Hexachlorobiphenyl Research Articles

Desorption of chlorinated hydrocarbons from phytoplankton

Studies were conducted to investigate the desorptive characteristics of hexachlorobenzene (HCB) and 2,4,5,2′,4′,5′-hexachlorobiphenyl (HCBP) sorbed to algal and sediment suspensions by using three approaches: batch equilibration, air purging and competitive exchange. In both short-term and extended batch desorptions, sorption partition coefficients were consistently greater than desorption partition coefficients. Air-purging removed virtually all of the sorbed HCBP within 2 h, exhibiting a two-component release pattern for all tested sorbents. In competitive sorption tests, HCB originally sorbed to suspended sediments was released to be taken up by algae. These studies indicate the importance of sorbent type in the magnitude and strength of sorption for two hydrophobic compounds.

Characterization of an inducible aryl hydrocarbon receptor-like protein in rat liver

The individual pretreatment of Sprague-Dawley rats with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) has been previously shown to result in the "induction" of [3H]TCDD specific binding activity in hepatic tissue. In the present work, the coadministration of TCDD and HCB increased the concentration of hepatic proteins capable of binding [3H]TCDD specifically by at least 2-3-fold. This increase was shown not to be the result of activation, by HCB, of a form of the receptor having low affinity toward [3H]TCDD into a form with high affinity. Kinetic analysis of the time course of binding of [3H]TCDD to induced cytosol was consistent with the presence of an "inducible" binding protein in addition to the "constitutive" aryl hydrocarbon (Ah) receptor present in cytosol from untreated animals. The liganded ([3H]TCDD) form of the inducible binding component lost its ligand much faster than the liganded form of the constitutive Ah receptor at 37 degrees C; apparent first order rate constants for loss of [3H]TCDD were 0.55 min-1 and less than 0.0024 min-1, respectively. Conversely, the unliganded form of the induced binding component was slightly more stable (approximately 2-fold) toward thermal inactivation than the unbound constitutive Ah receptor. The [3H]TCDD-bound protein(s) in uninduced and induced cytosols behaved identically in a sucrose gradient; 8.7-8.9 S in the absence of salt, shifted to 5.5 S by 0.4 M KCl. They were also indistinguishable by gel permeation chromatography, and by photoaffinity labeling their TCDD-binding subunits, approximate molecular weights 105,000. These results show the hepatic TCDD-binding protein(s) induced upon pretreatment of Sprague-Dawley rats with TCDD/HCB to be kinetically distinct from the Ah receptor, but structurally very similar.

Mysis relicta Assimilation of Hexachlorobiphenyl from Sediments

Assimilation and retention of a PCB congener, 2,4,5,2′,4′,5′-hexachlorobiphenyl (HCBP), by the oppossum shrimp, Mysis relicta, was followed during sediment ingestion and defecation using a 14C label. Uptake was rapid and essentially linear, with mysids reaching a [14C]HCBP specific activity equivalent to that in the labeled sediment within 2 wk. Assimilation efficiencies calculated from the relative depletion of the label in the fecal material averaged 53% (±7%). Sediment ingestion rates calculated from the activity required to supply the label retained were approximately 1 mg∙mysid−1∙d−1, in agreement with published estimates. The distribution of HCBP as a function of particle size indicates that particle-size selective feeding by detritivores can have a significant effect on calculated assimilation efficiencies. Given the vertical migration behavior of M. relicta, sediment ingestion is a potentially important pathway for the reintroduction of sediment-associated contaminants into the pelagic environment of deep lakes.

Effects of polychlorinated biphenyls with Ah receptor affinity on lymphoid development in the thymus and the bursa of Fabricius of chick embryos in Ovo and in mouse thymus anlagen in vitro

