Evaluation of serum and liver toxicokinetics for furan and liver DNA adduct formation in male Fischer 344 rats

Furan is a food processing contaminant found in many common cooked foods that induces liver toxicity and liver cancer in animal models treated with sufficient doses. The metabolism of furan occurs primarily in the liver where CYP 2E1 produces a highly reactive bis-electrophile, cis-2-butene-1,4-dial (BDA). BDA reacts with nucleophilic groups in amino acids and DNA in vitro to form covalent adducts. Evidence for BDA-nucleoside adduct formation in vivo is limited but important for assessing the carcinogenic hazard of dietary furan. This study used controlled dosing with furan in Fischer 344 rats to measure serum and liver toxicokinetics and the possible formation of BDA-nucleoside adducts in vivo. After gavage exposure, furan concentrations in the liver were consistently higher than those in whole blood (∼6-fold), which is consistent with portal vein delivery of a lipophilic compound into the liver. Formation of BDA-2'-deoxycytidine in furan-treated rat liver DNA was not observed using LC/MS/MS after single doses as high as 9.2 mg/kg bw or repeated dosing for up to 360 days above a consistent background level (1-2 adducts per 10(8) nucleotides). This absence of BDA-nucleoside adduct formation is consistent with the general lack of evidence for genotoxicity of furan in vivo.

The effects of chronic methylphenidate administration on operant test battery performance in juvenile rhesus monkeys

Methylphenidate (MPH) is an amphetamine derivative widely prescribed for the treatment of attention deficit-hyperactivity disorder. Recent concern over its genotoxic potential in children [11] spurred a study on the effects of chronic MPH treatment in a nonhuman primate population and the studies reported here were conducted in conjunction with that study in the same animals. Here, the focus was on the ability of juvenile rhesus monkeys to learn how to perform a battery of operant behavioral tasks while being treated chronically with MPH. Performance of the National Center for Toxicological Research (NCTR) Operant Test Battery (OTB) was used to quantify the learning of tasks thought to model specific aspects of cognitive function including learning, motivation, color and position discrimination, and short-term memory. The OTB tasks designed to assess these specific behaviors included Incremental Repeated Acquisition (IRA), Progressive Ratio (PR), Conditioned Position Responding (CPR), and Delayed Matching-to-Sample (DMTS), respectively. Juvenile males (n=10/group) pressed levers and press-plates for banana-flavored food pellets. Subjects were treated orally, twice a day, five days per week (M-F) for 66 weeks with escalating doses (0.15 mg/kg initially, increased to 2.5 mg/kg for the low dose group and to 12.5 mg/kg for the high dose group) and tested in OTB tasks 30 to 60 min after the morning dose. The findings indicate that MPH at doses up to 2.5 mg/kg twice per day were well tolerated (performance was no different than controls) but at doses of 12.5 mg/kg twice per day there was a significant decrement in OTB performance, characterized by decreases in both percent task completed and response rates for all tasks. These effects of MPH seem primarily due to decreases in motivation to perform for food, which is not surprising given the well known appetite suppressing effects of amphetamine-like stimulants. Thus, the current data do not strongly suggest cognitive impairments following chronic MPH administration.

Effect of dietary genistein on cell replication indices in C57BL6 mice

The phytoestrogen and isoflavone, genistein, inhibited the activity of the DNA synthesis-related enzyme, topoisomerase-II (topo-II), altered cell-cycle traverse and produced cell death in cell culture models. In order to examine the potential effects of genistein on cell replication and cell death in an animal model, 8-week-old C57BL6 mice were fed either a control diet or one containing one of five doses (100-2000 ppm) of genistein for 28 days. At the end of the feeding period, both male and female mice were sacrificed and the serum isoflavone and aglycone levels determined by liquid chromatography with electrospray tandem mass spectrometry (LC-ES/MS/MS). Immunohistochemistry was utilized to measure the cell replication and cell death rates in the small intestine. Total isoflavone concentration increased from below the limits of detection (0.001 microM) in control animals to 0.28 microM in male and 0.54 microM in female mice fed the 2000 ppm diet. A decrease in the percentage of cells in G(0) and an increase in the percentage of cells in S-phase, consistent with topo-II-induced S-phase arrest, was found in the duodenum and jejunum of the small intestine. Thus, genistein appears to accumulate to a sufficient level to affect topo-II activity in the small intestine.

Analysis of DNA adducts from chemical carcinogens and lipid peroxidation using liquid chromatography and electrospray mass spectrometry

The identification and dosimetry of DNA adducts are cornerstones of research on cancer etiology in experimental animals and humans. DNA adducts can result from exposure to exogenous chemical carcinogens or through reactions with endogenous by-products of oxidative metabolism. An important research need is high throughput methodology for quantification of any and all adducts that are present at trace amounts in DNA derived from target tissues of animals and humans. This review describes some recent progress made through applications of liquid chromatography coupled with mass spectrometry to structural characterization of unknown DNA adducts and highly sensitive quantitative analysis of target adducts.

Degradation of biphenyl by Mycobacterium sp. strain PYR-1

The metabolism of biphenyl by Mycobacterium sp. PYR-1 was investigated. The Mycobacterium sp. degraded >98% of the biphenyl added within 72 h. Analysis of ethyl acetate extracts of the culture medium by HPLC indicated that benzoic acid was the major metabolite. Other products were 4-hydroxybiphenyl, 4-hydroxybenzoic acid, and 5-oxo-5-phenylpentanoic acid. The metabolites were characterized by mass and 1H NMR spectrometry. Identification of benzoic acid and 5-oxo-5-phenylpentanoic acid indicates that biphenyl degradation by Mycobacterium sp. PYR-1 is generally similar to known pathways. A novel alternative metabolic pathway consisted of monooxygenation at C-4 of biphenyl to give 4-hydroxybiphenyl, with subsequent degradation via ring cleavage to 4-hydroxybenzoic acid.

Physiological concentrations of dietary genistein dose-dependently stimulate growth of estrogen-dependent human breast cancer (MCF-7) tumors implanted in athymic nude mice

Previously our laboratory has shown that the soy isoflavone, genistein, stimulates growth of human breast cancer (MCF-7) cells in vivo and in vitro. In this study, the dose-response analysis of genistein at the physiologically achievable concentration range between 125 and 1,000 microg/g in the diet was conducted in ovariectomized athymic nude mice implanted with MCF-7 cells. We hypothesized that genistein at this concentration range can stimulate dose-dependently the breast tumor growth, cell proliferation and an estrogen-responsive pS2 gene induction. Tumor size and body weight were monitored weekly. At completion of the study, we analyzed cellular proliferation of tumors using incorporation of BrdU, pS2 expression of tumors using a Northern blot analysis and total genistein level in plasma using liquid chromatography-isotope dilution mass spectrometry (LC-ES/MS). Dietary genistein (> or = 250 microg/g) increased tumor size in a dose-dependent manner [8.4x the negative control (NC) group in the 250 microg/g group, 12.0x in the 500 microg/g group, 20.2x in the 1,000 microg/g group and 23.2x in the positive control (PC) group]. The percentage of proliferating cells was significantly increased by genistein at and above 250 microg/g (5.3x the NC group in the 250 microg/g, 5.6x in the 500 microg/g, 5.0x in the 1,000 microg/g and 4.8x in the PC group). Expression of pS2 mRNA was also significantly increased with increasing dietary genistein levels (11.25x the NC group in the 500 microg/g group and 15.84x in the 1,000 microg/g group). Total plasma genistein concentrations were between 0.39 and 3.36 micromol/L in mice fed between 125 and 1,000 microg/g genistein. In conclusion, dietary treatment with genistein at physiological concentrations produces blood levels of genistein sufficient to stimulate estrogenic effects, such as breast tumor growth, cellular proliferation and pS2 expression in athymic mice in a dose-responsive manner similar to that seen in vitro.

Effect of soy isoflavone supplementation on markers of oxidative stress in men and women

Dietary intake of soy has been linked with decreased cancer risk, and the active compounds in soy that have been identified include the isoflavones genistein and daidzein. Since these compounds have antioxidant properties, we examined levels of oxidative damage in blood of six women and six men before and during soy supplementation using Novasoy tablets. Blood samples were obtained at weekly intervals for 3 weeks from the women taking 50-mg isoflavones once daily and the men taking 50-mg isoflavones twice daily. Plasma levels of genistein and daidzein increased after supplementation with maximal levels occurring at 2 weeks for the women while levels in men kept increasing over the 3 weeks of study. There was wide variation between individuals in the levels of isoflavones achieved. Mean levels of 5-hydroxymethyl-2'-deoxyuridine (5-OHmdU) in DNA from nucleated blood cells decreased after 1 week of supplementation in the women, with a decrease of 47% in mean 5-OHmdU levels after 3 weeks. In men, mean 5-OHmdU levels did not decrease until after 3 weeks of supplementation, at which there was 61% decrease. Mean plasma levels of 8-isoprostanes were not changed appreciably in either men or women. These pilot results suggest that soy isoflavone supplementation decreases levels of oxidative DNA damage in humans, and this may be a mechanism behind the cancer-preventive effects of soy isoflavones.

