Styrene metabolism in man: gas chromatographic separation of mandelic acid enantiomers in the urine of exposed persons

Formation of styrene dependent on fermentation management during wheat beer production

Styrene is formed by the thermal decarboxylation of cinnamic acid during wort boiling or by enzymatic decarboxylation during fermentation. The enzymatic reactions proceed in parallel to the decarboxylation of ferulic- and p-cumaric acid to 4-vinylguaiacol and 4-vinylphenol by the same decarboxylase enzyme. However, the formation of styrene occurs much faster and all available cinnamic acid in wort was converted completely within a few hours. Moreover, the comparison of various manufacturing parameters shows that a higher fermentation temperature of 25 °C compared to 16 °C and an open fermentation management lead to a rapid decrease of styrene. This allows minimising the content of styrene in beer while maintaining the typical wheat beer flavours.

Isolation and characterization of styrene metabolism genes from styrene-assimilating soil bacteria Rhodococcus sp. ST-5 and ST-10

Styrene metabolism genes were isolated from styrene-assimilating bacteria Rhodococcus sp. ST-5 and ST-10. Strain ST-5 had a gene cluster containing four open reading frames which encoded styrene degradation enzymes. The genes showed high similarity to styABCD of Pseudomonas sp. Y2. On the other hand, strain ST-10 had only two genes which encoded styrene monooxygenase and flavin oxidoreductase (styAB). Escherichia coli transformants possessing the sty genes of strains ST-5 and ST-10 produced (S)-styrene oxide from styrene, indicating that these genes function as styrene degradation enzymes. Metabolite analysis by resting-cell reaction with gas chromatography-mass spectrometry revealed that strain ST-5 converts styrene to phenylacetaldehyde via styrene oxide by styrene oxide isomerase (styC) reaction. On the other hand, strain ST-10 lacked this enzyme, and thus accumulated styrene oxide as an intermediate. HPLC analysis showed that styrene oxide was spontaneously isomerized to phenylacetaldehyde by chemical reaction. The produced phenylacetaldehyde was converted to phenylacetic acid (PAA) in strain ST-10 as well as in strain ST-5. Furthermore, phenylacetic acid was converted to phenylacetyl-CoA by the catalysis of phenylacetate-CoA ligase in strains ST-5 and ST-10. This study proposes possible styrene metabolism pathways in Rhodococcus sp. strains ST-5 and ST-10.

Comparison and evaluation of analysis procedures for the quantification of (2-methoxyethoxy)acetic acid in urine

Several extraction and derivatization procedures were evaluated for the quantification of (2-methoxyethoxy)acetic acid (MEAA) in urine. MEAA is a metabolite and a biomarker for exposure to 2-(2-methoxyethoxy)ethanol, a glycol ether with widespread use in various industrial applications and the specific use as an anti-icing additive in the military jet fuel formulation JP-8. Quantification of glycol ether biomarkers is an active area of analytical research. Various sample preparation procedures were evaluated: liquid-liquid extraction (LLE) using ethyl acetate yielded the highest recovery, and solid-phase extraction (SPE) gave low recovery of MEAA. Two derivatization procedures were thoroughly investigated and validated, namely, silylation of MEAA with N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA), and esterification of MEAA using ethanol. Quantification was performed by gas chromatography (GC) with a mass spectrometer as detector and using a polydimethylsiloxane (HP-1) capillary column. Deuterated 2-butoxyacetic acid (d-BAA) was used as an internal standard. Recovery studies of spiked human urine demonstrated the accuracy and precision of both procedures. The limit of detection (LOD) and other figures of merit for both derivatization procedures will be discussed in detail. Applications of these analysis procedures are also discussed.