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) and its congeners bind to the Ah receptor and are known to cause thymic atrophy in most experimental animal species and also to inhibit lymphoid development in the embryonic thymus (T-cells) and in the bursa of Fabricius of chick embryos (B-cells). The coplanar polychlorinated biphenyls (PCBs) 3,3′,4,4′-tetrachlorobiphenyl (TCB), 3,3′,4,4′,5-pentachlorobiphenyl (PeCB), and 3,3′,4,4′,5,5′-hexachlorobiphenyl (HCB) (relatively strong Ah receptor ligands) and the mono- ortho-chlorinated analogues of TCB and PeCB (relatively weak Ah receptor ligands) were administered to chick embryos by air chamber injection on Day 13 of incubation. The numbers of lymphoid cells (on Day 19) in the thymus and the bursa of Fabricius were lower, in a dose-dependent manner, in embryos treated with the coplanar PCBs compared with controls. Approximate ED50 values for inhibition of bursal cell development were 4 μg for PeCB, 50 μg for TCB, and 300 μg/kg egg for HCB. The most immunotoxic of the mono- ortho-chlorinated analogues of TCB and PeCB were about 1000 times less potent than PeCB. The in vitro effects of the PCBs were studied in organ cultures of thymi from 15-day-old mouse fetuses. The three coplanar chlorobiphenyls inhibited lymphoid development in this culture system in a dose-dependent manner. PeCB was only about 10 times less potent (EC50 ≈ 2 × 10 −9 m) than TCDD (EC50 ≈ 2 × 10 −10 m), whereas HCB and TCB were about 100 times less toxic than PeCB. No inhibition of lymphoid development by the mono- ortho-chlorinated PCBs was observed using concentrations as high as 10 −6 m.

Studies of the antenna effect in polymer molecules. 23. Photosensitized dechlorination of 2,2',3,3',6,6'-hexachlorobiphenyl solubilized in an aqueous solution of poly(sodium styrenesulfonate-co-2-vinylnaphthalene)

Photodechlorination of 2,2{prime},3,3{prime},6,6{prime}-hexachlorobiphenyl (HCB) solubilized in an aqueous solution of poly(sodium styrenesulfonate-co-2-vinylnaphthalene) (PSSS-VN) was studied with use of solar-simulated radiation. The reaction was found to be photosensitized by the naphthalene antenna units present in the copolymer. Studies performed in a low molecular weight model system have shown that dechlorination of HCB may occur via an exciplex intermediate. Exciplex formation in the system is efficient because of the high local concentration of HCB in proximity to the naphthalene polymeric units.

Chlorinated biphenyls induce cytochrome P450IA2 and uroporphyrin accumulation in cultures of mouse hepatocytes

Previous enzymatic and immunological studies from this laboratory have indicated a critical role for cytochrome P450IA2-catalyzed uroporphyrinogen oxidation in the development of uroporphyria caused by halogenated aromatic hydrocarbons. To extend these studies, we investigated whether primary cultures of mammalian hepatocytes which are inducible for cytochrome P450IA2 are also inducible for chemically mediated uroporphyria. Hepatocytes were isolated from C57BL/6 mice and maintained on Matrigel, an extracellular matrix isolated from a mouse tumor. When these cultures were treated with 3,4,5,3′,4′,5′-hexachlorobiphenyl (HCB) and 5-aminolevulinic acid (ALA), they accumulated cytochrome P450IA2 as well as uroporphyrin (URO) and heptacarboxyporphyrin for up to 12 days. Cultures treated with ALA alone accumulated no P450IA2 and very little URO. Neither URO accumulation nor the level of P450IA2 was affected by addition of iron as the nitrilotriacetate complex. Other inducers of P450IA2 in vivo (3,4,5,3′, 4′-pentachlorobiphenyl, 3,4,3′,4′-tetrachlorobiphenyl, and 3-methylcholanthrene) also increased P450IA2 in the cultures and caused URO accumulation in the presence of added ALA. The tetrachlorobiphenyl and methylcholanthrene caused these effects only when given repeatedly. Inducers of other forms of P450 failed to cause URO accumulation in the presence of ALA and iron. Cultures of hepatocytes from DBA mice (which are resistant to the uroporphyria in vivo) accumulated much less P450IA2 or URO when treated with HCB and ALA. These primary cultures of mammalian hepatocytes represent a new experimental model to investigate the role of cytochrome P450IA2 in the mechanism of chemically induced uroporphyria.

