Human benzene metabolism following occupational and environmental exposures

We previously reported evidence that humans metabolize benzene via two enzymes, including a hitherto unrecognized high-affinity enzyme that was responsible for an estimated 73% of total urinary metabolites [sum of phenol (PH), hydroquinone (HQ), catechol (CA), E,E-muconic acid (MA), and S-phenylmercapturic acid (SPMA)] in nonsmoking females exposed to benzene at sub-saturating (ppb) air concentrations. Here, we used the same Michaelis-Menten-like kinetic models to individually analyze urinary levels of PH, HQ, CA and MA from 263 nonsmoking Chinese women (179 benzene-exposed workers and 84 control workers) with estimated benzene air concentrations ranging from less than 0.001-299 ppm. One model depicted benzene metabolism as a single enzymatic process (1-enzyme model) and the other as two enzymatic processes which competed for access to benzene (2-enzyme model). We evaluated model fits based upon the difference in values of Akaike's Information Criterion (DeltaAIC), and we gauged the weights of evidence favoring the two models based upon the associated Akaike weights and Evidence Ratios. For each metabolite, the 2-enzyme model provided a better fit than the 1-enzyme model with DeltaAIC values decreasing in the order 9.511 for MA, 7.379 for PH, 1.417 for CA, and 0.193 for HQ. The corresponding weights of evidence favoring the 2-enzyme model (Evidence Ratios) were: 116.2:1 for MA, 40.0:1 for PH, 2.0:1 for CA and 1.1:1 for HQ. These results indicate that our earlier findings from models of total metabolites were driven largely by MA, representing the ring-opening pathway, and by PH, representing the ring-hydroxylation pathway. The predicted percentage of benzene metabolized by the putative high-affinity enzyme at an air concentration of 0.001 ppm was 88% based upon urinary MA and was 80% based upon urinary PH. As benzene concentrations increased, the respective percentages of benzene metabolized to MA and PH by the high-affinity enzyme decreased successively to 66 and 77% at 0.1 ppm, 20 and 58% at 1 ppm, and 2.7 and 17% at 10 ppm. This indicates that the putative high-affinity enzyme was active primarily below 1 ppm and favored the ring-opening pathway.

Using urinary biomarkers to elucidate dose-related patterns of human benzene metabolism

Although the toxicity of benzene has been linked to its metabolism, the dose-related production of metabolites is not well understood in humans, particularly at low levels of exposure. We investigated unmetabolized benzene in urine (UBz) and all major urinary metabolites [phenol (PH), E,E-muconic acid (MA), hydroquinone (HQ) and catechol (CA)] as well as the minor metabolite, S-phenylmercapturic acid (SPMA), in 250 benzene-exposed workers and 139 control workers in Tianjin, China. Median levels of benzene exposure were approximately 1.2 p.p.m. for exposed workers (interquartile range: 0.53-3.34 p.p.m.) and 0.004 p.p.m. for control workers (interquartile range: 0.002-0.007 p.p.m.). (Exposures of control workers to benzene were predicted from levels of benzene in their urine.) Metabolite production was investigated among groups of 30 workers aggregated by their benzene exposures. We found that the urine concentration of each metabolite was consistently elevated when the group's median benzene exposure was at or above the following air concentrations: 0.2 p.p.m. for MA and SPMA, 0.5 p.p.m. for PH and HQ, and 2 p.p.m. for CA. Dose-related production of the four major metabolites and total metabolites (micromol/l/p.p.m. benzene) declined between 2.5 and 26-fold as group median benzene exposures increased between 0.027 and 15.4 p.p.m. Reductions in metabolite production were most pronounced for CA and PH<1 p.p.m., indicating that metabolism favored production of the toxic metabolites, HQ and MA, at low exposures.

Determination of dihydroxynaphthalenes in human urine by gas chromatography-mass spectrometry

A gas chromatography-mass spectrometry (GC-MS) method was developed for measuring 1,2-dihydroxynaphthalene (1,2-DHN) and 1,4-dihydroxynaphthalene (1,4-DHN) in urine. The method involves enzymatic digestion of urinary conjugates to release the DHNs which were then analyzed as trimethylsilyl derivatives by GC-MS. For 1,2-DHN and 1,4-DHN, respectively, the assay limits of detection were 0.21 and 0.15 microg/l, the assay limits of quantitation were 0.69 and 0.44 microg/l, and the coefficients of variation were 14.7 and 10.9%. This method was successfully applied to determine urinary levels of 1,2-DHN and 1,4-DHN in coke workers (14 top workers and 13 side-bottom workers) and 21 matching control workers from the steel industry of northern China. The geometric mean (GM) levels of 1,2-DHN were approximately 100 and 30 times higher than those of 1,4-DHN in exposed and control subjects, respectively. The GM levels 1,2-DHN and 1,4-DHN were significantly higher for coke workers (1,2-DHN: top workers--552 microg/l, side-bottom workers--260 microg/l; 1,4-DHN: top workers--3.42 microg/l, side-bottom workers--3.56 microg/l) than for controls (1,2-DHN: 38.8 microg/l; 1,4-DHN: 1.21 microg/l) (p<or=0.0031). In each exposure category, levels of the DHNs were marginally greater in smokers than in nonsmokers (p=0.0646). Strong correlations were observed among 1,2-DHN and 1,4-DHN and previously measured urinary levels of naphthalene, 1-hydroxynaphthalene, and 2-hydroxynaphthalene in these subjects (rs>or=0.623; p<0.0001). Also, levels of 1,2-DHN were significantly correlated with those of serum albumin adducts of l,2-naphthoquinone (rs=0.492, p=0.0004). These results indicate that 1,2- and 1,4-DHN are good biomarkers for assessment of naphthalene exposure in coke workers. Since the DHNs are precursors of the naphthoquinones, which have been implicated as toxic products of naphthalene metabolism, measurements of urinary DHNs may have toxicological significance.

Metabolism and disposition of hydroquinone in Fischer 344 rats after oral or dermal administration

Studies were conducted to determine the absorption, tissue distribution, excretion, and metabolism of 14C-hydroquinone (HQ) in male and female rats following single oral, repeated oral, or 24-h dermal administration. The concentration of parent compound in blood was also determined following a single 50-mg/kg gavage administration. Absorption into the blood was rapid after oral dosing; the maximum concentration of parent compound was attained within 20 min after dosing, and the maximum concentration of total 14C was attained within 30 min. Parent compound represented 1% of total 14C in blood, indicative of extensive first-pass metabolism. Excretion was primarily via the urine within the first 8h of gavage. Typically, 87-94% of the 14C was excreted in urine. Dermal application of 14C-HQ (20 microCi) as a 5.4% aqueous solution resulted in near background levels of 14C in blood; the maximum mean blood concentration was 0.65 microg HQ equivalents/g in females and not quantifiable in males. The majority (61-71%) of the 14C was recovered from the skin surface by washing at 24 h. HQ was extensively metabolized following oral dosing with typically <3% of the dose excreted as parent compound. The major urinary metabolites of HQ were glucuronide and O-sulfate conjugates, which represented 45-53% and 19-33%, respectively, of an oral dose. A <5% metabolite was identified as a mercapturic acid conjugate of HQ.

Use of stable isotopically labeled benzene to evaluate environmental exposures

The use of stable, isotopically labeled compounds in controlled exposure experiments at environmentally relevant levels allows for the distinguishing of urinary metabolites associated with known exposure from background levels generally present in the urine. Exposures of volunteers to (13)C-benzene for 2 h at 40+/-10 p.p.b. were conducted after obtaining informed consent, and urinary phenol, catechol, hydroquinone and trans,trans- muconic acid were measured. Each isotopically labeled urinary metabolite was determined in the presence of significantly higher concentrations of the unlabeled metabolite. Following exposure, free and acid hydrolyzed phenol, acid hydrolyzed catechol and hydroquinone, and free trans,trans-muconic acid were determined by GC/MS. The percentage of trans,trans-muconic acid excreted was higher than reported following exposure at occupational levels. The use of isotopically labeled compounds has the potential to investigate the metabolism of common environmental contaminants for validation of toxicokinetic models and improve risk extrapolation from high concentration occupational exposures and animal studies to environmentally relevant pollutant levels.

