Determination of aromatic hydrocarbons and their metabolites in human blood and urine

Environmental chemical exposures in the urine of dogs and people sharing the same households

Introduction:

Urothelial carcinoma (UCC) develops in both humans and dogs and tracks to regions of high industrial activity. We hypothesize that dogs with UCC may act as sentinels for human urothelial carcinogen exposures. The aim of this pilot study was to determine whether healthy people and dogs in the same households share urinary exposures to potentially mutagenic chemical carcinogens.

Methods:

We measured urinary concentrations of acrolein (as its metabolite 3-HPMA), arsenic species, 4-aminobiphenyl, and 4-chlorophenol (a metabolite of the phenoxyherbicide 2,4-D) in healthy dogs and their owners. We assessed possible chemical sources through questionnaires and screened for urothelial DNA damage using the micronucleus assay.

Results:

Biomarkers of urinary exposure to acrolein, arsenic, and 4-chlorophenol were found in the urine of 42 pet dogs and 42 owners, with 4-aminobiphenyl detected sporadically. Creatinine-adjusted urinary chemical concentrations were significantly higher, by 2.8- to 6.2-fold, in dogs compared to humans. Correlations were found for 3-HPMA (r = 0.32, P = 0.04) and monomethylarsonic acid (r = 0.37, P = 0.02) between people and their dogs. Voided urothelial cell yields were inadequate to quantify DNA damage, and questionnaires did not reveal significant associations with urinary chemical concentrations.

Conclusions:

Healthy humans and pet dogs have shared urinary exposures to known mutagenic chemicals, with significantly higher levels in dogs. Higher urinary exposures to acrolein and arsenic in dogs correlate to higher exposures in their owners. Follow-up studies will assess the mutagenic potential of these levels in vitro and measure these biomarkers in owners of dogs with UCC.

Hollow fibre supported liquid membrane extraction for BTEX metabolites analysis in human teeth as biomarkers

The use of human teeth as biomarkers has been previously applied to characterize environmental exposure mainly to metal contamination. Difficulties arise when the contaminants are volatile or its concentration level is very low. This study presents the development of a methodology based on the transport through hollow fibre membrane liquid-phase microextraction (HF-LPME), followed by HPLC-UV measurement, to determine three different metabolites of BTEX contaminants, mandelic acid (MA), hyppuric acid (HA), and methylhippuric acid (4mHA). The driving force for the liquid membrane has been studied by using both non-facilitated (pH gradient 2-12) and facilitated transport (ionic and non-ionic carriers). Enrichment factors of several hundreds were accomplished. Different ionic and non-ionic water insoluble compounds were used as metabolite carriers for the facilitated transport at HF-LPME. Three organic solvents were used to constitute the liquid membrane, dodecane, dihexyl ether and n-decanol. Other parameters affecting the extraction process, such as extraction time, stirring speed, acceptor buffer and salt content were optimised in spiked solutions and selected those that presented the best enrichment factors for all analytes. Final conditions were established for donor solution as 20mL, pH2 of 0.5M NaCl, the OLM (Organic Liquid Membrane) as n-decanol and the acceptor solution as 40μL of 1M NaOH. The selected extraction time was 20h with stirring speed of 500rpm. Validation of the optimised method included the determination of individual linearity range (MA: 0.002-5.7μg; HA: 0.01-7.9μg; 4mHA 0.002-5.3μg), limits of detection (MA: 1.6ng; HA: 0.2ng; 4mHA 0.2ng), repeatability (RSD 7-10%) and reproducibility (5-8%). The developed method was applied to the analysis of MA, HA and 4mHA in teeth samples of 8 workers exposed to BTEX.

The use of biomonitoring data in exposure and human health risk assessment: benzene case study

Abstract A framework of "Common Criteria" (i.e. a series of questions) has been developed to inform the use and evaluation of biomonitoring data in the context of human exposure and risk assessment. The data-rich chemical benzene was selected for use in a case study to assess whether refinement of the Common Criteria framework was necessary, and to gain additional perspective on approaches for integrating biomonitoring data into a risk-based context. The available data for benzene satisfied most of the Common Criteria and allowed for a risk-based evaluation of the benzene biomonitoring data. In general, biomarker (blood benzene, urinary benzene and urinary S-phenylmercapturic acid) central tendency (i.e. mean, median and geometric mean) concentrations for non-smokers are at or below the predicted blood or urine concentrations that would correspond to exposure at the US Environmental Protection Agency reference concentration (30 µg/m(3)), but greater than blood or urine concentrations relating to the air concentration at the 1 × 10(-5) excess cancer risk (2.9 µg/m(3)). Smokers clearly have higher levels of benzene exposure, and biomarker levels of benzene for non-smokers are generally consistent with ambient air monitoring results. While some biomarkers of benzene are specific indicators of exposure, the interpretation of benzene biomonitoring levels in a health-risk context are complicated by issues associated with short half-lives and gaps in knowledge regarding the relationship between the biomarkers and subsequent toxic effects.