Procedure for the quantification of the biomarker (2-methoxyethoxy)acetic acid in human urine samples

An accurate and precise procedure was developed for the detection and quantification of (2-methoxyethoxy)acetic acid (MEAA), a metabolite and biomarker for human exposure to 2-(2-methoxyethoxy)ethanol. The compound 2-(2-methoxyethoxy)ethanol has a wide array of industrial applications including its use as an additive in military jet fuel. Exposure to 2-(2-methoxyethoxy)ethanol is a health concern owing to its toxicity which includes developmental and teratogenic properties. Sample preparation consisted of liquid-liquid extraction (LLE) and esterification of MEAA to produce the ethyl ester. Measurement was by a gas chromatograph (GC) equipped with a mass selective detector (MSD) using a HP-1 capillary column. Recovery studies of spiked blank urine demonstrated good accuracy and precision; recovery varied between 95 and 103% with relative standard deviations of 8.6% and less. The limit of detection (LOD) for this procedure was found to range from 0.02 to 0.08 microg/ml equivalent levels of MEAA in urine. These data and other aspects of the validation of this procedure will be discussed.

Stereochemistry of styrene biotransformation

Metabolism of styrene, an important industrial monomer, is reviewed. Attention is focused on the stereoselectivity of its oxidation to 7,8-styrene oxide as well as on further stereoselective biotransformation by hydrolytic and mercapturic acid pathway. Toxic effects such as mutagenicity, genotoxicity, hepatotoxicity, and pneumotoxicity may be related to the ratio of styrene oxide enantiomers at the target site. In rats formation of the less mutagenic (S)-styrene oxide and a faster detoxication of the (R)-enantiomer is favored. In mice metabolic activation of styrene favors the formation of (R)-styrene oxide but this more toxic enantiomer is detoxified faster, so that a nearly racemic styrene oxide results. Stereochemistry of biotransformation can contribute to the species differences in toxicity but can hardly be interpreted as a crucial factor. Due to lack of relevant data the stereochemistry of human metabolism cannot be interpreted in relation to the toxic effects.

Determination of styrene and styrene-7,8-oxide in human blood by gas chromatography-mass spectrometry

Methods of isotope-dilution gas chromatography-mass spectrometry (GC-MS) are described for the determination of styrene and styrene-7,8-oxide (SO) in blood. Styrene and SO were directly measured in pentane extracts of blood from 35 reinforced plastics workers exposed to 4.7-97 ppm styrene. Using positive ion chemical ionization, styrene could be detected at levels greater than 2.5 microg/l blood and SO at levels greater than 0.05 microg/l blood. An alternative method for measurement of SO employed reaction with valine followed by derivatization with pentafluorophenyl isothiocyanate and analysis via negative ion chemical ionization GC-MS-MS (SO detection limit=0.025 microg/l blood). The detection limits for SO by these two methods were 10-20-fold lower than gas chromatographic assays reported earlier, based upon either electron impact MS or flame ionization detection. Excellent agreement between the two SO methods was observed for standard calibration curves while moderate to good agreement was observed among selected reinforced plastics workers (n = 10). Levels of styrene in blood were found to be proportional to the corresponding air exposures to styrene, in line with other published relationships. Although levels of SO in blood, measured by the direct method, were significantly correlated with air levels of either styrene or SO among the reinforced plastics workers, blood concentrations were much lower than previously reported at a given exposure to styrene. The two assays for SO in blood appear to be unbiased and to have sufficient sensitivity and specificity for applications involving workers exposed to styrene and SO during the manufacture of reinforced plastics.

Gas chromatography-electron capture determination of styrene-7,8-oxide enantiomers

The enantiomers of styrene-7,8-oxide (phenyloxirane, SO) were determined using a method based on base catalysed hydrolysis with sodium methoxide. The oxirane ring opening resulted in formation, without racemisation, of the enantiomeric pairs of the two regional isomers, 2-methoxy-1-phenylethanol and 2-methoxy-2-phenylethanol. The structure of these regional isomers was confirmed by gas chromatography-mass spectrometry (GC-MS) and proton nuclear magnetic resonance (1H-NMR). To improve sensitivity of determination, the formed methoxy alcohols were subsequently derivatised with pentafluoropropionic anhydride enabling electron capture detection. This derivatization proceeded also without racemisation and the formed pentafluoropropionyl derivatives were separated on two serially coupled columns, a non-chiral AT 1705 and a chiral CP Chirasil-Dex-CB. As internal standard 2S,3S-(-)-2-methyl-3-phenyloxirane was used. The limit of quantitation of the method was 0.2 microM. The repeatability of the method was assessed at two concentration levels (2.5 and 25 microM) and ranged from 6 to 9% for both enantiomers. The method was applied to the determination of the rate and enantioselectivity of the cytochrome P-450 dependent oxidation of styrene to SO enantiomers in human liver microsomes.