Environmental benzene exposure assessment for parent-child pairs in Rouen, France

There is a lack of data on environmental benzene exposure in children. In this study, we compared personal benzene exposure and inhalation uptake in a group of children to those of their parents. We also compared levels of urinary benzene metabolites, trans,trans-muconic acid (MA) and hydroquinone (HQ), for those two groups, and assessed the correlation between personal benzene exposure and urinary MA and HQ concentrations. The study was performed on 21, 2-3-year-old children and their parents recruited on a voluntary basis among non-smokers from the three largest day-care centers of the town of Rouen in France. Average benzene concentrations were measured over 5 consecutive days with diffusive samplers. The following simultaneous measurements were carried out: personal exposure of the parents, concentrations inside and outside the day care centers, and inside the volunteer's bedrooms. Morning and evening urine samples were collected during the same period. Benzene personal exposure levels were 14.4+/-7.7 microg/m(3) and 11.09+/-6.15 microg/m(3) in parents and children, respectively. Benzene inhalation uptake estimates were 2.51+/-1.23 microg/kg/day in the group of parents and 5.68+/-3.17 microg/kg/day in the group of children. Detectable levels of MA and HQ were found in 85% and 100% of the samples, respectively. Intra-individual variation of urinary MA and HQ concentrations expressed as a coefficient of variation (CV) ranged from 63 to 232% and from 13 to 144%, respectively. Mean values of MA and HQ (in mg/g creatinine) were 1.6- and 1.8-fold higher in the group of children than in the group of parents (P=0.008 and P<0.0001, respectively). Significant correlations between metabolites levels and benzene were not found.

Bacterial deconjugation of arbutin by Escherichia coli

Cystinol akut containing arbutin was developed as an antiseptic since it liberates hydroquinone in the urine. The in vivo release of hydroquinone from arbutin with or without addition of glusulase or an E. coli suspension was investigated in 4 volunteers. They ingested 6 dragees Cystinol akut (420 mg arbutin equivalent to 168 mg hydroquinone), urine was sampled and assayed by a validated HPLC method.

RESULTS

In comparison to incubation with glusulase the E. coli-suspension resulted in a 2.3 fold higher increase in free hydroquinone. When separating bacteria from the urine, the hydroquinone concentration in bacteria was 20 fold higher than in the supernatant.

CONSEQUENCES

Glucuronic acid or sulfuric acid conjugates of hydroquinones obviously are taken up, enriched and metabolized to hydroquinone by bacteria. Deconjugation of hydroquinone likely is catalyzed by intracellular enzymes presumably present in bacterial cytoplasm; comparable activities in eucaryotic cells usually are localized in lysosomes. Alkalization of the urine seems not to be a prerequisite to improve the antiseptic properties of hydroquinone released from arbutin.

Urinary excretion and metabolism of arbutin after oral administration of Arctostaphylos uvae ursi extract as film-coated tablets and aqueous solution in healthy humans

Bearberry leaves and preparations made from them are traditionally used for urinary tract infections. The urinary excretion of arbutin metabolites was examined in a randomized crossover design in 16 healthy volunteers after the application of a single oral dose of bearberry leaves dry extract (BLDE). There were two groups of application using either film-coated tablets (FCT) or aqueous solution (AS). The urine sample analysis was performed by a validated HPLC coolarray method (hydroquinone) and a validated capillary electrophoresis method (hydroquinone-glucuronide, hydroquinone-sulfate). The total amounts of hydroquinone equivalents excreted in the urine from BLDE were similar in both groups. With FCT, 64.8% of the arbutin dose administered was excreted; with AS, 66.7% was excreted (p = 0.61). The maximum mean urinary concentration of hydroquinone equivalents was a little higher and peaked earlier in the AS group versus the FCT group, although this did not reach statistical significance (Cur max = 1.6893 micromol/ml vs. 1.1250 micromol/ml, p = 0.13; tmax (t midpoint) = 3.60 h vs. 4.40 h, p = 0.38). The relative bioavailability of FCT compared to AS was 103.3% for total hydroquinone equivalents. There was substantial intersubject variability. No significant differences between the two groups were found in the metabolite patterns detected (hydroquinone, hydroquinone-glucuronide, and hydroquinone-sulfate).

Validated methods for direct determination of hydroquinone glucuronide and sulfate in human urine after oral intake of bearberry leaf extract by capillary zone electrophoresis

Bearberry leaf extracts are used in herbal medicinal products for the treatment of lower urinary tract infections. Two metabolites of the major phenolic constituent in the extract, arbutin (hydroquinone-1-O-beta-D-glucoside), must be assumed to be precursors of the active disinfectant principle hydroquinone. In order to assay the renal elimination of these two metabolites. i.e., hydroquinone conjugates with glucuronic and sulfuric acid, two separate capillary electrophoresis methods have been developed. Both methods were validated according to the criteria for validation of pharmaceutical bioanalytical methods as drafted by the US Department of Health and Human Services. 1998. As there is little sample preparation necessary, both methods are very suitable for urine analysis with large sample numbers as frequently coming up in the course of pharmaceutical bioavailability, bioequivalence and pharmacokinetic studies.

Validated method for the determination of hydroquinone in human urine by high-performance liquid chromatography-coulometric-array detection

The paper describes the computer aided method development and validation for the determination of hydroquinone in human urine from a clinical study on renal excretion of hydroquinone metabolites and the release of free hydroquinone in the urinary tract in order to evaluate the proposed urine disinfecting concept. The presented method uses high-performance liquid chromatography on reversed-phase material with a polar endcapping (Aqua-C18, 250x4.6 mm). Selective and sensitive determination (LOQ= 12.5 ng on-column) of the target compound was achieved by electrochemical array detection (CoulArray). Gradient and parameter optimization were supported by DryLab software in order to minimize efforts of the expensive and time-consuming method development. Specificity and selectivity were carried out by separation experiments involving the prodrug arbutin and the metabolites hydroquinone, hydroquinone glucuronide, and hydroquinone sulfate, respectively. Hydroquinone glucuronide reference standard was obtained from in vitro glucuronidation in a rat liver microsomes assay. The method was validated according to the criteria for validation of pharmaceutical bioanalytical methods as drafted by the US Department of Health and Human Services, 1998.

Validation of biomarkers in humans exposed to benzene: urine metabolites

BACKGROUND

The present study was conducted among Chinese workers employed in glue- and shoe-making factories who had an average daily personal benzene exposure of 31+/-26 ppm (mean+/-SD). The metabolites monitored were S-phenylmercapturic acid (S-PMA), trans, trans-muconic acid (t,t-MA), hydroquinone (HQ), catechol (CAT), 1,2, 4-trihydroxybenzene (benzene triol, BT), and phenol.

METHODS

S-PMA, t,t-MA, HQ, CAT, and BT were quantified by HPLC-tandem mass spectrometry. Phenol was measured by GC-MS.

RESULTS

Levels of benzene metabolites (except BT) measured in urine samples collected from exposed workers at the end of workshift were significantly higher than those measured in unexposed subjects (P < 0.0001). The large increases in urinary metabolites from before to after work strongly correlated with benzene exposure. Concentrations of these metabolites in urine samples collected from exposed workers before work were also significantly higher than those from unexposed subjects. The half-lives of S-PMA, t,t-MA, HQ, CAT, and phenol were estimated from a time course study to be 12.8, 13.7, 12.7, 15.0, and 16.3 h, respectively.

