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D C Liebler

Publications and source records attributed to D C Liebler.

At least 37 records · Page 2Linked to original sources
PubMed1999

alpha,beta-unsaturated aldehydes accelerate oxymyoglobin oxidation.

This study investigates the potential basis for enhancement of oxymyoglobin (OxyMb) oxidation by lipid oxidation products. Aldehydes known to be formed as secondary lipid oxidation products were combined with OxyMb in aqueous solution at 37 degrees C and pH 7.4. Metmyoglobin (MetMb) formation was greater in the presence of alpha,beta-unsaturated aldehydes than their saturated counterparts of equivalent carbon chain length. Additionally, increasing chain length from hexenal through nonenal resulted in increased MetMb formation (P < 0.05). Electrospray ionization mass spectrometry (ESI-MS) revealed that OxyMb incubated with 4-hydroxynonenal (HNE) at pH 7.4 at 37 degrees C yielded myoglobin molecules adducted with one to three molecules of HNE from 0.5 to 2 h of incubation, respectively. A prooxidant effect of HNE was noted at pH 7.4 but was not apparent at pH 5.6 when compared to the control (P < 0.05). This appeared to be due to rapid OxyMb autoxidation at this pH compared to pH 7.4. ESI-MS demonstrated that adduction of HNE to OxyMb occurred at pH 5.6. This research demonstrates that alpha, beta-unsaturated aldehydes accelerate OxyMb oxidation and appear to do so via covalent attachment.

Aldehydes↗
PubMed1999

UV-B-Induced photooxidation of vitamin E in mouse skin.

Topically applied alpha-tocopherol (alpha-TH, vitamin E) inhibits UV-B (290-320 nm) photocarcinogenesis, yet alpha-TH undergoes rapid photooxidation by UV-B in vitro. To examine the effect of UV-B on alpha-TH in vivo, we studied the fate of alpha-TH in UV-B-irradiated mouse skin. [14C]-alpha-TH was applied to mouse skin at various times prior to UV-B irradiation. UV-B irradiation for 1 h at a dose rate of 2.6-2.9 J m-2 s-1 resulted in consumption of 40-60% of the applied dose and formation of oxidation products. The major product fraction formed in UV-B-irradiated mice treated topically with alpha-TH contained an alpha-TH dihydroxy dimer and its two-electron oxidation product, a spirodimer. Products previously identified as being derived from photochemical or peroxyl radical scavenging reactions of alpha-TH were also observed, including alpha-tocopherolquinone (alpha-TQ), alpha-tocopherolquinone 2, 3-epoxide (alpha-TQE 1), alpha-tocopherolquinone 5,6-epoxide (alpha-TQE 2), and 8a-(hydroperoxy)epoxytocopherones. These results indicate that topically applied alpha-TH is extensively oxidized in skin and suggest that alpha-TH photoproducts may be involved in the observed effects of topically applied vitamin E in UV-B-irradiated skin.

Animals↗
PubMed1999

Reactions of beta-carotene with cigarette smoke oxidants. Identification of carotenoid oxidation products and evaluation of the prooxidant/antioxidant effect.

Recent intervention trials reported that smokers given dietary beta-carotene supplementation exhibited an increased risk of lung cancer and overall mortality. beta-Carotene has been hypothesized to promote lung carcinogenesis by acting as a prooxidant in the smoke-exposed lung. We have examined the interactions of cigarette smoke with beta-carotene in model systems. Both whole smoke and gas-phase smoke oxidized beta-carotene in toluene to several products, including carbonyl-containing polyene chain cleavage products and beta-carotene epoxides. A major product of the reaction was identified as 4-nitro-beta-carotene, which was formed by nitrogen oxides in smoke. Both cis and all-trans isomers of 4-nitro-beta-carotene were detected. The hypothesis that smoke-driven beta-carotene autoxidation exerts prooxidant effects was tested in a liposome system. Lipid peroxidation in dilinoleoylphosphatidylcholine liposomes exposed to gas-phase smoke was modestly inhibited by the incorporation of 0.1 mol % beta-carotene. Both the lipid soluble antioxidant alpha-tocopherol and the water soluble antioxidant ascorbate were oxidized more slowly by gas-phase smoke exposure in liposomes containing beta-carotene. These data indicate that beta-carotene exerts weak antioxidant effects against smoke-induced oxidative damage in vitro. It is unlikely that a prooxidant effect of beta-carotene occurs under biologically relevant conditions or is responsible for an increased incidence of lung cancer observed in smokers who consume beta-carotene supplements.