Metabolism of anthracene by a Rhodococcus species

A Rhodococcus sp. isolated from contaminated river sediment was investigated to determine if the isolate could degrade high molecular mass polycyclic aromatic hydrocarbons. The Rhodococcus sp. was able to utilize anthracene (53%), phenanthrene (31%), pyrene (13%), and fluoranthene (5%) as sole source of carbon and energy, but not naphthalene or chrysene. In a study of the degradation of anthracene by a Rhodococcus sp., the identification of ring-fission products indicated at least two ring-cleavage pathways. One results in the production of 6,7-benzocoumarin, previously shown to be produced chemically from the product of meta cleavage of 1,2-dihydroxyanthracene, a pathway which has been well established in Gram-negative bacteria. The second is an ortho cleavage of 1,2-dihydroxyanthracene that produces 3-(2-carboxyvinyl)naphthalene-2-carboxylic acid, a dicarboxylic acid ring-fission product. This represents a novel metabolic pathway only identified in Gram-positive bacteria.

Biotransformation of malachite green by the fungus Cunninghamella elegans

The filamentous fungus Cunninghamella elegans ATCC 36112 metabolized the triphenylmethane dye malachite green with a first-order rate constant of 0.029 micromol x h(-1) (mg of cells)(-1). Malachite green was enzymatically reduced to leucomalachite green and also converted to N-demethylated and N-oxidized metabolites, including primary and secondary arylamines. Inhibition studies suggested that the cytochrome P450 system mediated both the reduction and the N-demethylation reactions.

Molecular cloning, nucleotide sequence, and expression of genes encoding a polycyclic aromatic ring dioxygenase from Mycobacterium sp. strain PYR-1

Mycobacterium sp. strain PYR-1 degrades high-molecular-weight polycyclic hydrocarbons (PAHs) primarily through the introduction of both atoms of molecular oxygen by a dioxygenase. To clone the dioxygenase genes involved in PAH degradation, two-dimensional (2D) gel electrophoresis of PAH-induced proteins from cultures of Mycobacterium sp. strain PYR-1 was used to detect proteins that increased after phenanthrene, dibenzothiophene, and pyrene exposure. Comparison of proteins from induced and uninduced cultures on 2D gels indicated that at least six major proteins were expressed (105, 81, 52, 50, 43, and 13 kDa). The N-terminal sequence of the 50-kDa protein was similar to those of other dioxygenases. A digoxigenin-labeled oligonucleotide probe designed from this protein sequence was used to screen dioxygenase-positive clones from a genomic library of Mycobacterium sp. strain PYR-1. Three clones, each containing a 5,288-bp DNA insert with three genes of the dioxygenase system, were obtained. The genes in the DNA insert, from the 5' to the 3' direction, were a dehydrogenase, the dioxygenase small (beta)-subunit, and the dioxygenase large (alpha)-subunit genes, arranged in a sequence different from those of genes encoding other bacterial dioxygenase systems. Phylogenetic analysis showed that the large alpha subunit did not cluster with most of the known alpha-subunit sequences but rather with three newly described alpha subunits of dioxygenases from Rhodococcus spp. and Nocardioides spp. The genes from Mycobacterium sp. strain PYR-1 were subcloned and overexpressed in Escherichia coli with the pBAD/ThioFusion system. The functionality of the genes for PAH degradation was confirmed in a phagemid clone containing all three genes, as well as in plasmid subclones containing the two genes encoding the dioxygenase subunits.

Gender-based differences in rats after chronic dietary exposure to genistein

Gender-based differences can be observed from pharmacokinetic, behavioral, or anatomical assessments. No single assessment tool will provide a complete answer, but the use of a variety of indices, each with known gender-related outcome differences, can reveal agent-induced gender-based alterations. In a series of initial range-finding studies in rats conducted at the National Center for Toxicological Research (NCTR), the effects of dietary exposure to the weak estrogen, genistein, have been assessed using a number of techniques with validated gender-related outcome measures. The findings indicated that (1) the internal dose of genistein was higher in females than males after equivalent dietary exposure and this was consistent with the faster rate of genistein elimination in males; (2) in behavioral assessments, males and females in the high-dose dietary genistein group consumed more of a sodium-flavored solution; however, no genistein-related changes were observed in open field or running wheel activity, play behavior, or intake of a saccharin-flavored solution; and (3) dose-related alterations of the volume of the sexually dimorphic nucleus of the medial preoptic area were observed in genistein-exposed male rats but not females. These observations describe the utility of a variety of gender-based assessment tools and indicate that dose-related effects of developmental and chronic dietary exposure to genistein can be observed in the rodent. Additional studies, perhaps in nonhuman primates, are necessary to further predict the effect(s) of genistein on human gender-based development.

Degradation of phenanthrene and anthracene by cell suspensions of Mycobacterium sp. strain PYR-1

Cultures of Mycobacterium sp. strain PYR-1 were dosed with anthracene or phenanthrene and after 14 days of incubation had degraded 92 and 90% of the added anthracene and phenanthrene, respectively. The metabolites were extracted and identified by UV-visible light absorption, high-pressure liquid chromatography retention times, mass spectrometry, (1)H and (13)C nuclear magnetic resonance spectrometry, and comparison to authentic compounds and literature data. Neutral-pH ethyl acetate extracts from anthracene-incubated cells showed four metabolites, identified as cis-1,2-dihydroxy-1,2-dihydroanthracene, 6,7-benzocoumarin, 1-methoxy-2-hydroxyanthracene, and 9,10-anthraquinone. A novel anthracene ring fission product was isolated from acidified culture media and was identified as 3-(2-carboxyvinyl)naphthalene-2-carboxylic acid. 6,7-Benzocoumarin was also found in that extract. When Mycobacterium sp. strain PYR-1 was grown in the presence of phenanthrene, three neutral metabolites were identified as cis- and trans-9,10-dihydroxy-9,10-dihydrophenanthrene and cis-3,4-dihydroxy-3,4-dihydrophenanthrene. Phenanthrene ring fission products, isolated from acid extracts, were identified as 2,2'-diphenic acid, 1-hydroxynaphthoic acid, and phthalic acid. The data point to the existence, next to already known routes for both gram-negative and gram-positive bacteria, of alternative pathways that might be due to the presence of different dioxygenases or to a relaxed specificity of the same dioxygenase for initial attack on polycyclic aromatic hydrocarbons.

Placental transfer of the soy isoflavone genistein following dietary and gavage administration to Sprague Dawley rats

Genistein, the principal soy isoflavone, has estrogenic activity and is widely consumed by humans for putative beneficial health effects. The goal of the present study was to measure placental transfer of genistein in rats as a possible route of developmental exposure. Pregnant Sprague-Dawley rats were administered genistein orally, either by diet or by gavage. Concentrations of genistein aglycone and conjugates were measured in maternal and offspring serum and brain using HPLC with isotope dilution electrospray tandem mass spectrometry. Although fetal or neonatal serum concentrations of total genistein were approximately 20-fold lower than maternal serum concentrations, the biologically active genistein aglycone concentration was only 5-fold lower. Fetal brain contained predominately genistein aglycone at levels similar to those in the maternal brain. These studies show that genistein aglycone crosses the rat placenta and can reach fetal brain from maternal serum genistein levels that are relevant to those observed in humans.

Quantitative analysis of etheno-2'-deoxycytidine DNA adducts using on-line immunoaffinity chromatography coupled with LC/ES-MS/MS detection

Etheno DNA adducts, including 3,N4-etheno-2'-deoxycytidine (etheno-dC), are promutagenic lesions present in normal animal and human tissues. These DNA adducts are believed to be important in the etiology of cancer. Existing methods for quantifying etheno-dC use 32p. postlabeling. Although highly sensitive, postlabeling requires the use of an energetic radioisotope and considerable time and effort. The new methodology reported here permits automated quantification of trace levels of etheno-dC in crude DNA hydrolysates on the order of 5 adducts in 10(8) normal nucleotides from 100-microg samples of DNA. This was accomplished by using on-line immunoaffinity chromatography, a reverse-phase LC separation on graphitized carbon, tandem mass spectrometric detection, and an isotopically labeled internal standard. The automated procedures permitted analysis of 4 DNA hydrolysates/hr. The sensitivity using immunoaffinity cleanup was approximately 100-fold greater than that observed when using a silica-based trapping system. The validated method was applied to the analysis of etheno-dC in commercial calf thymus DNA, untreated mouse liver, and untreated rat liver DNA. The demonstrated level of performance suggests future applicability of this method in studies of cancer in humans and experimental animals.

Development of a (32)P-postlabeling/HPLC method for detection of dehydroretronecine-derived DNA adducts in vivo and in vitro

Pyrrolizidine alkaloids are naturally occurring genotoxic chemicals produced by a large number of plants. Metabolism of pyrrolizidine alkaloids in vivo and in vitro generates dehydroretronecine (DHR) as a common reactive metabolite. In this study, we report the development of a (32)P-postlabeling/HPLC method for detection of (i) two DHR-3'-dGMP and four DHR-3'-dAMP adducts and (ii) a set of eight DHR-derived DNA adducts in vitro and in vivo. The approach involves (1) synthesis of DHR-3'-dGMP, DHR-3'-dAMP, and DHR-3',5'-dG-bisphosphate standards and characterization of their structures by mass and (1)H NMR spectral analyses, (2) development of optimal conditions for enzymatic DNA digestion, adduct enrichment, and (32)P-postlabeling, and (3) development of optimal HPLC conditions. Using this methodology, we have detected eight DHR-derived DNA adducts, including the two epimeric DHR-3',5'-dG-bisphosphate adducts both in vitro and in vivo.

