The in vivo turnover of rat liver microsomal epoxide hydrolase and both the apoprotein and heme moieties of specific cytochrome P-450 isozymes

Protein turnover measurement using selected reaction monitoring-mass spectrometry (SRM-MS)

Protein turnover represents an important mechanism in the functioning of cells, with deregulated synthesis and degradation of proteins implicated in many diseased states. Therefore, proteomics strategies to measure turnover rates with high confidence are of vital importance to understanding many biological processes. In this study, the more widely used approach of non-targeted precursor ion signal intensity (MS1) quantification is compared with selected reaction monitoring (SRM), a data acquisition strategy that records data for specific peptides, to determine if improved quantitative data would be obtained using a targeted quantification approach. Using mouse liver as a model system, turnover measurement of four tricarboxylic acid cycle proteins was performed using both MS1 and SRM quantification strategies. SRM outperformed MS1 in terms of sensitivity and selectivity of measurement, allowing more confident determination of protein turnover rates. SRM data are acquired using cheaper and more widely available tandem quadrupole mass spectrometers, making the approach accessible to a larger number of researchers than MS1 quantification, which is best performed on high mass resolution instruments. SRM acquisition is ideally suited to focused studies where the turnover of tens of proteins is measured, making it applicable in determining the dynamics of proteins complexes and complete metabolic pathways.This article is part of the themed issue 'Quantitative mass spectrometry'.

Cellular proteolytic systems in P450 degradation: evolutionary conservation from Saccharomyces cerevisiae to mammalian liver

Mammalian hepatic cytochromes P450 (P450s) are endoplasmic reticulum (ER)-anchored haemoproteins with the bulk of their catalytic domains exposed to the cytosol and engaged in the metabolism of numerous xeno- and endobiotics. The native P450s exhibit widely ranging half-lifes and predominantly turn over via either autophagic-lysosomal degradation (ALD) or ubiquitin-dependent 26S proteasomal degradation (UPD). The basis for this heterogeneity and differential proteolytic targeting is unknown. On the other hand, structurally/functionally inactivated P450s are predominantly degraded via UPD in a process known as ER-associated degradation (ERAD). ALD/UPD/ERAD pathways are evolutionarily highly conserved. The availability of Saccharomyces cerevisiae mutants with specific genetic defects/deletions in various ALD/UPD/ERAD-associated proteins and corresponding isogenic wild-type strains has enabled the molecular dissection of the degradation pathways for heterologously expressed mammalian P450s, leading to the identification of specific protein participants. These findings collectively attest to a highly versatile cellular system for the physiological disposal of native, senescent and/or inactivated, structurally damaged mammalian liver P450s.

CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Cytochrome P450 (CYP) induction-mediated interaction is one of the major concerns in clinical practice and for the pharmaceutical industry. There are two major issues associated with CYP induction: a reduction in therapeutic efficacy of comedications and an induction in reactive metabolite-induced toxicity. Because CYP induction is a metabolic liability in drug therapy, it is highly desirable to develop new drug candidates that are not potent CYP inducer to avoid the potential of CYP induction-mediated drug interactions. For this reason, today, many drug companies routinely include the assessment of CYP induction at the stage of drug discovery as part of the selection processes of new drug candidates for further clinical development. The purpose of this article is to review the molecular mechanisms of CYP induction and the clinical implications, including pharmacokinetic and pharmacodynamic consequences. In addition, factors that affect the degree of CYP induction and extrapolation of in vitro CYP induction data to in vivo situations will also be discussed. Finally, assessment of the potential of CYP induction at the drug discovery and development stage will be discussed.

A mechanism-based integrated pharmacokinetic enzyme model describing the time course and magnitude of phenobarbital-mediated enzyme induction in the rat

PURPOSE

To characterize the magnitude, time course, and specificity of phenobarbital (PB)-mediated enzyme induction, and further, to develop an integrated pharmacokinetic (PK)-enzyme model describing the changes in the activities of CYP enzymes as well as in the PK of PB.

METHODS

PB plasma concentrations and in vitro activities of several CYP enzymes were measured in rats treated with PB between 0 and 14 days. A PB PK-enzyme induction model was developed using the program NONMEM: .