Animals↗
PubMed1999

Antioxidant chemistry of green tea catechins. Identification of products of the reaction of (-)-epigallocatechin gallate with peroxyl radicals.

(-)-Epigallocatechin gallate (EGCG), isolated from green tea, displays antioxidant properties and is thought to act as an antioxidant in biological systems. However, the specific mechanisms of its antioxidant actions remain unclear. In this study, we have isolated and identified for the first time two reaction products of EGCG derived from its reaction with peroxyl radicals generated by thermolysis of the initiator 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). The products include a seven-membered B-ring anhydride and a novel dimer. The identification of these products provides the first unambiguous proof that the principal site of antioxidant reactions on the EGCG molecule is the trihydroxyphenyl B ring, rather than the 3-galloyl moiety. In contrast to phenoxyl radicals from simple phenolic antioxidants, an initially formed EGCG phenoxyl radical apparently does not form stable addition products with AMVN-derived peroxyl radicals. Characteristic reaction products may provide novel markers for EGCG antioxidant reactions in living systems.

Antioxidants↗
PubMed1998

Photoprotective actions of natural and synthetic melanins.

Melanins are thought to be important modulators of photochemistry in skin. Eumelanin, a black-brown pigment, is believed to protect against UV-induced photodamage, whereas pheomelanin, a red-yellow pigment, is believed to possess photosensitizing properties. To investigate the hypothesized dichotomy of melanins as both photoprotectants and photosensitizers, we examined the effects of melanins on UV-induced liposomal lipid peroxidation. Sepia melanin, a representative eumelanin, and both red hair pheomelanin and synthetic pheomelanin were employed in these studies. Both eumelanin and pheomelanin inhibited UVA/B- and UVA-induced liposomal lipid peroxidation in a concentration-dependent manner as measured by inhibition of conjugated diene formation. No change in protective properties of the melanins was observed in the presence of saturating levels of O2 during UVA irradiation. Pheomelanin irradiated with UVA/B or UVA induced superoxide-catalyzed reduction of nitroblue tetrazolium, whereas eumelanin did not. Melanins are known to bind various metals, and we examined the effect of iron on the photoproperties of melanins. Eumelanin complexed with Fe(III) did not inhibit UVA/B-induced lipid peroxidation, whereas pheomelanin complexed with Fe(III) stimulated UVA/B-induced lipid peroxidation. Thus, complexation with iron reversed the antioxidant effect of eumelanin and converted pheomelanin into a prooxidant. Analysis of lipid peroxidation products indicated that the oxidation was mediated by free radicals rather than by singlet oxygen. These data indicate that both eumelanin and pheomelanin exert antioxidant effects against UV-induced lipid peroxidation but that the prooxidant activities of pheomelanin result from pheomelanin-metal complexation.

Hair↗
PubMed1998

Effect of UVB on hydrolysis of alpha-tocopherol acetate to alpha-tocopherol in mouse skin.

