Effect of uroporphyrin on the spectral measurement of cytochrome P450

Uncoupling of cytochrome P450 1A and stimulation of reactive oxygen species production by co-planar polychlorinated biphenyl congeners

The non-ortho-polychlorinated biphenyl (PCB) congener 3,3'4,4'-tetrachlorobiphenyl (PCB 77) can uncouple the catalytic cycle of fish (scup) cytochrome P4501A (CYP1A) and mammalian (rat, human) CYP1A1, stimulating release of reactive oxygen species (ROS). PCB 77 also inactivates CYP1A in an NADPH-, oxygen-, and time-dependent process, linked to uncoupling. We addressed a hypothesis that planar halogenated hydrocarbons generally will uncouple CYP1A. Thus, additional PCB congeners including non-ortho-3,3',4,4',5'-pentachlorobiphenyl (PCB 126) and 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169), mono-ortho-2,3,3',4,4'-pentachlorobiphenyl (PCB 105) and di-ortho-2,2',5,5'-tetrachlorobiphenyl (PCB 52), as well as the polycyclic aromatic hydrocarbon benzo[a]pyrene (B[a]P), were examined for their ability to stimulate microsomal ROS production and to inactivate CYP1A. Incubated without NADPH, non-ortho-PCB 126 and -PCB 169 both inhibited microsomal CYP1A activity (ethoxyresorufin O-deethylase; EROD). When NADPH was included, these congeners caused a progressive inactivation of CYP1A, in addition to the inhibition. The determined K(Inact) values for inactivation were 0.14 and 0.08 microM, respectively, for PCB 126 and PCB 169, similar to the 0.05 microM for PCB 77 previously reported. The mono-ortho-PCB 105 weakly inhibited and weakly inactivated CYP1A. The di-ortho-PCB 52 neither inhibited nor inactivated CYP1A. Alone, B[a]P strongly inhibited CYP1A, but when NADPH was added that inhibition was reversed, apparently by metabolic depletion of the substrate, and there was no inactivation. PCB 126 and PCB 169 stimulated release of ROS from induced liver microsomes, while B[a]P, PCB 52 and PCB 105 did not. ROS release and CYP1A inactivation stimulated by the non-ortho-PCB 126 and PCB 169 indicate an uncoupling of CYP1A like that previously shown with PCB 77. The uncoupling and release of ROS further suggest a participation of CYP1A in the oxidative stress associated with some planar halogenated aryl hydrocarbon receptor agonists.

Induction and suppression of cytochrome P450 1A by 3,3',4,4',5-pentachlorobiphenyl and its relationship to oxidative stress in the marine fish scup (Stenotomus chrysops)

The planar polychlorinated biphenyl (PCB) 3,3',4,4'-tetrachlorobiphenyl (TCB) causes dose-dependent induction and post-transcriptional suppression of hepatic cytochrome P450 1A (CYP1A) in the marine teleost scup (Stenotomus chrysops). That suppression is linked to inhibition and oxidative inactivation of CYP1A by TCB. Other planar PCBs, including 3,3',4,4',5-pentachlorobiphenyl (PeCB), inactivate scup CYP1A in vitro leading us to hypothesize that PeCB also will suppress CYP1A in vivo. We examined induction and suppression of CYP1A by PeCB in scup, as related to oxidative stress. PeCB at a low dose (0.01 mg/kg) induced hepatic microsomal spectral P450 and CYP1A protein and catalytic activities (ethoxyresorufin o-deethylase (EROD) and methoxyresorufin o-demethylase (MROD)) over an 18 day period. A high dose (1 mg PeCB/kg) only minimally induced hepatic spectral P450 and CYP1A content, and EROD and MROD rates remained at control levels at all sampling times, while CYP1A mRNA expression was induced strongly (up to 35-fold) at both doses. High dose PeCB had minimal effects on content of P450A (a CYP3A protein), P450B (a CYP2B-like protein) and cytochrome b5 in scup liver, suggesting that the suppression was specific for CYP1A. High dose PeCB suppressed EROD but not CYP1A protein in the kidney but did not strongly suppress either CYP1A or EROD in the heart or gill. PeCB stimulated ROS production (oxidation of dihydroethidium) by liver microsomes from the low dose but not the high dose fish, and the rate of PeCB-stimulated ROS production was correlated with EROD activity (r(2)=0.641, P<0.0005). Oxidative stress, indicated by increased levels of catalase, glutathione peroxidase, glutathione reductase and superoxide dismutase activities, was stimulated in the liver by low dose but not high dose PeCB. The results support a hypothesis that many PHAH can inactivate teleost CYP1A in vivo, and that CYP1A is a source of ROS. However, there appears to be a complex balance between the effects of PeCB on the levels of active CYP1A, ROS release and oxidative stress.