RESULTS

PB treatment both induces and reduces the activity of CYP enzymes by stimulating the enzymes' formation or elimination rates. Certain CYP enzymes affected the PB PK through autoinduction. The half-life of the induction process was estimated to be 2 days for CYP1A2, CYP3A1/2, and CYP2B1/2, and 3 days for androstenedione producing enzymes. The CYP2C11 activity was rapidly reduced by PB treatment. A lag time for the PB autoinduction was observed. This lag time is explained by the rate difference between induction and reduction in CYP activities.

CONCLUSION

To our knowledge, this is the first example of an induction model that simultaneously describes plasma PK and in vitro data. It does so by integrating the bidirectional interaction between drug and enzymes in a mechanistic manner.

Hepatic cytochrome P450 degradation: mechanistic diversity of the cellular sanitation brigade

Hepatic cytochromes P450 (P450s) are monotopic endoplasmic reticulum (ER)-anchored hemoproteins that exhibit heterogenous physiological protein turnover. The molecular/cellular basis for such heterogeneity is not well understood. Although both autophagic-lysosomal and nonlysosomal pathways are available for their cellular degradation, native P450s such as CYP2B1 are preferentially degraded by the former route, whereas others such as CYPs 3A are degraded largely by the proteasomal pathway, and yet others such as CYP2E1 may be degraded by both. The molecular/structural determinants that dictate this differential proteolytic targeting of the native P450 proteins remain to be unraveled. In contrast, the bulk of the evidence indicates that inactivated and/or otherwise posttranslationally modified P450 proteins undergo adenosine triphosphate-dependent proteolytic degradation in the cytosol. Whether this process specifically involves the ubiquitin (Ub)-/26S proteasome-dependent, the Ub-independent 20S proteasome-dependent, or even a recently characterized Ub- and proteasome-independent pathway may depend on the particular P450 species targeted for degradation. Nevertheless, the collective evidence on P450 degradation attests to a remarkably versatile cellular sanitation brigade available for their disposal. Given that the P450s are integral ER proteins, this mechanistic diversity in their cellular disposal should further expand the repertoire of proteolytic processes available for ER proteins, thereby extending the currently held general notion of ER-associated degradation.

Expression and regulation of cytochrome P450 enzymes in primary cultures of human hepatocytes

The aim of this study was to test suitable culture conditions for maintaining normal cellular cytoarchitecture and inducibility of P450 enzymes in primary cultures of human hepatocytes by prototypical inducers. The selectivity and sensitivity of a sandwich culture system were determined by treating cultures with a number of clinically relevant drugs that are known to be inducers of either rodent and/or human P450 enzymes. The results showed that considerable induction of CYP3A4 activity is observed at DMSO concentrations greater than 0.1% (v/v). No differences in P450 induction response were observed between cultures maintained under different matrix conditions. However, the matrix condition considered to be optimal for maintaining cellular integrity, protein yields, and P450 enzyme induction was a sandwich configuration in combination with modified Chee's medium containing insulin (6.25 microg/mL) and dexamethasone (< or =0.1 microM). Under these conditions, induction of CYP3A4 occurred at clinically relevant drug concentrations, and maximal activities were achieved after 3 days of exposure. Overall, the response of human hepatocyte cultures to treatment with both positive and negative modulators was found to reflect that observed in vivo with respect to both enzyme specificity and potency of enzyme induction, although considerable sample-to-sample variability was observed in the magnitude of induction.

Determination of parameters responsible for pharmacokinetic behavior of TCDD in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic member of a class of planar and halogenated chemicals. Improvements in exposure assessment of TCDD require scientific information on the distribution of TCDD in target tissues and cellular responses induced by TCDD. Since 1980, several physiologically based pharmacokinetic (PBPK) models for TCDD and related compounds have been reported. Some of these models incorporated the induction of a hepatic binding protein in response to interactions of TCDD, the Ah receptor, and DNA binding sites and described the TCDD disposition in a biological system for certain data sets. Due to the limitations of the available experimental data, different values for the same physical parameters of these models were obtained from the different studies. The inconsistencies of the parameter values limit the application of PBPK models to risk assessment. Therefore, further refinement of previous models is necessary. This paper develops an improved PBPK model to describe TCDD disposition in eight target tissues. The interaction of TCDD with the Ah receptor and with hepatic inducible CYP1A2 were also incorporated into the model. This model accurately described the time course distribution of TCDD following a single oral dose of 10 microg/kg, as well as the TCDD concentration on Day 3 after six different doses, 0.01, 0.1, 0.3, 1, 10, and 30 microg TCDD/kg, in target tissues. This study extends previous TCDD models by illustrating the validity and the limitation of the model and providing further confirmation of the potential PBPK model for us in optimal experimental design and extrapolation across doses and routes of exposure. In addition, this study demonstrated some critical issues in PBPK modeling.

