Biotransformation of diethenylbenzenes. III: Identification of metabolites of 1,3-diethenylbenzene in rat

Glutathione pathway in ethylbenzene metabolism: novel biomarkers of exposure in the rat

Glutathione pathway was specifically studied in rats exposed by inhalation to a range of ethylbenzene vapours (5-2000 ppm). Urines were collected during exposure (6h) and over the 18 h following the exposure. The potential metabolites coming from either side-chain or ring oxidation were synthesized: 1-, 2-phenylethylmercapturic acids (1-, and 2-PEMA) and 2-, 3- and 4-ethylphenylmercapturic acids (2-, 3-, and 4-EPMA). Their synthesis was fully described and the molecules characterized. Urine samples were analysed using a selective HPLC-fluorescence method. Among the five metabolites, 2-PEMA was never observed in any urine sample. By contrast, 1-PEMA was discovered in its two diastereomeric forms, and it was shown that one of them was mainly present. 2-EPMA, 3-EPMA and 4-EPMA (in the ratio 1:2:6) were also found, and their combined excretion levels were similar to that of 1-PEMA. The atmospheric concentrations and urinary excretions yielded very close correlations which allow us to consider these mercapturic acids as novel ethylbenzene exposure biomarkers.

Biotransformation of diethenylbenzenes, V. Identification of urinary metabolites of 1,2-diethenylbenzene in the rat

1. Biotransformation of 1,2-diethenylbenzene (1) in rat was studied. Five urinary metabolites were isolated by extraction of acid hydrolysed urine and identified by nmr and mass spectroscopy, namely, 1-(2-ethenylphenyl)ethane-1,2-diol (2) 2-ethenylmandelic acid (3), 2-ethenylphenylglyoxylic acid (4), 2-ethenylphenylacetylglycine (5) N-acetyl-S-[1-(2-ethenylphenyl)-2-hydroxyethyl]cysteine (6) and N-acetyl-S-[2-(2-ethenylphenyl)-2-hydroxy-ethyl]cysteine (7). 2. In addition, minor metabolites, namely, 2-ethenylbenzoic acid (8) and 2-ethenylphenyl-acetic acid (9) were identified by glc-mass spectral analysis of the hydrolysed urine extract treated subsequently with diazomethane, hydroxylamine and a trimethyl-silylating reagent. Several compounds, which could arise from biotransformation of both ethenyl groups in the molecule of 1, were detected but not identified unequivocally. 3. A glucuronide was detected by tlc analysis of urine as a blue spot after spraying with naphthoresorcinol. Compounds showing molecular fragments indicating the glucuronide moiety were also detected by glc-mass spectroscopy in non-hydrolysed urine samples. 4. The total thioether excretion amounted to 5.3 +/- 2.4, 5.1 +/- 3.4 and 5.0 +/- 1.9% of the dose at 500, 300 and 100 mg/kg, respectively (mean +/- SD; n = 5). 5. Like styrene and other diethenylbenzene isomers, 1,2-diethenylbenzene is metabolically activated to a reactive epoxide intermediate, 2-ethenylphenyloxirane (10), which is further converted to the urinary metabolites mentioned above. The main detoxification pathways are hydrolysis to the glycol 2 followed by several oxidation steps, and conjugation with glutathione. The latter reaction is both regioselective and stereoselective. 6. The ratio of mercapturic acids 6:7 was 83:17. Each regioisomer consists of two diastereomers which show distinct resonance signals in the 13C-nmr. The diastereomer ratio was 82:28 and 79:21 for 6 and 7 respectively.