CONCLUSIONS

All metabolites, except BT, are good markers for benzene exposure at the observed levels; however, due to their high background, HQ, CAT, and phenol may not distinguish unexposed subjects from workers exposed to benzene at low ambient levels. S-PMA and t,t-MA are the most sensitive markers for low level benzene exposure.

Modifications in the metabolic pathways of benzene in streptozotocin-induced diabetic rat

Benzene is a ubiquitous environmental pollutant primarily metabolized by a cytochrome P-450 (CYP-450) isoenzyme, CYP-450 IIE1. A consistent induction of CYP450 IIE1 has been observed in both rat and human affected by diabetes mellitus. The aim of this study was to evaluate whether streptozotocin (STZ)-induced diabetes determines modifications in the metabolic pathways of benzene in rat. Benzene (100 mg/kg per day, dissolved in corn oil) was administered i.p. once a day for 5 days. Urine samples were collected every day in STZ-treated and normoglycaemic animals, treated and untreated with benzene (n = 10). Urinary levels of trans,trans-muconic acid and of phenol, catechol and hydroquinone (free and conjugated with sulphuryl and glucuronic group) were measured by high-performance liquid chromatography (HPLC). In normoglycaemic rats during the 5 days of treatment with benzene we observed a progressive and significant decrement in the urinary excretion of phenol, phenyl sulphate and glucuronide, catechol, catechol glucuronide, hydroquinone, hydroquinone glucuronide and t,t-muconic acid (P < 0. 05). In the diabetic animals, conversely, the same metabolites showed progressively increasing urinary levels (P < 0.05). Catechol sulphate and hydroquinone sulphate levels were below the instrument's detection limit. In the comparison between diabetic and normoglycaemic benzene treated rats, the inter-group difference was significant (P < 0.05) from day 3 of treatment for t,t-muconic acid, and from day 1 for free and conjugated phenol, free and glucuronide catechol and free hydroquinone. In the normoglycaemic rat exposed to benzene the decreasing trend observed in urinary excretion of free and conjugated metabolites may be due to their capability to reduce cytochromial activity. Conversely, in the diabetic rat, urinary levels of benzene metabolites tended to increase progressively, probably due to the consistent induction of CYP-450 IIE1 observed in diabetes, which would overwhelm the inhibition of this isoenzyme caused by phenolic metabolites. Furthermore, the metabolic switch towards detoxification metabolites observed after administration of high doses of benzene is not allowed in the diabetic because of reduced glutathione-S-transferase activity. As a consequence, higher levels of hydroquinone, phenol and catechol, considered the actual metabolites responsible for benzene toxicity, will accumulate in the diabetic rat. Extrapolating these data to human, we may thus suggest that occupational exposure to benzene of a diabetic subject poses a higher risk level, as his metabolism tends to produce and accumulate higher levels of reactive benzene catabolites.

Development of liquid chromatography-electrospray ionization-tandem mass spectrometry methods for determination of urinary metabolites of benzene in humans

To investigate the ways in which different levels of exposure affect the metabolic activation pathways of benzene in humans, and to examine the relationship between urinary metabolites and other biological markers, we have developed two sensitive and specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays for quantitation of the benzene metabolites trans,transmuconic acid (t,t-MA), S-phenylmercapturic acid (S-PMA), hydroquinone (HQ), catechol (CAT), and for estimation of 1,2,4-trihydroxybenzene (BT). In our first assay, urinary S-PMA and t,t-MA were measured simultaneously by liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring (LC-ESI-MS/MS-SRM) in the negative ionization mode. In this assay, the metabolites [13C6]-S-PMA and [13C6]-t,t-MA were used as internal standards. The efficacy of this specific assay was evaluated in human urine specimens from 28 smokers and 18 nonsmokers serving as the benzene-exposed and nonexposed groups, respectively. The coefficient of variation (CV) of analyses on different days (n = 8) for S-PMA was 7% for samples containing 9.4 micrograms/L urine, and for t,t-MA was 10% for samples containing 0.07 mg/L. The mean levels of S-PMA and t,t-MA in smokers were 1.9-fold (p = 0.02) and 2.1-fold (p = 0.03) higher, respectively, than those in nonsmokers.

Direct high-performance liquid chromatography determination of propofol and its metabolite quinol with their glucuronide conjugates and preliminary pharmacokinetics in plasma and urine of man

Propofol (P) is metabolized in humans by oxidation to 1,4-di-isopropylquinol (Q). P and Q are in turn conjugated with glucuronic acid to the respective glucuronides, propofol glucuronide (Pgluc), quinol-1-glucuronide (Q1G) and quinol-4-glucuronide (Q4G). Propofol and quinol with their glucuronide conjugates can be measured directly by gradient high-performance liquid chromatographic analysis without enzymic hydrolysis. The glucuronide conjugates were isolated by preparative HPLC from human urine samples. The glucuronides of P and Q were present in plasma and urine, P and Q were present in plasma, but not in urine. Quinol in plasma was present in the oxidised form, the quinone. Calibration curves of the respective glucuronides were constructed by enzymic deconjugation of isolated samples containing different concentrations of the glucuronides. The limit of quantitation of P and quinone in plasma are respectively 0.119 and 0.138 microg/ml. The limit of quantitation of the glucuronides in plasma are respectively: Pgluc 0.370 microg/ml, Q1G 1.02 microg/ml and Q4G 0.278 microg/ml. The corresponding values in urine are: Pgluc 0.264 microg/ml, Q1G 0.731 microg/ml and Q4G 0.199 microg/ml. A pharmacokinetic profile of P with its metabolites is shown, and some preliminary pharmacokinetic parameters of P and Q glucuronides are given.

Influence of gender and acetone pretreatment on benzene metabolism in mice exposed by nose-only inhalation

Benzene (BZ) requires oxidative metabolism catalyzed by cytochrome P-450 2E1 (CYP 2E1) to exert its hematotoxic and genotoxic effects. We previously reported that male mice have a two-fold higher maximum rate of BZ oxidation compared with female mice; this correlates with the greater sensitivity of males to the genotoxic effects of BZ as measured by micronuclei induction and sister chromatid exchanges. The aim of this study was to quantitate levels of BZ metabolites in urine and tissues, and to determine whether the higher maximum rate of BZ oxidation in male mice would be reflected in higher levels of hydroxylated BZ metabolites in tissues and water-soluble metabolites in urine. Male and female B6C3F, mice were exposed to 100 or 600 ppm 14C-BZ by nose-only inhalation for 6 h. An additional group of male mice was pretreated with 1% acetone in drinking water for 8 d prior to exposure to 600 ppm BZ; this group was used to evaluate the effect of induction of CYP 2E1 on urine and tissue levels of BZ and its hydroxylated metabolites. BZ, phenol (PHE), and hydroquinone (HQ) were quantified in blood, liver, and bone marrow during exposure and postexposure, and water-soluble metabolites were analyzed in urine in the 48 h after exposure. Male mice exhibited a higher flux of BZ metabolism through the HQ pathway compared with females after exposure to either 100 ppm BZ (32.0 2.03 vs. 19.8 2.7%) or 600 ppm BZ (14.7 1.42 vs. 7.94 + 0.76%). Acetone pretreatment to induce CYP 2E1 resulted in a significant increase in both the percent and mass of urinary HQ glucuronide and muconic acid in male mice exposed to 600 ppm BZ. This increase was paralleled by three- to fourfold higher steady-state concentrations of PHE and HQ in blood and bone marrow of acetone-pretreated mice compared with untreated mice. These results indicate that the higher maximum rate of BZ metabolism in male mice is paralleled by a greater proportion of the total flux of BZ through the pathway for HQ formation, suggesting that the metabolites formed along this pathway may be responsible for the genotoxicity observed following BZ exposure.