We have assessed the hydrolysis of alpha-tocopherol acetate (alpha-TAc) to the active antioxidant alpha-tocopherol (alpha-TH) in mouse epidermis and in supernatant from epidermal homogenates. Topically administered alpha-TH prevents UVB photocarcinogenesis in C3H mice, whereas alpha-TAc does not. Hydrolysis in skin was monitored in mice treated topically with deuterium labeled alpha-TAc (d3-alpha-TAc). Epidermal samples were isolated from mice and analyzed for endogenous (d0-alpha-TAc) and d3-alpha-TH by gas chromatography-mass spectrometry. Within 24 h, the levels of d3-alpha-TH increased up to 10-fold over endogenous d0-alpha-TH levels; however, in mice irradiated with UVB prior to the application of d3-alpha-TAc, levels of d3-alpha-TH increased up to 30-40-fold over endogenous d0-alpha-TH. This enhancement of alpha-TAc hydrolysis increased with increasing UVB dose. Prior UVB exposure may increase hydrolysis of alpha-TAc by increasing epidermal esterase activity. Nonspecific esterase activity was measured in the 2000 x g supernatant from epidermis of unirradiated and irradiated mice. Alpha-napthyl acetate, a nonspecific esterase substrate, was converted to alpha-napthol in supernatants from unirradiated mice. Hydrolysis to alpha-napthol increased approximately 3-fold in supernatants from irradiated mice. Hydrolysis of alpha-TAc to alpha-TH also occurred in supernatant from unirradiated mice, and this hydrolysis increased approximately 3-fold in supernatant from irradiated animals. These data indicate that nonspecific esterase activity was increased by UVB in the skin, that alpha-TAc is converted to alpha-TH in the homogenate fraction containing nonspecific esterase, and that UVB exposure modulates the metabolism of alpha-TAc to alpha-TH in vivo.

Animals↗
PubMed1997

Determination of singlet oxygen-specific versus radical-mediated lipid peroxidation in photosensitized oxidation of lipid bilayers: effect of beta-carotene and alpha-tocopherol.

Photosensitized oxidation reactions damage tissue by catalyzing the formation of oxyradicals and singlet oxygen. beta-Carotene is hypothesized to exert photoprotective effects by quenching singlet oxygen formed by Type II reactions and by scavenging free radicals formed by Type I reactions. beta-Carotene antioxidant mechanisms were studied in a phospholipid membrane model of photooxidation with a new isotope dilution gas chromatography-mass spectrometry (GC-MS) assay that quantitatively distinguishes singlet oxygen-mediated and radical-mediated lipid peroxidation. This assay measures 9- and 10-hydroxylinoleate methyl esters and was used to generate photooxidation profiles for the photosensitizers methylene blue, Rose Bengal, and tetraphenylporphine. These profiles indicate a shift from Type II to Type I photooxidation mechanisms in later stages of photooxidation. beta-Carotene (0.45 mol %) inhibited singlet oxygen-mediated lipid peroxidation at early stages of methylene blue-sensitized photooxidation. Production of radical-mediated products increased faster than singlet oxygen-mediated products at later stages. beta-Carotene-5,8-endoperoxide, a specific marker for singlet oxygen oxidation of beta-carotene in solution, was unstable under the incubation conditions and was not detected in this system. alpha-Tocopherol (0.45 mol %) was ineffective in inhibiting photosensitized lipid peroxidation, whereas 4.5 mol % alpha-tocopherol inhibited almost all radical-mediated lipid peroxidation as well as early-stage singlet oxygen-mediated lipid peroxidation. Cumene hydroperoxide stimulated radical-mediated lipid peroxidation, indicating that accumulation of hydroperoxides from Type II photooxidation may enhance Type I reactions. These data suggest that singlet oxygen quenching, rather than radical scavenging reactions, accounts for the photoprotective actions of beta-carotene.

Chromatography, High Pressure Liquid↗
PubMed1997

Antioxidant reactions of vitamin E in the perfused rat liver: product distribution and effect of dietary vitamin E supplementation.