Determination of mandelic acid enantiomers in urine by gas chromatography and electron-capture or flame ionisation detection

A sensitive and stereospecific GC method was developed for the analysis of R- and S-enantiomers of mandelic acid (MA) in urine, using a chiral CP Chirasil-Dex-CB column. The enantiomers of MA were derivatised with isopropanol into their corresponding isopropyl esters and determined either directly with flame ionisation detection (FID) or after subsequent derivatisation of a hydroxy group with pentafluoropropionic anhydride with electron-capture detection (ECD). Both derivatisation steps proceeded with negligible inversion of enantiomers (<1%). The limit of detection of the FID determination was 8 and 5 mg/l for R-MA and S-MA, respectively and of the ECD determination 1 mg/l for both enantiomers. Repeatability (within-day precision) and reproducibility (day-to-day precision) was for both enantiomers below 7.5% for the FID and below 5.8% for the ECD analysis. The method was applied to urine of volunteers exposed to 105 and 420 mg styrene/m3 air. In the urine of the exposed volunteers, the S-enantiomer showed higher excretion compared to that of the R-enantiomer, with marked interindividual differences in excretion of both enantiomers.

Review: biocatalytic transformations of ferulic acid: an abundant aromatic natural product

In this review we examine the fascinating array of microbial and enzymatic transformations of ferulic acid. Ferulic acid is an extremely abundant, preformed phenolic aromatic chemical found widely in nature. Ferulic acid is viewed as a commodity scale, renewable chemical feedstock for biocatalytic conversion to other useful aromatic chemicals. Most attention is focused on bioconversions of ferulic acid itself. Topics covered include cinnamoyl side-chain cleavage; nonoxidative decarboxylation; mechanistic details of styrene formation; purification and characterization of ferulic acid decarboxylase; conversion of ferulic acid to vanillin; O-demethylation; and reduction reactions. Biotransformations of vinylguaiacol are discussed, and selected biotransformations of vanillic acid including oxidative and nonoxidative decarboxylation are surveyed. Finally, enzymatic oxidative dimerization and polymerization reactions are reviewed.

A note on individual differences in the urinary excretion of optical enantiomers of styrene metabolites and of styrene-derived mercapturic acids in humans

Urine samples from 20 male workers in the polyester industry exposed by inhalation to styrene concentrations ranging from 29 to 41 ppm were investigated. Excretion products of styrene metabolism, mandelic acid and mercapturic acids, were purified from the urine over an extraction column packed with Porapak Q, with subsequent ether elution. The optical enantiomers R- and S-mandelic acid were then determined by thin layer chromatography (TLC) using chiral plate material and selective staining with vanadium pentoxide. Quantitative analysis of these compounds was performed using commercial reference substances. Styrene-specific mercapturic acids were analyzed by a modified TLC method, using synthesized reference substances. The concentration of racemic mandelic acid in the individual urine samples ranged from 80 to 1610 mg/l, and the ratio of the R- and S-enantiomers ranged from 0.7 to 2.2. These individual variations are not explained by differences in individual styrene exposure levels, or by differences in the concentration of the urine samples (in relation to creatinine excretion). Styrene-specific mercapturic acids were detected in the urine of only 1 of the 20 workers, at a concentration much lower than expected from previous investigations by others in humans and laboratory animals, in which less specific analytical methods had been used. The results point to marked interindividual differences in metabolism of styrene, probably related to enzyme polymorphisms.

Styrene-7,8-oxide in blood of workers exposed to styrene

A field study was carried out on 13 workers exposed to styrene vapors at time-weighted average concentrations between 10 and 73 ppm. The reactive intermediate styrene-7,8-oxide was determined in blood samples using a direct gas chromatographic method. Styrene-7,8-oxide concentrations were in the range between 0.9 and 4.1 micrograms/l blood. Linear correlations were found between styrene-7,8-oxide in blood and styrene in ambient air and blood. For an exposure concentration of 20 ppm styrene (German MAK value) a steady-state level of about 1 microgram styrene-7,8-oxide/l blood was calculated.