Induction of CYP1A mRNA and catalytic activity in gizzard shad (Dorosoma cepedianum) after waterborne exposure to benzo[a]pyrene

The purpose of this study was to examine the concentration and time-dependent response of hepatic CYP1A mRNA levels and ethoxyresorufin O-deethylase (EROD) activity in gizzard shad (Dorosoma cepedianum) after waterborne exposure to the polycyclic aromatic hydrocarbon benzo[a]pyrene (BaP). Gizzard shad hepatic CYP1A mRNA levels and EROD activity were increased within 6 hr by waterborne concentrations of 1.01 microgram BaP/l and were highest after 72 hr of exposure. The median effective time to reach 50% of maximum induction (ET50) values for CYP1A mRNA and EROD activity was 12 +/- 9 and 34 +/- 3 hr, respectively. The rise of CYP1A mRNA levels ahead of the rise of EROD activity showed that the CYP1A gene has the general pattern of a gene that is under transcriptional control. Although CYP1A mRNA levels were maximally induced after 72 hr of exposure, they returned to basal levels between 72 and 120 hr of exposure. Despite a decrease in CYP1A mRNA levels between 72 and 120 hr of exposure, EROD activity did not significantly change between 72 and 120 hr of exposure. These results indicate that hepatic CYP1A mRNA levels in gizzard shad can only feasibly identify short-term exposures to BaP concentrations of approximately 1 microgram/l and that are not longer than 72 hr. Consequently, hepatic CYP1A catalytic activity (e.g., EROD) and CYP1A protein levels should be measured in addition to hepatic CYP1A mRNA levels when biomonitoring aquatic systems for certain types of chemical contamination.

Regional hepatic CYP1A1 and CYP1A2 induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin evaluated with a multicompartment geometric model of hepatic zonation

A physiologically based pharmacokinetic (PBPK) model for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was combined with a five-compartment geometric model of hepatic zonation to predict both total and regional induction of CYP450 proteins within the liver. Three literature studies on TCDD pharmacokinetics and protein induction in female rats were analyzed. In simulating low-dose behavior for mRNA in whole liver and, particularly, in representing immunohistochemical observations, the five-compartment model was more successful than conventional homogeneous one-compartment liver models. The five-compartment liver model was used with the affinity of TCDD for the Ah receptor (AhR) held constant across all the liver (Kb = 0.2 nM). The presumed affinities of the AhR-TCDD complex for TCDD responsive elements in the CYP1A1 (Kd1) and CYP1A2 (Kd2) genes varied between adjacent compartments by a factor of 3. This parameterization leads to predicted 81-fold differences in affinities between the centrilobular and the periportal regions. The affinities used for AhR-TCDD complex binding to TCDD response elements for CYP1A2 in compartment 3 (the midzonal area) ranged from 0.08 to 1.0 nM in the three studies modeled. For CYP1A1 the corresponding dissociation constant in compartment 3 varied from 0.6 to 2.0 nM. In each compartment, the Hill coefficient for induction had to be 4 or greater to match the immunohistochemical results. This multi-compartment liver model is consistent with data on protein and mRNA induction throughout the liver and on the regional distribution of these proteins. No previous model has incorporated regional variations in induction. The PBPK analysis based on the multicompartment liver model suggests that the low-dose behavior for hepatic CYP1A1/CYP1A2 induction by TCDD is highly non-linear.