Effects of two prototypic polychlorinated biphenyls (PCBs) on lipid composition of rat liver and serum

Mature male Sprague-Dawley rats received a single IP injection of either 2,2′,4,4′,5,5′-hexachlorobiphenyl (HCB), 3,3′,4,4′-tetrachlorobiphenyl (TCB) (300 μm/kg) in corn oil (10 ml/kg) or the corn oil vehicle alone, and were killed four days later after having been fasted overnight. The vehicle control group consisted of rats which were allowed free access to feed as well as pair-fed animals. Lipid analyses were conducted on liver, hepatic microsomes and serum. TCB- (but no HCB-) treatment resulted in a statistically significant increase in total liver lipids and triglycerides. Liver phospholipids remained unchanged. Both PCBs increased the cholestrol and phospholipids content of the liver microsomal fraction. Serum lipids measured were not statistically different from control values. While HCB had little effect on the fatty acid composition of liver lipids, TCB caused an increase in C 18:1 (n-9) and a decrease in C 20:4 (n-6). Both PCBs increased C 18:0 in the hepatic microsomal fraction, but TCB also decreased C 16:0. Neither PCB altered the fatty acid composition of serum total lipids. These data are consistent with the concept that specific alterations in lipid metabolism are dependent on the structure of the PCB.

Role of the Ah locus in suppression of cytotoxic T lymphocyte activity by halogenated aromatic hydrocarbons (PCBs and TCDD): Structure-activity relationships and effects in [formula omitted] mice congenic at the Ah locus

Previous studies have shown that the generation of cytotoxic T lymphocytes (CTL) following allogeneic tumor challenge is suppressed in Ah-responsive C57Bl 6 mice treated with a single oral dose of the toxic, Ah receptor-binding 3,4,5,3′,4′,5′-hexachlorobiphenyl (HxCB). The present studies have examined the specific role of the Ah receptor in this immunotoxic response by utilizing HxCB isomers of known, varied affinity for the Ah receptor as well as by comparing effects of high-affinity Ah receptor ligands (3,4,5,3′,4′,5′-HxCB and 2,3,7,8-tetrachlorodibenzo- p-dioxin [TCDD]) on the CTL response of mice that differ only at the Ah locus, that is, Ah-responsive (Ah bb) and Ah-nonresponsive (Ah dd) congenic C57Bl 6 mice. Correlative changes in thymic weight, serum corticosterone (CS) levels, and spleen cellularity were also measured. The potency of HxCB congeners (3,4,5,3′,4′,5′-; 2,3,4,5,3′,4′-; 2,4,5,2′,4′,5′-) and 2,3,7,8-TCDD to suppress the CTL response, to reduce spleen cellularity, to cause thymic atrophy, and to elevate serum CS levels was directly correlated with the binding affinity of the congener for the Ah receptor. Furthermore, these parameters of immunotoxicity in Ah dd C57Bl 6 mice were significantly more resistant to alterations induced by either 3,4,5,3′,4′,5′-HxCB or 2,3,7,8-TCDD as compared to Ah bb C57Bl 6 mice. These results strongly support an Ah receptor-dependent immunotoxic mechanism in suppression of the CTL response following acute exposure to halogenated aromatic hydrocarbons.

Partitioning of 2,4,5,2′,4′,5′-hexachlorobiphenyl between seawater and air

PCBs are generally characterized by low vapor pressures; they would not usually be considered volatile. However, they are also characterized by low water solubilities. Studies addressing PCB toxicology have usually employed open aquaria. Therefore, the objective of this investigation was to describe the gas-liquid partitioning of a single PCB isomer, 2,4,5,2{prime},4{prime}5{prime}-hexachlorobiphenyl (HCBP), in seawater within glass aquaria commonly used for toxicological testing. Partitioning may reduce desired exposure levels during testing, seriously effecting results. However, it may be utilized to realistically model environmental conditions during testing. Therefore, the measured H{prime} will be compared to others obtained with specialized purging equipment. 2,4,5,2{prime},4{prime},5{prime}-HCBP was chosen for its presence in common PCB mixtures and high chlorine content, thus low water solubility and high bioaccumulation potential.

Role of the Ah Locus in Suppression of Cytotoxic T Lymphocyte Activity by Halogenated Aromatic Hydrocarbons (PCBs and TCDD): Structure-Activity Relationships and Effects in C57BI/6 Mice Congenic at the Ah Locus

