Effect of benzo(a)pyrene exposure on fluoranthene metabolism by mouse adipose tissue microsomes

Abstract

The present study has been undertaken to examine whether exposure to benzo(a)pyrene (BaP), a polycyclic aromatic hydrocarbon (PAH) compound, influences the metabolism of fluoranthene (FLA), another PAH compound. Microsomes were isolated from the adipose tissue of mice that received 50 microg/kg BaP and incubated with FLA (3 microM) alone; FLA in combination with BaP at equimolar concentrations, and a control group that received nothing. Post-incubation, samples were extracted with ethyl acetate and analyzed for FLA metabolites by reverse-phase HPLC with fluorescence detection. The rate of FLA metabolism (pmol of metabolite/min/mg protein) was increased when microsomes from BaP-treated mice were exposed to FLA alone and FLA in combination with BaP, compared to controls. On the other hand, the difference in FLA metabolic rate between microsomes that were exposed to FLA + BaP was higher than the ones that received FLA. The microsomes from BaP-pre-treated mice produced a considerably higher proportion of FLA 2, 3-diol, and 2, 3 D FLA when microsomes were incubated with FLA. There were no differences in the FLA metabolite types formed when BaP-pre-treated mice were co-incubated with BaP and FLA than with FLA alone. The enhanced biotransformation of FLA as a result of prior and concomitant exposure to BaP may have implications for assessment of risks arising from human exposure to PAH mixtures.

Figure 1

Rate of fluoranthene metabolism in adipose microsomes of Apc^Min mice pre-exposed to benzo(a)pyrene. Explanations of the abbreviations are as follows; FL-no BaP pretr: FLA metabolite concentrations from microsomes of unexposed mice incubated with 3μM FLA alone; FL + BaP-no BaP pretr: FLA metabolite concentrations from microsomes of unexposed mice incubated with 3μM FLA + 3μM BaP; FL-BaP pretr: FLA metabolite concentrations from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM of FLA alone; FLA + BaP-BaP pretr: FLA metabolite concentrations from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM each of FLA and BaP. Values are expressed as mean concentration of total metabolites ± SE. Data from triplicate determination (variability was <10%) of five individual animals from each treatment group were compared. Asterisks denote statistical significance (*** p < 0.001, ** p < 0.01) in FLA metabolite concentrations among the treatment categories.

Figure 2

Rate of benzo(a)pyrene metabolism in adipose microsomes of Apc^Min mice pre-exposed to benzo(a)pyrene. Explanations of the abbreviations are as follows; BaP-no pretr: BaP metabolite concentrations from microsomes of unexposed mice incubated with 3μM BaP alone; BaP + FL-no BaP pretr: BaP metabolite concentrations from microsomes of unexposed mice incubated with 3μM FLA + 3μM BaP; BaP-BaP pretr: BaP metabolite concentrations from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM of BaP alone; BaP + FLA-BaP pretr: BaP metabolite concentrations from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM each of BaP and FLA. Values are expressed as mean concentration of total metabolites ± SE. Data from triplicate determination (variability was <10%) of five individual animals from each treatment group were compared. Asterisks denote statistical significance (*** p < 0.001, ** p < 0.01) in BaP metabolite concentrations among the treatment categories.

Figure 3

Metabolite composition of fluoranthene in adipose microsomes of Apc^Min mice pre-exposed to benzo(a)pyrene. Values are expressed as the percentage of individual metabolite types among the total metabolites (sum of individual concentrations of FLA2, 3-diol, trans-2,3-dihydroxy-1,10b-epoxy-1,2,3,10b tetrahydro FLA [2, 3D FLA], 3-hydroxy FLA, and 8-hydroxy FLA) formed (n = 5 for each treatment category). Explanations of the abbreviations are as follows; FL-no BaP pretr: FLA metabolite types from microsomes of unexposed mice incubated with 3μM FLA alone; FL + BaP-no BaP pretr: FLA metabolite types from microsomes of unexposed mice incubated with 3μM FLA + 3μM BaP; FL-BaP pretr: FLA metabolite types from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM of FLA alone; FLA + BaP-BaP pretr: FLA metabolite types from microsomes of BaP (50 μ/kg)-pretreated mice incubated with 3μM each of FLA and BaP.

Figure 4

Metabolite composition of benzo(a)pyrene in adipose microsomes of Apc^Min mice pre-exposed to benzo(a)pyrene. Values are expressed as the percentage of individual metabolite types among the total metabolites (sum of individual concentrations of BaP 9, 10-diol; BaP 4,5-diol; BaP 7,8-diol; 3-hydroxy and 9-hydroxy BaP) formed (n = 5 for each treatment category). Explanations of the abbreviations are as follows; BaP-no pretr: BaP metabolite types from microsomes of unexposed mice incubated with 3μM BaP alone; BaP + FL-no BaP pretr: BaP metabolite types from microsomes of unexposed mice incubated with 3μM FLA + 3μM BaP; BaP-BaP pretr: BaP metabolite types from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM of BaP alone; BaP + FLA-BaP pretr: BaP metabolite types from microsomes of BaP (50 μg/kg)-pretreated mice incubated with 3μM each of BaP and FLA.