Differential time-course and dose-response relationships of TCDD-induced CYP1B1, CYP1A1, and CYP1A2 proteins in rats

This study examined the relationship between dose- and time-dependent hepatic localization of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and expression of CYP1B1, CYP1A1 and CYP1A2 proteins. A dose-dependent increase in hepatic TCDD in female Sprague-Dawley rats treated with 0.01-30.0 microg TCDD/kg was observed. TCDD induced CYP1A1 protein in rats treated with 0.3 microg TCDD/kg or higher. TCDD induced CYP1A2 and CYP1B1 proteins in rats treated with 1.0 microg TCDD/kg or higher. The in vivo ED50 (microg TCDD/kg) for TCDD-induced CYP1A1, CYP1A2 and CYP1B1 proteins were 0.22, 0.40 and 5.19, respectively. Hepatic accumulation of TCDD reached a maximum at 8 hours post dosing with a t1/2 of approximately 10 days. TCDD-induced CYP1A1/CYP1A2 protein expression was increased time-dependently, reaching a maximum at 3 days after dosing and remaining elevated for 35 days. In contrast, TCDD-induced CYP1B1 protein showed significant expression at 3 days after dosing and decreased to basal concentrations by 35 days. This study demonstrates that TCDD exhibits differential dose-response and time-course relationships on hepatic localization and cytochrome P-450 protein expression.

Halogenated aromatic hydrocarbon-mediated porphyrin accumulation and induction of cytochrome P4501A in chicken embryo hepatocytes

Concentration-dependent induction of cytochrome P4501A (CYP1A) and intracellular porphyrin accumulation were observed following treatment of chicken embryo hepatocyte (CEH) cultures with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 3,3',4,4'-tetrachlorobiphenyl (PCB 77, IUPAC nomenclature), 2,3',4,4',5-pentachlorobiphenyl (PCB 118), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169), and a commercial mixture of PCBs (Aroclor 1254). For these halogenated aromatic hydrocarbons (HAHs), or mixture, maximal CYP1A activity [measured as ethoxyresorufin-O-deethylase (EROD) activity] and immunodetectable protein were observed at concentrations just prior to, or coincident with, the concentrations at which porphyrin accumulation became evident. Both immunodetectable CYP1A protein and catalytic activity decreased at high concentrations of these compounds, but the rate and extent of decrease of immunodetectable CYP1A protein varied. Time-course studies with PCB 77 indicated a decrease in potency and an increase in maximal CYP1A induction between 24 and 48 hr of exposure which may indicate in vitro metabolism of this HAH. Intracellular accumulation of total porphyrins without CYP1A induction, was observed for 2,2',5,5'-tetrachlorobiphenyl (PCB 52), 2,2',6,6'-tetrachlorobiphenyl (PCB 54), 2,2',3,5',6-pentachlorobiphenyl (PCB 95), 2,2',4,5,5'-pentachlorobiphenyl (PCB 101), 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136), and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153). Overall, these results are consistent with a role for CYP1A induction and/or Ah receptor activation in porphyrin accumulation mediated by HAHs with a planar configuration, whereas those that are not planar may mediate porphyrin accumulation by a mechanism not involving induction of CYP1A.

Evidence against a role for serine 129 in determining murine cytochrome P450 Cyp2e-1 protein levels

The cytochrome P450 CYP2E subfamily plays a central role in drug and carcinogen metabolism. The cellular content of this protein is regulated at both the transcriptional and posttranslational levels. CYP2E1 is degraded by both rapid and slow acting proteolytic systems. In the presence of a substrate, CYP2E1 becomes stabilized, and the contribution of the rapid actinig proteolytic pathway to its destruction decreases. It has been suggested that phosphorylation at serine 129 acts as a switch to initiate the fast acting degradative pathway. Phosphorylation at serine 129 has also been suggested to be the point at which hormones, such as insulin, exert actions on the stability of this protein. In order to investigate the role of phosphorylation in determining murine Cyp2e-1 levels, serine 129 was changed by site-directed mutagenesis to amino acids that could not be phosphorylated and the recombinant proteins expressed in COS 7 cells. Replacement of serine 129 with alanine and glycine does not lead to Cyp2e-1 accumulation. In the presence of insulin, although Cyp2e-1 levels increase slightly, specific stabilization of the wild-type protein relative to the two mutant forms is not observed. These observations provide evidence that insulin can act by stabilization of Cyp2e-1 protein but suggest that the phosphorylation of serine 129 is not the molecular basis of stabilization observed.