Role of the Ah Locus in Suppression of Cytotoxic T Lymphocyte Activity by Halogenated Aromatic Hydrocarbons (PCBs and TCDD): Structure-Activity Relationships and Effects in C57B1/6 Mice Congenic at the Ah Locus. KERKVLIET, N. I., BAECHER-STEPPAN, L., SMITH, B. B., YOUNGBERG, J. A., HENDERSON, M. C, AND BUHLER, D. R. (1990). Fundam. Appl. Toxicol. 14, 532–541. Previous studies have shown that the generation of cytotoxic T lymphocytes (CTL) following allogeneic tumor challenge is suppressed in Ah-responsive C57B1/6 mice treated with a single oral dose of the toxic, Ah receptor-binding 3,4,5,3′,4′,5′-hexachlorobiphenyl (HxCB). The present studies have examined the specific role of the Ah receptor in this immuno-toxic response by utilizing HxCB isomers of known, varied affinity for the Ah receptor as well as by comparing effects of high-affinity Ah receptor ligands (3,4,5,3′,4′,5′-HxCB and 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) on the CTL response of mice that differ only at the Ah locus, that is, Ah-responsive (Ah1*) and Ah-nonresponsive (Ahdd) congenic C57B1/6 mice. Correlative changes in thymic weight, serum corticosterone (CS) levels, and spleen cellularity were also measured. The potency of HxCB congeners (3,4,5,3′,4′,5′-; 2,3,4,5,3′,4′-; 2,4,5,2′,4′,5′-) and 2,3,7,8-TCDD to suppress the CTL response, to reduce spleen cellularity, to cause thymic atrophy, and to elevate serum CS levels was directly correlated with the binding affinity of the congener for the Ah receptor. Furthermore, these parameters of immunotoxicity in Ah*1 C57B1/6 mice were significantly more resistant to alterations induced by either 3,4,5,3′,4′,5′-HxCB or 2,3,7,8-TCDD as compared to Ah1* C57B1/6 mice. These results strongly support an Ah receptor-dependent immunotoxic mechanism in suppression of the CTL response following acute exposure to halogenated aromatic hydrocarbons.

In vivo and in vitro dermal penetration of 2,4,5,2′,4′,5′-Hexachlorobiphenyl in young and adult rats

Penetration of 2,4,5,2',4',5'-[14C]hexachlorobiphenyl (HCB) through skin of young (33 days) and adult (82 days) female Fischer 344 rats was determined in vivo and by two in vitro methods. In vivo dermal penetration at 120 hr was 45% in young and 43% in adults. At 72 hr in vivo dermal penetration was 35% in young and 26% in adults compared to 1.5% for young and 1.0% for adult as measured with a continuous flow in vitro system and 2.9% for young and 1.9% for adults as measured with a static in vitro system. Most of the dermally absorbed HCB remained in the body as only 4.9 and 2.6% of that absorbed was excreted by young and adult rats, respectively, at the end of 120 hr. Significant differences in dermal penetration and kinetics of HCB between young and adult female rats were observed. The elimination of HCB-derived material was approximately six times higher in feces than in urine. A physiological pharmacokinetic model was fitted to the organ and tissue radioactivity distribution data. Parameters in the model determined from dermal dosing of female Fischer 344 rats were in reasonable agreement with those reported in the literature for adult male Sprague-Dawley rats (iv dose). The rat constant for dermal penetration was 0.83 x 10(-4) min-1 for adults and 0.96 x 10(-4)min-1 for young. The delay or lag time parameter for dermal penetration was 4.4 hr in adults and 1.1 hr in young.

Suppression of male-specific cytochrome P450 2c and its mRNA by 3,4,5,3′,4′,5′-hexachlorobiphenyl in rat liver is not causally related to changes in serum testosterone

Rat cytochrome P450 2c (P450 gene IIC11) is a constitutive, male-specific hepatic enzyme which is suppressed >90% by treatment with 3,4,5,3′,4′,5′-hexachlorobiphenyl (HCB) [H. N. Yeowell et al. (1987) Mol. Pharmacol. 32, 340–347]. HCB also decreases serum testosterone levels in adult male rats (>98% loss). The present study assesses whether the suppression of P450 2c by HCB is a direct result of its effects on serum testosterone levels. Further, the site along the hypothalamic-pituitary-testicular axis at which HCB acts to depress testosterone secretion was examined. Administration of the synthetic androgen methyltrienolone to HCB-treated rats failed to prevent the suppression of P450 2c mRNA and its associated microsomal steroid 16α-hydroxylase activity under conditions where it effectively reversed the large decrease in P450 2c mRNA and steroid 16α-hydroxylase activity produced by castration. Hepatic steroid 6β-hydroxylase activity, which is catalyzed primarily by P450 2a (P450 gene IIIA2), was also suppressed by HCB and was not protected by methyltrienolone. Administration of either human chorionic gonadotropin, an analog of pituitary-derived luteinizing hormone, or the hypothalamic luteinizing hormone releasing hormone elevated serum testosterone levels to a much smaller extent in HCB-treated rats than in control rats. These results indicate that the effects of HCB on serum testosterone levels reflect its effects on testicular function rather than the pituitary or hypothalamus. However, the present study demonstrates that the consequential reduction in serum testosterone levels in HCB-treated rats is not causally related to the reduction in hepatic P450 2c levels. Thus, HCB must also act on some other regulatory mechanism involved in the expression of this protein.