Evaluation of metabolism and disposition of GDC-0152 in rats using 14C labeling strategy at two different positions: a novel formation of hippuric acid from 4-phenyl-5-amino-1,2,3-thiadiazole

The compound (S)-1-[(S)-2-cyclohexyl-2-([S]-2-[methylamino]propanamido)acetyl]-N-(4-phenyl-1,2,3-thiadiazol-5-yl)pyrrolidine-2-carboxamide (GDC-0152) is a peptidomimetic small molecule antagonist of inhibitor of apoptosis (IAP) proteins with antitumor activity. The mass balance, pharmacokinetics, tissue distribution and metabolism of GDC-0152 was investigated in rats following intravenous administration of 15 mg/kg of [(14)C]GDC-0152, labeled either at the terminal phenyl ring (A) or at the carbonyl of the 2-amino-2-cyclohexylacetyl moiety (B). In rats, 92.2%-95.1% of the radiolabeled GDC-0152 dose was recovered. Approximately 62.3% and 25.1% of A was excreted in urine and feces, respectively. By contrast, B was excreted almost equally in urine (27.2%), feces (32.2%), and expired air (27.5%). GDC-0152 underwent extensive metabolism, with less than 9% of the dose recovered as parent in excreta. Similarly, in plasma, GDC-0152 represented 16.7% and 7.5% of the area under the curve of the total radioactivity for A and B, respectively. The terminal half-life (t(1/2)) for total radioactivity was longer for B (21.2 hours) than for A (4.59 hours). GDC-0152 was highly metabolized via oxidation and amide hydrolysis, followed by subsequent sulfation and glucuronidation. The most abundant circulating metabolites were the amide hydrolyzed products, M26, M28, M30, M31, and M34, which ranged from 3.5% to 9.0% of total radioactivity. In quantitative whole-body autoradiography studies, the residence of radioactivity in tissues was longer for B than for A, which is consistent with the t(1/2) of the total radioactivity in circulation. A novel 4-phenyl-5-amino-1,2,3-thiadiazole (M28) oxidative cleavage resulted in the formation of hippuric acid (M24). This biotransformation was also observed in rat hepatocyte incubations with para-substituted M28 analogs. In addition, the formation of M24 was inhibited by 1-aminobenzotriazole, which points to the involvement of P450 enzymes.

Nuclear magnetic resonance-based metabolomics for prediction of gastric damage induced by indomethacin in rats

Non-steroidal anti-inflammatory drugs (NSAIDs) have side effects including gastric erosions, ulceration and bleeding. In this study, pattern recognition analysis of the (1)H-nuclear magnetic resonance (NMR) spectra of urine was performed to develop surrogate biomarkers related to the gastrointestinal (GI) damage induced by indomethacin in rats. Urine was collected for 5 h after oral administration of indomethacin (25 mg kg(-1)) or co-administration with cimetidine (100 mg kg(-1)), which protects against GI damage. The (1)H-NMR urine spectra were divided into spectral bins (0.04 ppm) for global profiling, and 36 endogenous metabolites were assigned for targeted profiling. The level of gastric damage in each animal was also determined. Indomethacin caused severe gastric damage; however, indomethacin administered with cimetidine did not. Simultaneously, the patterns of changes in their endogenous metabolites were different. Multivariate data analyses were carried out to recognize the spectral pattern of endogenous metabolites related to indomethacin using partial least square-discrimination analysis. In targeted profiling, a few endogenous metabolites, 2-oxoglutarate, acetate, taurine and hippurate, were selected as putative biomarkers for the gastric damage induced by indomethacin. These metabolites changed depending on the degree of GI damage, although the same dose of indomethacin (10 mg kg(-1)) was administered to rats. The results of global and targeted profiling suggest that the gastric damage induced by NSAIDs can be screened in the preclinical stage of drug development using a NMR based metabolomics approach.