We have investigated the relationship between vitamin E (alpha-tocopherol, TH) oxidation and antioxidant protection in a perfused rat liver model. Perfusion of a male Sprague-Dawley rat liver with 2 mM tert-butylhydroperoxide (t-BuOOH) for 10 min resulted in lipid peroxidation and metabolic changes reflecting oxidative stress. Mitochondria isolated from the liver exhibited increases in state 3 and state 4 respiration and a decline in the respiratory control ratio. In livers from rats given supplementary vitamin E in the diet, TH content was 7- to 10-fold higher than in controls and lipid peroxidation and metabolic changes induced by t-BuOOH were decreased. In mitochondria from these vitamin E-supplemented livers, the t-BuOOH-induced increase in state 4 respiration was reduced and the respiratory control ratio was maintained. In livers from unsupplemented rats, t-BuOOH induced oxidation of TH to alpha-tocopherolquinone, alpha-tocopherolhydroquinone, 2,3-epoxy-alpha-tocopherolquinone, and 5,6-epoxy-alpha-tocopherolquinone, as determined by gas chromatography-mass spectrometry analysis. Yields of these products were approximately doubled by treatment of samples with dilute acid, which indicated the presence of tocopherone and epoxytocopherone precursors. Oxidation of TH in vitamin E-supplemented livers yielded the same products and the relative extent of TH oxidation appeared similar to that in unsupplemented livers. In livers from both unsupplemented and vitamin E-supplemented animals, the distribution of oxidation products was similar in whole liver and isolated mitochondria. These data provide the first simultaneous documentation of TH antioxidant reactions and antioxidant effects in an intact organ system during oxidative stress.

Animals↗
PubMed1997

Antioxidant actions of beta-carotene in liposomal and microsomal membranes: role of carotenoid-membrane incorporation and alpha-tocopherol.

beta-Carotene and other carotenoids are widely regarded as biological antioxidants. However, recent clinical trials indicate that beta-carotene supplements are not effective in disease prevention and raise questions about the biological significance of carotenoid antioxidant actions. To further explore this issue, we have reevaluated the antioxidant actions of beta-carotene in liposomal and biological membrane systems. In dilinoleoylphosphatidylcholine liposomes in which 0.35 mol % beta-carotene was incorporated into the bilayer during liposome preparation, the carotenoid inhibited lipid peroxidation initiated by 10 mm azobis[amidinopropane HCl] (AAPH). In carotenoid-free liposome suspensions to which the same amount of beta-carotene was added, no antioxidant effect was observed. Supplementation of rat liver microsomes with beta-carotene in vitro yielded microsomes containing 1.7 nmol beta-carotene mg-1 and 0.16 nmol alpha-tocopherol mg-1 microsomal protein. In beta-carotene supplemented microsomes incubated with 10 mm AAPH under an air atmosphere, lipid peroxidation did not occur until alpha-tocopherol was depleted by approximately 60%. beta-Carotene exerted no apparent antioxidant effect and was not significantly depleted in the incubations. Similar results were obtained when the incubation was done at 3.8 torr O2. In liver microsomes from Mongolian gerbils fed beta-carotene-supplemented diets, beta-carotene levels were 16-37% of alpha-tocopherol levels. The kinetics of AAPH-induced lipid peroxidation were no different in beta-carotene-supplemented microsomes than in microsomes from unsupplemented animals, although the kinetics of beta-carotene and alpha-tocopherol depletion were similar. The results indicate that beta-carotene is ineffective as an antioxidant when added to preformed lipid bilayer membranes and that alpha-tocopherol is a much more effective membrane antioxidant than beta-carotene, regardless of the method of carotenoid-membrane incorporation. These results support a reevaluation of the proposed antioxidant role for beta-carotene in biological membranes.

Animals↗
PubMed1997

beta-carotene: friend or foe?

This symposium focused on the research which documents benefit and toxicity in beta-carotene supplementation. Reflecting on past and current studies, the panel of experts discussed: (1) the potential harm of a high intake of beta-carotene on selected populations, (2) biochemical antioxidant/prooxidant mechanisms of beta-carotene at the cellular level, (3) potential benefits of other carotenoids and antioxidants, and (4) future directions for research in beta-carotene and other antioxidants.

