Hydrolytic Cleavage Products of Globin Adducts in Urine as Possible Biomarkers of Cumulative Dose: Proof of Concept Using Styrene Oxide as a Model Adduct-Forming Compound

Toxicokinetic relationship between the adducts in globin and their cleavage products in the urine: Implications for human biomonitoring

Globin adducts of various chemicals, persisting in organism over the whole lifetime of erythrocytes, have been used as biomarkers of cumulative exposures to parent compounds. After removal of aged erythrocytes from the bloodstream, cleavage products of these adducts are excreted with urine as alternative, non-invasively accessible biomarkers. In our biomonitoring studies on workers exposed to ethylene oxide, its adduct with globin, N-(2-hydroxyethyl)valine, and the related urinary cleavage product N-(2-hydroxyethyl)-L-valyl-L-leucine have been determined. To describe a toxicokinetic relationship between the above types of biomarkers, a general compartmental model for simulation of formation and removal of globin adducts has been constructed in the form of code in R statistical computing environment. The essential input variables include lifetime of erythrocytes, extent of adduct formation following a single defined exposure, and parameters of exposure scenario, while other possible variables are optional. It was shown that both biomarkers reflect the past exposures differently as the adduct level in globin is a mean value of adduct levels across all compartments (subpopulations of erythrocytes of the same age) while excretion of cleavage products reflects the adduct level in the oldest compartment. Application of the model to various scenarios of continuous exposure demonstrated its usefulness for human biomonitoring data interpretation.

New Aminobiphenylcysteine Derivatives in Globin and Urine of Rats Dosed with 4-Aminobiphenyl, a Tobacco Smoke Carcinogen

The 4-biphenylnitrenium ion (BPN), a reactive metabolic intermediate of the tobacco smoke carcinogen 4-aminobiphenyl (4-ABP), can react with nucleophilic sulfanyl groups in glutathione (GSH) as well as in proteins. The main site of attack of these S-nucleophiles was predicted using simple orientational rules of aromatic nucleophilic substitution. Thereafter, a series of presumptive 4-ABP metabolites and adducts with cysteine were synthesized, namely, S-(4-amino-3-biphenyl)cysteine (ABPC), N-acetyl-S-(4-amino-3-biphenyl)cysteine (4-amino-3-biphenylmercapturic acid, ABPMA), S-(4-acetamido-3-biphenyl)cysteine (AcABPC), and N-acetyl-S-(4-acetamido-3-biphenyl)cysteine (4-acetamido-3-biphenylmercapturic acid, AcABPMA). Then, globin and urine of rats dosed with a single ip dose of 4-ABP (27 mg/kg b.w.) was analyzed by HPLC-ESI-MS². ABPC was identified in acid-hydrolyzed globin at levels of 3.52 ± 0.50, 2.74 ± 0.51, and 1.25 ± 0.12 nmol/g globin (mean ± S.D.; n = 6) on days 1, 3, and 8 after dosing, respectively. In the urine collected on day 1 (0-24 h) after dosing, excretion of ABPMA, AcABPMA, and AcABPC amounted to 1.97 ± 0.88, 3.09 ± 0.75, and 3.69 ± 1.49 nmol/kg b.w. (mean ± S.D.; n = 6), respectively. On day 2, excretion of the metabolites decreased by one order of magnitude followed by a slower decrease on day 8. Regarding the possible formation of AcABPC from ABPC, N-acetylation of the amino group at the biphenyl moiety prior to that at cysteine appears to be very unlikely. Thus, the structure of AcABPC indicates the involvement of N-acetyl-4-biphenylnitrenium ion (AcBPN) and/or its reactive ester precursors in in vivo reactions with GSH and protein-bound cysteine. ABPC in globin might become an alternative biomarker of the dose of toxicologically relevant metabolic intermediates of 4-ABP.

HPLC-ESI-HRMS2 Determination of N-(2-Hydroxyethyl)-l-valyl-l-leucine in Human Urine: Method Validation