Metabolic activation of the tumorigenic pyrrolizidine alkaloid, riddelliine, leading to DNA adduct formation in vivo

Riddelliine is a representative naturally occurring genotoxic pyrrolizidine alkaloid. We have studied the mechanism by which riddelliine induces hepatocellular tumors in vivo. Metabolism of riddelliine by liver microsomes of F344 female rats generated riddelliine N-oxide and dehydroretronecine (DHR) as major metabolites. Metabolism was enhanced when liver microsomes from phenobarbital-treated rats were used. Metabolism in the presence of calf thymus DNA resulted in eight DNA adducts that were identical to those obtained from the reaction of DHR with calf thymus DNA. Two of these adducts were identified as DHR-modified 7-deoxyguanosin-N(2)-yl epimers (DHR-3'-dGMP); the other six were DHR-derived DNA adducts, but their structures were not characterized. A similar DNA adduct profile was detected in the livers of female F344 rats fed riddelliine, and a dose-response relationship was obtained for the level of the total (eight) DHR-derived DNA adducts and the level of the DHR-3'-dGMP adducts. These results suggest that riddelliine induces liver tumors in rats through a genotoxic mechanism and the eight DHR-derived DNA adducts are likely to contribute to liver tumor development.

Quantification of etheno-DNA adducts using liquid chromatography, on-line sample processing, and electrospray tandem mass spectrometry

Etheno-DNA adducts are promutagenic lesions present in normal animal and human tissues that are believed to be important in the etiology of cancer related to diet and lifestyle. A method has been developed for the quantification of trace levels of etheno-DNA adducts using on-line sample preparation coupled with liquid chromatography and electrospray tandem mass spectrometry. The use of automated solid-phase extraction and stable labeled internal standards permitted the robust determination of ethenodeoxyadenosine contained in crude DNA hydrolysates from untreated rodent and human tissues at levels on the order of one adduct in 10(8) normal nucleotides from 100 microg of DNA. Inherent analyte response and matrix interference made sensitivity for simultaneous determination of ethenodeoxycytidine approximately 5-fold lower. The method was applied to the analysis of liver DNA from untreated and urethane-treated B6C3F1 mice, untreated rat liver, human placenta, and several commercial DNA preparations. Some sources of potential artifactual formation of etheno-DNA adducts were investigated.

Dietary genistein inactivates rat thyroid peroxidase in vivo without an apparent hypothyroid effect

Biological effects of genistein are currently under investigation by the National Toxicology Program because of widespread and increasing soy consumption by humans and evidence for modulation of endocrine function. Rats were exposed to genistein aglycone in soy-free feed fortified at 0, 5, 100, and 500 ppm starting in utero through 20 weeks. Thyroid glands and serum were analyzed for total genistein (aglycone + conjugates) using HPLC with electrospray mass spectrometric detection. Microsomal thyroid peroxidase (TPO) activity was measured spectrophotometrically. The total genistein content in rat serum was as high as 8 microM, and significant dose-dependent increases of genistein in thyroid tissue up to 1 pmol/mg were found in male and female rats. The activity of TPO in male and female rats was found to be reduced by up to 80% in a dose-dependent manner. Male and female rats consuming a standard soy-based rodent diet (NIH 31) had TPO activity approximately 50% lower than rats consuming a soy-free diet and this loss was commensurate with measured serum levels of isoflavones. Suicide inactivation of rat, porcine, and human TPO was observed in vitro at concentrations of genistein aglycone comparable to those measured in rat thyroids. Thyroid hormone levels (T3, T4, TSH) in serum, thyroid weights, and histopathology showed no differences between treated and untreated groups. These findings suggest that, even though substantial amounts of TPO activity are lost concomitant to soy isoflavone consumption by normal rats, the remaining enzymatic activity is sufficient to maintain thyroid homeostasis in the absence of additional perturbations.

DNA adduct formation by Fusarium culture extracts: lack of role of fusarin C

Fusarium fungi have been shown to infect corn and other crops worldwide, and have a significant impact on human health through loss of crops or contamination of food with mycotoxins. Isolates of Fusarium fungi from an area of South Africa with high incidence of esophageal cancer have been shown to induce esophageal and liver cancer in rats. Several isolates of Fusarium fungi were grown on corn to determine if genotoxic products were produced. We report the incubation of methanol extracts of Fusarium verticillioides cultures with DNA in the presence of rat liver fractions (S9) resulted in the formation of a unique DNA adduct that was detected by (32)P-postlabeling. Fusarin C was purified from cultures of Fusarium verticillioides RRC 415, and was not responsible for the formation of the DNA adduct. Treatment of the methanolic extracts with ultraviolet B radiation reduced the fusarin C content in the extract; however, this had no effect on the formation of the DNA adduct following incubation of the extract with DNA and S9. The unique DNA adduct was formed following the incubation of several Fusarium verticillioides isolates from the US and South Africa, while extracts of cultures of Fusarium graminearium and Fusarium sacchari isolates formed very little of the DNA adduct when incubated with DNA and S9. These data suggest that neither fusarin C nor any of its metabolites are responsible for formation of the DNA adduct, and that an unidentified compound is present in F. verticillioides cultures that forms a DNA adduct, and may be important in the etiology of human esophageal cancer.

A simple colorimetric assay for phenotyping the major human thermostable phenol sulfotransferase (SULT1A1) using platelet cytosols

A thermostable phenol sulfotransferase, SULT1A1, has been implicated in numerous detoxification and bioactivation pathways; however, little is known regarding its endogenous function or its putative role in mediating risk for human environmental disease. A simple endpoint colorimetric assay is described that can be used for rapid phenotyping of SULT1A1 activity in human populations. The assay utilizes a microtiter-plate format and relatively small amounts of platelet cytosol-derived enzyme. The enzyme catalyzes the synthesis of 2-naphthylsulfate from 2-naphthol and 5'-phosphoadenosine 3'-phosphosulfate (PAPS), whereas addition of p-nitrophenyl sulfate to the assay contributes to an effective PAPS-regenerating system. In contrast to other sulfotransferase assay methods, 3'-phosphoadenosine 5'-phosphate (PAP) does not accumulate during the incubation to interfere with enzyme activity, but instead serves as a cofactor to cause the removal of sulfate from p-nitrophenyl sulfate to regenerate PAPS. This reaction concomitantly results in generation of p-nitrophenol that can be quantified colorimetrically at 405 nm (epsilon = 18,200 M(-1)) to give an indirect measure of sulfotransferase activity. Using platelet enzyme preparations from adult human subjects, sulfation rates of two prototypical thermostable phenol sulfotransferase substrates (2-naphthol and p-nitrophenol) and one thermolabile phenol sulfotransferase substrate (dopamine) were determined using standard radiochemical protocols. These data were then compared with results from the colorimetric assay using 2-naphthol as substrate. There was a good correlation between the phenotyping assay and radiochemical assays for both 2-naphthol sulfotransferase and p-nitrophenol sulfotransferase activity (r = 0.85 and 0.69, respectively). However, SULT1A1 activity was approximately 10 to 20 times higher with the colorimetric determination. As anticipated, there was no correlation between SULT1A1 activity and dopamine sulfotransferase activity (r = 0.07) in these human platelet preparations. This inexpensive and rapid method for phenotyping SULT1A1 activity may help investigators assess a role for this enzyme in disease susceptibility.

Mass spectrometric determination of Genistein tissue distribution in diet-exposed Sprague-Dawley rats

Genistein, the principal soy isoflavone, was administered in the diet to male and female Sprague-Dawley rats as part of a multigeneration study of potential endocrine modulation. The rats were exposed to genistein in utero, through maternal milk, and as adults through postnatal d 140 via essentially isoflavone-free feed (approximately 0.5 microg/g) fortified at 5, 100 and 500 microg/g with genistein aglycone. Analytical methods based on liquid chromatography, mass spectrometry and the use of deuterated genistein were developed and validated for use in measuring genistein in serum and tissues. Pharmacokinetic analysis of serum genistein showed a significant difference (P < 0.001) in the elimination half-life and area under the concentration-time curve between male [2.97 +/- 0.14 h and 22.3 +/- 1.2 micromol/(L. h), respectively] and female rats [4.26 +/- 0.29 h and 45.6 +/- 3.1 micromol/(L. h), respectively, +/- SEM]. Endocrine-responsive tissues including brain, liver, mammary, ovary, prostate, testis, thyroid and uterus showed significant dose-dependent increases in total genistein concentration. Female liver contained the highest amount of genistein (7.3 pmol/mg tissue) and male whole brain contained the least (0.04 pmol/mg). The physiologically active aglycone form was present in tissues at fractions up to 100%, and the concentration was always greater than that observed in serum in which conjugated forms predominated (95-99%). These results for measured amounts of genistein, present as aglycone and conjugates, in putative target tissues provide a link with other studies in which blood concentrations and physiologic effects of genistein are measured.