Metabolism of styrene by Rhodococcus rhodochrous NCIMB 13259

Rhodococcus rhodochrous NCIMB 13259 grows on styrene, toluene, ethylbenzene, and benzene as sole carbon sources. Simultaneous induction tests with cells grown on styrene or toluene showed high rates of oxygen consumption with toluene cis-glycol and 3-methylcatechol, suggesting the involvement of a cis-glycol pathway. 3-Vinylcatechol accumulated when intact cells were incubated with styrene in the presence of 3-fluorocatechol to inhibit catechol dioxygenase activity. Experiments with 18O2 showed that 3-vinylcatechol was produced following a dioxygenase ring attack. Extracts contained a NAD-dependent cis-glycol dehydrogenase, which converted styrene cis-glycol to 3-vinylcatechol. Both catechol 1,2- and 2,3-dioxygenase activities were present, and these were separated from each other and from the activities of cis-glycol dehydrogenase and 2-hydroxymuconic acid semialdehyde hydrolase by ion-exchange chromatography of extracts. 2-Vinylmuconate accumulated in the growth medium when cells were grown on styrene, apparently as a dead-end product, and extracts contained no detectable muconate cycloisomerase activity. 3-Vinylcatechol was cleaved by catechol 2,3-dioxygenase to give a yellow compound, tentatively identified as 2-hydroxy-6-oxoocta-2,4,7-trienoic acid, and the action of 2-hydroxymuconic acid semialdehyde hydrolase on this produced acrylic acid. A compound with the spectral characteristics of 2-hydroxypenta-2,4-dienoate was produced by the action of 2-hydroxymuconic acid semialdehyde hydrolase on the 2,3-cleavage product of 3-methylcatechol. Extracts were able to transform 2-hydroxypenta-2,4-dienoate and 4-hydroxy-2-oxopentanoate into acetaldehyde and pyruvate.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromatographic determination of volatile solvents and their metabolites in urine for monitoring occupational exposure

The determination of volatile solvents and their metabolites in biological materials such as expired air, blood or urine allows the estimation of the degree of exposure of these chemicals. Chromatographic methods are now universally employed for this purpose and numerous analytical procedures are available for the determination of the most commonly used volatile solvents and their metabolites in urine. GC methods appear well adapted to the determination of the parent volatile solvents in blood and urine and may be used for the determination of their urinary metabolites, but these methods often require several prechromatographic steps. However, HPLC is becoming a powerful tool for the accurate and easy determination of urinary metabolites of volatile solvents, considering its decisive advantages for routine monitoring. Further, recent developments in HPLC could widen the usefulness of this method for most complex analytical problems that could be encountered during this measurement. However, despite the relative neglect of planar chromatography in this area of concern and considering the great interest in methods that could permit the simultaneous assay of numerous samples often required by routine monitoring, new approach using improved methods such as overpressured TLC could be very fruitful in the future.

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

Methods for the biological monitoring of aromatic hydrocarbons and their metabolites in the human blood and urine are reviewed. For the determination of the unchanged aromatic hydrocarbon in blood, gas chromatographic head-space analysis is recommended. The metabolites can be monitored by photometric, thin-layer chromatographic, high-performance liquid chromatographic and gas chromatographic methods. For the assessment of health risks caused by aromatic hydrocarbons, reference values and occupational limit values, expressed as biological tolerance values and biological exposure indices, have to be considered.

