Potential Metabolic Activation of a Representative C2-Alkylated Polycyclic Aromatic Hydrocarbon 6-Ethylchrysene Associated with the Deepwater Horizon Oil Spill in Human Hepatoma (HepG2) Cells.

Exposure to polycyclic aromatic hydrocarbons (PAHs) is the major human health hazard associated with the Deepwater Horizon oil spill. C2-Chrysenes are representative PAHs present in crude oil and could contaminate the food chain. We describe the metabolism of a C2-chrysene regioisomer, 6-ethylchrysene (6-EC), in human HepG2 cells. The structures of the metabolites were identified by HPLC-UV-fluorescence detection and LC-MS/MS. 6-EC-tetraol isomers were identified as signature metabolites of the diol-epoxide pathway. O-Monomethyl-O-monosulfonated-6-EC-catechol, its monohydroxy products, and N-acetyl-l-cysteine(NAC)-6-EC-ortho-quinone were discovered as signature metabolites of the ortho-quinone pathway. Potential dual metabolic activation of 6-EC involving the formation of bis-electrophiles, i.e., a mono-diol-epoxide and a mono-ortho-quinone within the same structure, bis-diol-epoxides, and bis-ortho-quinones was observed as well. The identification of 6-EC-tetraol, O-monomethyl-O-monosulfonated-6-EC-catechol, its monohydroxy products, and NAC-6-EC-ortho-quinone supports potential metabolic activation of 6-EC by P450 and AKR enzymes followed by metabolic detoxification of the ortho-quinone through interception of its redox cycling capability by catechol-O-methyltransferase and sulfotransferase enzymes. The tetraols and catechol conjugates could be used as biomarkers of human exposure to 6-EC resulting from oil spills.

Introduction

The Deepwater Horizon oil spill in the Gulf of Mexico in 2010 was the largest release of crude oil in U.S. history.[1,2] Polycyclic aromatic hydrocarbons (PAHs), which are suspected human carcinogens, are among the most toxic and persistent components of crude oil.[3] According to their origins, PAHs are classified into pyrogenic PAHs arising from fossil fuel combustion and petrogenic PAHs that are unique to crude oil. Petrogenic PAHs differ in structure from that of pyrogenic PAHs (unsubstituted) in that they are either extensively alkylated or oxygenated to yield PAH-quinones.

Contamination of the food chain with petrogenic PAHs is a major hazard that would impact human health.[4] It is widely recognized that pyrogenic PAHs themselves are biologically inert and that their carcinogenic effects require metabolic activation to generate biologically reactive intermediates to form DNA adducts that result in mutations.[5] However, there is a paucity of information on the toxicological properties of petrogenic PAHs. In our previous studies, we described metabolic activation of a representative alkylated PAH 5-methylchrysene (regioisomer of C1-chrysenes) and a representative oxygenated PAH phenanthrene-9,10-quinone in human hepatoma (HepG2) cells.[6,7]

Alkylated PAH 6-ethylchrysene (6-EC) is a representative regioisomer of C2-chrysenes detected in the crude oil released from the Deepwater Horizon oil spill.[8,9] To our knowledge, there are no published accounts of the metabolic activation of 6-EC. In view of the fact that human liver is the major target organ for exposure to 6-EC following ingestion, we studied the metabolism of 6-EC in human HepG2 cells as a model to predict metabolism in primary human hepatocytes.

Four possible routes for the potential metabolic activation of 6-EC were considered to predict and identify its metabolites (Scheme ). First, we predicted the diol-epoxide pathway, involving conversion of 6-EC to trans-dihydrodiols followed by formation of diol-epoxides, which can form DNA adducts or be hydrolyzed to the corresponding tetraols. Second, we predicted the ortho-quinone pathway, involving conversion of trans-dihydrodiols to catechols followed by either formation of conjugates, or their oxidation to ortho-quinones, which could be reduced back to the catechols to establish redox cycling that leads to oxidative DNA damage. The electrophilic ortho-quinone could also react with DNA or the most abundant cellular nucleophile, glutathione (GSH), to form the GSH conjugates, which could be further metabolized into the cysteinylglycine (Cys-Gly) conjugates, cysteine (Cys) conjugates, and eventually N-acetyl-l-cysteine (NAC) conjugates. Third, as 6-EC contains two bay regions within its structure, a combination of these two potential metabolic activation pathways was also predicted. Fourth, we also predicted that hydroxylation on the side chain of the 6-ethyl group followed by formation of sulfate conjugates resulting in DNA adducts could be another potential metabolic activation pathway.

Scheme 1

Possible Metabolic Activation Pathways of 6-EC

It was found that the metabolism of 6-EC involved the formation of mono-diol-epoxides, mono-ortho-quinones, and bis-electrophiles. Bis-electrophiles involved the formation of a mono-diol-epoxide and a mono-ortho-quinone on different terminal rings within the same structure, the formation of bis-diol-epoxides, and the formation of bis-ortho-quinones. We conclude that the diol-epoxide pathway, the ortho-quinone pathway, and their combination likely lead to the potential metabolic activation of 6-EC following ingestion. 6-EC-tetraols and 6-EC-catechol conjugates represent human exposure biomarkers for 6-EC that may result from consuming seafood contaminated by crude oil spills.