Analysis of soy isoflavone conjugation in vitro and in human blood using liquid chromatography-mass spectrometry

Soybean products containing isoflavones are widely consumed in Western and Asian diets for putative health benefits, but adverse effects are also possible. The conjugated forms of isoflavones present in a soy nutritional supplement (predominately acetyl glucosides) and in blood from two human volunteers after consuming the supplement (7- and 4'-glucuronides and sulfates) were identified using liquid chromatography coupled with electrospray/tandem mass spectrometry. Circulating conjugates of genistein and daidzein were quantified using selective enzymatic hydrolysis and deuterated internal standards for liquid chromatography-electrospray/mass spectrometry. The levels of isoflavone glucuronides were much greater than the corresponding sulfates or aglycones. The substrate activities of genistein and daidzein were evaluated with recombinant human UDP glucuronosyl transferase (UGT) and sulfotransferase (SULT) by using enzyme kinetics. The SULTs 1A1*2, 1E, and 2A1 catalyzed formation of a single genistein sulfate; however, SULTs 1A2*1 and 1A3 had no observed activity. None of the SULTs showed activity with daidzein. Although several UGTs (1A1, 1A4, 1A6, 1A7, 1A9, and 1A10) catalyzed 7- and 4'-glucuronidation of genistein or daidzein, the UGT 1A10 isoform, which is found in human colon but not liver, was found to be specific for genistein. Glucuronidation of only genistein was observed in human colon microsomes, although nearly equal activity was observed for daidzein in human liver and kidney microsomes. These findings suggest a prominent role for glucuronidation of genistein in the intestine concomitant with absorption, although hepatic glucuronidation of absorbed genistein and daidzein aglycones is also likely.

Quantitative analysis of interindividual variation of glutathione S-transferase expression in human pancreas and the ambiguity of correlating genotype with phenotype

Analysis of glutathione S-transferases (GSTs) of the alpha, mu, and pi classes by reverse-phase high-performance liquid chromatography and electrospray-ionization mass spectrometry in 43 samples of normal human pancreas demonstrated a wide variation in expression of subunits P1, A1, A2, A4, M1, M2, and M3 and the presence of a novel form designated GST "A5." GSTA2 consisted of three forms that were differentially expressed between individuals in a manner consistent with allelic polymorphism at the hGSTA2 locus. Expression, in terms of microg GST subunit/mg cytosolic protein, varied by 6-15-fold for subunits P1, A2, and M3 and 17-30-fold in the case of GSTs A1 and M2. Less consistently expressed were GSTs M1a, M1b, A4, and A5. Among these, GSTM1 expression (excluding M1-null samples) varied 12-fold between samples, whereas GST A4 and A5 expression varied approximately 50-100-fold between samples, well beyond the range of other subunits, suggesting that their expression is highly inducible. Linear correlations (P < 0.001-0.003) existed between levels of the most consistently expressed GST, GSTP1, and total GSTs, GSTA2 and M3, and in GSTM1-positive samples, between GSTM1, M3, and P1. The correlation between GST subunits P1 and M3 was bimodal according to M1 genotype, reflecting the presence of the regulatory element in hGSTM3*B that is linked with the hGSTM1*A genotype. It is concluded that although a degree of regulation of expression of GSTs occurs in human pancreas, the variability of phenotype is high and might obscure the effects of genetic polymorphisms on individual cancer susceptibility. Interindividual variation of GST expression is, therefore, a factor that should be taken account of in epidemiological studies.

On-line sample preparation using restricted-access media in the analysis of the soy isoflavones, genistein and daidzein, in rat serum using liquid chromatography electrospray mass spectrometry

Soy isoflavones are the subject of many investigations in experimental animals and humans regarding possible modulation of endocrine activity and chemoprevention of carcinogenesis. Genistein and daidzein, the principal biologically active isoflavones in soy, were measured using on-line solid-phase extraction (SPE) and liquid chromatography electrospray mass spectrometry (LC/ES-MS) detection in serum of rats consuming a common open-formula (NIH 31) chow that contained approximately 30 microg each of genistein and daidzein per gram of feed and a specially designed 'soy-free' chow that contained approximately 60-fold lower isoflavones. The use of a restricted-access/reverse phase trap cartridge and automated column switching permitted rapid and robust analytical performance with many injections of plasma onto a reverse phase LC column. Enzymatic deconjugation and a single centrifugation step were the only sample preparation steps required. The limit of detection for the isoflavones, based on the MS responses observed in serum from male and female rats consuming the soy-free chow, was 0.020 microM. The method, which uses deuterated isoflavones as internal standards, was determined to be accurate using spiked control serum (102-110% of added amounts) and precise using spiked control serum and incurred serum (<6% relative standard deviation). The average genistein and daidzein levels were determined in female (0.62 and 0.25 microM, respectively) and male rats (0.35 and 0.20 microM, respectively) consuming the standard diet. The sex difference observed for serum genistein concentrations was statistically significant (p < 0.0001). These results underscore the potential impact of standard open-formula diets on the results from rodent bioassays of biological activity.

Analysis of erythromycin by liquid chromatography/mass spectrometry using involatile mobile phases with a novel atmospheric pressure ionization source

A critical limitation of electrospray ionization (ESI) liquid chromatography/mass spectrometry (LC/MS) sources is the susceptibility to blockage of interface orifices due to the deposition of involatile components from the sample and/or mobile phase. These components, including salts, buffers, and ion-pairing agents, can be essential to the performance of the chosen analytical method. We report here the performance enhancements provided by a novel atmospheric pressure ionization (API) source in the analysis of erythromycin A (ERY) using mobile phases that contain involatile components. The enhanced robustness of the new source is derived from the use of a continuous flow of aqueous solvent at the sampling cone orifice that maintains unobstructed ion transmission. The ESI mass spectral responses measured for ERY, using an LC separation that incorporates 10 mM sodium phosphate with and without 10 mM octane sulfonate, were monitored by repeated injections over 13-15 h total analysis time. Minimal effects on ESI mass spectral responses (integrated peak area) or chromatographic performance (peak shape, retention time) were observed during these studies. In the absence of the aqueous cleaning flow, complete loss of mass spectral responses and total blocking of the sampling cone was observed in less than 30 min. Responses for ERY spiked into chicken and beef liver, and catfish muscle at or below the regulatory level of interest (100 ppb), were quantified by internal standard calibration using this procedure. These results demonstrate the ability of a novel API-MS ion source to perform analyses that require the use of involatile mobile phase additives.

Analysis of methylphenidate and its metabolite ritalinic acid in monkey plasma by liquid chromatography/electrospray ionization mass spectrometry

Methylphenidate (MP, Ritalin) is a psychotropic drug widely prescribed to children for treating the symptoms of attention deficit disorder with and without hyperactivity. Because little information exists about the effects of chronic MP administration on cognitive function in children, measures of behavior changes in non-human primates are important surrogates. An essential component of such studies is the determination of MP plasma levels under chronic and acute dosing conditions. An analytical method was developed that provided sufficient sensitivity to measure low levels of the active parent drug (lower limit of quantitation = 0.25 ng/mL) and the inactive metabolite, ritalinic acid (RA), in monkey plasma as well as the ability to conveniently analyze large numbers of samples. The method uses a polymeric reversed-phase sorbent for solid phase extraction, an efficient reverse-phase high performance liquid chromatography (HPLC) separation, deuterated internal standards for isotope dilution quantification of MP and RA, and detection by sensitive electrospray ionization mass spectrometry (ES-MS) with a single quadrupole instrument. The method responses are linear over the range of plasma concentrations of MP and RA observed in monkeys, gives respective analyte recoveries of 75 and 60% with reasonable precision and accuracy, and demonstrates robust MS performance for rapid determination of MP/RA plasma levels. The average peak MP concentration (ca. 16 ng/mL) and half-lives for MP and RA elimination in monkeys (1.79 and 2.31 h, respectively) were not significantly different under acute vs. chronic dosing conditions and were comparable to values previously reported from human studies.

Inactivation of Coprinus cinereus peroxidase by 4-chloroaniline during turnover: comparison with horseradish peroxidase and bovine lactoperoxidase

The peroxidase from Coprinus cinereus (CPX) catalyzed oxidative oligomerization of 4-chloroaniline (4-CA) forming several products: N-(4-chlorophenyl)-benzoquinone monoamine (dimer D), 4,4'-dichloroazobenzene (dimer E); 2-(4-chloroanilino)-N-(4-chlorophenyl)-benzoquinone (trimer F); 2-amino-5-chlorobenzoquinone-di-4-chloroanil (trimer G); 2-(4-chloroanilino)-5-hydroxybenzoquinone-di-4-chloroanil (tetramer H) and 2-amino-5-(-4-chlroanilino)-benzoquinone-di-4-chloroanil (tetramer 1). In the presence of 4-CA and H2O2, CPX was irreversibly inactivated within 10 min. Inactivation of CPX in the presence of H2O2 was a time-dependent, first-order process when the concentration of 4-CA was varied between 0 and 2.5 mM. The apparent dissociation constant (Ki) for CPX and 4-CA was 0.71 mM. The pseudo-first order rate constant for inactivation (k(inact)), was 1.15 x 10(-2) s(-1). Covalent incorporation of 20 mole 14C-4-CA per mole of inactivated CPX was observed. The partition ratio was about 2200 when either 4-CA or H2O2 was used as the limiting substrate. These results show that 4-CA is a metabolically activated inactivator (i.e. a suicide substrate). Unmodified heme and hydroxymethyl heme were isolated from native, 4-CA-inactivated and H2O2-incubated CPX. Inactivation resulted in significant losses in both heme contents. Analysis of tryptic peptides from 4-CA-inactivated CPX by MALDI-TOF/ MS and UV-VIS spectrophotometry suggested that trimer G and tetramer H were the major 4-CA derivatives that were covalently bound, including to a peptide (MGDAGF-SPDEVVDLLAAHSLASQEGLNSAIFR) containing the heme binding site. These studies show that heme destruction and covalent modification of the polypeptide chain are both important for the inactivation of CPX. These results were compared with similar studies on 4-CA-inactivated horseradish peroxidase (HRP) and bovine lactoperoxidase (LPO) during the oxidation of 4-CA.