Biomonitoring of human exposure to reactive electrophilic chemicals such as ethylene oxide (EO) has been commonly based on the determination of adducts with N-terminal valine in blood protein globin, but a systematic search has also been undertaken to find surrogate markers enabling non-invasive sampling. Recently, N-(2-hydroxyethyl)-L-valyl-L-leucine (HEVL) has been identified as an ultimate cleavage product of EO-adducted globin in the urine of occupationally exposed workers. Herein, full validation of the analytical procedure consisting of solid-phase extraction of HEVL from urine samples (2 mL) followed by high-performance liquid chromatography-electrospray ionization-high-resolution mass spectrometry determination using deuterium-labeled HEVL as an internal standard (IS) is described. Method limit of quantitation is 0.25 ng/mL, and its selectivity is excellent as demonstrated by the invariable ratio of the qualifier and quantifier ion intensities across diverse urine samples and synthetic standard. The linear calibration model was applicable over the whole concentration range tested (0.25-10 ng/mL). The method accuracy assessed as a recovery of HEVL using a spiking experiment was 98-100%. Within-day precision of the method ranged from 1.8% to 3.0%, while the results from consecutive analytical runs conducted within 1 week or within 10-150 weeks differed in the range of 2.2-9.7%. The stability study on urine samples (-20°C up to 3 years, freeze-and-thaw up to 10 cycles) as well as on aqueous solutions (5°C up to 4 months) indicated no relevant changes in HEVL concentration (≤4%) over the time tested. Analytical responses of both HEVL and IS correlated with urinary creatinine as an index of matrix composition, but this matrix effect was mostly eliminated using the HEVL/IS peak area ratio, attaining the IS-normalized relative matrix effect <3%. In conclusion, the method complied successfully with the bioanalytical method validation criteria, making it a reliable tool for HEVL determination in human biomonitoring.

Hemoglobin adducts of acrylamide in human blood - What has been done and what is next?

Acrylamide forms in many commonly consumed foods. In animals, acrylamide causes tumors, neurotoxicity, developmental and reproductive effects. Acrylamide crosses the placenta and has been associated with restriction of intrauterine growth and certain cancers. The impact on human health is poorly understood and it is impossible to say what level of dietary exposure to acrylamide can be deemed safe as the assessment of exposure is uncertain. The determination of hemoglobin (Hb) adducts from acrylamide is increasingly being used to improve the exposure assessment of acrylamide. We aim to outline the literature on Hb adduct levels from acrylamide in humans and discuss methodological issues and research gaps. A total of 86 studies of 27,966 individuals from 19 countries were reviewed. Adduct levels were highest in occupationally exposed individuals and smokers. Levels ranged widely from 3 to 210 pmol/g Hb in non-smokers and this wide range suggests that dietary exposure to acrylamide varies largely. Non-smokers from the US and Canada had slightly higher levels as compared with non-smokers from elsewhere, but differences within studies were larger than between studies. Large studies with exposure assessment of acrylamide and related adduct forming compounds from diet during early-life are encouraged for the evaluation of health effects.

Quo vadis blood protein adductomics?

Chemicals are measured regularly in air, food, the environment, and the workplace. Biomonitoring of chemicals in biological fluids is a tool to determine the individual exposure. Blood protein adducts of xenobiotics are a marker of both exposure and the biologically effective dose. Urinary metabolites and blood metabolites are short term exposure markers. Stable hemoglobin adducts are exposure markers of up to 120 days. Blood protein adducts are formed with many xenobiotics at different sites of the blood proteins. Newer methods apply the techniques developed in the field of proteomics. Larger adducted peptides with 20 amino acids are used for quantitation. Unfortunately, at present the methods do not reach the limits of detection obtained with the methods looking at single amino acid adducts or at chemically cleaved adducts. Therefore, to progress in the field new approaches are needed.

Novel aminoarylcysteine adducts in globin of rats dosed with naphthylamine and nitronaphthalene isomers

Novel aminonaphthylcysteine (ANC) adducts, formed via naphthylnitrenium ions and/or their metabolic precursors in the biotransformation of naphthylamines (NA) and nitronaphthalenes (NN), were identified and quantified in globin of rats dosed intraperitoneally with 0.16 mmol/kg b.w. of 1-NA, 1-NN, 2-NA and 2-NN. Using HPLC-ESI-MS2 analysis of the globin hydrolysates, S-(1-amino-2-naphthyl)cysteine (1A2NC) together with S-(4-amino-1-naphthyl)cysteine (4A1NC) were found in rats given 1-NA or 1-NN, and S-(2-amino-1-naphthyl)cysteine (2A1NC) in those given 2-NA or 2-NN. The highest level of ANC was produced by the most mutagenic and carcinogenic isomer 2-NA (35.8 ± 5.4 nmol/g globin). The ratio of ANC adduct levels for 1-NA, 1-NN, 2-NA and 2-NN was 1:2:100:3, respectively. Notably, the ratio of 1A2NC:4A1NC in globin of rats dosed with 1-NA and 1-NN differed significantly (2:98 versus 16:84 respectively), indicating differences in mechanism of the adduct formation. Moreover, aminonaphthylmercapturic acids, formed via conjugation of naphthylnitrenium ions and/or their metabolic precursors with glutathione, were identified in the rat urine. Their amounts excreted after dosing rats with 1-NA, 1-NN, 2-NA and 2-NN were in the ratio 1:100:40:2, respectively. For all four compounds tested, haemoglobin binding index for ANC was several-fold higher than that for the sulphinamide adducts, generated via nitrosoarene metabolites. Due to involvement of electrophilic intermediates in their formation, ANC adducts in globin may become toxicologically more relevant biomarkers of cumulative exposure to carcinogenic or non-carcinogenic arylamines and nitroarenes than the currently used sulphinamide adducts.