Materials and Methods

Caution!All PAHs are potentially hazardous and should be handled in accordance with the National Institutes of Health Guidelines for the Laboratory Use of Chemical Carcinogens.

Chemicals and Reagents

Cell culture media and reagents were all obtained from Invitrogen Co. (Carlsbad, CA) except for fetal bovine serum, which was purchased from Hyclone (Logan, UT). 6-EC was purchased from AccuStandard Inc. (New Haven, CT). All other chemicals used were of the highest grade available, and all solvents were HPLC grade.

Cell Culture

HepG2 (human hepatocellular carcinoma) cells were obtained from American Type Culture Collection and maintained as previously described.[6] Cultured cells with a passage number of 10–20 were used in the experiments to reduce variability due to long-term culture conditions. Cultured cells were authenticated by short-terminal repeat DNA analysis and were mycoplasma-free (DNA Diagnostics Center Medical, Fairfield, OH).

Detection and Identification of 6-EC Metabolites in HepG2 Cells

Confluent HepG2 cells were plated in a 6-well plate (∼5 × 106). The cells were washed twice and then treated with MEM (without phenol red) containing 10 mM glucose and 1 μM 6-EC (DMSO, 0.2% v/v). The culture media were collected at 0 and 24 h and subsequently acidified with 0.1% formic acid before extraction as previously described.[6] The extract from the cell culture media was reconstituted in 150 μL of methanol.

For HPLC-UV-FLR analysis, a 10 μL aliquot of the reconstituted extract was analyzed on a tandem Waters Alliance 2695 chromatographic system with a Waters 2996 photodiode array (PDA) detector and a Waters 2475 multi λ fluorescence (FLR) detector (Waters Corporation, Milford, MA). Separations were accomplished on a Zorbax-ODS C18 analytical column (5 μm, 4.6 mm × 250 mm) with a Zorbax-ODS analytical guard column (5 μm, 4.6 mm × 12.5 mm) (DuPont Co., Wilmington, DE) at ambient temperature. The mobile phase consisted of 5 mM ammonium acetate and 0.1% trifluoroacetic acid (TFA) (v/v) in H2O (solvent A) and 5 mM ammonium acetate and 0.1% TFA in acetonitrile (solvent B) and was delivered at a flow rate of 0.5 mL/min. The linear gradient elution program was as follows: 5 to 95% B over 30 min, followed by an isocratic hold at 95% B for another 10 min. At 40 min, B was returned to 5% in 1 min, and the column was equilibrated for 19 min before the next injection. The total run time for each analysis was 60 min. Eluants from the column were introduced sequentially into the PDA and FLR detectors. Excitation (λex) and emission (λem) wavelengths for the FLR detector were set at 269 and 366 nm, respectively, based on the spectral properties of 6-EC (Figure S1). The optimal pair of λex and λem of 6-EC was employed to detect its metabolites based on the assumption that most 6-EC metabolites show fluorescence signals at these wavelengths.

For ion trap LC-MS/MS analysis, a 10 μL aliquot of the reconstituted extract was analyzed on a Waters Alliance 2690 HPLC system (Waters Corporation, Milford, MA) coupled to a Finnigan LTQ linear ion trap mass spectrometer (Thermo Scientific, San Jose, CA). The column, mobile phase, flow rate, and linear gradient elution program were the same as those described above. During LC-MS/MS analysis, up to 10 min of the initial flow was diverted to waste before evaluation of eluants. The mass spectrometer was operated in both positive and negative ion modes with an electrospray ionization (ESI) source. Eluants were monitored on the LTQ using product ion scan (MS2), subsequent MS/MS/MS (MS3), and pseudo selected reaction monitoring (SRM) modes. The mass spectrometry parameters included spray voltage (3 kV in positive ion mode and 5 kV in negative ion mode), sheath gas flow rate (40 arbitrary units in both ion modes), auxiliary gas flow rate (15 arbitrary units in both ion modes), capillary temperature (275 °C in both ion modes), capillary voltage (38 V in positive ion mode and −19 V in negative ion mode), and tube lens (20 V in positive ion mode and −22.05 V in negative ion mode). An isolation width of 3 bracketed around the m/z of interest, activation Q of 0.25, and activation time of 30 ms were used for data acquisition. Xcalibur software, version 2.0 (Thermo Scientific, San Jose, CA), was used to control the LC-MS/MS system and to process data. The preliminary information on metabolite structures was obtained by interpreting the corresponding MS2 and MS3 spectra of 6-EC metabolites from ion trap LC-MS/MS.