An FDA review of sulfamethazine toxicity

Recently, changes have been proposed in the criteria historically used in the evaluation of the applicability to humans of some of the results obtained from the rodent carcinogenicity bioassay data. These questions center on the suitability of the rodent model for agents that exert their toxic effects via specific enzyme interactions and endocrine mechanisms which appear to be inoperative within humans. Within the U.S. Food and Drug Administration (FDA), this issue has been brought to the forefront of concern with the recent application for a New Animal Drug Application for sulfamethazine (SMZ). A panel of FDA experts from the National Center for Toxicological Research (NCTR), the Center for Veterinary Medicine (CVM), and the Center for Food Safety and Applied Nutrition has reviewed the sum of the scientific evidence available on the toxicology of SMZ. They noted that, in previous feeding studies at NCTR, high doses of SMZ were associated with significant incidences of thyroid tumors in mice and rats. The panel also notes that the tumorigenic activity of SMZ in rodents was due to its goitrogenic activity, resulting in constant stimulation of the thyroid by TSH. Humans, on the other hand, were found to be insensitive to the SMZ-like inhibition of thyroid function. Further, apart from X-irradiation and radioactive iodine, there are no other physical or chemical agents known to cause thyroid tumors in humans. Thus, the expert panel concludes that the best scientific information available indicates that elevated levels of TSH and the consequent thyroid tumors would not be produced under approved use conditions of SMZ. This conclusion is in agreement with recommendations made by three other panels, viz. the World Health Organization, the U.S. Environmental Protection Agency, and CVM, which also evaluated the public health risk of SMZ.

Toxicity and metabolism of malachite green and leucomalachite green during short-term feeding to Fischer 344 rats and B6C3F1 mice

Malachite green, an N-methylated diaminotriphenylmethane dye, has been widely used as an antifungal agent in commercial fish hatcheries. Malachite green is reduced to and persists as leucomalachite green in the tissues of fish. Female and male B6C3F1 mice and Fischer 344 rats were fed up to 1200 ppm malachite green or 1160 ppm leucomalachite green for 28 days to determine the toxicity and metabolism of the dyes. Apoptosis in the transitional epithelium of the urinary bladder occurred in all mice fed the highest dose of leucomalachite green. This was not observed with malachite green. Hepatocyte vacuolization was present in rats administered malachite green or leucomalachite green. Rats given leucomalachite green also had apoptotic thyroid follicular epithelial cells. Decreased T4 and increased TSH levels were observed in male rats given leucomalachite green. A comparison of adverse effects suggests that exposure of rats or mice to leucomalachite green causes a greater number of and more severe changes than exposure to malachite green. N-Demethylated and N-oxidized malachite green and leucomalachite green metabolites, including primary arylamines, were detected by high performance liquid chromatography/mass spectrometry in the livers of treated rats. 32P-Postlabeling analyses indicated a single adduct or co-eluting adducts in the liver DNA. These data suggest that malachite green and leucomalachite green are metabolized to primary and secondary arylamines in the tissues of rodents and that these derivatives, following subsequent activation, may be responsible for the adverse effects associated with exposure to malachite green.

Quantification of soy isoflavones, genistein and daidzein, and conjugates in rat blood using LC/ES-MS

Genistein is the principal soy isoflavone to which the putative beneficial effects of soy consumption have been attributed; however, the possibility of adverse biological effects (e.g., estrogenic, antithyroid) has also been raised. This paper describes development and validation of a simple and sensitive analytical method for the determination of genistein in the blood of rats receiving dietary genistein (<0.5-1250 microg of genistein aglycone/g of chow). The method uses serum/plasma deproteination, liquid-liquid extraction, deuterated genistein and daidzein internal standards, isocratic LC separation, and electrospray mass spectrometric quantification using selected ion monitoring. Extraction efficiency is approximately 85%, the detection limits for genistein and daidzein from 50 microL of rat blood are approximately 5 nM, and the limit of quantification is approximately 15 nM. Interassay precision (relative standard deviation 4.5-4.6%) and intraassay precision (3.3-6.7%) were determined from replicate analysis of a spiked control and an incurred serum sample. The distribution of conjugated and unconjugated forms of genistein in the blood of rats was determined using selective enzyme hydrolysis. The glucuronide was the predominant metabolite (>90%), and only small amounts of the sulfate conjugate and the aglycone were observed at all dose levels. No evidence for additional metabolites was obtained. The 7- and 4'-glucuronide conjugates of genistein were identified using electrospray mass spectrometry and (1)H NMR. Total blood genistein ranged from <15 nM in animals fed soy-free control diet to as high as 8.9 microM in male rats fed 1250 microg of genistein/g of chow and encompasses blood isoflavone levels observed in humans consuming a typical Asian diet and nutritional supplements (0.1-1 microM) and infants consuming soy formulas (2-7 microM).

Quantitative analysis of 4-aminobiphenyl-C8-deoxyguanosyl DNA adducts produced in vitro and in vivo using HPLC-ES-MS

Electrospray mass spectrometry (ES-MS) is a powerful tool for analysis of carcinogen-adducted DNA. In this study, we developed a quantitative isotope dilution method for analysis of N-(deoxyguanosine-8-yl)-4-aminobiphenyl (dG-C8-4-ABP), the principal nucleoside adduct derived from enzymatic hydrolysis of 4-aminobiphenyl (4-ABP)-modified DNA. The method used column switching valves to perform on-line sample concentration and cleanup, which permitted direct analysis of enzymatic DNA hydrolysates using narrow-bore liquid chromatography (LC). ES-MS detection was performed using a single quadrupole instrument by monitoring M+H+ and two fragment ions characteristic for dG-C8-4-ABP, along with M+H+ and a fragment ion for the deuterated internal standard. The detection limit for dG-C8-4-ABP in DNA hydrolysates was approximately 10 pg on-column, equivalent to 0.7 dG-C8-4-ABP adducts in 10(7) normal nucleotides for a sample containing 100 microg DNA. The method was applied to the analysis of calf thymus DNA modified in vitro through reaction with N-hydroxy-4-ABP and of hepatic DNA isolated from mice treated in vivo with two dose levels of 4-ABP.

Synthesis, characterization, and quantitation of a 4-aminobiphenyl-DNA adduct standard

32P-Postlabeling is a powerful technique for the detection of DNA adducts; however, quantitation of DNA adducts by this method can result in errors due to differences in hydrolysis and labeling efficiencies between adducted and normal nucleotides. We have synthesized a DNA sample modified with 4-aminobiphenyl to serve as a quantitation standard for 32P-postlabeling and other DNA adduct detection methodologies. [2,2'-3H]-N-Hydroxy-4-aminobiphenyl was reacted with calf thymus DNA at pH 5 to give 62 +/- 0.8 adducts/10(8) nucleotides (mean +/- SD) on the basis of 3H content. HPLC analyses following enzymatic hydrolysis to nucleosides indicated one major adduct, N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-4-ABP). The adduct identity was confirmed by HPLC/electrospray ionization mass spectrometry, which indicated a modification level of 19 +/- 1.7 dG-C8-4-ABP/10(8) nucleotides. 32P-Postlabeling analysis gave a value of 0.84 dG-C8-4-ABP/10(8) nucleotides, while a dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA) indicated levels of 82 +/- 26 and 63 +/- 20 dG-C8-4-ABP/10(8) nucleotides after enzymatic hydrolysis to nucleotides and nucleosides, respectively. The utility of the DNA adduct standard was determined by assessing the level of dG-C8-4-ABP in liver DNA from mice treated with [2,2'-3H]-4-aminobiphenyl. 32P-Postlabeling analyses, based upon measuring the extent of the 32P incorporation, underestimated the levels of dG-C8-4-ABP, while DELFIA, using a G-C8-4-ABP quantitation standard, overestimated the adduct levels. The adduct levels determined by HPLC/electrospray ionization mass spectrometry best reflected those obtained from 3H incorporation. When the 32P-postlabeling analyses and the DELFIA were conducted using the DNA modified in vitro with dG-C8-4-ABP as a quantitation standard, accurate estimations of the extent of in vivo formation of dG-C8-4-ABP were obtained.