Bioavailability and toxicokinetics of polycyclic aromatic hydrocarbons sorbed to sediments for the amphipod Pontoporeia hoyi

The accumulation kinetics, by the benthic amphipod, Pontoporeia hoyi, were measured for sediment-associated, selected polycyclic aromatic hydrocarbons (PAHs) and 2,4,5,2',4',5'-hexachlorobiphenyl (HCB). The kinetics data suggest that uptake occurs largely via the sediment interstitial water and is kinetically controlled by desorption from sediment particles and dissolved organic matter. Assimilation from ingested material may be significant for the more strongly sorbed compounds such as benzo[a]pyrene and HCB. The desorption rate of contaminants from the sediment matrix appears to determine whether the major sediment contaminant source is interstitial water or ingested particles. The log of the contaminant uptake clearance is inversely proportional to the log octanol-water partition coefficient for PAHs. Bioavailability of sediment-sorbed contaminants declined as the contact time between the sediment and contaminant increased. Chemical extractability remained high even though bioavailability was reduced. A conceptual model to describe accumulation of organic contaminants from sediments is described.

2,2′,4,4′,5,5′-Hexachlorobiphenyl as a 2,3,7,8-tetrachlorodibenzo- p-dioxin antagonist in [formula omitted] mice

At doses as high as 750 to 1000 μmol/kg, 2,2′,4,4′,5,5′-hexachlorobiphenyl (HCBP) did not cause fetal cleft palate, suppress the splenic plaque-forming cell response to sheep red blood cells, or induce hepatic microsomal ethoxyresorufin O-deethylase (EROD) in C57BL 6J mice. Despite the lack of activity of HCBP in eliciting any of these aryl hydrocarbon (Ah) receptor-mediated responses, competitive binding studies indicated that HCBP competitively displaced 2,3,7,8-[ 3 H] tetrachlorodibenzo-p- dioxin (TCDD) from the murine hepatic cytosolic receptor. Cotreatment of C57BL 6J mice with TCDD (3.7 nmol/kg) and HCBP or 4,4′-diiodo-2,2′,5,5′-tetrachlorobiphenyl (I 2-TCBP) (400 or 1000 μmol/kg) showed that both compounds partially antagonized TCDD-mediated cleft palate and immunotoxicity (i.e., suppression of the splenic plaque-forming cell response to sheep red blood cells), and HCBP antagonized TCDD-mediated hepatic microsomal EROD induction. Thus, HCBP and I 2-TCBP, like the commercial polychlorinated biphenyl mixture Aroclor 1254, were partial antagonists of TCDD action in C57BL 6J mice; however, it was also apparent from the results that Aroclor 1254 was the more effective antagonist at lower doses. Using [ 3 H] TCDD , it was also shown that some of the effects of HCBP on TCDD-mediated cleft palate may be due to the decreased levels of TCDD found in the fetal palates after cotreatment with TCDD and HCBP. 4,4′-[ 125 I 2] diiodo-2,2′,5,5′- tetrachlorobiphenyl ([ 125I 2] TCBP ) of high specific activity (3350 Ci/mmol) was synthesized and used to investigate the direct binding of this compound to the murine hepatic Ah receptor or other cytosolic proteins. No direct specific binding was observed between 125 I 2- TCBP and any cytosolic proteins using a sucrose density gradient assay procedure. These results contrasted with previous studies with Aroclor 1254 that suggested that this mixture acted as a competitive Ah receptor antagonist.