Induction of cytochrome P450 1A in the American Eel by model halogenated and non-halogenated aryl hydrocarbon receptor agonists

Eels (Anguilla sp.) have phylogenetic, life history, and morphological characteristics which distinguish them from many other species that have been examined for cytochrome P450 1A (CYP1A) induction. Members of the family Anguillidae often occur in regions of the coastal environment that are heavily impacted by chemical contamination. Although eels have been suggested to be a useful species for biomonitoring, the sensitivity with which eel CYP1A is induced by aryl hydrocarbon receptor (AHR) agonists is not known. We investigated the dose-dependent induction of hepatic CYP1A in the American eel (Anguilla rostrata). Eels from an uncontaminated site were injected intra-peritoneally with the model AHR agonists ß-naphthoflavone (BNF), benzo[a]pyrene (B[a]P) or 3,3',4,4-tetrachlorobiphenyl (TCB) at increasing doses (BNF at 0.1, 1, 10 and 100 mg/kg, B[a]P at 0.1, 1 and 10 mg/kg, and TCB at 0.1, 1, 10 and 20 mg/kg). All three compounds produced dose-dependent induction of CYP1A content and catalytic activity. An estimated ED(50) for induction of liver microsomal EROD activity by TCB was approximately 5 mg/kg, indicating only moderate sensitivity. At comparable doses of 1 and 10 mg/kg (or 3-4 and 30-40 µmol/kg), BNF and B[a]P had 2-3-fold greater effect than TCB in eliciting hepatic CYP1A induction. Injection of radiolabeled B[a]P and TCB resulted in similar dose-dependent concentrations of these compounds in eel liver, and the hepatic inducer concentrations and CYP1A levels were correlated positively. Eels collected from New Bedford Harbor (NBH), a Superfund site highly contaminated by polycyclic aromatic hydrocarbons and polychlorinated biphenyls, had levels of microsomal CYP1A protein and EROD activity that were equivalent to the highest levels induced experimentally. Eels from less contaminated sites had correspondingly less CYP1A expression. The responses to B[a]P or BNF as compared to TCB suggest a lower efficacy and/or potency for CYP1A induction by TCB which could involve differences in the mechanisms of responses to these compounds in eels. However, the moderate sensitivity and the CYP1A induction in NBH eels support suggestions that eels may be useful in monitoring more contaminated regions.

Oxidative inactivation of cytochrome P-450 1A (CYP1A) stimulated by 3,3',4,4'-tetrachlorobiphenyl: production of reactive oxygen by vertebrate CYP1As