In some instances, another 5 μL aliquot of the reconstituted extract was analyzed on a nano-Acquity ultra-performance liquid chromatography (UPLC) system (Waters Corporation, Milford, MA) coupled to a LTQ Orbitrap XL mass spectrometer (Thermo Scientific, San Jose, CA). Separations were accomplished on a nano-UPLC C18 column (1.7 μm BEH130, 150 μm × 100 mm) (Waters Corporation, Milford, MA) at 50 °C. The mobile phase consisted of 0.1% formic acid (v/v) in H2O (solvent A) and 0.1% formic acid (v/v) in acetonitrile (solvent B) and was delivered at a flow rate of 1.6 μL/min. The linear gradient elution program was as follows: an isocratic hold at 5% B for 5 min, 5 to 95% B over 30 min, followed by an isocratic hold at 95% B for another 10 min. At 46 min, B was returned to 5% in 2 min, and the column was equilibrated for 12 min before the next injection. The total run time for each analysis was 60 min. The mass spectrometer was operated in positive and negative ion modes with a nano-electrospray ionization (nano-ESI) source after accurate calibration with the manufacturer’s calibration mixture. The ionization voltage was set to 1.5 kV, and the capillary temperature was set to 200 °C. Full scan spectra were acquired with a resolving power of 60 000 full width at half-maximum in the mass range from m/z 100 to 600. Lists of accurate masses of the potential 6-EC metabolites were used to detect the formation of bis-electrophiles containing both a diol-epoxide and an ortho-quinone within the same structure in Table , the formation of bis-diol-epoxides in Table , and the formation of bis-ortho-quinones in Table . Xcalibur software, version 2.0 (Thermo Scientific, San Jose, CA), was used to control the Orbitrap mass spectrometer and to process data.

Table 1

Accurate Masses of Potential 6-EC Metabolites in HepG2 Cells That Result from the Formation of Bis-Electrophiles Containing Both a Diol-Epoxide and an Ortho-Quinone

6-EC metabolites	molecular formula	positive mode	negative mode
tetraol + O-quinone	C20H18O6	355.1182	353.1025
monodehydrated tetraol + O-quinone	C20H16O5	337.1076	335.0919
bis-dehydrated tetraol + O-quinone	C20H14O4	319.0970	317.0814
tetraol + O-methyl catechol	C21H22O6	371.1495	369.1338
monodehydrated tetraol + O-methyl catechol	C21H20O5	353.1389	351.1232
bis-dehydrated tetraol + O-methyl catechol	C21H18O4	335.1283	333.1127
tetraol + O-sulfonated catechol	C20H20O9S	437.0906	435.0750
monodehydrated tetraol + O-sulfonated catechol	C20H18O8S	419.0801	417.0644
bis-dehydrated tetraol + O-sulfonated catechol	C20H16O7S	401.0695	399.0538
tetraol + O-glucuronosyl catechol	C26H28O12	533.1659	531.1503
monodehydrated tetraol + O-glucuronosyl catechol	C26H26O11	515.1553	513.1397
bis-dehydrated tetraol + O-glucuronosyl catechol	C26H24O10	497.1448	495.1291
tetraol + O-methyl-O-sulfonated catechol	C21H22O9S	451.1063	449.0906
monodehydrated tetraol + O-methyl-O-sulfonated catechol	C21H20O8S	433.0957	431.0801
bis-dehydrated tetraol + O-methyl-O-sulfonated catechol	C21H18O7S	415.0851	413.0695

Table 2

Accurate Masses of Potential 6-EC Metabolites in HepG2 Cells That Result from the Formation of Bis-Diol-Epoxides

6-EC metabolites	molecular formula	positive mode	negative mode
tetraol + tetraol	C20H24O8	393.1549	391.1393
monodehydrated tetraol + tetraol	C20H22O7	375.1444	373.1287
monodehydrated tetraol + monodehydrated tetraol	C20H20O6	357.1338	355.1182
(or bis-dehydrated tetraol + tetraol)
monodehydrated tetraol + bis-dehydrated tetraol	C20H18O5	339.1232	337.1076
bis-dehydrated tetraol + bis-dehydrated tetraol	C20H16O4	321.1127	319.0970
tetraol + O-sulfonated tetraol	C20H24O11S	473.1118	471.0961
monodehydrated tetraol + O-sulfonated tetraol	C20H22O10S	455.1012	453.0855
monodehydrated tetraol + monodehydrated O-sulfonated tetraol	C20H20O9S	437.0906	435.0750
(or bis-dehydrated tetraol + O-sulfonated tetraol)
monodehydrated tetraol + bis-dehydrated O-sulfonated tetraol	C20H18O8S	419.0801	417.0644
bis-dehydrated tetraol + bis-dehydrated O-sulfonated tetraol	C20H16O7S	401.0695	399.0538
tetraol + O-glucuronosyl tetraol	C26H32O14	569.1870	567.1714
monodehydrated tetraol + O-glucuronosyl tetraol	C26H30O13	551.1765	549.1608
monodehydrated tetraol + monodehydrated O-glucuronosyl tetraol	C26H28O12	533.1659	531.1503
(or bis-dehydrated tetraol + O-glucuronosyl tetraol)
monodehydrated tetraol + bis-dehydrated O-glucuronosyl tetraol	C26H26O11	515.1553	513.1397
bis-dehydrated tetraol + bis-dehydrated O-glucuronosyl tetraol	C26H24O10	497.1448	495.1291

Table 3

Accurate Masses of Potential 6-EC Metabolites in HepG2 Cells That Result from the Formation of Bis-Ortho-Quinones