Fast determination of urinary S-phenylmercapturic acid (S-PMA) and S-benzylmercapturic acid (S-BMA) by column-switching liquid chromatography-tandem mass spectrometry

Benzene and toluene are important industrial chemicals and ubiquitous environmental pollutants. The urinary mercapturic acids of benzene and toluene, S-phenylmercapturic acid (S-PMA) and S-benzylmercapturic acids (S-BMA) are specific biomarkers for the determination of low-level exposures. We have developed and validated a fast, specific and very sensitive method for the simultaneous determination of S-PMA and S-BMA in human urine using an automated multidimensional LC-MS-MS-method that requires no additional sample preparation. Analytes are stripped from urinary matrix by online extraction on a restricted access material, transferred to the analytical column and subsequently determined by tandem mass spectrometry using isotopically labelled S-PMA as internal standard. The lower limit of quantification (LLOQ) for both analytes was 0.05 microg/L urine and sufficient to quantify the background exposure of the general population. Precision within series and between series for S-PMA and S-BMA ranged from 1.0% to 12.2%, accuracy was 108% and 100%, respectively. We applied the method on spot urine samples of 30 subjects of the general population with no known exposure to benzene or toluene. Median levels (range) for S-PMA and S-BMA in non-smokers (n=15) were 0.14 microg/L (<0.05-0.26 microg/L) and 8.2 (1.6-77.4 microg/L), respectively. In smokers (n=15), median levels for S-PMA and S-BMA were 1.22 microg/L (0.17-5.75 microg/L) and 11.5 microg/L (0.9-51.2 microg/L), respectively. Due to its automation, our method is well suited for application in large environmental studies.

Biologic monitoring of exposure to environmental chemicals throughout the life stages: requirements and issues for consideration for the National Children's Study

Biomonitoring of exposure is a useful tool for assessing environmental exposures. The matrices available for analyses include blood, urine, breast milk, adipose tissue, and saliva, among others. The sampling can be staged to represent the particular time period of concern: preconceptionally from both parents, from a pregnant woman during each of the three trimesters, during and immediately after childbirth, from the mother postnatally, and from the child as it develops to 21 years of age. The appropriate sample for biomonitoring will depend upon matrix availability, the time period of concern for a particular exposure or health effect, and the different classes of environmental chemicals to be monitored. This article describes the matrices available for biomonitoring during the life stages being evaluated in the National Children's Study; the best biologic matrices for exposure assessment for each individual chemical class, including consideration of alternative matrices; the analytical methods used for analysis, including quality control procedures and less costly alternatives; the costs of analysis; optimal storage conditions; and chemical and matrix stability during long-term storage.

Bioavailability of flavonoids and potential bioactive forms in vivo

Flavonoids are powerful antioxidants in vitro, but their overall functions in vivo have yet to be clarified, whether antioxidant, anti-inflammatory, enzyme inhibitor or inducer, or some other role. The reducing properties of flavonoids might also contribute to redox regulation in cells independently of their antioxidant properties. However, in order to understand their bioactivity in vivo, it is necessary to understand the factors influencing the absorption of flavonoids by the gastrointestinal tract, the nature of the conjugates and metabolites in the circulation and how this influences their antioxidant activities.

Determination of monobromobimane derivatives of phenylmercapturic and benzylmercapturic acids in urine by high-performance liquid chromatography and fluorimetry