Animals↗
PubMed1997

UVB induced photooxidation of vitamin E.

The photochemistry of alpha-tocopherol (alpha-TH, vitamin E) may contribute to its inhibition of UVB (290-320 nm) photocarcinogenesis. Photochemical reactions of alpha-TH were studied by monitoring the fate of alpha-TH in UVB irradiated liposomes and solution. Soy phosphatidylcholine (SPC) and dioleoylphosphatidylcholine (DOPC) liposomes were supplemented with alpha-TH (1.0 mol % alpha-TH/phospholipid) and irradiated with UVB at a dose rate of 6.0 J m-2s-1 for up to 90 min. alpha-TH was rapidly depleted in UVB irradiated liposomes. Oxidative damage, assessed by monitoring lipid peroxidation, was suppressed in SPC liposomes until alpha-TH was depleted to 20% of initial levels. alpha-TH also was rapidly depleted by UVB irradiation in acetonitrile/H2O (4:1 v/v) solution. In SPC liposomes, products previously identified as marker products for peroxyl radical scavenging by alpha-TH were observed, including alpha-tocopherol quinone, 5,6-epoxy-alpha-tocopherol quinone, and 2,3-epoxy-alpha-tocopherol quinone. These products also were formed in DOPC liposomes, which are resistant to lipid peroxyl radical formation. In addition, an alpha-tocopherol dihydroxy dimer and several 8a-(hydroperoxy)epoxytocopherones were identified by HPLC and HPLC-MS. The dimer appears to result from recombination of photoinduced tocopheroxyl radicals. Products associated with peroxyl radical scavenging (quinones, epoxyquinones, 8a-(hydroperoxy)epoxytocopherones) and with UVB dependent production of tocopheroxyl radicals (dihydroxy dimer) also were found when alpha-TH was oxidized by UVB in acetonitrile. Because the acetonitrile contained no autoxidizable substrate, formation of peroxyl radical derived products may occur via intermediate tocopherone peroxyl radicals. These results indicate that alpha-TH photooxidation proceeds via competing reactions of UVB induced tocopheroxyl radicals.

Carbon Radioisotopes↗
PubMed1997

Reactions of peroxynitrite with gamma-tocopherol.

The reaction of peroxynitrite with gamma-tocopherol (gamma-TH) in a methanol/potassium phosphate buffer solution results in the formation of four major products, which were identified as 2,7,8-trimethyl-2-(4,8,12-trimethyldecyl)-5-nitro-6-chromanol++ + (NGT), 2,7,8-trimethyl-2-(4,8,12-trimethyldecyl)-5,6-chromaquinone (tocored), and two diastereomers of 8a-(hydroxy)-gamma-tocopherone. NGT was the major product formed in these reactions, and its formation was modestly increased by increasing amounts of Fe(3+)-EDTA. Tocored and NGT also were formed when gamma-TH was exposed to 3-morpholinosydnonimine (SIN-1), a compound that decomposes to form peroxynitrite. When gamma-TH reacted with the nitrating agent NO2+BF4- in acetonitrile or methanol/potassium phosphate buffer, NGT and tocored also were formed, but the major product detected was gamma-tocopherol quinone (gamma-TQ). This product was not detected in reactions involving peroxynitrite. Oxidation of gamma-TH by peroxynitrite involves nitration and electron transfer reactions. Since the product distribution in oxidations with NO2+BF4- differed substantially from that in oxidations with peroxynitrite and SIN-1, NO2+ appeared not to be the principal species involved in NGT formation. Nitration of gamma-TH may involve either peroxynitrite or some peroxynitrite-derived oxidant other than NO2+. Because of its stability and formation as a novel product of the reaction between gamma-TH with peroxynitrite, NGT may be a useful in vivo marker for peroxynitrite interactions with lipid structures that contain gamma-TH.