Effect of hexachlorobiphenyl on vitamin A homeostasis in the rat

Vitamin A status and turnover were examined in rats that had been exposed to chronic dietary treatment of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), 1 mg/kg diet. HCB caused hepatic depletion and renal accumulation of vitamin A, and a 1.7-fold increase in the serum retinol concentration. Intravenously administered [3H]retinol bound to retinol binding protein-transthyretin complex (RBP-TTR complex) was used to study the dynamics of circulatory retinol in these rats. In HCB-treated rats, the plasma turnover rate of retinol was increased compared to vitamin A-adequate untreated controls. HCB caused a 50% reduction of total radioactivity in liver, and, except for 0.5 h after the [3H]retinol-RBP-TTR dose, the specific activity of the hepatic retinyl ester pool was greater compared to control rats. The kidneys of HCB-treated rats accumulated radioactivity in the retinyl ester fraction. HCB also caused a 50% reduction in adrenal radioactivity compared with control rats. Urinary and fecal excretion of radioactivity was 3-fold higher in HCB-treated rats as compared to controls. Our findings demonstrate that chronic HCB feeding results in expansion of plasma vitamin A mass, in changes of liver and kidney retinol and retinyl ester pool dynamics and in an increased metabolism of vitamin A.

Toxicokinetics of DDE, Benzo(a)Pyrene, and 2, 4, 5, 2′, 4′, 5′-Hexachlorobiphenyl in Pontoporeia Hoyi and Mysis Relicta

The toxicokinetics of DDE, benzo(a)pyrene (BaP), and 2, 4, 5, 2’, 4’, 5’-hexachlorobiphenyl (HB) were followed for the amphipod, Pontoporeia hoyi, and the mysid, Mysis relicta. Pontoporeia and Mysis had similar uptake clearances (K u) for DDE (mean = 79.2 mL/g/h and 46.0 mL/g/h, respectively), BaP (mean = 75.9 mL/g/h and 39.9 mL/g/h, respectively), and HB (mean = 53.5 mL/g/h and 57.5 mL/g/hr, respectively) compounds with log octanol-water partition coefficients (K ow's) ranging from 5.7 to 6.7. Amphipods and mysids were most efficient at eliminating BaP (mean K d = −0.0017/h and −0.0047/h, respectively) and least efficient at eliminating HB (mean = −0.0008/h and −0.0001/h respectively). Amphipods were more efficient than mysids in eliminating DDE (mean = −0.0010/h and −0.0005/h, respectively) and HB while mysids were more efficient at eliminating BaP. Because K d's for DDE, BaP, and HB were substantially different between P. hoyi and M. relicta, there were substantial differences in the calculated bioconcentration factors (BCFs). Amphipods tended toward larger BCFs than mysids for BaP (mean = 48,582 and 8,496, respectively) but lower BCFs for DDE (mean = 95,629 and 138,760, respectively) and HB (mean = 101,663 and 442,231, respectively) than mysids.

Toxicity and 7-ethoxyresorufin O-deethylase-inducing potency of coplanar polychlorinated biphenyls (PCBs) in chick embryos.

The toxicities of the coplanar polychlorinated biphenyls 3,3',4,4'-tetrachlorobiphenyl (TCB), 3,3',4,4',5-pentachlorobiphenyl (PeCB) and 3,3',4,4',5,5'-hexachlorobiphenyl (HCB) were compared in a 72-h study on chick embryos. The substances were injected into the air sacs of hens' eggs preincubated for 7 days. Mortality was measured 72 h later and corresponding LD50 values were calculated. The rank order of toxicity was PeCB greater than TCB greater than HCB. Using the same injection procedure, the potencies of these chlorobiphenyls with regard to their induction of hepatic 7-ethoxyresorufin O-deethylase activity were compared. The ranking order of the substances as inducers was the same as their order when ranked according to toxicity. The three coplanar chlorobiphenyls were considerably more toxic and potent as inducers than the nonplanar 2,2',4,4',5,5'-hexachlorobiphenyl. In a 2-week toxicity study, PeCB and HCB were injected into the yolks of hens' eggs preincubated for 4 days. PeCB was about 50-fold more potent than HCB in causing embryonic death. Both substances caused abnormalities, including edema, liver lesions, microphthalmia and beak deformities.