Microsomal cytochrome P-450 1A (CYP1A) in a vertebrate model (the teleost fish scup) is inactivated by the aryl hydrocarbon receptor agonist 3,3',4,4'-tetrachlorobiphenyl (TCB). Here, the mechanism of CYP1A inactivation and its relationship to reactive oxygen species (ROS) formation were examined by using liver microsomes from scup and rat and expressed human CYP1As. In vitro inactivation of scup CYP1A activity 7-ethoxyresorufin O-deethylation by TCB was time dependent, NADPH dependent, oxygen dependent, and irreversible. TCB increased microsomal NADPH oxidation rates, and CYP1A inactivation was lessened by adding cytochrome c. CYP1A inactivation was accompanied by loss of spectral P-450, a variable loss of heme and a variable appearance of P-420. Rates of scup liver microsomal metabolism of TCB were < 0.5 pmol/min/mg, 25-fold less than the rate of P-450 loss. Non-heme iron chelators, antioxidant enzymes, and ROS scavengers had no influence on inactivation. Inactivation was accelerated by H(2)O(2) and azide but not by hydroxylamine or aminotriazole. TCB also inactivated rat liver microsomal CYP1A, apparently CYP1A1. Adding TCB to scup or rat liver microsomes containing induced levels of CYP1A, but not control microsomes, stimulated formation of ROS; formation rates correlated with native CYP1A1 content. TCB stimulated ROS formation by baculovirus-expressed human CYP1A1 but not CYP1A2. The results indicate that TCB uncouples the catalytic cycle of CYP1A, ostensibly CYP1A1, resulting in formation of ROS within the active site. These ROS may inactivate CYP1A or escape from the enzyme. ROS formed by CYP1A1 may contribute to the toxicity of planar halogenated aromatic hydrocarbons.

Arachidonic acid metabolism in the marine fish Stenotomus chrysops (Scup) and the effects of cytochrome P450 1A inducers

Cytochrome P450-mediated arachidonic acid (AA) metabolism was investigated in the marine fish scup, Stenotomus chrysops. Liver microsomes incubated with AA and NADPH produced epoxyeicosatrienoic acids (EETs) and their hydration products (dihydroxyeicosatrienoic acids, DHETs), midchain conjugated dienols (midchain HETEs), and C16-through C20-alcohols of AA (omega-terminal HETEs), all identified by HPLC and GC/MS. Gravid females had 4-fold lower AA metabolism rates than males but identical metabolite profiles. The 5,6-EET (inferred from stable metabolites) was most abundant (47% of total EETs) followed by 14,15-, 11,12-, and 8,9-EET (27, 13, and 13%, respectively). The 12-HETE represented 25% of total HETEs followed in abundance by 16-, 15-, 11-, 19-, 20-, 8-, and 9-HETE. Antibodies against scup CYP1A and a scup CYP2B-like protein inhibited liver microsomal AA metabolism by 30 and 46%, respectively. GC/MS analysis revealed EETs and DHETs as endogenous constituents in scup liver; the predominant EETs were 8,9- and 14,15-EET, followed by a lesser amount of 11,12-EET. Chiral analysis showed a preference for the S,R-enantiomers of endogenous 8,9-, 11,12-, and 14,15-EET (optical purities 80, 64, and 64%, respectively). Treatment of scup with the CYP1A inducer benzo(a)pyrene (BP) increased liver microsomal formation of EETs and HETEs by 2.7-fold in spring and 1.7-fold in summer. BP treatment did not affect microsomal EET regioselectivity, but shifted hydroxylation in favor of 19-HETE and induced 17-HETE formation. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) treatment in summer did not induce liver microsomal AA metabolism rates, yet BP and TCDD both increased endogenous EET content of liver (5- and 3-fold, respectively), with a shift to 14,15-EET. BP treatment increased the selectivity for the S,R-enantiomers of endogenous 8,9-, 11,12-, and 14,15-EET (optical purities 91, 84, and 83%, respectively). Kidney, gill, and heart microsomes all metabolized AA, at rates 10- to 30-fold less than liver microsomes. Similar amounts of endogenous 8,9- and 14,15-EET and less 11,12-EET were detected in heart and kidney, and there was a strong enantioselectivity for 8(R),9(S)-EET in heart (optical purity 78%) but not in kidney. BP treatment did not alter the total EET content in these organs but did shift the regiochemical profile in heart to favor 14,15-EET. Thus, scup liver and extrahepatic organs metabolize AA via multiple cytochrome P450 (CYP) forms to eicosanoids in vitro and in vivo. BP or TCDD induced endogenous AA metabolism in liver, altering EET regioselectivity and, with BP, stereoselectivity. While AhR agonists alter metabolism of AA in early diverging vertebrates expressing both CYP1A and AhR, the magnitude of effects may depend upon the type of inducer.