6-EC metabolites	molecular formula	positive mode	negative mode
O-quinone + O-quinone	C20H12O4	317.0814	315.0657
O-quinone + O-methyl catechol	C21H16O4	333.1127	331.0970
O-quinone + O-sulfonated catechol	C20H14O7S	399.0538	397.0382
O-quinone + O-glucuronosyl catechol	C26H22O10	495.1291	493.1135
O-quinone + O-methyl-O-sulfonated catechol	C21H16O7S	413.0695	411.0538
O-methyl catechol + O-methyl catechol	C22H20O4	349.1440	347.1283
O-methyl catechol + O-sulfonated catechol	C21H18O7S	415.0851	413.0695
O-methyl catechol + O-glucuronosyl catechol	C27H26O10	511.1604	509.1448
O-sulfonated catechol + O-sulfonated catechol	C20H16O10S2	481.0263	479.0107
O-sulfonated catechol + O-glucuronosyl catechol	C26H24O13S	577.1016	575.0859
O-glucuronosyl catechol + O-glucuronosyl catechol	C32H32O16	673.1769	671.1612

Results

Detection of 6-EC Metabolites in HepG2 Cells by HPLC-UV-FLR

Comparison of UV chromatograms at λmax 269 nm at 0 h (Figure A) and 24 h (Figure B) showed that nine metabolites of 6-EC were detected in the organic phase of the ethyl acetate-extracted acidified media from HepG2 cells. The peak attributed to 6-EC at 0 h was almost completely absent at 24 h, suggesting that 6-EC was rapidly metabolized by HepG2 cells over this time course. The corresponding UV spectra of the nine metabolites were extracted from the PDA detector and are shown in Figure S2. The UV spectra of metabolites 4–8 were similar to that of 6-EC, showing that aromaticity has been retained, whereas metabolites 2 and 3 were predicted to have lost their aromaticity.

Figure 1

HPLC detection of 6-EC metabolites in human HepG2 cells. (A) UV chromatogram at λmax 269 nm at 0 h. (B) UV chromatogram at λmax 269 nm at 24 h. (C) FLR chromatogram at λex 269 nm and λem 366 nm at 0 h. (D) FLR chromatogram at λex 269 nm and λem 366 nm at 24 h. Human HepG2 cells (∼5 × 106) were treated with 6-EC (1 μM, 0.2% (v/v) DMSO) in MEM (without phenol red) containing 10 mM glucose. The cell media were collected at 0 and 24 h and subsequently acidified with 0.1% formic acid before extraction with ethyl acetate. The extracts were analyzed by HPLC-UV-FLR.

Comparison of FLR chromatograms at λex 269 nm and λem 366 nm at 0 h (Figure C) and 24 h (Figure D) showed that there were seven fluorescence peaks in Figure D, corresponding to metabolites 2–8 in Figure B, thus validating the presence of a fluorophore. The peaks corresponding to metabolites 1 and 9 were detected only in the UV chromatogram (Figure B) but not in the FLR chromatogram (Figure D), suggesting a loss of the 6-EC ring fluorophore.

Evidence for the Diol-Epoxide Pathway

Identification of O-monosulfonated-6-EC-dihydrodiol, 6-EC-dihydrodiol, and 6-EC-tetraol indicated the occurrence of the diol-epoxide pathway as a potential metabolic route of activation of 6-EC in HepG2 cells.

A single isomer of monodehydrated O-monosulfonated-6-EC-dihydrodiol with the retention time of 23.42 min was detected by comparing the pseudo SRM chromatograms (m/z 351 → 271) at 0 h (Figure A) and 24 h (Figure B) in the negative ion mode. The corresponding MS2 spectrum (m/z 351) of this metabolite showed the characteristic loss of the sulfate group (80 amu) from the deprotonated molecular ion (Figure C), and the MS3 spectrum (m/z 351 → 271 → ) of this metabolite showed the subsequent loss of one CH3 group from the alkyl side chain (Figure D). The specific position of the O-monosulfonated-dihydrodiol could not be assigned using mass spectrometry.

Figure 2

Detection of monodehydrated O-monosulfonated-6-EC-dihydrodiol in human HepG2 cells. (A) Extracted ion chromatogram of pseudo SRM transition at 0 h. (B) Extracted ion chromatogram of pseudo SRM transition at 24 h. (C) MS2 spectrum of the peak at 23.42 min. (D) MS3 spectrum of the peak at 23.42 min. The samples were prepared as described in the caption to Figure and were subsequently analyzed on an ion trap LC-MS/MS.

Three isomers of 6-EC-dihydrodiols and three isomers of monodehydrated 6-EC-dihydrodiols were detected in HepG2 cells with identical retention times by monitoring the extracted ion chromatograms of the Orbitrap full scan at 0 and 24 h in the positive ion mode (Figure S3). MS spectra of these isomers provided the accurate masses and molecular formulas of 6-EC-dihydrodiols and their monodehydrated products with acceptable ppm values (Figure S3). The presence of these isomers suggests saturation of 6-EC on both terminal benzo rings.

Two isomers of bis-dehydrated 6-EC-tetraols at 17.72 and 19.04 min were detected by monitoring the MS2 chromatograms (m/z 289) at 0 and 24 h in the positive ion mode (Table ). The corresponding MS2 spectra (m/z 289) of these metabolites showed the sequential loss of H2O (18 amu), CO (28 amu), and CH3 (15 amu) from the protonated molecular ion (Table ). Comparison of the retention times of bis-dehydrated 6-EC-tetraols with metabolites 2 and 3 by HPLC-UV-FLR in Figure showed good agreement.