Distribution and excretion of radiolabelled tert-butylhydroquinone in Fischer 344 rats

Uniformly 14C-ring-labelled tert-butylhydroquinone (TBHQ) was diluted with non-radioactive TBHQ and administered orally (for excretion studies) to Fischer 344 rats. An average of 72.9% and 10.6% of the administered radioactivity was recovered in the urine and faeces, respectively, of male rats, and 77.3% and 8.2% in the urine and faeces, respectively, of female rats in 4 days. No significant sex-related differences were found in either excretion, tissue distribution or urinary metabolites of TBHQ-derived radiolabel. For distribution studies, intraperitoneal doses were administered to female rats, and tissue levels of radiolabel were determined at various times after dosing. The parent compound quickly disappeared from tissue in vivo. The highest concentrations of radiolabel were found in the liver and kidneys. The urinary metabolites consisted of conjugated TBHQ and unidentified polar substance(s).

Urinary excretion of phenol, catechol, hydroquinone, and muconic acid by workers occupationally exposed to benzene

OBJECTIVES

Animal inhalation studies and theoretical models suggest that the pattern of formation of benzene metabolites changes as exposure to benzene increases. To determine if this occurs in humans, benzene metabolites in urine samples collected as part of a cross sectional study of occupationally exposed workers in Shanghai, China were measured.

METHODS

With organic vapour monitoring badges, 38 subjects were monitored during their full workshift for inhalation exposure to benzene. The benzene urinary metabolites phenol, catechol, hydroquinone, and muconic acid were measured with an isotope dilution gas chromatography mass spectroscopy assay and strongly correlated with concentrations of benzene air. For the subgroup of workers (n = 27) with urinary phenol > 50 ng/g creatinine (above which phenol is considered to be a specific indicator of exposure to benzene), concentrations of each of the four metabolites were calculated as a ratio of the sum of the concentrations of all four metabolites (total metabolites) and were compared in workers exposed to > 25 ppm v < or = 25 ppm.

RESULTS

The median, 8 hour time weighted average exposure to benzene was 25 ppm. Relative to the lower exposed workers, the ratio of phenol and catechol to total metabolites increased by 6.0% (p = 0.04) and 22.2% (p = 0.007), respectively, in the more highly exposed workers. By contrast, the ratio of hydroquinone and muconic acid to total metabolites decreased by 18.8% (p = 0.04) and 26.7% (p = 0.006), respectively. Similar patterns were found when metabolite ratios were analysed as a function of internal benzene dose (defined as total urinary benzene metabolites), although catechol showed a more complex, quadratic relation with increasing dose.

CONCLUSIONS

These results, which are consistent with previous animal studies, show that the relative production of benzene metabolites is a function of exposure level. If the toxic benzene metabolites are assumed to be derived from hydroquinone, ring opened products, or both, these results suggests that the risk for adverse health outcomes due to exposure to benzene may have a supralinear relation with external dose, and that linear extrapolation of the toxic effects of benzene in highly exposed workers to lower levels of exposure may underestimate risk.

Biological monitoring of vehicle mechanics and other workers exposed to low concentrations of benzene

It has been suggested that the threshold limit value (TLV) for the time-weighted average (TWA), of benzene be lowered because of its possible leukemogenic effect at low exposure concentrations. This requires the development of new methods of biological monitoring. In this cross-sectional study the diagnostic power of blood and breath benzene and of urinary phenol, catechol, hydroquinone, S-phenylmercapturic acid, and muconic acid were compared in a population of 410 male workers exposed to benzene in garages, in two coke plants, and in a by-product plant. Benzene exposure was assessed by personal air sampling (charcoal tube and passive dosimeter). In all, 95% of the workers were exposed to less than 0.5 ppm benzene. According to the multiple regression equation, the muconic acid and S-phenylmercapturic acid concentrations detected in nonsmokers exposed to 0.5 ppm benzene were 0.3 mg/g and 6 micrograms/g, respectively (range 0.2-0.6 mg/g and 1.2-8.5 micrograms/g, respectively). With muconic acid very few false-positive test results were found, and this determination remained reliable even around a cutoff level of 0.1 ppm benzene. Moreover, the diagnostic power of this test proved to be good even when diluted or concentrated urine samples were not excluded. S-Phenylmercapturic acid (S-PMA) also performed fairly well. Blood and breath benzene as well as urinary phenol (PH) and hydroquinone (HQ) were clearly less suitable biomarkers than muconic acid (MA). Catechol (CA) was not associated with occupational benzene exposure. According to the results of biological monitoring, the skin resorption of benzene from gasoline or other fuels seems negligible. Correlation, multiple regression, and likelihood ratios consistently showed that MA and S-PMA concentrations were fairly good indicators of benzene exposure in the 0.1- to 1-ppm range, even in a population comprising both smokers and nonsmokers. PH, HQ, CA, and blood and breath benzene were less suitable, if at all, in the same exposure range.

Biomarkers of exposure to low concentrations of benzene: a field assessment

OBJECTIVE

To carry out a comprehensive field investigation to evaluate various conventional and recently developed biomarkers for exposure to low concentrations of benzene.

METHODS

Analyses were carried out on environmental air, unmetabolised benzene in blood and urine, urinary trans, transmuconic acid, and three major phenolic metabolites of benzene: phenol, catechol, and hydroquinone. Validations of these biomarkers were performed on 131 never smokers occupationally exposed to the time weighed average benzene concentration of 0.25 ppm (range, 0.01 to 3.5 ppm).

RESULTS

Among the six biomarkers studied, unmetabolised benzene in urine correlated best with environmental benzene concentration (correlation coefficient, r = 0.76), followed by benzene in blood (r = 0.64). When urinary metabolites were compared with environmental benzene, trans, trans-muconic acid showed a close correlation (r = 0.53) followed by hydroquinone (r = 0.44), and to a lesser extent with urinary phenol (r = 0.38). No correlation was found between catechol and environmental benzene concentrations. Although unmetabolised benzene in urine correlates best with benzene exposure, owing to serious technical drawbacks, its use is limited. Among the metabolites, trans, trans-muconic acid seems to be more reliable than other phenolic compounds. Nevertheless, detailed analyses failed to show that it is specific for monitoring benzene exposures below 0.25 ppm.

CONCLUSION

The overall results suggest that most of the currently available biomarkers are unable to provide sufficient specificity for monitoring of low concentrations of benzene exposure. If a lower occupational exposure limit for benzene is to be considered, the reliability of the biomarker and the technical limitations of measurements have to be carefully validated.

Metabolism of tert-butylhydroquinone to S-substituted conjugates in the male Fischer 344 rat

tert-Butyl-4-hydroxyanisole (BHA) and its demethylated analog, tert-butyl-hydroquinone (TBHQ), are antioxidants used in food. Both BHA and TBHQ have been shown to promote kidney and bladder carcinogenesis in the rat. We have previously demonstrated that glutathione (GSH) conjugates of a variety of hydroquinones are nephrotoxic and proposed that GSH conjugation serves to target these compounds to the kidney. In the present study, we examined the metabolism of TBHQ, focusing on the formation of potentially nephrotoxic sulfur-containing metabolites. 2-tert-Butyl-5-glutathion-S-ylhydroquinone, 2-tert-butyl-6-glutathion-S-ylhydroquinone, and 2-tert-butyl-3,6-bisglutathion-S-ylhydroquinone were identified as biliary metabolites of TBHQ (1.0 mmol/kg, ip) in male F344 rats, accounting for 2.2% of the dose. Liquid chromatography/mass spectroscopic analysis of urine also revealed the presence of additional sulfur-containing metabolites, tentatively identified as 2,5-dihydroxy-3-tert-butyl-thiophenol, 2,5-dihydroxy-4-tert-butylthiophenol, and their S-methyl derivatives. No mercapturic acids of TBHQ were found in the urine. The major biliary and urinary metabolites were TBHQ-glucuronide and TBHQ-sulfate, with a trace of TBHQ excreted unchanged. The results indicate that TBHQ undergoes oxidation and GSH conjugation in vivo in the male F344 rat. These conjugates are excreted into bile and undergo further metabolism prior to excretion in urine. Formation of the S-containing metabolites of TBHQ may occur in amounts sufficient to play a role in the toxicity of TBHQ to kidney and bladder.