Mechanism for inhibition of thyroid peroxidase by leucomalachite green

The triphenylmethane dye, malachite green (MG), is used to treat and prevent fungal and parasitic infections in the aquaculture industry. It has been reported that the reduced metabolite of MG, leucomalachite green (LMG), accumulates in the tissues of fish treated with MG. MG is structurally related to other triphenylmethane dyes (e.g., gentian violet and pararosaniline) that are carcinogenic in the liver, thyroid, and other organs of experimental animals. The ability of LMG to inhibit thyroid peroxidase (TPO), the enzyme that catalyzes the iodination and coupling reactions required for thyroid hormone synthesis, was determined in this study. LMG inhibited TPO-catalyzed tyrosine iodination (half-maximal inhibition at ca. 10 microM). LMG also inhibited the TPO-catalyzed formation of thyroxine in low-iodine human goiter thyroglobulin (half-maximal inhibition at ca. 10 microM) using a model system that measures simultaneous iodination and coupling. Direct inhibition of the coupling reaction by LMG was shown using a coupling-only system containing chemically preiodinated thyroglobulin as the substrate. Incubation of LMG with TPO, iodide, and tyrosine in the presence of a H2O2-generating system yielded oxidation products that were identified by using on-line LC/APCI-MS as desmethyl LMG, 2desmethyl LMG, 3desmethyl LMG, MG, and MG N-oxide. Similar products from LMG were observed in incubations with TPO and H2O2 alone. These findings suggest that the anti-thyroid effects (increased serum thyroid-stimulating hormone and decreased serum thyroxine) observed in rats treated with LMG result from blockade of hormone synthesis through alternate substrate inhibition and that chronic exposure could cause thyroid follicular cell tumors through a hormonal mechanism. The observed TPO-catalyzed oxidative demethylation of LMG to a primary arylamine also suggests a genotoxic mechanism for tumor formation is possible.

An improved 32P-postlabeling/high-performance liquid chromatography method for the analysis of the malondialdehye-derived 1, N2-propanodeoxyguanosine DNA adduct in animal and human tissues

Malondialdehyde (MDA) is a major lipid peroxidation product that is mutagenic and tumorigenic. The MDA-modified DNA adduct, 3-(2-deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1, 2-alpha]purin-10(3H)-one (M1G), has been detected in human tissues and may be a marker of human cancer risk. In this paper, we describe an improved 32P-postlabeling/HPLC method for sensitive detection and quantitation of this MDA-modified 2'-deoxyribonucleotide adduct. Specific improvements include (i) unequivocal structural identification of the postlabeling products, both the 3', 5'-bisphosphate of M1G (MDA-3',5'-dGDP) and the 5'-monophosphate of M1G (MDA-5'-dGMP); (ii) efficient separation of the 32P-postlabeling products by HPLC; and (iii) the incorporation of a synthetically prepared MDA-modified DNA (or the 3'-monophosphate of M1G) with a known modification level as an internal standard. This improved quantitative methodology provides high intra- and inter-assay reproducibility and has been applied to the analysis of this adduct in rodent and human samples.

Fungal metabolism of nitrofluoranthenes

Metabolism of 2-nitrofluoranthene (2-NFA), one of the most abundant and genotoxic environmental pollutants in air, and of a mixture of 2-nitrofluoranthene and 3-nitrofluoranthene (3-NFA) was studied using (1) the fungus Cunninghamella elegans ATCC 36112 and (2) rat liver microsomes. The fungal metabolites were separated by reversed-phase high-performance liquid chromatography (HPLC) and identified by 1H nuclear magnetic resonance (NMR) spectrometry, ultraviolet (UV)-visible spectroscopy, and online atmospheric-pressure chemical ionization/mass spectrometry (APCI/MS). The fungus metabolized 82% of 2-nitro-[3H]-fluoranthene to 2-nitrofluoranthene 8-sulfate and 2-nitrofluoranthene 9-sulfate. Metabolism of a mixture of 2- and 3-nitrofluoranthene by C. elegans similarly produced 2-nitrofluoranthene 8- and 9-sulfate and 3-nitrofluoranthene 8- and 9-sulfate as major metabolites. In addition, a glucoside conjugate of 3-hydroxy-2-nitrofluoranthene was tentatively identified by APCI/MS analysis. When rat liver microsomes were incubated with a mixture of 2- and 3-nitrofluoranthene for 1 h, in addition to the trans-7,8- and 9,10-dihydrodiols reported previously for 2-nitrofluoranthene, several novel metabolites were produced including 2-nitrofluoranthene trans-4,5-dihydrodiol and 2-nitrofluoranthene trans-8,9-dihydrodiol, the trans-4,5-dihydrodiol of 3-nitrofluoranthene, and phenolic products of both 2- and 3-nitrofluoranthene. The fungal metabolism of the 2- and 3-nitrofluoranthene mixture was similar to the metabolism of individual nitrofluoranthenes; however, the mammalian metabolism of the nitrofluoranthene mixture showed differences in regioselectivity at positions C4, C5, C8, and C9.

Mass spectrometric analysis of 2-deoxyribonucleoside and 2'-deoxyribonucleotide adducts with aldehydes derived from lipid peroxidation

An important emerging issue in chemical carcinogenesis is the role that products of endogenous metabolism play in formation of covalently modified DNA. One example is the formation of alpha, beta-unsaturated aldehydes as a result of endogenous and drug-stimulated lipid peroxidation. Malondialdehyde (MDA), crotonaldehyde (CR), 2-hexenal (HX), and 4-hydroxy-2-nonenal (HNE) react covalently with 2'-deoxyguanosine (dG) and 2'-deoxyadenosine (dA) residues on DNA to form promutagenic cyclic adducts that may be important in the etiology of cancer in humans and animals. The accurate quantification of such adducts provides a powerful tool in molecular epidemiology for assessing carcinogenic risks from various lifestyle choices (e.g. diet, drug use) in humans. 32P-Postlabeling is recognized as one of the most sensitive methods available for detection of DNA adducts in human tissues, but without adequate validation such methodology can yield inaccurate quantitative measurements. We have used LC separations in conjunction with electrospray ionization MS and tandem MS (triple quadrupole and hybrid quadrupole-orthogonal acceleration time of flight analyzers) to characterize MDA-, CR-, HX- and HNE-modified dG and nucleotide (3'- and 5'-monophosphate; 3',5'-bisphosphate) adducts. These data have been used to validate 32P-postlabeling methods for quantification of low level MDA-dG adducts formed in DNA of human and animal tissues. Availability of reliable methods for quantification of endogenous DNA damage in humans and animals is essential for determining unknown etiologies of cancer and for the assessment of cancer risks in humans.

Analysis of malachite green and metabolites in fish using liquid chromatography atmospheric pressure chemical ionization mass spectrometry

Malachite green (MG), a traditional agent used in aquaculture, is structurally related to other carcinogenic triphenylmethane dyes. Although MG is not approved for use in aquaculture, its low cost and high efficacy make illicit use likely. We developed sensitive and specific methods for determination of MG and its principal metabolite, leucoMG (LMG), in edible fish tissues using isotope dilution liquid chromatography atmosphere pressure chemical ionization mass spectrometry. MG and LMG concentrations were measured in filets from catfish treated with MG under putative use conditions (ca. 250 and 1000 ppb, respectively) and from commercial trout samples (0-3 and 0-96 ppb, respectively). Concentrations of LMG in edible fish tissues always exceeded those of MG. A rapid cone voltage switching acquisition procedure was used to simultaneously produce molecular ions for quantification and diagnostic fragment ions for confirmation of MG and metabolites. The accurate and precise agreement between diagnostic ion intensity ratios produced by LMG in authentic standards and incurred fish samples was used to unambiguously confirm the presence of LMG in edible fish tissue. This suggested the validity of using LMG as a marker residue for regulatory determination of MG misuse. Additional metabolites derived from oxidative metabolism of MG or LMG (demethylation and N-oxygenation) were identified in catfish and trout filets, including a primary arylamine which is structurally related to known carcinogens. The ability to simultaneously quantify residues of MG and LMG, and to confirm the chemical structure of a marker residue by using LC/MS, suggests that this procedure may be useful in monitoring the food supply for the unauthorized use of MG in aquaculture.

Anti-thyroid isoflavones from soybean: isolation, characterization, and mechanisms of action

The soybean has been implicated in diet-induced goiter by many studies. The extensive consumption of soy products in infant formulas and in vegetarian diets makes it essential to define the goitrogenic potential. In this report, it was observed that an acidic methanolic extract of soybeans contains compounds that inhibit thyroid peroxidase- (TPO) catalyzed reactions essential to thyroid hormone synthesis. Analysis of the soybean extract using HPLC, UV-VIS spectrophotometry, and LC-MS led to identification of the isoflavones genistein and daidzein as major components by direct comparison with authentic standard reference isoflavones. HPLC fractionation and enzymatic assay of the soybean extract showed that the components responsible for inhibition of TPO-catalyzed reactions coeluted with daidzein and genistein. In the presence of iodide ion, genistein and daidzein blocked TPO-catalyzed tyrosine iodination by acting as alternate substrates, yielding mono-, di-, and triiodoisoflavones. Genistein also inhibited thyroxine synthesis using iodinated casein or human goiter thyroglobulin as substrates for the coupling reaction. Incubation of either isoflavone with TPO in the presence of H2O2 caused irreversible inactivation of the enzyme; however, the presence of iodide ion in the incubations completely abolished the inactivation. The IC50 values for inhibition of TPO-catalyzed reactions by genistein and daidzein were ca. 1-10 microM, concentrations that approach the total isoflavone levels (ca. 1 microM) previously measured in plasma from humans consuming soy products. Because inhibition of thyroid hormone synthesis can induce goiter and thyroid neoplasia in rodents, delineation of anti-thyroid mechanisms for soy isoflavones may be important for extrapolating goitrogenic hazards identified in chronic rodent bioassays to humans consuming soy products.