Table 4

Mass Transitions for 6-Ethylchrysene Metabolites in HepG2 Cells

metabolite no.	6-EC metabolites	retention time (min)	mode	m/z
−	dehydrated O-sulfonated dihydrodiol	23.42	negative	351 [M – H]−, 271 [M – H – SO3]−, 256 [M – H – SO3 – CH3]−
2, 3	bis-dehydrated tetraol	17.72, 19.04	positive	289 [M + H]+, 271 [M + H – H2O]+, 243 [M + H – H2O – CO]+, 228 [M + H – H2O – CO – CH3]+, 215 [M + 2H – H2O – CO – CH2CH3]+
7	O-sulfonated catechol [O-sulfonated bis-phenol]	24.38	negative	367 [M – H]−, 287 [M – H – SO3]−, 272 [M – H – SO3 – CH3]−
6	O-methyl-O-sulfonated catechol	21.12, 24.05	negative	381 [M – H]−, 301 [M – H – SO3]−, 286 [M – H – SO3 – CH3]−, 257 [M – H – SO3 – CH3 – CH2CH3]−
4, 5, 8	monohydroxy-O-methyl-O-sulfonated catechol	19.93, 20.47, 25.18	negative	397 [M – H]−, 317 [M – H – SO3]−, 302 [M – H – SO3 – CH3]−, 273 [M – H – SO3 – CH3 – CH2CH3]−
−	quinone	34.06	positive	287 [M + H]+, 269 [M + H – H2O]+, 259 [M + H – CO]+, 244 [M + H – CO – CH3]+, 231 [M + H – 2CO]+
−	monohydroxy-quinone	29.58	positive	302 [M + H – •H]+, 274 [M + H – CO – •H]+, 246 [M + H – 2CO – •H]+, 218 [M + H – 3CO – •H]+
−	NAC-ortho-quinone	32.99	negative	446 [M – H]−, 418 [M – H – CO]−, 317 [M – H – CH2=C(NHCOCH3)COOH]−, 288 [M – H – CH2=C(NHCOCH3)COOH–CH2CH3]−

A single isomer of a monodehydrated 6-EC-tetraol and a single isomer of a bis-dehydrated 6-EC-tetraol were detected in HepG2 cells with different retention times by monitoring the extracted ion chromatograms of the Orbitrap full scan at 0 and 24 h in the positive ion mode (Figure S4). Because 6-EC is asymmetric, there are two possible regioisomeric 6-EC-tetraols: those that come from 6-EC-trans-1,2-dihydrodiol and those that come from 6-EC-trans-7,8-dihydrodiol. The resulting tetraols and their absolute configurations cannot be assigned using mass spectrometry.

Evidence for the Ortho-Quinone Pathway

Identification of 6-EC-dihydrodiol, O-monosulfonated-6-EC-catechol, O-monomethyl-O-monosulfonated-6-EC-catechol, monohydroxy-O-monomethyl-O-monosulfonated-6-EC-catechol, 6-EC-dione, monohydroxy-6-EC-dione, and NAC-6-EC-ortho-quinone indicated the occurrence of the ortho-quinone pathway as a potential route of metabolic activation of 6-EC in HepG2 cells.

A single isomer at 24.38 min corresponding to either an O-monosulfonated-6-EC-catechol or an O-monosulfonated-6-EC-bis-phenol was detected by monitoring the pseudo SRM chromatograms (m/z 367 → 287) at 0 h (Figure A) and 24 h (Figure B) in the negative ion mode. The corresponding MS2 spectrum (m/z 367) of this metabolite showed the characteristic loss of the sulfate group (80 amu) from the deprotonated molecular ion (Figure C), and the MS3 spectrum (m/z 367 → 287 →) of this metabolite showed the subsequent loss of one CH3 group (Figure D). Comparison of the retention time of this peak with that of metabolite 7 by HPLC-UV-FLR in Figure showed that they were in agreement. Mass spectrometry cannot distinguish a catechol from a bis-phenol. However, we favor formation of the O-monosulfonated-6-EC-catechol isomer because it would be derived from the reduction of either 6-EC-1,2-dione or 7,8-dione. Both the intermediate 6-EC-dihydrodiol and 6-EC-dione were detected in this study.

Figure 3

Detection of either an O-monosulfonated-6-EC-catechol or an O-monosulfonated-6-EC-bis-phenol in human HepG2 cells. (A) Extracted ion chromatogram of pseudo SRM transition at 0 h. (B) Extracted ion chromatogram of pseudo SRM transition at 24 h. (C) MS2 spectrum of the peak at 24.38 min. (D) MS3 spectrum of the peak at 24.38 min. The samples were prepared as described in the caption to Figure and were subsequently analyzed on an ion trap LC-MS/MS.