N-(2-Hydroxyethyl)-l-valyl-l-leucine: a novel urinary biomarker of ethylene oxide exposure in humans

Ethylene oxide (EO), a carcinogenic chemical used as an industrial intermediate and sterilant, forms covalent adducts with DNA and proteins. The adduct with N-terminal valine [N-(2-hydroxyethyl)-l-valine, HEV] in blood protein globin has been employed as a principal biomarker of cumulative exposures to EO. However, as sampling of blood is inconvenient in routine occupational health practice, a non-invasive alternative to globin analysis has been investigated. Following identification of N-(2-hydroxyethyl)-l-valyl-l-leucine (HEVL) as ultimate cleavage product of EO-adducted globin excreted in the rat urine, here we report for the first time on the presence of HEVL in the urine of humans. In 18 sterilization workers, urinary HEVL ranged from 0.67 to 11.98 μg/g creatinine (mean ± SD: 5.04 ± 3.14 μg/g creat) and correlated with HEV: HEVL (μg/g creat) = 0.833 HEV (nmol/g globin) + 1.19 (R² = 0.45). As unexpectedly high levels of urinary HEVL were found also in controls (mean ± SD: 0.97 ± 0.37 μg/g creat, n = 32), HEVL is not proposed for the accurate assessment of sub-ppm exposures to EO. On the other hand, non-invasive sampling and facile work-up procedure predetermine HEVL for screening purposes to identify subjects approaching to or exceeding occupational exposure limit for EO (1.8 mg/m³) to be re-examined by the more sensitive reference analysis for HEV.

Dilysine-Methylene Diphenyl Diisocyanate (MDI), a Urine Biomarker of MDI Exposure?

Biomonitoring of methylene diphenyl diisocyanate (MDI) in urine may be useful in industrial hygiene and exposure surveillance approaches toward disease (occupational asthma) prevention and in understanding pathways by which the internalized chemical is excreted. We explored possible urine biomarkers of MDI exposure in mice after respiratory tract exposure to MDI, as glutathione (GSH) reaction products (MDI-GSH), and after skin exposure to MDI dissolved in acetone. LC-MS analyses of urine identified a unique m/ z 543.29 [M + H]+ ion from MDI-exposed mice but not from controls. The m/ z 543.29 [M + H]+ ion was detectable within 24 h of a single MDI skin exposure and following multiple respiratory tract exposures to MDI-GSH reaction products. The m/ z 543.29 [M + H]+ ion possessed properties of dilysine-MDI, including (a) an isotope distribution pattern for a molecule with the chemical formula C27H38N6O6, (b) the expected collision-induced dissociation (CID) fragmentation pattern upon MS/MS, and (c) a retention time in reversed-phase LC-MS identical to that of synthetic dilysine-MDI. Further MDI-specific Western blot studies suggested albumin (which contains multiple dilysine sites susceptible to MDI carbamylation) as a possible source for dilysine-MDI and the presence of MDI-conjugated albumin in urine up to 6 days after respiratory tract exposure. Two additional [M + H]+ ions ( m/ z 558.17 and 863.23) were found exclusively in urine of mice exposed to MDI-GSH via the respiratory tract and possessed characteristics of previously described cyclized MDI-GSH and oxidized glutathione (GSSG)-MDI conjugates, respectively. Together the data identify urinary biomarkers of MDI exposure in mice and possible guidance for future translational investigation.