Human exposure to naturally occurring hydroquinone

Hydroquinone (HQ) is a nonvolatile chemical used in the photographic, rubber, chemical, and cosmetic industries. HQ is also known to occur in nature as the beta-D-glucopyranoside conjugate (arbutin), and free HQ is a known component of cigarette smoke. Low concentrations of HQ have been detected in the urine and plasma of humans with no occupational or other known exposure to HQ. The studies reported here investigate dietary and other potential sources of HQ and their contribution to HQ concentrations in the plasma and urine of human volunteers. Analysis of possible food sources of HQ by GC indicated significant amounts of arbutin in wheat products (1-10 ppm), pears (4-15 ppm), and coffee and tea (0.1 ppm). Free HQ was found in coffee (0.2 ppm), red wine (0.5 ppm), wheat cereals (0.2-0.4 ppm), and broccoli (0.1 ppm). After consuming a meal including arbutin- and HQ-containing foods, volunteers showed significant increases in plasma and urinary levels of HQ and its conjugated metabolites (total HQ). Mean plasma concentrations of total HQ peaked at 5 times background levels at 2 h after the completion of the meal, and mean urinary excretion rates of total HQ peaked at 12 times background at 2-3 h after the meal. Immediately after smoking four cigarettes in approximately 30 min, mean plasma concentrations of total HQ were maximally 1.5 times background levels; mean urinary excretion rates of total HQ peaked at 2.5 times background at 1-3 h after smoking. These data indicate that considerable human exposure to HQ can result from plant-derived dietary sources and, to a lesser extent, from cigarette smoke.

Evaluation of biomarkers for occupational exposure to benzene

OBJECTIVE

To evaluate the relations between environmental benzene concentrations and various biomarkers of exposure to benzene.

METHODS

Analyses were carried out on environmental air, unmetabolised benzene in urine, trans, trans-muconic acid (ttMA), and three major phenolic metabolites of benzene; catechol, hydroquinone, and phenol, in two field studies on 64 workers exposed to benzene concentrations from 0.12 to 68 ppm, the time weighted average (TWA). Forty nonexposed subjects were also investigated.

RESULTS

Among the five urinary biomarkers studied, ttMA correlated best with environmental benzene concentration (correlation coefficient, r = 0.87). When urinary phenolic metabolites were compared with environmental benzene, hydroquinone correlated best with benzene in air. No correlation was found between unmetabolised benzene in urine and environmental benzene concentrations. The correlation coefficients for environmental benzene and end of shift catechol, hydroquinone, and phenol were 0.30, 0.70, and 0.66, respectively. Detailed analysis, however, suggests that urinary phenol was not a specific biomarker for exposure below 5 ppm. In contrast, ttMA and hydroquinone seemed to be specific and sensitive even at concentrations of below 1 ppm. Although unmetabolised benzene in urine showed good correlation with atmospheric benzene (r = 0.50, P < 0.05), data were insufficient to suggest that it is a useful biomarker for exposure to low concentrations of benzene. The results from the present study also showed that both ttMA and hydroquinone were able to differentiate the background level found in subjects not occupationally exposed and those exposed to less than 1 ppm of benzene. This suggests that these two biomarkers are useful indices for monitoring low concentrations of benzene. Furthermore, these two metabolites are known to be involved in bone marrow leukaemogenesis, their applications in biological monitoring could thus be important in risk assessment.

CONCLUSION

The good correlations between ttMA, hydroquinone, and atmospheric benzene, even at concentrations of less than 1 ppm, suggest that they are sensitive and specific biomarkers for benzene exposure.

Disposition and metabolism of radiolabelled pentachloroanisole in rats and rabbits

Male Sprague-Dawley rats and New Zealand White rabbits were administered 14C-labelled pentachloroanisole (PCA) in corn oil by gavage as single doses of 25 mg/kg and were then placed in individual metabolism cages for as long as 4 days. Peak blood level of radioactivity occurred 6 hr after administration of the dose to rats and between 3 and 4 hr in rabbits; the blood elimination half-life ranged from 8 to 15 hr in rats and averaged 6 hr in rabbits. Rats excreted an average of 54.2% of the administered radiolabel in the urine and 32.4% in the faeces during the 96 hr following the dose; rabbits excreted an average of 84.2 and 13.1% of the radiolabel in the urine and faeces, respectively, during this time. Examination of the metabolites in the rat showed that 60% of the urinary radioactivity was attributable to tetrachlorohydroquinone (TCH), 3% to free pentachlorophenol (PCP) and 29% to conjugated PCP; faecal metabolites were PCP (85.7%), TCH (4.3%) and polar metabolite(s) (10%). In the rabbit, 58% of the urinary radioactivity was attributable to TCH, 8% to free PCP and 34% to conjugated PCP. Faecal metabolites consisted of PCP and conjugated material.

Age-related changes in benzene disposition in male C57BL/6N mice described by a physiologically based pharmacokinetic model

A physiologically based pharmacokinetic (PBPK) model was developed to describe the disposition of benzene in 3- and 18-month C57BL/6N mice and to examine the relevant physiologic and/or biochemical parameters governing previously observed age-related changes in the disposition of benzene. The model developed was based on that of Medinsky et al. (Toxicol. Appl. Pharmacol. 99 (1989) 193-206), with the inclusion of an additional rate constant for urinary elimination of benzene metabolites. Experimentally determined tissue partition coefficients for benzene in 3- and 18-month mice, as well as actual body weights and fat compartment volumes, were included as part of the model. Model simulations were conducted for oral exposure of 3-month mice to 10 and 200 mg benzene/kg and for oral exposure of 18-month mice to 10 and 150 mg benzene/kg. Total amount of benzene metabolized, as well as metabolism of benzene to specific metabolites and their elimination, was simulated. Modeling results for total amount of benzene metabolites eliminated in urine over a 24-h period at 10 mg/kg showed that a greater total amount of benzene metabolites would be excreted by 18-month versus 3-month old mice. At saturating doses of 150 and 200 mg/kg, total amount of benzene metabolites excreted 24 h post-dose was predicted to be equivalent in 18-month mice and 3-month old mice, but the rate of elimination over time was shown to be decreased in 18-month vs. 3-month mice. Decreased urinary elimination of total benzene metabolites was simulated by a smaller renal elimination rate constant in 18-month vs. 3-month mice, which is consistent with decreased renal blood flow noted in aging rodents. These model predictions were consistent with observed in vitro and in vivo experimental data. Model simulations for production of specific metabolites from benzene and elimination in urine agreed well with experimental data in showing no significant age-related changes in formation of benzene metabolites, with the exception of hydroquinone conjugates. Model simulations and experimental data showed decreased total urinary elimination of hydroquinone conjugates in 18-month vs. 3-month mice. The change in hydroquinone conjugate elimination with age was simulated in modeling experiments as an age-related increase in Km for production of hydroquinone conjugates from benzene. The results of this study indicate that age-related changes in physiology are primarily responsible for altered disposition of benzene in aged mice and suggest that concentrations for toxicity of benzene and/or metabolites may differ in target tissues of aged mice.