Two isomers of O-monomethyl-O-monosulfonated-6-EC-catechols at 21.12 and 24.05 min were detected by monitoring the pseudo SRM chromatograms (m/z 381 → 301) at 0 h (Figure A) and 24 h (Figure B) in the negative ion mode, which showed the loss of the sulfate group (80 amu) from the deprotonated molecular ion. The MS3 spectrum (m/z 381 → 301 →) of the isomer peak at 24.05 min showed the characteristic losses of CH3 and CH2CH3 groups (Figure C), suggesting that the CH3 group was not cleaved from the CH2CH3 side chain of 6-EC and was due to the occurrence of O-methylation. The unique biotransformation of O-methylation strongly indicated the formation of the catechol, thus confirming the potential metabolic activation of 6-EC via the ortho-quinone pathway. Although the specific position of catechol conjugation, the methyl group, and the sulfate group on 6-EC-catechols could not be assigned using mass spectrometry, it is likely that only 6-EC-1,2-catechols or 7,8-catechols were formed. Comparison of the retention time of the major isomer peak at 24.05 min with that of metabolite 6 by HPLC-UV-FLR in Figure showed good concordance.

Figure 4

Detection of O-monomethyl-O-monosulfonated-6-EC-catechols in human HepG2 cells. (A) Extracted ion chromatogram of pseudo SRM transition at 0 h. (B) Extracted ion chromatogram of pseudo SRM transition at 24 h. (C) MS3 spectrum of the peak at 24.05 min. The samples were prepared as described in the caption to Figure and were subsequently analyzed on an ion trap LC-MS/MS.

Five isomers of monohydroxy-O-monomethyl-O-monosulfonated-6-EC-catechols at 19.51, 19.93, 20.47, 24.05, and 25.18 min were detected by monitoring the pseudo SRM chromatograms (m/z 397 → 317) at 0 h (Figure A) and 24 h (Figure B) in the negative ion mode, which showed the loss of the sulfate group (80 amu) from the deprotonated molecular ion. The MS3 spectrum (m/z 397 → 317 →) of the isomer peak at 19.93 min showed the characteristic losses of CH3 and CH2CH3 (Figure C), suggesting that the CH3 group was not cleaved from the CH2CH3 side chain of 6-EC and was due to the occurrence of O-methylation. The unique biotransformation of O-methylation strongly indicated the formation of the catechol, thus confirming the potential metabolic activation of 6-EC by the ortho-quinone pathway. As the loss of CH2CH2OH was not observed, this ruled out activation of 6-EC by a combination of the ortho-quinone pathway and side chain hydroxylation. Although the specific position of catechol conjugation, the hydroxyl group, the methyl group, and the sulfate group on 6-EC-catechols could not be assigned using mass spectrometry, it is likely that only 6-EC-1,2-catechols or 7,8-catechols were formed. Comparison of the retention times of the isomer peaks at 19.93, 20.47, 25.18 min with those of metabolites 4, 5, and 8 by HPLC-UV-FLR in Figure showed that they were in agreement.

Figure 5

Detection of monohydroxy-O-monomethyl-O-monosulfonated-6-EC-catechol isomers in human HepG2 cells. (A) Extracted ion chromatogram of pseudo SRM transition at 0 h. (B) Extracted ion chromatogram of pseudo SRM transition at 24 h. (C) MS3 spectrum of the peak at 19.93 min. Similar MS3 spectra were obtained for the remaining isomers. The samples were prepared as described in the caption to Figure and were subsequently analyzed on an ion trap LC-MS/MS.

Two peaks at 24.41 and 34.06 min corresponding to 6-EC-dione were detected by comparing the pseudo SRM chromatograms (m/z 287 → 259) at 0 h (Figure A) and 24 h (Figure B) in the positive ion mode. The respective MS2 spectra (m/z 287) of these two metabolites showed the characteristic loss of CO (28 amu) from the protonated molecular ion, and a representative MS2 spectrum of the metabolite with a retention time of 34.06 min is shown in Figure C. The MS3 spectrum (m/z 287 → 259 →) of this metabolite showed the characteristic loss of a second CO and the loss of CH3 (Figure D). The sequential loss of two CO groups supported the presence of either an ortho-quinone or a remote quinone, which cannot be distinguished on the basis of mass spectrometry.

Figure 6

Detection of 6-EC-dione in human HepG2 cells. (A) Extracted ion chromatogram of pseudo SRM transition at 0 h. (B) Extracted ion chromatogram of pseudo SRM transition at 24 h. (C) MS2 spectrum of the peak at 34.06 min. (D) MS3 spectrum of the peak at 34.06 min. The samples were prepared as described in the caption to Figure and were subsequently analyzed on an ion trap LC-MS/MS. Another peak with a retention time of 24.41 min with a relatively high polarity could be an isomer of O-monosulfonated-6-EC-catechol, which could undergo cleavage of its sulfate conjugate in the mass spectrometer, followed by auto-oxidation, and thus result in the detection of quinone instead.

A single isomer of monohydroxy-6-EC-dione at 29.58 min was detected by comparing the pseudo SRM chromatograms (m/z 303 → 274) at 0 h (Figure A) and 24 h (Figure B) in the positive ion mode. The corresponding MS2 spectrum (m/z 303) of this metabolite (Figure C) showed the characteristic loss of CO (28 amu) plus one hydrogen from the protonated molecular ion. The MS3 spectrum (m/z 303 → 274 →) of this metabolite (Figure D) showed another characteristic loss of CO. Monohydroxy-6-EC-dione could be derived from either an ortho-quinone or a remote quinone, and these alternatives cannot be distinguished on the basis of mass spectrometry. As the loss of CH2CH2OH was not observed, this ruled out activation of 6-EC by a combination of the ortho-quinone pathway and side chain hydroxylation.