The effects of total parenteral nutrition on the hepatic handling of bilirubin in the rat

Disturbances of bilirubin metabolism such as jaundice or pigment gallstone formation occur during total parenteral nutrition (TPN). We have studied the effects of TPN on bile flow and bile acid secretion and on the hepatobiliary transport of bilirubin in rats. Animals on parenteral nutrition for 5 days received 4.8 g of amino acids and 6.9 g of glucose daily. Controls were orally fed animals. Bile flow and bile acid secretion were not significantly modified by TPN. Serum bile acid and alkaline phosphatase levels were significantly increased in TPN animals when compared with the controls (+98% and +38%, respectively), which points to a relative cholestasis in the TPN rats. The biliary excretion of bilirubin monoconjugates and bilirubin diconjugates was significantly increased (+72% and +78%, respectively). This provides evidence for enhanced production of the pigment. Serum concentration of total bilirubin was enhanced in the TPN rats (+240%). The esterified/total bilirubin ratio in serum increased, whereas the bilirubin diconjugates/bilirubin monoconjugates ratio decreased. These facts, together with the minor reduction of hepatic bilirubin UDP glucuronosyltransferase activity (-12%), suggest that hyperbilirubinemia would be a consequence of both cholestasis and increased bilirubin production. The alterations reported here could contribute to the explanation of hyperbilirubinemia and pigment gallstone formation in patients maintained on parenteral nutrition.

Degradation of cytochrome P450 2E1: selective loss after labilization of the enzyme

Mechanism-based inactivation of cytochrome P450 can result in the chemical modification of the heme, the protein, or both as a result of covalent binding of modified heme to the protein. In the present study we took advantage of different modes of inactivation of P450 2E1 by CCl4, 1-aminobenzotriazole, or 3-amino-1,2,4-triazole to investigate parameters which target P450 2E1 for proteolysis from the microsomal membrane. Treatment of mice with CCl4 at the point of maximal induction of P450 2E1 after a single oral dose of acetone resulted in the complete loss of P450 2E1-dependent p-nitrophenol hydroxylation and a 75% loss of immunochemically detectable protein within 1 h of administration. Treatment with 1-aminobenzotriazole at the point of maximal induction caused a complete loss of P450 2E1-dependent p-nitrophenol hydroxylation but only a 12% loss of immunochemically detectable protein 1 h after administration. Treatment of mice with 3-amino-1,2,4-triazole caused a rapid loss of both catalytic activity and microsomal p-nitrophenol hydroxylase activity. However, unlike CCl4 treatment, the activity and enzyme level rebounded 5 and 9 h after treatment. The P450 2E1 ligand, 4-methylpyrazole, administered at the point of maximal induction maintained the acetone-induced catalytic and immunochemical level of P450 2E1. These results suggest that differentially modified forms of P450 2E1 show a characteristic susceptibility to degradation. While there are many potential pathways for protein degradation, the loss of P450 2E1 was associated with increased formation of high molecular weight microsomal ubiquitin conjugates. The formation of ubiquitin-conjugated microsomal protein which correlates with P450 2E1 loss suggests that ubiquitination may represent a proteolytic signal for the rapid and selective proteolysis of certain labilized conformations of P450 2E1 from the endoplasmic reticulum.

The effect of IFN-alpha-Con1 on hepatic cytochrome P-450 and protein synthesis and degradation in hepatic microsomes

Interferon and its inducers are well known to depress drug biotransformation in the liver by decreasing the levels of cytochrome P-450 in that organ. We now report that IFN-alpha-Con1, which was constructed from the most frequently observed amino acid sequences in human alpha-interferon subtypes, causes a loss in cytochrome P-450 which could be prevented by pretreating animals with either puromycin or actinomycin D. This suggests that the loss in drug biotransformation is mediated via the production of an intermediate protein. When the turnover of microsomal protein was examined this interferon appeared to depress the synthesis of proteins with molecular weights 46-60 kd and had little effect on the synthesis of other proteins. The in vitro translation of proteins of molecular weights 45-60 kd was also depressed in an in vitro translation system using mRNA isolated from the livers of interferon treated hamsters. Interferon had no effect on the degradation of microsomal proteins of all molecular weights. It is concluded that interferon probably depresses the levels of cytochrome P-450 in the liver by decreasing the synthesis of the apoprotein and that interferon has little effect on the degradation of the hemoprotein.