A method was developed for the determination in human urine of S-phenylmercapturic (PMA) and S-benzylmercapturic (BMA) acids, metabolites respectively of benzene and toluene. PMA and BMA were determined, after alkaline hydrolysis, to give respectively thiophenol and benzylmercaptan, and coupling of the thiol-containing compounds with monobromobimane (MB), by reversed-phase HPLC on a diphenyl-silica bonded cartridge (100 x 4.6 mm I.D., 5 microm particle size) with fluorimetric detection. Wavelengths for excitation and emission were 375 and 480 nm, respectively. The recovery of PMA and BMA from spiked urines was >90% in the 10-500 microg/l range; the quantification limits were respectively 1 and 0.5 microg/l; day-to-day precision at 42 microg/l was C.V. <7%. The suitability of the proposed procedure for the biological monitoring of exposure to low-level airborne concentrations of benzene and toluene, was evaluated by analyzing the urinary excretion of PMA and BMA in subjects exposed to different sources of aromatic hydrocarbons, namely occupationally-unexposed referents (non-smokers, n=15; moderate smokers, n=8; mean number of cigarettes smoked per-day=17 cig/day) and non-smoker workers occupationally exposed to toluene in maintenance operations of rotogravure machines (non-smokers, n=17). Among referents, non-smokers showed values of PMA ranging from <1 to 4.6 microg/l and BMA from 1.0 to 10.4 microg/l; in smokers, PMA values ranging from 1.2 to 6.7 microg/l and BMA from 9.3 to 39.9 microg/l, were observed. In occupationally exposed non-smoker subjects, BMA median excretion value (23.6 microg/l) was higher than in non-smoker referents (3.5 microg/l) (P<0.001) and individual BMA values (y, microg/l) were associated and increased with airborne toluene concentration (x, mg/m3) according to the equation y=6.5+0.65x (r=0.69, P<0.01, n=17). The proposed analytical method appears to be a sensitive and specific tool for biological monitoring of low-level exposure to benzene and toluene mixtures in occupational and environmental toxicology laboratory.

Gas chromatography-electron-capture detection of urinary methylhippuric acid isomers as biomarkers of environmental exposure to xylene

Methylhippuric acid isomers (MHAs), urinary metabolites of xylenes, were determined, after clean-up by C18-SPE and esterification with hexafluoroisopropanol and diisopropylcarbodiimide, by GC with ECD detection, on an SPB-35 capillary column (30 m, 0.32 mm I.D., 0.25 microm film thickness, beta = 320). S-benzyl-mercapturic acid was used for internal standardization. Chromatographic conditions were: oven temperature 162 degrees C, for 14.2 min; ramp by 30 degrees C/min to 190 degrees C, for 3.5 min; ramp by 30 degrees C/min to 250 degrees C, for 4 min; helium flow rate: 1.7 ml/min; detector and injector temperature: 300 degrees C. The sample (1 microl) was injected with a split injection technique (split ratio 5:1). MHA recovery was >95% in the 0.5-20 micromol/l range; the limit of detection was <0.25 micromol/l; day-to-day precision, at 2 micromol/l, was Cv<10%. Urinary MHAs were determined in subjects exposed to different low-level sources of xylenes: (a) tobacco smoking habit and (b) BTX urban air pollution (airborne xylene ranging from 0.1 to 3.7 micromol/m3). Study (a) showed a significant difference between urinary MHA median excretion values of nonsmokers and smokers (4.6 micromol/l vs. 8.1 micromol/l, p<0.001). Study (b) revealed a significant difference between indoor workers and outdoor workers (4.3 micromol/l vs. 6.9 micromol/l, p<0.001), and evidenced a relationship between MHAs (y, micromol/mmol creatinine) and airborne xylene (x, micromol/m3) (y = 0.085+0.34x; r = 0.82, p<0.001, n = 56). Proposed biomarkers could represent reliable tools to study very low-level exposure to aromatic hydrocarbons such as those observed in the urban pollution due to vehicular traffic or in indoor air quality evaluation.

Effect of diet on the urinary excretion of hippuric acid and other dietary-derived aromatics in rat. A complex interaction between diet, gut microflora and substrate specificity

1. A combined in vivo and in vitro study has been devised to investigate an observation, obtained by 1H NMR of urine, that Alp:AprSD (Wistar derived) rats kept under standard husbandry conditions did not excrete urinary hippuric acid (HA). meta-(hydroxyphenyl)-propionic acid ¿m-HPPA¿ was identified as the major aromatic component in urine samples lacking HA. 2. Examination of urine from Alp:APrSD and Zücker (obese negative) rats fed various diets showed that the lack of HA/presence of m-HPPA was due to diet and not to the strain of animal. This observation was reinforced by the demonstration that the administration of benzoic acid (BA) to rats not previously excreting urinary HA resulted in the return of this component to the urinary excretion profile. Thus rats receiving the standard diet were still capable of glycine conjugation. 3. Changing the diet of rats excreting m-HPPA led to the cessation of m-HPPA excretion and the return of HA urine excretion. Interestingly, switching back to the original diet did not cause the loss of HA and the re-emergence of m-HPPA. 4. In vitro studies on the two enzyme systems responsible for glycine conjugation (benzoyl CoA:synthetase and benzoyl CoA:glycine N-acyltransferase) in isolated liver mitochondria showed that m-HPPA did not inhibit either enzyme. However, m-HPPA was not found to be a substrate for the first reaction step explaining why it was found in the urine as the free acid and not as a glycine conjugate. 5. The absence and presence of m-HPPA and hippuric acid is suggested to be due to a combination of differences in dietary precursors of substrates for glycine conjugation and a dietary dependent redistribution of the intestinal microflora responsible for breakdown of plant phenolics and aromatic amino acids. Taken collectively this study emphasises how a simple diet change can cause a profound change in metabolism.