Synthesis and 32P-postlabeling/high-performance liquid chromatography separation of diastereomeric 1,N2-(1,3-propano)-2'-deoxyguanosine 3'-phosphate adducts formed from 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (HNE), a major electrophilic byproduct of lipid peroxidation, is mutagenic and cytotoxic. The two pairs of HNE-derived diastereomeric 1,N2-propanodeoxyguanosine 3'-monophosphate adducts were synthesized from reaction of HNE with 2'-deoxyguanosine 3'-monophosphate. After HPLC separation, these adducts were characterized by UV-visible absorption and negative ion electrospray ionization MS/MS analysis. To further characterize the structures, these adducts were dephosphorylated to the corresponding HNE-modified deoxyguanosine adducts and their HPLC retention times and UV spectra were compared with those of the synthetic standards prepared from reaction of HNE with 2'-deoxyguanosine. Separation of these adducts by 32P-postlabeling/HPLC was developed. Reaction of HNE with calf thymus DNA resulted in only one pair of diastereomeric adducts, with one adduct predominantly formed with a modification level of 1.2 +/- 0.5 adducts/10(7) nucleotides.

Identification of the colored guaiacol oxidation product produced by peroxidases

Oxidation of guaiacol by peroxidases in the presence of H2O2 is the basis for a widely used colorimetric assay. However, the nature of the assay product, which has an absorption maximum around 470 nm, had not been determined. In the present study, we combined HPLC with a rapid scanning uv-visible detector and observed a single product with a spectrum identical to the assay product from the reaction catalyzed by lactoperoxidase. Analysis of the reaction product using on-line HPLC with atmospheric pressure chemical ionization detection (LC-APCI/MS) yielded a mass spectrum consistent with 3,3 '-dimethoxy-4,4'-biphenylquinone. A minor reaction product was observed with mass spectrum consistent with 3,3'-dimethoxy-4,4'-dihydroxybiphenyl. The presence of a catechol impurity in guaiacol was previously shown to yield an additional product from peroxidase-mediated oxidation based on its visible absorption (Taurog et al., 1992 Anal. Biochem. 205, 271-277). When such an incubation mixture was analyzed using LC-APCI/MS, a product with mass spectrum consistent with 3-methoxy-2',3',4-trihydroxybiphenyl was observed. Identification of such a heterodimeric product supports the previously proposed mechanism for catechol interference in the guaiacol assay as well as the radical nature of peroxidase-catalyzed oxidation of phenols.

Metabolism of metolachlor by the fungus Cunninghamella elegans

The metabolism of metolachlor[2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-met hyl ethyl)acetamide]by the fungus Cunninghamella elegans ATCC 36112 was determined. Thesix metabolites identified comprised 81% of the total[14C]-metolachlor metabolized by C. elegans. Thesemetabolites were separated by reversed-phase high-performance liquidchromatography and identified by 1H nuclear magnetic resonance, UV, and atmospheric pressure chemical ionization (APCI) mass spectraltechniques. Metabolites I and II were identified as stereoismers of2-chloro-N-[2-ethyl-6-hydroxymethylphenyl)]-N-(2-hydroxy-1-me thylet hyl)acetamide. Metabolites III and IV have been tentatively identified as stereoismers of2-chloro-N-[2-(1-hydroxyethyl)-6-methylphenyl]-N-(2-methoxy-1-++ +methy lethyl)acetamide. Metabolites V and VI were identified as stereoismers of2-chloro-N-(2-ethyl-6-hydroxy-methylphenyl)-N-(2-methoxy-1-me thylet hyl)acetamideand 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-hydroxy-1-methylethyl)acetam ide, respectively. The fungus Cunninghamellaelegans was able to biotransform metolachlor. Multiple site oxidation ofmetolachlor by C. elegans occurred predominantly byO-demethylation of the N-alkyl side chain and benzylichydroxylation of the arylalkyl side chain.

Mechanism for the anti-thyroid action of minocycline

Administration of minocycline (MN), a tetracycline antibiotic, produces a black pigment in the thyroids of humans and several species of experimental animals and antithyroid effects in rodents. We have previously shown that these effects appear to be related to interactions of MN with thyroid peroxidase (TPO), the key enzyme in thyroid hormone synthesis. In the present study, the mechanisms for inhibition of TPO-catalyzed iodination and coupling reactions by MN were investigated. MN was stable in the presence of TPO and H2O2, but adding iodide or a phenolic cosubstrate caused rapid conversion to several products. TPO-dependent product formation, characterized by on-line LC-APCI/MS and 1H-NMR, involved oxidative elimination to form the corresponding benzoquinone with subsequent dehydrogenation at the aliphatic 4-(dimethylamino) group. Addition of thiol-containing polymers (bovine serum albumin or thiol-agarose chromatographic beads) had a minimal effect on MN oxidation by TPO, but substantially reduced product formation and produced concomitant losses in free thiols. Covalent bonding through a thioether linkage of a reactive intermediate, the benzoquinone iminium ion, was inferred from these findings. Iodide- and phenolic cosubstrate-dependent oxidation of tetracycline to demethylated and dehydrogenated products was also observed, although at a slower rate than MN. The products and kinetics observed with MN were consistent with oxidation of MN by either the enzymatic iodinating species formed by reaction of TPO compound I with iodide or phenoxyl radicals/cations generated by TPO-mediated oxidation of a phenolic cosubstrate. The proposed reaction mechanism is consistent with alternate substrate inhibition of TPO-catalyzed iodination of tyrosyl residues in thyroglobulin (Tg) by MN, as previously reported. Furthermore, the observed phenoxyl radical-mediated oxidation of MN is consistent with its previously reported potent inhibition of the coupling of hormonogenic iodotyrosine residues in Tg in the reaction that forms thyroid hormones. The proposed reaction mechanism also implicates a reactive benzoquinone iminium ion intermediate that could be important in toxicity of MN.

Quantification of heterocyclic amine carcinogens in cooked meats using isotope dilution liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry

Heterocyclic aromatic amines (HAAs) are formed in cooked meats through pyrolysis reactions of different amino acids in the presence or absence of creatine/creatinine and sugars. HAAs are mutagens, colon/mammary gland carcinogens in rodents, and are suspected in the etiology of human cancers. In this study, cooked meats containing incurred HAAs as well as control (microwave) meat, were spiked with four labeled HAA internal standards (MeIQx, IQ, AAC and PhIP) and extracted using a liquid/liquid cleanup procedure. Isotope dilution measurements were made using on-line liquid chromatography atmosphere pressure chemical ionization tandem mass spectrometry with multiple reaction monitoring to provide the sensitivity and specificity needed for trace analysis in these complex matrices. The procedure was validated using control meat spiked with the four native HAAs at 0-50 ppb. The levels of HAAs found in cooked meats ranged from non-detectable (limit of detection 0.1-1.0 ppb) in microwave-cooked hamburger to 226 ppb PhIP and 104 ppb AAC in well-done grilled chicken. This methodology has the potential to provide accurate data on the consumption of HAAs in the diet for use in human cancer risk assessment.

Identification of ceramides in human cells using liquid chromatography with detection by atmospheric pressure chemical ionization-mass spectrometry

Ceramides are intermediates in the biosynthesis of membrane sphingolipids. These biomolecules are also important as second messengers in signal transduction pathways controlling cell growth. We have developed two reversed-phase high pressure liquid chromatography (RPHPLC) techniques for identification and quantification of ceramides from mammalian cells. One method was based on atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) detection of ceramides and had the advantage of requiring minimal sample preparation, yielding significant structural information, and affording high sensitivity. The second method relied on perbenzoylation of the ceramides and detection at 230 nm. The predominant ceramides detected in the human leukemic HL-60 cell were N-(palmitoyl)-sphingosine, N-(nervonyl)-sphingosine, and N-(lignoceroyl)-sphingosine. When selected ion monitoring was used with RPHPLC/APCI-MS, approximately 2.2 pmol N-(palmitoyl)-sphingosine and 1.7 pmol N-(nervonyl)-sphingosine were observed in an extract from 40,000 HL-60 cells. Perbenzoylation with benzoyl chloride permitted RPHPLC separation and 230 nm UV absorbance detection of the trisbenzoyl derivatives of sphingosine, N-(palmitoyl)-sphingosine, N-(nervonyl)-sphingosine, and N-(lignoceroyl)-sphingosine in the HL-60 cells. These results demonstrate the utility of utilizing two different methods coupled with APCI-MS for the quantification and identification of ceramides in biological samples.