N-(2-Hydroxyethyl)-L-valyl-L-leucine in rat urine as a hydrolytic cleavage product of ethylene oxide adduct with globin

Ethylene oxide (EO), a genotoxic industrial chemical and sterilant, forms covalent adducts with DNA and also with nucleophilic amino acids in proteins. The adduct with N-terminal valine in globin [N-(2-hydroxyethyl)valine (HEV)] has been used in biomonitoring of cumulative exposures to EO. Here we studied in rats the fate of EO-adducted N-termini of globin after life termination of the erythrocytes. Rat erythrocytes were incubated with EO to produce the HEV levels in globin at 0.4-13.2 µmol/g as determined after acidic hydrolysis. Alternative hydrolysis of the isolated globin with enzyme pronase afforded N-(2-hydroxyethyl)-L-valyl-L-leucine (HEVL) and N-(2-hydroxyethyl)-L-valyl-L-histidine (HEVH), the EO-adducted N-terminal dipeptides of rat globin α- and β-chains, respectively. The ratio of HEVL/HEVH (1:3) reflected higher reactivity of EO with the β-chain. The EO-modified erythrocytes were then given intravenously to the recipient rats. HEVL and HEVH were found to be the ultimate cleavage products excreted in the rat urine. Finally, rats were dosed intraperitoneally with EO, 50 mg/kg. Herein, the initial level of globin-bound HEVL (11.7 ± 1.3 nmol/g) decreased almost linearly over 60 days corresponding to the life span of rat erythrocytes. Daily urinary excretion of HEVL was almost constant for 30-40 days, decreasing faster in the subsequent phase of elimination. Recoveries of the total urinary HEVL from its globin-bound form were 84 ± 6% and 101 ± 17% after administrations of EO and the EO-modified erythrocytes, respectively. In conclusion, urinary HEVL appears to be a promising novel non-invasive biomarker of human exposures to EO.

S-(3-Aminobenzanthron-2-yl)cysteine in the globin of rats as a novel type of adduct and possible biomarker of exposure to 3-nitrobenzanthrone, a potent environmental carcinogen

3-Nitrobenzanthrone (3-NBA), a potent environmental mutagen and carcinogen, is known to be activated in vivo to 3-benzanthronylnitrenium ion which forms both NH and C2-bound adducts with DNA and also reacts with glutathione giving rise to urinary 3-aminobenzanthron-2-ylmercapturic acid. In this study, acid hydrolysate of globin from rats dosed intraperitoneally with 3-NBA was analysed by HPLC/MS to identify a novel type of cysteine adduct, 3-aminobenzanthron-2-ylcysteine (3-ABA-Cys), confirmed using a synthesised standard. The 3-ABA-Cys levels in globin peaked after single 3-NBA doses of 1 and 2 mg/kg on day 2 to attain 0.25 and 0.49 nmol/g globin, respectively, thereafter declining slowly to 70-80% of their maximum values during 15 days. After dosing rats for three consecutive days with 1 mg 3-NBA/kg a significant cumulation of 3-ABA-Cys in globin was observed. 3-ABA-Cys was also found in the plasma hydrolysate. Herein, after dosing with 1 and 2 mg 3-NBA/kg the adduct levels peaked on day 1 at 0.15 and 0.51 nmol/ml plasma, respectively, thereafter declining rapidly to undetectable levels on day 15. In addition, sulphinamide adducts were also found in the exposed rats, measured indirectly as 3-aminobenzanthrone (3-ABA) split off from globin by mild acid hydrolysis. Levels of both types of adducts in the globin samples parallelled very well with 3-ABA/3-ABA-Cys ratio being around 1:8. In conclusion, 3-ABA-Cys is the first example of arylnitrenium-cysteine adduct in globin representing a new promising class of biomarkers to assess cumulative exposures to aromatic amines, nitroaromatics and heteroaromatic amines.

Biological fate of styrene oxide adducts with globin: Elimination of cleavage products in the rat urine

The in vivo fate of globin adducts with styrene 7,8-oxide (SO), an electrophilic metabolic intermediate of styrene, was studied in male Wistar rats dosed intraperitoneally with racemic SO, 100mg/kg b.w. Regioisomeric hydroxy(phenyl)ethyl (HPE) adducts at Cys, N-terminal Val, Lys and His in globin were determined and their elimination from blood was followed during 60days, corresponding to life span of rat erythrocytes. In the rat urine, Nα-acetylated products of hydrolytic cleavage of the HPE adducts with Cys, Lys and His were determined. On the first day post-exposure, abundant Nα-acetyl-HPE-Cys adducts (mercapturic acids) formed via direct conjugation of SO with hepatic glutathione were excreted rapidly, but then a much slower phase of elimination reflecting formation of Nα-acetyl-HPE-Cys via cleavage of the adducted globin was observed. A two-phase elimination occurred also in urinary Nα-acetyl-HPE adducts with His and Lys. While a decline by 75-85% during the first 7days post-exposure most likely reflected elimination of adducted albumin, the subsequent slow decline until day 60 corresponded to elimination kinetics of the adducted globin. Thus, the study not only provided original data on the fate of SO-globin adducts but also allowed to reveal general toxicokinetics properties of the urinary cleavage products as a novel type of chemical exposure biomarkers.