Stereometabolism of ethylbenzene in man: gas chromatographic determination of urinary excreted mandelic acid enantiomers and phenylglyoxylic acid and their relation to the height of occupational exposure

Ethylbenzene is an important industrial solvent and a key substance in styrene production. Ethylbenzene metabolism leads to the formation of mandelic acid, which occurs in two enantiomeric forms, and phenylglyoxylic acid. To decide which enantiomer is preferably formed, 70 urine samples of exposed workers were taken at the end of shifts and--after 3-pentyl ester derivatisation--gas chromatographically analysed. The R/S ratio of mandelic acid enantiomers in urine amounts to 19:1, which means that R-mandelic acid is a major metabolite and S-mandelic acid is one of the minor urinary metabolites of ethylbenzene in man. The R/S ratio is independent of ambient air concentration of ethylbenzene within the investigated range. Compared to an ethylbenzene monoexposure the height of total mandelic acid excretion is decreased in the case of coexposure to other aromatic solvents.

DNA adduct formation by 12 chemicals with populations potentially suitable for molecular epidemiological studies

DNA adduct formation, route of absorption, metabolism and chemistry of 12 hazardous chemicals are reviewed. Methods for adduct detection are also reviewed and approaches to sensitivity and specificity are identified. The selection of these 12 chemicals from the Environmental Protection Agency list of genotoxic chemicals was based on the availability of information and on the availability of populations potentially suitable for molecular epidemiological study. The 12 chemicals include ethylene oxide, styrene, vinyl chloride, epichlorohydrin, propylene oxide, 4,4'-methylenebis-2-chloroaniline, benzidine, benzidine dyes (Direct Blue 6, Direct Black 38 and Direct Brown 95), acrylonitrile and benzyl chloride. While some of these chemicals (styrene and benzyl chloride, possibly Direct Blue 6) give rise to unique DNA adducts, others do not. Potentially confounding factors include mixed exposures in the work place, as well the formation of common DNA adducts. Additional research needs are identified.

Metabolism of inhaled styrene in acetone-, phenobarbital- and 3-methylcholanthrene-pretreated rats: stimulation and stereochemical effects by induction of cytochromes P450IIE1, P450IIB and P450IA

1. The effect of various cytochrome P-450 inducers, namely acetone, phenobarbital (PB) and 3-methylcholanthrene (MC), on the pharmacokinetics of styrene metabolism was studied. 2. Styrene metabolism in vivo was studied measuring phenylglyoxylic acid (PGA), the enantiomers of mandelic acid (MA), and total thioethers excreted in the urine during a 24 h period of airborne exposure to styrene at 500 cm3/m3 (2100 mg/m3). In acetone-pretreated rats, PGA and MA and thioether formation were elevated 30-50%. The R/S ratio of MA enantiomers was about two in all styrene-exposed groups except PB-pretreated rats, which showed a ratio of four. 3. Styrene metabolism in liver microsomes measured in vitro was increased by styrene 140%, acetone plus styrene by 190%, methylcholanthrene plus styrene by 180% and phenobarbital plus styrene by 250%. 4. N-Nitrosodimethylamine demethylation (NDMAD) and 7-pentoxyresorufin dealkylation (PROD) in liver microsomes were enhanced 100-150% by styrene inhalation. The metabolism of 7-ethoxyresorufin was not significantly enhanced. 5. Monoclonal antibodies to P-450 IA1, IA2, IIB1 and IIE1 were utilized to identify cytochrome P-450s by Western blot analysis. These studies showed clearly that styrene inhalation induced principally cytochrome P450IE1, whereas styrene given by gavage at a high narcotic dosage induced both P450IIE1 (NDMAD, 60%) and P450IIB (PROD, 3000%). 6. Our conclusions are that styrene metabolism in vivo in both autoinduced and induced by other foreign compounds, that cytochrome P450IIE1 induction has a major impact on styrene metabolism and that P450IIB1 induction yields an altered MA metabolite enantiomer ratio.