Magnetic Resonance Spectroscopy↗
PubMed1997

Inhibition of UVB induced DNA photodamage in mouse epidermis by topically applied alpha-tocopherol.

Ultraviolet B (UVB, 290-320 nm) exposure results in a variety of cellular insults including induction of cyclobutane pyrimidine dimers in DNA. Accumulation of these lesions can lead to mutations in critical genes and contribute to the development of nonmelanoma skin cancer. Topically applied alpha-tocopherol (vitamin E) has previously been shown to prevent the induction of skin tumors in UVB irradiated female C3H/HeNTac mice. We hypothesized that alpha-tocopherol, which absorbs strongly in the UVB, may act as a sunscreen to prevent photodamage. To explore possible mechanisms of photoprotection, we topically applied alpha-tocopherol dispersed in a neutral cream vehicle to the dorsal epidermis of female C3H/HeNTac mice and exposed them to 2.5 J/m2/s of UVB for 60 min. Immediately after exposure, we analyzed thymine dimer levels in DNA by capillary gas chromatography with electron capture detection. Epidermal DNA from mice receiving this UVB dose contained 247 +/- 42 pmol thymine dimers/micromol thymine. Topical application of alpha-tocopherol inhibited dimer formation in a dose-dependent manner. A 1% alpha-tocopherol dispersion inhibited the formation of thymine dimers to 43% of levels in vehicle controls. Several vitamin E compounds, including alpha-tocopherol acetate, alpha-tocopherol methyl ether, gamma-tocopherol, and delta-tocopherol also inhibited thymine dimer formation, but were five- to ten-fold less potent than alpha-tocopherol. A variety of commercially available sunscreens were also less potent than alpha-tocopherol in their ability to reduce dimer formation. These results suggest that DNA photoprotection is an important mechanism by which topically applied alpha-tocopherol can inhibit UVB induced skin cancer. Alpha-Tocopherol acetate, the most common form of vitamin E in commercial skin care products, conferred less protection, perhaps due to its lower absorptivity in the UVB. Our results further underscore the importance of determining which forms of vitamin E can inhibit specific lesions involved in photocarcinogenesis.

4-Aminobenzoic Acid↗
PubMed1997

The reduction of alpha-tocopherolquinone by human NAD(P)H: quinone oxidoreductase: the role of alpha-tocopherolhydroquinone as a cellular antioxidant.

alpha-Tocopherolquinone (TQ), a product of alpha-tocopherol oxidation, can function as an antioxidant after reduction to alpha-tocopherolhydroquinone (TQH2). We examined the ability of human NAD(P)H:quinone oxidoreductase (NQO1) to catalyze the reduction of TQ to TQH2 in cell-free and cellular systems. In reactions with purified human NQO1, TQ was reduced to TQH2. Kinetic parameters for the reduction of TQ by NQO1 (Km = 370 microM; k(cat) = 5.6 x 10(3) min(-1); k(cat)/Km = 15 min(-1) x microM(-1)) indicate that NQO1 can efficiently reduce TQ to TQH2. A comparison of the rate of reduction of TQ and coenzyme Q10 by NQO1 showed that TQ is reduced more efficiently than coenzyme Q10. Experiments with either Chinese hamster ovary (CHO) cells stably transfected with human NQO1 or CHO cell sonicates demonstrated a correlation between NQO1 activity and TQ reduction to TQH2. CHO cells with elevated NQO1 generated and maintained higher levels of TQH2 after treatment with TQ relative to NQO1-deficient CHO cells. TQH2 generated from NQO1-mediated reduction of TQ prevented cumene hydroperoxide-induced lipid peroxidation in rat liver microsomes. In addition, cumene hydroperoxide-induced lipid peroxidation was inhibited more efficiently by TQ in CHO cell lines with elevated NQO1 activity. These data demonstrate that NQO1 can reduce TQ to TQH2 and that TQH2 can function as an efficient antioxidant. This work suggests that one of the physiological functions of NQO1 may be to regenerate antioxidant forms of alpha-tocopherol.