Figure 7

Detection of monohydroxy-6-EC-dione in human HepG2 cells. (A) Extracted ion chromatogram of pseudo SRM transition at 0 h. (B) Extracted ion chromatogram of pseudo SRM transition at 24 h. (C) MS2 spectrum of the peak at 29.58 min. (D) MS3 spectrum of the peak at 29.58 min. The samples were prepared as described in the caption to Figure and were subsequently analyzed on an ion trap LC-MS/MS.

A single isomer of NAC-6-EC-ortho-quinone at 32.99 min was detected by monitoring the MS2 chromatograms (m/z 446) at 0 and 24 h in the negative ion mode (Table ). The corresponding MS2 spectrum (m/z 446) of this metabolite showed the characteristic loss of CO (28 amu) from the deprotonated molecular ion and the characteristic loss of 129 amu resulting from a cleavage of the thioether bond,[10] followed by the loss of CH2CH3 side chain (Table ). The unique biotransformation of NAC conjugation strongly indicated the formation of the ortho-quinone, thus confirming a potential route of metabolic activation of 6-EC via the ortho-quinone pathway. The specific position of NAC conjugation on 6-EC-ortho-quinone could not be assigned using mass spectrometry.

Evidence for the Bis-Electrophile Pathway

Evidence for potential dual metabolic activation of 6-EC in HepG2 cells to form a bis-electrophile containing a diol-epoxide and an ortho-quinone within the same structure was obtained. Two isomers of tetrahydroxy-O-monomethyl-6-EC-catechol were detected by comparing the extracted ion chromatograms of the Orbitrap full scan at 0 and 24 h in the negative ion mode (Figure ). In particular, the unique biotransformation of O-methylation strongly indicated the formation of the catechol, thus confirming the occurrence of ortho-quinone pathway. The specific positions of the hydroxy groups of the tetraol and O-monomethyl-catechol could not be assigned using mass spectrometry.

Figure 8

Detection of tetrahydroxy-O-monomethyl-6-EC-catechols in human HepG2 cells. (A) Extracted ion chromatogram of Orbitrap full scan at 0 h. (B) Extracted ion chromatogram of Orbitrap full scan at 24 h. (C) MS spectrum of the peak at 16.35 min. (D) MS spectrum of the peak at 16.93 min.

A bis-tetraol was detected by identification of two bis-dehydrated-tetraols in HepG2 cells by comparing the extracted ion chromatograms of Orbitrap full scan at 0 and 24 h in the positive ion mode (Figure S5), indicating the occurrence of activation on the two terminal benzo rings to give a bis-diol-epoxide.

O-Monomethyl-catechol-6-EC-ortho-quinone was also detected in HepG2 cells by comparing the extracted ion chromatograms of Orbitrap full scan at 0 and 24 h in the positive ion mode (Figure S6), indicating the occurrence of the formation of a bis-ortho-quinone. In particular, the unique biotransformation involving O-methylation strongly indicated the formation of the catechol, thus confirming the occurrence of the ortho-quinone pathway. Further evidence for the formation of a bis-ortho-quinone came from the detection of 6-EC-bis-ortho-quinone in HepG2 cells when the extracted ion chromatograms of Orbitrap full scan at 0 and 24 h in the positive ion mode were compared (Figure S7).

Discussion

6-EC is a representative regioisomer of C2-chrysenes present in crude oil that may enter the food chain after oil spills. We examined its metabolism in human HepG2 cells knowing that human exposure pathways involve ingestion. Human HepG2 cells were selected as an alternative to human hepatocytes due to the limitations of the latter, for instance, scarce availability, shorter life span, phenotypic instability, and higher individual variability.

We proposed four potential pathways of 6-EC metabolic activation: formation of diol-epoxides, ortho-quinones, bis-electrophiles, and ethanesulfonates. We found evidence for the first three pathways but no evidence for activation on the ethyl side chain involving hydroxylation and subsequent sulfonation. Representative metabolites of 6-EC from the pathways of activation were detected and identified by HPLC-UV-FLR and LC-MS/MS (Table ). Metabolites 2 and 3 in Figure were identified as isomers of a tetraol. Metabolites 4, 5, and 8 in Figure were identified as isomers of a monohydroxy-O-monomethyl-O-monosulfonated-catechol. Metabolite 6 in Figure was identified as an O-monomethyl-O-monosulfonated-catechol. Metabolite 7 in Figure was identified as an O-monosulfonated-catechol or an O-monosulfonated-bis-phenol. Metabolites 1 and 9 in Figure remain unassigned.

On the basis of the peak areas of the metabolites on UV and FLR chromatograms, the major potential metabolic activation pathways of 6-EC in human HepG2 cells involved formation of diol-epoxides and ortho-quinones (Scheme ). We also found evidence for the potential metabolic activation of 6-EC on both bay regions to form bis-electrophiles containing a mono-diol-epoxide and a mono-ortho-quinone within the same structure, bis-diol-epoxides, and bis-ortho-quinones (Scheme ). We suspect that these metabolites were underdetected probably due to the potential of bis-electrophiles to form protein and DNA cross-links.