Metabolism of 2-(glutathion-S-yl)hydroquinone and 2,3,5- (triglutathion-S-yl)hydroquinone in the in situ perfused rat kidney: relationship to nephrotoxicity

2,3,5-(Triglutathion-S-yl)hydroquinone [2,3,5-(triGSyl)HQ] (20 mumol/kg) and 2-(glutathion-S-yl)hydroquinone [2-(GSyl)-HQ] (250 mumol/kg) both cause nephrotoxicity when administered to male rats, although the former is considerably more potent than the latter. To address the issue of the differential potency of these conjugates we investigated the metabolism and toxicity of 2,3,5-(triGSyl)HQ and 2-(GSyl)HQ in the in situ perfused rat kidney. Infusion of 5 and 10 mumol 2,3,5-(triGSyl)HQ into the right renal artery caused a time-dependent elevation in gamma-glutamyl transpeptidase (gamma-GT) excretion into urine produced by both the perfused and the contralateral kidneys. At the lower concentration, gamma-GT excretion was greater from the perfused kidney, whereas gamma-GT excretion from the perfused and contralateral kidneys was the same at the higher concentration. Using HPLC-EC to analyze urine and bile, metabolites of 2,3,5-(triGSyl)HQ (10 mumol) were observed only within the first 30 min of perfusion. At the lower dose (5 mumol) neither parent compound nor metabolites were found in urine or bile. Infusion of 40 mumol 2-(GSyl)HQ into the right renal artery also caused a time-dependent excretion of gamma-GT into urine: excretion being greater from the perfused kidney. HPLC-EC analysis of urine and bile from 2-(GSyl)HQ perfused kidneys demonstrated the formation of three known metabolites; 2-(N-acetyl-cystein-S-yl)HQ (9.2 +/- 0.5 mumol). 2-(cystein-S-ylglycine)HQ (0.8 +/- 0.3 mumol), and 2-(cystein-S-yl)HQ (1.3 +/- 0.3 mumol). Unchanged 2-(GSyl)HQ was detected in the urine and bile (0.8 +/- 0.1 mumol). A greater fraction of the dose (74%) was recovered in urine following infusion of 40 mumol 2-(GSyl)[14C]HQ than of 10 mumol 2,3,5-(triGSyl)[14C]HQ (29%). In contrast, a greater fraction of the dose was retained by the kidney following treatment with 10 mumol 2,3,5-(triGSyl)[14C]HQ than following treatment with 40 mumol 2-(GSyl)[14C]HQ (36 and 11%, respectively). This result suggests that metabolites derived from 2,3,5-(triGSyl)[14C]HQ are more reactive than those derived from 2-(GSyl)[14C]HQ, which is consistent with the finding that 2,3,5-(tricystein-S-yl)hydroquinone exhibits a lower oxidation potential than 2-(cystein-S-yl)hydroquinone. Differences in the reactivity of the metabolites derived from 2,3,5-(triGSyl)[14C]HQ and 2-(GSyl)[14C]HQ probably account for the more potent nephrotoxicity of 2,3,5-(triGSyl)HQ.

Comparative metabolism of the renal carcinogen 1,4-dichlorobenzene in rat: identification and quantitation of novel metabolites

1. The metabolism of 1,4-dichlorobenzene has been studied in the male and female Fisher 344 rat over 72 h after oral administration of 14C-1,4-dichlorobenzene (900 mg = 96.8 microCi/kg). No covalent binding of radioactivity could be detected in samples of liver, kidney, lung and spleen. The major route of excretion was with urine accounting for 41.3% of the dose for male and 37.8% of the dose for female rat within 72 h after dosing. 2. Urinary metabolites of 1,4-dichlorobenzene were identified and quantified. The major metabolites identified in the urine of both the male and female rat, were the sulphate and glucuronide of 2,5-dichlorophenol. Minor amounts of 2,5-dichlorohydroquinone were excreted as an unidentified conjugate. 3. 2-(N-acetyl-cysteine-S-yl)-1,4-dichlorobenzene and 2-(N-acetyl-cysteine-S-yl)-2,3-dihydro-3-hydroxy-1,3-hydroxy-1,4-dich lorobenzen e were minor metabolites excreted in the urine of both sexes. 4. A novel biotransformation pathway for 1,4-dichlorobenzene may be postulated, leading to the urinary excretion of a mercapturic acid of chlorophenol. 5. No marked differences in the distribution and excretion of metabolites of 1,4-dichlorobenzene were observed between the male and female Fisher 344 rat.

Pharmacokinetic interaction between benzene metabolites, phenol and hydroquinone, in B6C3F1 mice

There is strong evidence that metabolites are responsible for adverse effects of benzene. Benzene myelotoxicity, reproduced by coadministering phenol (PH) and hydroquinone (HQ) but not when these benzene metabolites were administered alone, has been postulated to be induced by PH stimulating the myeloperoxidase-mediated oxidation of HQ to the toxic 1,4-benzoquinone in bone marrow. A pharmacokinetic interaction between PH and HQ is also hypothesized to contribute to the observation. Both metabolites are sulfoconjugated and glucuronoconjugated. Sulfoconjugation of phenolic substrates has been shown to approach saturation at high concentrations in rats. Thus, more PH may be converted to HQ and HQ conjugation may be diminished. These effects would increase the amounts of PH and HQ present and result (by further oxidation) in the formation of more 1,4-benzoquinone. To test this hypothesis, we investigated the pharmacokinetics in blood and the recovery of hydroquinone and phenol in urine when the metabolites were administered intraperitoneally alone or in combination at 75 mg/kg each to B6C3F1 mice. The combination resulted in a 2.6-fold increase in the area under the blood concentration-time curve (AUC) of HQ compared to the sum of AUC values observed after administration of each compound alone. The half-life of HQ was also increased from 9 +/- 2 to 15 +/- 3 min. The AUC of PH was increased by a factor of 1.4. The clearance of phenol decreased from 89 +/- 13 ml/min per kilogram when injected alone to 62 +/- 7 ml/min per kilogram after coadministration. A decreased clearance of formation of each conjugate demonstrated that both conjugation pathways were diminished. This interaction may contribute to the observed production of myelotoxicity when these metabolites are coadministered.

Simultaneous determination of hydroquinone, catechol and phenol in urine using high-performance liquid chromatography with fluorimetric detection

A method was developed for simultaneous determination of urinary hydroquinone, catechol and phenol using high-performance liquid chromatography (HPLC) with variable-wavelength fluorimetric detection. Urine samples, after acid hydrolysis, were saturated with sodium sulphate and extracted by diethyl ether. The two buffers used for gradient elution were (A) 10 mM sodium acetate containing 0.5% (v/v) acetic acid and (B) the same as buffer A but containing an additional 20% (v/v) acetonitrile. Hydroquinone, catechol and phenol were separated in a C18 column and detected at 2.9, 6.8 and 13.6 min, respectively. The recovery and reproducibility were generally over 90%. Over 300 extracted samples were analysed and no change in column efficiency was noted. Comparisons were also made with HPLC using ultraviolet (UV) detection and with gas chromatography (GC). The proposed method appears to be more sensitive and reliable than other existing methods. This new method was also validated with urine samples collected from cigarette smokers and from refinery workers exposed to low concentrations of benzene.

Disposition, bioavailability, and serum protein binding of pentachlorophenol in the B6C3F1 mouse

The toxicokinetics of pentachlorophenol (PCP) were studied in B6C3F1 mice, a strain in which PCP was previously found to be carcinogenic. In a crossover design, doses of 15 mg/kg were given intravenously (bolus) and orally (gastric intubation) to six animals. Concentrations of PCP in blood, urine, and feces were measured by capillary gas chromatography with electron-capture detection. After intravenous administration, the values of clearance and volume of distribution were 0.057 +/- 0.007 L/hr/kg and 0.43 +/- 0.06 L/kg, respectively. These two parameters exhibited low intermouse variability (coefficients of variation less than 14%). The elimination half-life was 5.2 +/- 0.6 hr. After oral administration, the PCP peak plasma concentration (28 +/- 7 micrograms/ml) occurred at 1.5 +/- 0.5 hr and absorption was complete (bioavailability = 1.06 +/- 0.09). The elimination half-life was 5.8 +/- 0.6 hr. Only 8% of the PCP dose was excreted unchanged by the kidney. PCP was primarily recovered in urine as conjugates. A portion of the dose was recovered in urine as the mutagen, tetrachlorohydroquinone (5%) (TCHQ), and its conjugates (15%). For both PCP and TCHQ, sulfates accounted for 90% or more of the total conjugates (glucuronides and sulfates).