Animals↗
PubMed1996

Gas chromatography-mass spectrometry analysis of vitamin E and its oxidation products.

To facilitate studies of vitamin E ( alpha-tocopherol) antioxidant actions in tissues, we have developed a stable isotope dilution capillary gas chromatography-mass spectrometry assay for alpha-tocopherol and its three principal oxidation products, alpha-tocopherolquinone, 5,6-epoxy-alpha-tocopherolquinone, 2, 3-epoxy-alpha-tocopherolquinone, and for alpha-tocopherolhydroquinone, a reduction product of alpha-tocopherolquinone. Deuterium-labeled internal standards (5, 7-[2H3-methyl]-alpha-tocopherol, 2,6-[2H3-methyl]- alpha-tocopherolquinone, 2,6-[2H3-methyl]-5,6-epoxy- alpha-tocopherolquinone, 2,6-[2H3-methyl]-2,3-epoxy- alpha-tocopherolquinone, and 2-[2H3-methyl]- alpha-tocopherolhydroquinone are added to cell or tissue homogenates. The products then are extracted and converted to O-trimethylsilyl derivatives. Products are analyzed by capillary gas chromatography with on-column injection and detected by selected ion monitoring of characteristic fragment ions in electron ionization mode. Standard curves were linear from 25 fmol to 2 pmol for products and from 25 fmol to 4 pmol for alpha-tocopherol. The use of 2H6- and 2H3-internal standards for alpha-tocopherolquinone and alpha-tocopherolhydroquinone permits simultaneous analysis of both products despite possible redox interconversion during sample workup. alpha-Tocopherol oxidized in microsomes treated with azo-bis(amidinopropane HCl) was quantitatively accounted for as the epoxyquinones, alpha-tocopherolquinone, and alpha-tocopherolhydroquinone. However, over half of the oxidation products were present in microsomes as acid-labile tocopherone precursors. This method permits comprehensive assessment of vitamin E status in tissues and quantitative studies of vitamin E antioxidant actions and turnover.

Animals↗
PubMed1996

3,3',4,4'-tetrachloroazobenzene absorption, disposition, and metabolism in male Fischer 344 rats.

3,3',4,4'-Tetrachloroazobenzene (TCAB) is a contaminant generated during the synthesis of 3,4-dichloroaniline and 3,4-dichloroaniline-derived pesticides. TCAB is isosteric to 2,3,7,8-tetrachlorodibenzo-p-dioxin and has been shown to bind to the Ah receptor. Following oral administration of [14C]TCAB (3.2 and 32 mg/kg), 39-45% of the dosed radioactivity was excreted into the urine and 53-56% was recovered in the feces within 48 hr. Less than 6% of the dosed radioactivity remained in the tissues examined at 96 hr. After intravenous administration (3.2 mg/kg), 33% of the dose was excreted in the bile during 6 hr. TCAB metabolites in urine were identified using LC/MS. The major metabolites were sulfate ester conjugates of hydroxylated mono- or dichloroaniline derivatives. Some of these metabolites were also acetylated. After intravenous administration, the disappearance of [14C]TCAB from blood was monitored, and the pharmacokinetic profile was consistent with a two-compartment model. Pharmacokinetic parameters reveal that the compound is readily cleared from the blood with a t1/2 of 4.0 hr, clearance of 12.3 ml/min.kg, and an apparent volume of distribution of 4.3 liters/kg. The absolute oral bioavailability was determined to be 30%. The extensive azo reduction of TCAB decreases its systemic absorption after oral administration and thereby limits the amount of parent compound available to interact with the Ah receptor and decreases the Ah receptor-mediated toxicity.

Administration, Oral↗