The stereoselectivity of 1,2-phenylethanediol and mandelic acid metabolism and disposition in the rat

1. The steps involved in determining the chirality of the mandelic acid excreted by rats after administration of ethylbenzene and styrene were investigated by studying the fate of racemic, (R)- and (s)1,2-phenylethanediol, a precursor of mandelic acid. These investigations indicate the occurrence of two alternative routes of metabolism for 1,2-phenylethanediol, one involving retention of configuration and the other resulting in the loss of the chiral centre. 2. The stereoselectivity of the disposition of mandelic acid was investigated; rats were dosed with mandelic acid either as the racemate or as the individual enantiomers, G.1.c.-mass spectrometry and h.p.l.c. were used to determine the enantiomers of mandelic acid. 3. There were at least two routes by which mandelic acid could be metabolized and/or excreted; there is a stereoselective pathway in rat for (s)-mandelic acid, which gives rise to phenylglyoxylic acid. 4. The chiral inversion of (s)-mandelic acid to (R)-mandelic acid is reported; although this has been observed in bacteria it has not previously been observed in mammals. 5. The extent to which mandelic acid is metabolized to phenylglyoxylic acid is dependent on the enantiomeric composition of the mandelic acid administered. There is no evidence to indicate significant ketone-alcohol conversion, that is phenylglyoxylic acid is not significantly reduced to mandelic acid in vivo.

The metabolism of ethylbenzene and styrene to mandelic acid: stereochemical considerations

1. The stereochemistry of mandelic acid, produced as a major urinary metabolite of ethylbenzene and styrene in rat and man has been investigated. Although these solvents are both achiral they are metabolized to chiral metabolites, via a series of chiral intermediates. 2. Analytical methods (g.l.c.-mass spectrometry, h.p.l.c. and 19F-n.m.r.) have been developed for the determination of the enantiomeric composition of mandelic acid in urine. 3. These methods have been applied to the study of the metabolic stereochemistry of ethylbenzene and styrene in rats dosed orally (100 mg/kg body weight) and in human volunteers exposed to atmospheres containing these solvents at the upper limits prescribed for workplaces by the UK Health and Safety Executive (100 ppm in air). 4. Results show that whereas only the R-enantiomer of mandelic acid was excreted after ethylbenzene exposure, the mandelic acid formed from styrene was essentially racemic. In three workers occupationally exposed to styrene, ratios of R to S isomers of 1.16, 1.27 and 1.14 were found. A synthetic R/S mixture of mandelic acid had an R/S ratio of 1.03. 5. The implications of these findings for the biological monitoring of workers occupationally exposed to stryrene and/or ethylbenzene are discussed.

Microbial metabolism of mandelate: a microcosm of diversity

This review highlights the diversity of prokaryotic and eukaryotic microorganisms that can metabolise mandelate and it describes how a wide range of compounds related to mandelate is formed in many environments. The chief aspects that are summarised include the various pathways whereby mandelate and its structural analogues are converted into catechol or protocatechuate, the properties of the enzymes that are involved in the pathways, and the regulation and genetics of the pathways. The review incorporates the idea that the study of peripheral metabolic pathways is particularly useful for illuminating evolutionary speculations and it concludes with a list of questions that need to be answered.

Stereometabolism of styrene in man. Urinary excretion of chiral styrene metabolites

Chiral styrene metabolites obtained during initial styrene exposure of test persons were determined in urine samples using capillary gas chromatography. A typical time-dependent urinary concentration profile of one person over a 49-h period is presented and compared with the results of a previous study of occupationally exposed workers and an unexposed control group. Maximum levels of excretion of all styrene metabolites were observed at about the end of a 9-h workshift. Forty hours after exposure, the L/D-ratio of mandelic acid had subsided to the initial value, and the L/D-ratio of phenylethylene glycol to a value equal or slightly above the initial value.

Stereometabolism of styrene in man: gas chromatographic determination of phenylethyleneglycol enantiomers and phenylethanol isomers in the urine of occupationally-exposed persons

A gas chromatographic procedure for the determination of phenylethyleneglycol enantiomers and phenylethanol isomers is described and applied to the investigation of urine samples from occupationally styrene-exposed workers (11 males, six females) and an unexposed control group. Phenylethyleneglycol enantiomers and 2-phenylethanol were present in the urine samples of exposed and unexposed individuals whereas 1-DL-phenylethanol was not found in control urine. The L/D enantiomer ratio of phenylethyleneglycol was found to be approximately 3 in the exposed group and 1.5 in the control group. Because of the close structural relation of these metabolites to the primarily formed epoxide, the results give further insight into the stereotoxicity of styrene in man.