Activation of benzo(a)pyrene-7,8-dihydrodiol in rat uterus: an in vitro study

Alteration of pregnancy related hormones and fetal survival in F-344 rats exposed by inhalation to benzo(a)pyrene

The objective of this study was to evaluate the effect of subacute exposure to inhaled benzo(a)pyrene (BaP) on fetal survival and luteal maintenance using timed-pregnant Fisher 344 rats. Prior to assignment of pregnant rats to treatment and control groups, numbers of implantation sites were determined on gestation day (GD) 8 via midventral laparotomy. Subsequently, animals were assigned randomly to three treatment groups and two control groups. Treatment consisted of subacute exposure of rats via inhalation to BaP 25, 75, and 100 micro g/m(3), 4h daily for 10 days (GD-11-20). Control animals were either sham exposed to carbon black (CB) to control for inert BaP carrier or remained unexposed (UNC). Blood samples were collected on days 15 and 17 of gestation via sinus orbital veini-puncture for plasma. Number of pups per litter was determined postpartum and fetal survival rate was expressed as a percentage of the corresponding implantation sites. Radioimmunoassays were used to determine plasma progesterone, estrogen, and prolactin (indirect measurement of decidual luteotropin) concentrations. Fetal survival among BaP-treated rats declined in a dose-dependent manner (25 micro g/m(3), 78.3% per litter; 75 micro g/m(3), 38.0% per litter; 100 micro g/m(3), 33.8% per litter; P<0.05) compared with CB (96.7% per litter) and UNC (98.9% per litter). Plasma progesterone, estrogen, and prolactin concentrations also declined as a result of subacute exposure of rats to BaP compared to controls. These data suggest that inhaled BaP compromised fetal survival and consequently luteotropic activity in the exposed animals.

Xenobiotic metabolism in humans during early pregnancy: peroxidase-mediated oxidation and bioactivation of 2-aminofluorene

Arylamines such as 2-aminofluorene (2-AF) are known teratogens and transplacental carcinogens in laboratory animal species. Although exposure of women to arylamines is likely to occur during pregnancy, how these chemicals are metabolized by the enzymes from the human conceptual tissues is currently unknown. Highly purified preparations of peroxidase isolated from human intrauterine conceptual tissues at 8 weeks of gestation were used to study in vitro metabolism of 2-AF. The oxidation of 2-AF was examined spectrophotometrically whereas the bioactivation was assessed from the covalent binding to protein and DNA using [3H] 2-AF. Using guaiacol as a model substrate, the purified preparations of peroxidase used exhibited a specific activity of 15-20 micromol/min/mg protein. 2-AF oxidation was found to be enzymatic in nature. Kinetic data obtained under optimal assay conditions yielded a Km = 41 microM for 2-AF, 8.33 microM for H2O2, and a Vmax=1.2 micromol 2-AF oxidized/min/mg protein. Under optimal assay conditions, the covalent binding of reactive intermediates to protein and DNA (nmol equivalent/min/mg enzyme/mg bovine serum albumin or calf thymus DNA) was observed at the rate of about 3.75 +/- 0.39 and 1.90 +/- 0.11 respectively. A significant decline in the rate of both oxidation and bioactivation of 2-AF was observed in the presence of classical peroxidase inhibitors, KCN and NaN3.

2-Aminofluorene bioactivation by human term placental peroxidase

Earlier investigations from our laboratory demonstrated that human term placental peroxidase (HTPP) is capable of metabolism of xenobiotics and endogenous compounds. In this study, purified HTPP was found to bioactivate 2-aminofluorene (2-AF) in the presence of H2O2. 2-AF oxidation was studied spectrophotometrically while radiometry was employed to assess the bioactivation. The rate of oxidation and covalent binding to protein and DNA was dependent upon the pH of the reaction medium and the concentration of 2-AF, the enzyme, and H2O2. To observe maximal enzyme velocity of oxidation, the presence of 16.5 microM H2O2, 100 microM 2-AF, 37 micrograms of the enzyme protein/ml, and pH 7.2 was required. Under optimal assay conditions, the range of specific activity between 130 and 165 nmol of 2-AF oxidized/min/mg HTPP was observed. Using similar assay conditions, the magnitude of covalent binding of [3H]-2-AF to protein (BSA) and calf thymus DNA was found to be about 508 pmol bound/min/mg HTPP/mg BSA and 84 pmol bound/min/mg HTPP/mg DNA, respectively. Potassium cyanide and sodium azide, the known inhibitors of different peroxidases, significantly blocked both the oxidation and covalent binding of 2-AF in a dose dependent manner. These results strongly suggest that peroxidase may be one of the important pathways responsible for the bioactivation of arylamines in human term placenta.

Xenobiotic oxidation during early pregnancy in man: peroxidase catalysed chemical oxidation in conceptual tissues

1. A membrane-bound peroxidase activity was isolated and purified from pooled samples of human intrauterine tissue (embryo, placenta, amnion, chorion and uterine decidua) of 4-6 weeks of gestation. 2. The H2O2-dependent guaiacol oxidation catalysed by the purified peroxidase exhibited a specific activity of 10.82 +/- 1.81 mumol/min/mg protein. 3. The optimum conditions required for guaiacol oxidation catalysed by the peroxidase included 18 mM guaiacol, 400 microM H2O2, pH 9.0, and 2 micrograms/ml enzyme protein. The guaiacol oxidation was inhibited by KCN and NaN3 with IC50s of 17.6 and 22 microM, respectively. 4. Several other endogenous and exogenous chemicals also underwent H2O2-dependent oxidation catalysed by the purified peroxidase. These results suggest that peroxidase may be one of the major pathways of xenobiotic oxidation present in organogenesis-stage human conceptual tissues influencing the toxicity of chemicals to which the developing embryo is exposed.

Bioactivation of benzo(a)pyrene-7,8-dihydrodiol catalyzed by lipoxygenase purified from human term placenta and conceptal tissues

Bioactivation of 14C-benzo(a)pyrene-7,8-dihydrodiol catalyzed by lipoxygenase purified from human term placenta of nonsmoking women and intrauterine conceptal tissues (at 4 weeks of gestation) was investigated. Incubation of 14C-benzo(a)pyrene-7,8-dihydrodiol with 3 mM linoleic acid in the presence of lipoxygenase purified from either human term placenta or intrauterine conceptal tissues resulted in co-oxidation generating several soluble and protein-bound metabolites of benzo(a)pyrene-7,8-dihydrodiol. The co-oxidation was inhibited significantly by the specific lipoxygenase inhibitor, nordihydroguaiaretic acid. Substitution for the active enzyme in the reaction mixture with heat denatured enzyme resulted in almost complete abolition of benzo(a)pyrene-7,8-dihydrodiol co-oxidation. These results suggest that lipoxygenase in the placentas and intrauterine conceptal tissues is capable of metabolizing benzo(a)pyrene-7,8-dihydrodiol to several reactive metabolites and may represent one of the major xenobiotic metabolizing pathways of bioactivating chemicals in the intrauterine compartment.