Inducers of cytochrome P-450d: Influence on microsomal catalytic activities and differential regulation by enzyme stabilization

The interaction of isosafrole, 3,4,5,3′,4′,5′-hexabromobiphenyl (HBB) and hexachlorobiphenyl (HCB) with cytochrome P-450d was evaluated by characterization of estradiol 2-hydroxylase activity. Displacement of the isosafrole metabolite from microsomal cytochrome P-450d derived from isosafrole-treated rats resulted in a 160% increase in estradiol 2-hydroxylase. The increase was fully reversed by incubation with 1 μ m HBB. Although isosafrole is capable of forming a complex with many different cytochrome P-450 isozymes, it appears to bind largely to cytochrome P-450d in vivo as was demonstrated by measuring the enzymatic activity of microsomal cytochromes P-450b, P-450c, and P-450d from isosafrole-treated rats. When estradiol 2-hydroxylase was measured in rats treated with increasing doses of HCB, there was a gradual decrease in microsomal enzyme activity despite a 20-fold increase in cytochrome P-450d. The ability of cytochrome P-450d ligands to stabilize the enzyme was investigated in two ways. First, cytochromes P-450c and P-450d were quantitated immunochemically in microsomes from rats treated with 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD), at a dose which maximally induced total cytochrome P-450, followed by a single dose of a second inducer. The specific content of cytochrome P-450d was significantly increased when isosafrole or HCB was the second inducer but not when 3-methylcholanthrene was the second inducer. Second, the relative turnover of cytochrome P-450d was measured by the dual label technique. Following TCDD treatment, microsomal protein was labeled in vivo with [ 3H]leucine, the second inducer was given and protein was again labeled 3 days later with [ 14C]leucine. A higher ratio of 3 H 14 C in the cytochrome P-450d from isosafrole + TCDD- and HCB + TCDD-treated rats relative to TCDD (control)-treated rats suggested that isosafrole and HCB were able to retard the degradation of cytochrome P-450d, presumably by virtue of being tightly bound to the enzyme.

Nonadditive interactive effects of polychlorinated biphenyl congeners in rats: role of the 2,3,7,8-tetrachlorodibenzo-p-dioxin receptor.

Administration of 3,3',4,4',5,5'-hexa-,3,3',4,4',5-penta-, and 2,3,3'4,4'5-hexa-chlorobiphenyl to immature male Wistar rats caused a thymic atrophy at high dose levels (1.25, 1.0, and 100 mumol/kg, respectively) and induced the hepatic cytochrome P-448 dependent monooxygenases (benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase) at both high and low (0.25, 0.01, and 5 mumol/kg, respectively) doses. In contrast, 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP) (300 mumol/kg) did not elicit any of these effects but elevated hepatic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) cytosolic receptor protein levels (threefold) as previously reported. The effects of hepatic receptor modulation by 2,2',4,4',5,5'-HCBP (300 mumol/kg) on the enzyme induction activities of 3,3'4,4',5-penta-, 3,3'4,4',5,5'-hexa-, and 2,3,3',4,4',5-hexa-chlorobiphenyl were dose-dependent; no interactive effects were observed at high (toxic) doses of these compounds, whereas apparent synergistically increased hepatic microsomal monooxygenase induction activities were noted at the lower submaximal induction doses. It was concluded that the increased responsiveness of the rats was due to elevated hepatic 2,3,7,8-TCDD receptor levels.

Dual Tracer Studies of the Assimilation of an Organic Contaminant from Sediments by Deposit Feeding Oligochaetes

The assimilation of [14C]2,4,5,2′,4′,5′-hexachlorobiphenyl (HCBP) from Lake Michigan sediments by oligochaete worms was determined in laboratory microcosms using dual tracer techniques. Particle size selective feeding by oligochaetes makes single tracer calculations of assimilation from bulk feces subject to errors resulting from the changing distribution coefficients of adsorbed constituents as a function of particle size. 51Cr3+ adsorbed to sediments passes through the guts of worms without being assimilated and serves as a conservative tracer of ingestion. Assimilation efficiencies for HCBP decreased from 36 to 15% over the initial 10 d of active feeding and was inversely related to average defecation rate which increased from 0.05 to 0.25 mg sediment∙mg worm−1∙h−1 over the same period. In combination with measured defecation rates, assimilation efficiencies were used to estimate HCBP uptake rates of 3.9–8.1 pmol∙mg worm−1∙h−1. Assimilation efficiencies appear to be dependent upon gut clearing times which are a function of both gut volume and feeding rate and which are estimated to vary from <1 to >5 h.