Simultaneous determination of benzene, toluene, ethylbenzene, and xylenes in urine by thermal desorption-gas chromatography

The determination of metabolites of benzene, toluene, ethylbenzene, and xylenes in urine has been used to assess human exposure to these compounds. The analyses of urine samples for these metabolites are tedious and time consuming. The determination of unmetabolized individual compounds in urine has been studied previously with some success. A simultaneous determination of several unmetabolized VOC compounds in urine by thermal desorption-gas chromatography was conducted to assess the exposure of smokers and nonsmokers to these compounds. The method of thermal desorption-GC was sensitive enough to detect a significant difference in exposure levels due to the contribution of light smoking in the environmentally-exposed group.

On-line supported liquid membrane-liquid chromatography with a phenol oxidase-based biosensor as a selective detection unit for the determination of phenols in blood plasma

The potential of on-line combination of supported liquid membrane extraction and column liquid chromatography with a phenol oxidase-based biosensor as a selective detection unit has been investigated for the determination of phenols in human plasma. The phenols are selectively extracted into a porous PTFE (polytetraflouroethene) membrane impregnated with a water-immiscible organic solvent and further into an alkaline acceptor phase. Via an ion-exchange interface, the analytes are transferred to a reversed-phase column where they are separated and detected using the biosensor. No sample pretreatment before the extraction, except centrifugation, is made. Due to the high selectivity both in the extraction and in the detection steps and to the fact that the demands on the chromatographic separation are low, a quick separation using an eluent with a low concentration of organic modifier can be made, without affecting the biosensor response. Detection limits below the 50 microg/l level in blood plasma were obtained for the three model compounds, phenol, p-cresol and 4-chlorophenol.

Gas chromatographic-mass spectrometric identification and quantitation of urinary phenols after derivatization with 4-carbethoxyhexafluorobutyryl chloride, a novel derivative

Urinary phenol is analyzed widely to determine benzene exposure in humans. Most methods utilize direct measurements of phenols after extraction from urine using gas chromatography or high-performance liquid chromatography. We describe a novel derivatization of urinary phenols using 4-carbethoxyhexafluorobutyryl chloride after extraction from urine and subsequent analysis by gas chromatography-mass spectrometry. The derivative elutes at significantly higher temperature than phenol and the method is free from interferences from more volatile components in urine. We also observed excellent chromatographic properties of these derivatives. In addition, we observed strong molecular ions for the 4-carbethoxyhexafluorobutyryl derivative of phenol (m/z 344), p-cresol (m/z 358) and the internal standard 3,4-dimethylphenol (m/z 372) and other characteristic ions in the electron ionization, thus aiding in unambiguous identification of these compounds. The protonated molecular ions (m/z 373 for derivatized phenol, m/z 359 for derivatized p-cresol and m/z 373 for the internal standard) were the base peaks (relative abundance 100%) in the chemical ionization, although other secondary peaks were less abundant. The assay is linear for phenol concentration of 1-100 mg/l. The within-run and between-run precisions were 4.8% (mean = 52.4, S.D. = 2.5 mg/l) and 8.1% (mean = 53.0, S.D. = 4.3 mg/l) respectively, and the detection limit was 0.5 mg/l.