Metabolism of [14C]benzene by cynomolgus monkeys and chimpanzees

Rodent bioassays indicate that B6C3F1 mice are more sensitive to the carcinogenicity of benzene than are rats. The urinary profile of benzene metabolites is different in rats vs mice. Mice produce higher proportions of hydroquinone conjugates and muconic acid, indicators of metabolism via pathways leading to putative toxic metabolites, than do rats. In both species, metabolism to hydroquinone and muconic acid is favored at low concentrations of benzene, indicating that these pathways are easily saturated. These species differences in the metabolism of benzene make it difficult to predict the health risk to humans and how this risk varies with dose. For this reason, the metabolism of [14C]benzene by cynomolgus monkeys and chimpanzees, animals phylogenetically closer to humans than rodents, was studied. Monkeys were dosed ip with 5, 50, or 500 mg [14C]benzene/kg body wt. Urine was collected for up to 24 hr following exposure and was analyzed for benzene metabolites. The proportion of the administered 14C excreted in the urine of monkeys decreased from approximately 50 to 15% as the dose increased. Phenyl sulfate was the major urinary metabolite. The proportion of hydroquinone conjugates and muconic acid in the monkey's urine decreased as the dose increased. The proportion of catechol conjugates was not affected by dose. The proportion of these metabolites in the urine was quite variable from animal to animal, but the proportion of muconic acid was consistently much lower in the monkey than in the mouse or rat. Three chimpanzees were administered 1 mg [14C]benzene/kg body wt, iv; essentially all of the injected 14C was recovered in the urine. Of the total urinary metabolites, 79% were accounted for by phenyl conjugates and less than 15% by hydroquinone conjugates or muconic acid. Catechol conjugates were not detected. The metabolism of benzene appeared to be qualitatively similar but quantitatively different in the species studied. The mouse, the sensitive rodent species, forms the highest levels of hydroquinone conjugates and muconic acid and the chimpanzee, the lowest. In all animal species studied for the effect of dose on benzene metabolism, as the dose decreased, a larger proportion of the benzene metabolites was represented by hydroquinone conjugates and muconic acid.

Determination of benzene metabolites in urine of mice by solid-phase extraction and high-performance liquid chromatography

A method was developed for quantitative measurement of trans,trans-muconic acid, catechol, hydroquinone and phenol in urine. Hydrolysis of esterified and glucuronized phenolic compounds was effected by specific enzymes. The hydrolysed mixture was purified and separated by solid-phase extraction with an anion exchanger, followed by extraction with diethyl ether. By using a clean-up procedure the natural background from mouse urine could be reduced, so that the detection limit of the metabolites was in the range 3-60 mg/l. Optimization of the chromatographic conditions resulted in a short high-performance liquid chromatography analysis time. Phenol had the longest retention time of about 10 min. The clean-up procedure could also be used for phenylmercapturic acid, an additional benzene metabolite, but for sensitive high-performance liquid chromatographic detection of phenylmercapturic acid other conditions are necessary.

Simultaneous assay of pentachlorophenol and its metabolite, tetrachlorohydroquinone, by gas chromatography without derivatization

A sensitive capillary gas chromatographic method was developed for the simultaneous determination of pentachlorophenol and its major metabolite, tetrachlorohydroquinone, in plasma, urine and feces. The method involved a simple one-step liquid-liquid extraction with diethyl ether and electron-capture detection gas chromatography on a fused-silica capillary column coated with 50% methylsilicone-50% trifluoropropylsilicone. The detection limit of both compounds was 50 ng/ml in plasma (from an initial volume of 0.1 ml), 100 ng/ml in urine and 100 ng/g in feces. Optimal conditions for both chemical and enzymatic hydrolysis were defined to measure conjugates of both pentachlorophenol and tetrachlorohydroquinone in urine. Tetrachlorohydroquinone was found to be unstable in plasma and urine; means to prevent its degradation during sample collection and storage by addition of ascorbic acid and ethylenediaminetetracetic acid are presented. This chromatographic method was shown to be precise, accurate and specific. It was successfully applied to toxicokinetic studies in rat.

Effect of repeated benzene inhalation exposures on benzene metabolism, binding to hemoglobin, and induction of micronuclei

Metabolism of benzene is thought to be necessary to produce the toxic effects, including carcinogenicity, associated with benzene exposure. To extrapolate from the results of rodent studies to potential health risks in man, one must know how benzene metabolism is affected by species, dose, dose rate, and repeated versus single exposures. The purpose of our studies was to determine the effect of repeated inhalation exposures on the metabolism of [14C]benzene by rodents. Benzene metabolism was assessed by characterizing and quantitating urinary metabolites, and by quantitating 14C bound to hemoglobin and micronuclei induction. F344/N rats and B6C3F1 mice were exposed, nose-only, to 600 ppm benzene or to air (control) for 6 hr/day, 5 days/week for 3 weeks. On the last day, both benzene-pretreated and control animals were exposed to 600 ppm, 14C-labeled benzene for 6 hr. Individual benzene metabolites in urine collected for 24 hr after the exposure were analyzed. There was a significant decrease in the respiratory rate of mice (but not rats) pretreated with benzene which resulted in lower levels of urinary [14C]benzene metabolites. The analyses indicated that the only effects of benzene pretreatment on the metabolite profile in rat or mouse urine were a slight shift from glucuronidation to sulfation in mice and a shift from sulfation to glucuronidation in rats. Benzene pretreatment also had no effect, in either species, on formation of [14C]benzene-derived hemoglobin adducts. Mice and rats had similar levels of hemoglobin adduct binding, despite the higher metabolism of benzene by mice. This indicates that hemoglobin adduct formation occurs with higher efficiency in rats. After 1 week of exposure to 600 ppm benzene, the frequency of micronucleated, polychromatic erythrocytes (PCEs) in mice was significantly increased. Exposure to the same level of benzene for an additional 2 weeks did not further increase the frequency of micronuclei in PCEs. These results indicate that repeated exposures to benzene, such as might be encountered by humans as a result of occupational or environmental exposures, are not likely to change or increase benzene metabolism.

Excretion of 1,2,4-benzenetriol in the urine of workers exposed to benzene

Urine samples were collected from 152 workers (64 men, 88 women) who had been exposed to benzene, 53 workers (men only) exposed to a mixture of benzene and toluene, and 213 non-exposed controls (113 men, 100 women). The samples were analysed for 1,2,4-benzentriol (a minor metabolite of benzene) by high performance liquid chromatography. The time weighted average solvent exposure of each worker was monitored by diffusive sampling technique. The urinary concentration of 1,2,4-benzentriol related linearly to the intensity of exposure to benzene both in men and women among workers exposed to benzene, and was suppressed by toluene co-exposure among male workers exposed to a mixture of benzene and toluene. A cross sectional balance study in men at the end of the shift of a workday showed that only 0.47% of benzene absorbed will be excreted into urine as 1,2,4-benzenetriol, in close agreement with previous results in rabbits fed benzene. The concentration of 1,2,4-benzenetriol in urine was more closely related to the concentration of quinol than that of catechol. The fact that phenol and quinol, but not catechol, are precursors of 1,2,4-benzentriol in urine was further confirmed by the intraperitoneal injection of the three phenolic compounds to rats followed by urine analysis for 1,2,4-benzenetriol.