Scheme 2

Proposed Metabolic Pathway of 6-EC in Human HepG2 Cells

The number of each metabolite corresponds to the metabolites labeled in the UV and fluorescence chromatograms in Figure .

We did not find any evidence for hydroxylation on the ethyl group side chain at C6 followed by formation of sulfate in human HepG2 cells, which is consistent with the previous findings that hydroxymethylation of 5-methylchrysene and 6-methylchrysene (6-MC) is not an important metabolic activation pathway in humans.[6,11] Monodehydrated O-monosulfonated-6-EC-dihydrodiol shown in Figure has an identical nominal mass as that of the sulfate conjugate of side chain monohydroxy-6-EC. However, the fragmentation pattern in Figure shows the loss of the sulfate group and the subsequent loss of the CH3 group, which rules out the possibility of side chain hydroxylation on 6-EC.

Metabolic activation of 6-MC via a diol-epoxide pathway has been previously characterized in mouse skin, human liver microsomes, and human lung microsomes.[11−14] In this pathway, 6-MC containing a methyl group located at a non-bay region is activated by P450 1A1 to yield 6-MC-1R,2R-diol, which is further converted to 6-MC-1R,2S-diol-3S,4R-epoxide with relatively weak DNA-binding and carcinogenic properties compared with those of the diol-epoxide generated from 5-methylchrysene containing a methyl group located at a bay region.[14−16] It was also reported that hydroxylation on the methyl group of 6-MC catalyzed by P450 3A4 and 1A2 leads to 6-hydroxy-6-MC without carcinogenic and tumor-initiating activities in mouse skin.[17] However, a large amount of 6-MC was left unmetabolized after incubation in human liver microsomes, and a few metabolites found in significant amounts were unidentified,[11] which may explain why only the diol-epoxide pathway of 6-MC activation was proposed previously. The structure of 6-EC is identical to that of 6-MC except for the length of the side chain; therefore, it was expected that the diol-epoxide pathway of 6-EC would be identified in the present study.

Multiple metabolites were detected which indicated that the ortho-quinone pathway was involved in the potential metabolic activation of 6-EC. Interestingly, O-monomethyl-O-monosulfonated-6-EC-catechols and their monohydroxy products were detected in human HepG2 cells, whereas O-monomethyl-6-EC-catechols were not detected, which could be explained if either the O-monomethyl catechol is rapidly sulfonated or sulfonation proceeds prior to O-methylation. We failed to observe the occurrence of O-monoglucuronidation of 6-EC-catechol, probably due to the low expression level of uridine 5′-diphospho-glucuronosyltransferases in human HepG2 cells.[18]

The enzymes responsible for the potential metabolic activation pathway of 6-EC in human HepG2 cells remain to be identified. Inducible cytochrome P450 1A1 present in human HepG2 cells is probably responsible for the formation of diol-epoxides.[19] Several members of the aldo-keto reductase (AKR) superfamily, including AKR1C1, AK1C2, and AKR1C3, present in human HepG2 cells would oxidize 6-EC trans-dihydrodiols to ortho-quinones.[20−22] However, liver-specific AKR1C4 is absent in human HepG2 cells.[22] AKRs, NAD(P)H:quinone oxidoreductase 1, and carbonyl reductase can all catalyze the redox cycling of ortho-quinones to form intermediate catechols.[23] Formation of 6-EC-diones indicates that subsequent conjugation of the catechols is not completely effective in preventing redox cycling, as predicted earlier in our studies of benzo[a]pyrene-7,8-dione.[24] Sulfotransferases (SULTs) and catechol-O-methyltransferase would be responsible for the formation of O-monomethyl-O-monosulfonated-catechols.[25,26] The SULTs that are expressed in human HepG2 cells and likely responsible for O-sulfonation are SULT1A1, 1A2, 1E1, and 2A1.[18] γ-Glutamyltranspeptidase, dipeptidase, and N-acetyl transferase would sequentially catalyze the degradation of a GSH-ortho-quinone conjugate to generate the NAC-ortho-quinone conjugate that was detected.[27] Potential dual metabolic activation of 6-EC on both terminal benzo rings was consistent with the previous findings of 5-methylchrysene,[6] which further confirmed that P450 1A1 and AKRs can work separately or together in the metabolic activation of chrysene derivatives containing two bay regions to form bis-electrophiles.

Elucidation of the metabolic pathways of 6-EC in human liver cells can be used to identify exposure biomarkers of 6-EC that could be used for biomonitoring human urine and plasma. Our earlier studies on the metabolic activation of 5-methylchrysene indicated metabolites in common with those found in the present study, i.e., tetraols and catechol conjugates. In particular, O-sulfonated-catechols observed in both studies could be biomarkers due to the intense molecular ions in the negative ion mode and the characteristic loss of the sulfate group (80 amu) in MS2 spectra. By developing a panel of biomarkers for petrogenic PAH exposure, requisite specificity and sensitivity will likely increase as opposed to measuring a single analyte.[28] These biomarkers have the potential for monitoring the consumption of oil-contaminated seafood in humans.