Minocycline and the thyroid: antithyroid effects of the drug, and the role of thyroid peroxidase in minocycline-induced black pigmentation of the gland

Minocycline (MN), a member of the tetracycline family of antibiotics, is known to induce a black discoloration of the thyroid in several species, including humans. Antithyroid effects of MN have also been reported. The aim of the present study was two-fold: (1) to determine whether thyroid peroxidase (TPO) is involved in the MN-induced black thyroid, and (2) to obtain information on the effect of MN on TPO-catalyzed iodination and coupling in model systems containing highly purified TPO. Treatment of MN with TPO in the presence of the H2O2 generating system, glucose-glucose oxidase, resulted in the formation of a black product (or products). In phosphate buffer, pH 7.0, the color intensity reached its peak in about 90 min. Control samples without TPO showed little or no color change during this interval. Formation of the black product(s) did not require the presence of iodide. Other members of the tetracycline family were not oxidized to dark products by the TPO system. These results provide definitive evidence that TPO is involved in the MN-induced black thyroid. MN is an inhibitor of TPO-catalyzed iodination in model systems, with a potency comparable to that of MMI and PTU. At low drug concentrations (approximately 25 microM), MN appeared to act as a competitive inhibitor, as previously shown for lower concentrations of MMI and PTU. However, when the drug concentration was increased, MN and the thioureylene drugs inhibited iodination by different mechanisms. With PTU and MMI, iodination was irreversibly inhibited through inactivation of TPO. However, inhibition of iodination by MN (100 microM) was not associated with inactivation of TPO and was at least partially reversible. The most potent inhibitory effect of MN was on TPO-catalyzed coupling. This was demonstrated both in a coupling test system, designed to measure coupling in the absence of iodination, and in an iodination system, in which iodination and coupling occurred simultaneously. In both systems, MN was several times more potent than PTU and MMI, or other tetracycline drugs. Based on the potent antithyroid effects of MN observed in our in vitro studies, it seems advisable to monitor thyroid function in patients receiving long-term MN therapy.

Mechanism of simultaneous iodination and coupling catalyzed by thyroid peroxidase

Thyroid peroxidase (TPO) simultaneously catalyzes two very different types of reaction in the thyroid gland- iodination and coupling. The present study addresses the mechanism of this simultaneous dual activity. Compound I, the two-electron oxidation product of TPO, exists in two different forms--an oxoferryl porphyrin pi-cation radical and an oxoferryl protein radical. It has been proposed that iodination is mediated by the porphyrin pi-cation radical form of TPO compound I, while coupling is mediated by the protein radical form. However, results obtained in the present study favor the view that both iodination and coupling are mediated by the porphyrin pi-cation radical form of compound I. In the first part of the study, we compared coupling and iodination activities of two peroxidases with very similar crystal structures--cytochrome c peroxidase (CcP) and lignin peroxidase (LiP). Although these two peroxidases have very similar three-dimensional structures, CcP forms a compound I only of the protein radical type, whereas compound I of LiP exists only as a porphyrin pi-cation radical. Comparison of the catalytic activities of the two enzymes showed that diiodotyrosine (DIT)-stimulated coupling activity of LiP was significantly greater than that of CcP. Moreover, lignin peroxidase displayed very significant iodinating activity at acid pHs, whereas iodination with CcP was negligible at all pHs tested. Our findings with these two structurally similar peroxidases suggested that TPO-catalyzed iodination and coupling could both be mediated by the porphyrin pi-cation radical form of compound I. More direct evidence in support of this view was obtained in the second part of this study, employing TPO and lactoperoxidase (LPO) model systems in which iodination and coupling occurred simultaneously. Heme spectral analysis was used to correlate formation of the protein radical form of compound I with the kinetics of the iodination and coupling reactions. Formation of the compound I protein radical was not observed until the iodination and coupling reactions had almost been completed. In separate experiments it was shown that the spontaneous conversion of the porphyrin pi-cation radical form of TPO or LPO compound I to the protein radical form was markedly inhibited by a low concentration of iodide, especially in the presence of an iodide acceptor. These studies provide compelling evidence that both iodination and coupling are mediated by the porphyrin pi-cation radical form of compound I. This was further substantiated by the finding that coupling was inhibited in the presence of excess iodide, an observation readily explained by competition between iodide and DIT residues in thyroglobulin for oxidation by the porphyrin pi-cation radical.

Detection and confirmation of beta-agonists in bovine retina using LC/APCI-MS

Beta-adrenergic receptor agonists are growth-promoting drugs with the potential for illegal use in livestock, and human toxicity has resulted from consumption of contaminated meat. On-line liquid chromatography with atmospheric pressure chemical ionization mass spectrometry (LC/APCI-MS) was used for sensitive detection of several beta-agonists in retina, a tissue reported to concentrate and retain such residues for extended periods. Multiresidue extraction, separation, detection, and confirmation procedures were developed for retinal tissue and applied to eyes from cattle treated with clenbuterol (69-201 ppb) and to control eyes spiked with salbutamol (100 ppb) and terbutaline (25-100 ppb). Rapid switching of the potential difference between sampling cone and skimmer in the transport region of the API source was used to optimize acquisition of the protonated molecules and characteristic fragment ions obtained by collision-induced dissociation reactions. The respective selected ions were simultaneously acquired using a single quadrupole mass spectrometer. The accurate and precise agreement observed for diagnostic ion intensity ratios between beta-agonists in retinal samples and authentic standards suggests that LC/APCI-MS can be used for confirmation of analyte structure at trace levels and does not require the use of a triple-stage quadrupole mass analyzer.

Confirmation of gentian violet and its metabolite leucogentian violet in catfish muscle using liquid chromatography combined with atmospheric pressure ionization mass spectrometry

Gentian violet (GV) is a triphenylmethane dye antiseptic with potential for illegal use in livestock production, especially aquaculture where the related malachite green has been widely used. This potential misuse has regulatory importance because of the observed rodent carcinogenicity of GV. This report describes the use of online LC-APCI/MS for confirmation of incurred GV residues, and those of its principal metabolite, LGV, in catfish muscle following treatment of live catfish with GV under putative use conditions. LC with APCI/MS detection provided sensitive analysis of GV and LGV with estimated detection limits of < 1 pg observed for both compounds. Fragmentation of GV and LGV via in-source CID was effected by varying the sampling cone-skimmer voltage. Ion intensity data were collected using a rapid cone voltage switching procedure that permits selected ion acquisition under optimal conditions for the parent molecule and several selected fragment ions. For GV, four ions including the ionized molecule were used and for LGV, six ions including the protonated molecule were used. The levels of GV and LGV in muscle from fish dosed with 10 micrograms/l in aquarium water were determined by LC/VIS to be 0.5 and 44 ppb, respectively. Analysis of these samples yielded ion intensity ratios that agreed precisely between injections (< 5%) and accurately with those generated by a comparable amount of authentic GV and LGV (< 10% deviation). These results show the utility of on-line LC-APCI/MS to do both sensitive confirmatory analyses of incurred drug residues for use in monitoring the food supply.

Inhibition of thyroid peroxidase by dietary flavonoids

Flavonoids are widely distributed in plant-derived foods and possess a variety of biological activities including antithyroid effects in experimental animals and humans. A structure-activity study of 13 commonly consumed flavonoids was conducted to evaluate inhibition of thyroid peroxidase (TPO), the enzyme that catalyzes thyroid hormone biosynthesis. Most flavonoids tested were potent inhibitors of TPO, with IC50 values ranging from 0.6 to 41 microM. Inhibition by the more potent compounds, fisetin, kaempferol, naringenin, and quercetin, which contain a resorcinol moiety, was consistent with mechanism-based inactivation of TPO as previously observed for resorcinol and derivatives. Other flavonoids inhibited TPO by different mechanisms, such as myricetin and naringin, showed noncompetitive inhibition of tyrosine iodination with respect to iodine ion and linear mixed-type inhibition with respect to hydrogen peroxide. In contrast, biochanin A was found to be an alternate substrate for iodination. The major product, 6,8-diiodo-biochanin A, was characterized by electrospray mass spectrometry and 1H-NMR. These inhibitory mechanisms for flavonoids are consistent with the antithyroid effects observed in experimental animals and, further, predict differences in hazards for antithyroid effects in humans consuming dietary flavonoids. In vivo, suicide substrate inhibition, which could be reversed only by de novo protein synthesis, would be long-lasting. However, the effects of reversible binding inhibitors and alternate substrates would be temporary due to attenuation by metabolism and excretion. The central role of hormonal regulation in growth and proliferation of thyroid tissue suggests that chronic consumption of flavonoids, especially suicide substrates, could play a role in the etiology of thyroid cancer.

Porphyrin pi-cation and protein radicals in peroxidase catalysis and inhibition by anti-thyroid chemicals

1. Thyroid peroxidase (TPO) catalyses the iodination and phenolic coupling reactions in the biosynthesis of thyroid hormones. 2. The two-electron oxidation of TPO by H2O2 produces an oxoferryl porphyrin pi-cation radical compound I that isomerizes spontaneously to a form of compound I that contains an oxoferryl haem and the second oxidizing equivalent as an amino acid radical. 3. The pi-cation radical compound I is the catalytic species that effects iodide ion oxidation and the protein radical compound I is most likely the catalytic species that catalyses coupling. 4. Methimazole, a therapeutic, anti-hyperthyroid drug, is a suicide substrate for TPO and effects irreversible inactivation by TPO-mediated S-oxygenation to a reactive sulphenic acid that binds covalently to the prosthetic haem. 5. Sulphamethazine and other arylamines containing electron-withdrawing substituents inhibit TPO compound I-mediated reactions by reversible, mixed-type inhibition. 6. Ethylenethiourea, a fungicide metabolite, blocks TPO-mediated iodination by reacting with the catalytic iodinating species as an alternate substrate. 7. Resorcinol and related dietary flavonoids are suicide substrates for TPO and act by covalent binding to amino acid residues, presumably those radical sites present in the compound I isomer. 8. Nitrosobenzene, a known radical-trapping agent, blocks TPO-mediated coupling but not iodination or phenolic oxidations presumably by interception of the 3,5-diiodotyrosyl radical species generated during the coupling reaction.