Determination of catechol and quinol in the urine of workers exposed to benzene

Time weighted average concentrations of benzene in breathing zone air (measured by diffusive sampling coupled with FID gas chromatography) and concentrations of catechol and quinol in the urine (collected at about 1500 in the second half of a working week and analysed by high performance liquid chromatography) were compared in 152 workers who were exposed to benzene (64 men, 88 women). The concentration of urinary metabolites was also determined in 131 non-exposed subjects (43 men, 88 women). There was a linear relation between the benzene concentrations in the breathing zone and the urinary concentrations of catechol and quinol (with or without correction for urine density) in both sexes. Neither catechol nor quinol concentration was able to separate those exposed to benzene at 10 ppm from those without exposure. The data indicated that when workers were exposed to benzene at 100 ppm about 25% of benzene absorbed was excreted into the urine as phenolic metabolites, of which 13.2%, 1.6%, and 10.2% are phenol, catechol, and quinol, respectively.

Identification of S-(2,5-dihydroxyphenyl)-cysteine and S-(2,5-dihydroxyphenyl)-N-acetyl-cysteine as urinary metabolites of acetaminophen in the mouse. Evidence for p-benzoquinone as a reactive intermediate in acetaminophen metabolism

S-(2,5-Dihydroxyphenyl)-cysteine and S-(2,5-dihydroxyphenyl)-N-acetyl-cysteine [the cysteine- and N-acetyl-cysteine adducts, respectively, of hydroquinone (HQ)] were identified and quantified in the urine of mice administered [ring-U-14C]acetaminophen [14C]APAP, 200 mg kg-1, i.p.). Urine was collected for 24 h and fractionated by HPLC to isolate the above adducts. These conjugates were then converted to a common derivative, viz. O,O',S-tris-acetyl-3-thio-hydroquinone, which was characterized by GC/MS. Neither of the HQ adducts was detected in the urine of control mice which had not received APAP. Quantification of urinary HQ-cysteine and HQ-N-acetyl-cysteine was performed by HPLC techniques, which indicated that these conjugates accounted for approx. 1.5% of the administered dose of APAP after 24 h, a figure which is equivalent to 6.3% of the corresponding APAP-thiol conjugates in the urine. These findings provide strong indirect evidence that p-benzoquinone is formed as a reactive, but apparently non-hepatotoxic, metabolite of APAP in vivo.

Mutual metabolic suppression between benzene and toluene in man

The exposure intensity during a shift and the metabolite levels in the shift-end urine were examined in male workers exposed to either benzene (65 subjects; the benzene group), toluene (35 subjects; the toluene group), or a mixture of both (55 subjects; the mixture group). In addition, 35 non-exposed male workers (the control group) were similarly examined for urinary metabolites to define background levels. A linear relationship was established between the intensity of solvent exposure and the corresponding urinary metabolite levels (i.e. phenol, catechol and quinol from benzene, and hippuric acid and o-cresol from toluene) in each case when one of the three exposed groups was combined with the control group for calculation. Comparison of regression lines in combination with regression analysis disclosed that urinary levels of phenol and quinol (but not catechol) were lower in the mixture group than in the benzene group when the intensities of exposure to benzene were comparable, indicating that the biotransformation of benzene to phenolic compounds (excluding catechol) in man is suppressed by co-exposure to toluene. Conversely, metabolism of toluene to hippuric acid was suppressed by benzene co-exposure. Conversion of toluene to o-cresol was also reduced by benzene, but to a lesser extent. The significance of the present findings on the mutual suppression of metabolism between benzene and toluene is discussed in relation to solvent toxicology and biological monitoring of exposure to the solvents.

Identification of Escherichia coli by detection of hydroquinone and uracil in the urine system

For rapid identification of Escherichia coli, changes of urinary metabolites incubated with E. coli were investigated by gas chromatography--mass spectrometry. Hydroquinone and uracil were detected and the normal urinary constituent 4-deoxythreonic acid was found to diminish in urine incubated with E. coli. Hydroquinone could not be detected in urine incubated with Klebsiella pneumoniae, Serratia marcescens or Pseudomonas aeruginosa. Although uracil was detected in normal urine, urine incubated with E. coli showed an increased uracil level. Urine incubated with K. pneumoniae, S. marcescens or P. aeruginosa evidenced no such change. A decrease of 4-deoxythreonic acid was noted in urine incubated with S. marcescens or P. aeruginosa. In 7.0 X 10(7) cells of E. coli, 0.33-2.36 micrograms of hydroquinone and 13.4-42.0 micrograms of uracil were detected after 3 h of incubation at 38 degrees C, and production was not changed after 4, 5 or 8 h of incubation. These results suggest that the detection of hydroquinone and uracil in urine is useful for rapid identification of E. coli.

trans,trans-Muconic acid, an open-chain urinary metabolite of benzene in mice. Quantification by high-pressure liquid chromatography

A sensitive h.p.l.c. method is described which separated urinary metabolites from benzene-treated male CD-1 mice. Phenol, trans,trans-muconic acid and quinol in the 48 h urine accounted, respectively, for 12.8-22.8, 1.8-4.7 and 1.5-3.7% of the orally administered single dose of benzene (880, 440 and 220 mg/kg body wt.). Catechol occurred in trace amounts. Ascorbic acid was used to adjust urine pH and increase the extraction efficiency of metabolites, especially muconic acid. It allowed an accurate estimation of quinol by preventing its auto-oxidation. trans,trans-Muconic acid was identified and was unique to benzene as none was detected in urine of mice dosed orally with phenol, catechol or quinol (250, 150 and 200 mg/kg, respectively). The potential existence of a toxic benzene metabolite in the form of an aldehyde precursor of muconic acid in vivo is discussed.

Inverse relationship between melanogenesis and endogenous hydroquinone

Inhibition or stimulation of melanogenesis have been found to occur as a result of the alteration of hydroquinone levels in the body. Substances which stimulate melanogenesis are found to lower the level of hydroquinone in amphibia, and evidence for the relationship is also given by mammalian experiments.

The metabolism of phenylmercury by the rat

The metabolism of [U-(14)C]phenylmercury acetate was studied in the rat. After a single subcutaneous dose a small proportion is excreted unchanged in urine, and a larger amount in bile with some resorption from the gut. The greater part of the dose is broken down in the tissues to yield inorganic mercury which is excreted mainly in faeces, and conjugates of phenol and quinol are excreted in urine. In experiments in vitro phenylmercury is broken down by liver homogenates to release inorganic mercury and benzene; this reaction is effected by the soluble, but not the microsomal, fraction and does not require NADPH or NADH. No elemental mercury is formed under these conditions. It is probable that this reaction occurs in vivo and the benzene produced is rapidly converted into phenol and quinol by microsomal enzymes.

Some new aspects of the metabolism of phenacetin in the rat

1. Four new metabolites of phenacetin in the urine of the rat are described; these are (i) N-acetyl-S-ethylcysteine, (ii) quinol, (iii) acetamide and (iv) probably N-acetyl-S-2-(4-ethoxyacetanilido)cysteine S-oxide. 2. Metabolites (i), (iii) and (iv) were characterized and estimated by g.l.c., by t.l.c., by paper chromatography, by chemical reactions or by radioactive techniques after administration to rats of [ethyl-(14)C]phenacetin and [acetyl-(3)H]phenacetin; metabolite (ii), which was excreted mainly as conjugates of sulphuric acid and glucosiduronic acid, was measured by paper chromatography and characteristic colour reactions after enzymic and chemical hydrolysis of the conjugates. 3. Small amounts of azoxy-4-[ethyl-(14)C]ethoxybenzene and an unknown metabolite were also found in the urine of rats after administration of [ethyl-(14)C]phenacetin. 4. The likely mechanisms and some biological implications of these metabolic reactions are discussed.