OH were identified. Ultimately, approximately 80-90% of the total DMPO/OH is derived from water by an iron-dependent nucleophilic addition reaction. Initially, a water-independent mechanism also significantly contributes to DMPO/OH formation. Regardless of its mechanism of formation, the 1:2:2:1 quartet radical adduct of DMPO formed during the reduction of MLOOH by Fe2+ is in fact DMPO/OH.

Linoleate-dependent co-oxygenation of benzo(a)pyrene and benzo(a)pyrene-7,8-dihydrodiol by rat cytosolic lipoxygenase

1. Co-oxygenation of 14C-labelled benzo(a)pyrene and benzo(a)pyrene-7,8-dihydrodiol was studied in rat lung cytosol, using linoleic acid as a co-substrate. Covalently bound and soluble metabolites were quantified by radiometry and h.p.l.c., respectively. 2. The co-oxygenation resulted in the production of reactive metabolites capable of protein binding as well as a series of soluble derivatives. 3. Co-oxygenation of benzo(a)pyrene yielded primarily a significant amount of benzo(a)pyrene-6,12-dione while benzo(a)pyrene-7,8-dihydrodiol led to a significant amount of benzo(a)pyrene-trans-anti-tetrol. 4. Their production was abolished by addition of 25 microM of the lipoxygenase inhibitor and antioxidant NDGA. 5. It is postulated that the linoleic acid peroxyl radicals, formed by rat lung lipoxygenase, initiate the one-electron oxidation of benzo(a)pyrene to its quinones, and epoxidation of benzo(a)pyrene-7,8-diol to the ultimate carcinogenic benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide.

Vanadium redox cycling, lipid peroxidation and co-oxygenation of benzo(a)pyrene-7,8-dihydrodiol

Mechanism of lipid peroxidation triggered by vanadium in human term placental microsomes was reinvestigated in vitro. Production of lipid peroxyl radicals was estimated from co-oxygenation of benzo(a)pyrene and benzo(a)pyrene-7,8-dihydrodiol. Vanadyl(IV), but not vanadate(V) caused a dose-dependent co-oxygenation. Vanadate(V) required the presence of reduced nicotinamide adenine dinucleotide phosphate to trigger co-oxygenation of benzo(a)pyrene-7,8-dihydrodiol. To determine the role of pre-formed lipid hydroperoxides, the results obtained with partially peroxidized linoleic acid were compared with those of fresh linoleate. Superoxide dismutase inhibited the co-oxygenation of reaction when fresh linoleic acid was used. To further characterize the role of superoxide anion-radical in the vanadium redox cycling, the increase of optical density of vanadate(V) dissolved in Tris buffer was measured at 328 nm during the addition of KO2. The rate of this reaction producing peroxy-vanadyl complex was decreased by superoxide dismutase, especially, in the presence of catalase. It is suggested that vanadium catalyzes two separate processes, both leading to enhanced lipid peroxidation: (i) initiation, dependent on superoxide and triggered by peroxy-vanadyl; (ii) propagation, dependent on pre-formed lipid hydroperoxide not sensitive to superoxide dismutase. It is postulated that the vanadium-triggered initiation of lipid peroxidation may be crucial for toxicity in organs with limited endogenous lipid peroxidation.

Superoxide and peroxyl radical generation from the reduction of polyunsaturated fatty acid hydroperoxides by soybean lipoxygenase

Soybean lipoxygenase is shown to catalyze the breakdown of polyunsaturated fatty acid hydroperoxides to produce superoxide radical anion as detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In addition to the DMPO/superoxide radical adduct, the adducts of peroxyl, acyl, carbon-centered, and hydroxyl radicals were identified in incubations containing linoleic acid and lipoxygenase. These DMPO radical adducts were observed just prior to the system becoming anaerobic. Only a carbon-centered radical adduct was observed under anaerobic conditions. The superoxide radical production required the presence of fatty acid substrates, fatty acid hydroperoxides, active lipoxygenase, and molecular oxygen. Superoxide radical production was inhibited when nordihydroguaiaretic acid, butylated hydroxytoluene, or butylated hydroxyanisole was added to the incubation mixtures. We propose that polyunsaturated fatty acid hydroperoxides are reduced to form alkoxyl radicals and that after an intramolecular rearrangement, the resulting hydroxyalkyl radical reacts with oxygen, forming a peroxyl radical which subsequently eliminates superoxide radical anion.

Lipoxygenase: a new pathway for 2-aminofluorene bioactivation

Soybean lipoxygenase mediated co-oxidation of 2-aminofluorene (2-AF), a putative carcinogen and teratogen, was examined. 2-AF metabolism was followed spectrally as a decrease in absorbance at 286 nm. The enzymatic oxidation displayed a pH optimum at 9.5 with a Km of 0.27 mM and specific activity of 521.7 +/- 46.6 nmol/min per nmol of enzyme. The generation of electrophilic 2-AF intermediate(s) capable of covalent binding to macromolecules was also investigated radiometrically. Significant binding to protein and calf thymus DNA was observed, suggesting clearly bioactivation of 2-AF. Several classical lipoxygenase inhibitors caused a marked inhibition of 2-AF oxidation as well as covalent binding to protein and DNA. These results strongly suggest that lipoxygenase is capable of 2-AF metabolism and this may represent another pathway for bioactivation of arylamines.