Determination of benzene and its metabolites: application in biological monitoring of environmental and occupational exposure to benzene

Methods for the biological monitoring of benzene and its metabolites in exhaled air, blood and urine are reviewed. Analysis of benzene in breath can be carried out by using an exhaled-air collection tube and direct analysis by GC or GC-MS; however, this technique is less reliable when compared to analysis using blood or urine. For the determination of non-metabolized benzene in blood and urine, GC head-space analysis is recommended. Phenol, the major metabolite of benzene can be monitored by either HPLC or GC methods. However, urinary phenol has proved to be a poor biomarker for low-level benzene exposure. Recent studies have shown that trans,trans-muconic acid, a minor metabolite of benzene can be determined using HPLC with UV detection. This biomarker can be used for detection of low-level benzene exposure. Urinary S-phenylmercapturic acid is another sensitive biomarker for benzene, but it can be detected only by GC-MS. Hydroquinone, catechol and 1,2,4-benzenetriol can be measured using HPLC with either ultraviolet or fluorimetric detection. Nevertheless, their use for low-level assessment requires further studies. Eventually, for the assessment of health risks caused by benzene, biological-exposure reference values need to be established before they can be widely used in a field setting.

Blood and urinary benzene determined by headspace gas chromatography with photoionization detection: application in biological monitoring of low-level nonoccupational exposure

A simple and sensitive gas chromatography (GC) headspace method was developed for the determination of benzene in blood and urine. 1.0 ml of venous blood or urine sample in a headspace vial containing chlorobenzene as an internal standard was incubated at 60 degrees C for 30 min and 0.5 ml headspace gas was used for GC analysis. Unmetabolized benzene in blood or urine was detected at 2.5 min using a silicone gum capillary column and a photoionization detector. The proposed method appears to be more sensitive and reliable than other existing methods, with recovery and reproducibility generally over 90% and a detection limit of 0.64 and 0.51 nmol/l for blood and urinary benzene, respectively. The proposed method was validated with blood and urine samples collected from 25 nonsmokers and 50 smokers. The blood and urine concentrations of benzene in nonsmokers were significantly lower (P < 0.001) than those in smokers: the mean concentrations for blood and urinary benzene, respectively, were 1.42 and 4.21 nmol/l for nonsmokers and 1.49 and 5.19 nmol/l for smokers. A significant correlation (r = 0.61, P < 0.001) was also found between benzene in blood and benzene in urine. These findings suggest that benzene in urine as well as benzene in blood can be used for the biological monitoring of low levels of benzene exposure. Although there was a close correlation between benzene in blood and benzene in urine, no correlation was found between benzene in blood or benzene in urine and the number of cigarettes smoked.

Determination of phenol in urine by high-performance liquid chromatography with on-line precolumn enzymatic hydrolysis of the conjugates

A precolumn enzyme reactor containing beta-glucosidase immobilized on LC-NH2 packed-material beads was used on-line with HPLC for determining the glucuronide/sulphate metabolites of benzene. After dilution with phosphate buffer (pH 6.8), the urine sample was injected into the HPLC system directly. Subsequently, after hydrolysis of the conjugates, phenol was produced in the enzyme reactor and was separated from other urinary components on a reversed-phase C18 column with fluorescence detection. A switching valve assembly was used to control the passage of the sample and the eluent into the reactor to prevent damage to the enzyme by the elution solvent. Factors affecting the enzymatic hydrolysis were investigated. The proposed method provides a simple and rapid procedure for urinary phenol determination. The calibration graph was linear in the range 0.25-5.0 ppm with a good correlation coefficient (r = 0.999), and in the range 0.05-1.0 ppm with r = 0.981. The detection limit was 10 ppb and the relative standard deviation was less than 2.27%. Application of the method is illustrated by the analysis of a urine sample collected from a gas station worker.

GC/MS determination of N-phenylvaline, a possible biomarker for benzene exposure in human hemoglobin by the "N-alkyl Edman method"

We report the application of a modified Edman degradation procedure to the analysis of benzene oxide adducts at the N-terminal valine of human hemoglobin (Hb). Benzene oxide is thought to be formed in the liver from benzene and adduct formation with macromolecules is therefore likely to occur. We assumed that benzene oxide could covalently bind to hemoglobin after leaving the hepatic tissue. The "N-alkyl Edman method" was adapted for the approach to investigate this hypothesis. Using capillary gas chromatography/mass spectrometry (GC/MS) with negative chemical ionization, we could not detect N-phenylvaline in blood samples from persons occupationally exposed to benzene. We conclude that adducts of benzene oxide to the N-terminal valine of Hb are not formed in detectable amounts in vivo and consequently are not suitable for biomonitoring purposes. This result clearly indicates that other reactive benzene metabolites have to be taken into account not only in the search for a biomarker but also as the ultimate carcinogenic species.