Radical adducts of nitrosobenzene and 2-methyl-2-nitrosopropane with 12,13-epoxylinoleic acid radical, 12,13-epoxylinolenic acid radical and 14,15-epoxyarachidonic acid radical. Identification by h.p.l.c.-e.p.r. and liquid chromatography-thermospray-m.s

Linoleic acid-derived radicals, which are formed in the reaction of linoleic acid with soybean lipoxygenase, were trapped with nitrosobenzene and the resulting radical adducts were analysed by h.p.l.c.-e.p.r. and liquid chromatography-thermospray-m.s. Three nitrosobenzene radical adducts (peaks I, II and III) were detected; these gave the following parent ion masses: 402 for peak I, 402 for peak II, and 386 for peak III. The masses of peaks I and II correspond to the linoleic acid radicals with one more oxygen atom [L(O).]. The radicals are probably carbon-centred, because the use of 17O2 did not result in an additional hyperfine splitting. Computer simulation of the peak I radical adduct e.p.r. spectrum also suggested that the radical is carbon-centred. The peak I radical was also detected in the reaction of 13-hydroperoxylinoleic acid with FeSO4. From the above results, peak I is probably the 12,13-epoxylinoleic acid radical. An h.p.l.c.-e.p.r. experiment using [9,10,12,13-2H4]linoleic acid suggested that the 12,13-epoxylinoleic acid radical is a C-9-centred radical. Peak II is possibly an isomer of peak I. Peak III, which was observed in the reaction mixture without soybean lipoxygenase, corresponds to a linoleic acid radical (L.). The 12,13-epoxylinoleic acid radical, 12,13-epoxylinolenic acid radical and 14,15-epoxyarachidonic acid radical were also detected in the reactions of linoleic acid, linolenic acid and arachidonic acid respectively, with soybean lipoxygenase using nitrosobenzene and 2-methyl-2-nitrosopropane as spin-trapping agents.

Mechanisms of 1,3-butadiene oxidations to butadiene monoxide and crotonaldehyde by mouse liver microsomes and chloroperoxidase

NADPH-dependent oxidation of 1,3-butadiene by mouse liver microsomes or H2O2-dependent oxidation by chloroperoxidase produced both butadiene monoxide and crotonaldehyde; methyl vinyl ketone and 2,3- and 2,5- dihydrofuran were not detected. The crotonaldehyde to butadiene monoxide ratio remained constant over time in both the microsomal and the chloroperoxidase reactions; however, much more crotonaldehyde was produced by chloroperoxidase than microsomes; crotonaldehyde was not detected when reference samples of butadiene monoxide were used in control incubations containing NADPH and microsomes or H2O2 and chloroperoxidase. Moreover, incubations of 1,3-butadiene with horseradish peroxidase and H2O2, or microsomes and H2O2 or arachidonic acid did not result in the oxidation of 1,3-butadiene. In microsomes, metabolite formation was dependent on incubation time, NADPH, and protein concentrations and did not change when the 1,3-butadiene pressure was varied between 24 and 52 cm Hg. Inclusion of the cytochrome P450 inhibitor 1-benzylimidazole inhibited 1,3-butadiene metabolism, but inclusion of KCN, catalase, or superoxide dismutase had no effect. These results support the role of cytochrome P450 in 1,3-butadiene oxidation by mouse liver microsomes. The formation of crotonaldehyde but not methyl vinyl ketone by cytochrome P450 or chloroperoxidase indicates regioselectivity in the oxygen transfer from the hemoproteins to 1,3-butadiene. The intermediates formed may undergo either ring closure to form butadiene monoxide or a hydrogen shift to form 3-butenal which tautomerizes to produce crotonaldehyde. Evidence for this tautomerization was obtained by the finding that 3-buten-1-ol, an alternative precursor of 3-butenal, was oxidized to crotonaldehyde under incubation conditions similar to that used for 1,3-butadiene.

Hydroperoxide-dependent epoxidation of unsaturated fatty acids in the broad bean (Vicia faba L.)

Incubation of linoleic acid with the 105,000g particle fraction of the homogenate of the broad bean (Vicia faba L.) led to the formation of the following products: 13(S)-hydroxy-9(Z),11(E)-octadecadienoic acid, 9,10-epoxy-12(Z)-octadecenoic acid (9(R),10(S)/9(S)/10(R), 80/20), 12,13-epoxy-9(Z)-octadecenoic acid (12(S),13(R)/12(R)/13(S), 64/36), and 9,10-epoxy-13(S)-hydroxy-11(E)-octadecenoic acid (9(S),10(R)/9(R),10(S), 91/9). Oleic acid incubated with the enzyme preparation in the presence of 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid or cumene hydroperoxide was converted into 9,10-epoxyoctadecanoic acid (9(R),10(S)/9(S),10(R), 79/21). Two enzyme activities were involved in the formation of the products, an omega 6-lipoxygenase and a hydroperoxide-dependent epoxygenase. The lipoxygenase, but not the epoxygenase, was inhibited by low concentrations of 5,8,11,14-eicosatetraynoic acid and nordihydroguaiaretic acid. In contrast, the epoxygenase, but not the lipoxygenase, was readily inactivated in the presence of 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid. Studies with 18O2-labeled 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid showed that the epoxide oxygens of 9,10-epoxyoctadecanoic acid and of 9,10-epoxy-13(S)-hydroxy-11(E)-octadecenoic acid were derived from hydroperoxide and not from molecular oxygen.