Mephenytoin stereoselective elimination in the rat: III. Stereoselective time course of induction during chronic hepatic portal vein administration

The blood concentrations of R- and S-mephenytoin were followed in seven rats over a 5-8 day period during a hepatic portal vein infusion of racemic mephenytoin. In all but two rats, both of the enantiomers achieved an initial steady-state level before a measurable change was observed in their blood concentrations. In each case, the decrease in the initial S-mephenytoin steady state blood level occurred after the change in R-mephenytoin was apparent. During the period of study, the mean +/- SD portal vein clearance of S-mephenytoin increased from 96 +/- 33 ml/h to 450 +/- 160 ml/h. The mean +/- SD portal vein clearance of R-mephenytoin increased from 170 +/- 50 ml/h to 2400 +/- 1600 ml/h. The larger increase in the portal vein clearance for the R-enantiomer resulted in the R/S-mephenytoin clearance ratio over this same time period changing from 1.8 +/- 0.2 to 5.2 +/- 1.8. Attempts to describe the time course of change in the clearance of S- or R-mephenytoin using a previously reported model of induction were unsuccessful. The induction time course did suggest, however, that the rate of induction may be similar for each enantiomer.

BCNU-induced quantitative and qualitative changes in hepatic cytochrome P-450 can be correlated with cholestasis

Male Sprague-Dawley rats were given single i.p. injections of 1,3-bis(2-chloroethyl)-1-Nitrosourea (BCNU) to investigate changes in hepatic microsomal cytochrome P-450 content and metabolic activity. On day 14 after treatment (20 mg/kg), cytochrome P-450 content had decreased by approximately 25% and ethylmorphine N-demethylase activity (nmol product/nmol P-450/min) had decreased by 36%. In contrast, ethylmorphine O-deethylase and 7-ethoxycoumarin O-deethylase activities were not significantly decreased by BCNU treatment. Hepatic delta-aminolevulinic acid synthetase activity was only 60% of control values, and microsomal heme oxygenase activity was slightly but not statistically elevated. Cytochrome P-450 content in control and BCNU-treated rats increased in a similar manner after phenobarbital or beta-naphthoflavone induction. Electrophoretic analysis of cytochrome P-450 proteins isolated from hepatic endoplasmic reticular membranes of treated and control rats suggested that alterations in these proteins occurred in BCNU-treated rats. These changes in cytochrome P-450 content and activity are very similar to those reported in isolated systems exposed to bile acids or in rats with experimentally produced cholestasis. BCNU has been shown to produce cholestasis, which precedes its effects on microsomal mixed-function oxygenase activity. Thus, the delayed effects of BCNU on microsomal drug metabolism are probably secondary to its interference with bile formation.

A model of cytochrome P-450-centered hepatic dysfunction in drug metabolism induced by cobalt-protoporphyrin administration

Cobalt-protoporphyrin treatment disrupts cytochrome P-450-centered drug metabolism and is known to decrease significantly the cytochrome P-450 content of the liver. This study assesses further the correlations between biochemical and functional changes induced by Co-protoporphyrin. Specifically, it confirmed the fall in cytochrome P-450 levels in liver and demonstrated that both NADPH-cytochrome P-450 reductase and NADH-cytochrome b5 reductase activities decreased in a dose-dependent manner, albeit to a lesser degree, upon Co-protoporphyrin administration. Furthermore, plasma clearance of the marker drug aminopyrine fell off abruptly with a minimal decrease in cytochrome P-450 content, and then monotonically with its further depletion. Both aminopyrine and caffeine demethylation, as measured by the amount of radiolabeled CO2 exhaled, also decreased with diminishing cytochrome P-450 content. With aminopyrine the decrease was abrupt but with caffeine biphasic, consistent with preferential isozyme depletion. The drop in oxidative drug metabolism measured by these two in vivo techniques occurred in the absence of organellar damage to hepatocytes, as observed by electron microscopy. In vitro studies of aminopyrine metabolism in microsomes prepared from rats with and without Co-protoporphyrin injection proved to be consistent with the in vivo studies. Moreover aminopyrine Vmax decreased and Km increased with decreasing cytochrome P-450 content, suggesting preferential isozyme depletion. Furthermore, the changes in aminopyrine intrinsic clearance predicted by the in vitro Vmax and Km values agreed with those measured by in vivo plasma clearance. Taken together, these data suggest that Co-protoporphyrin treatment can be used to produce a model of altered cytochrome P-450-centered drug metabolism, as measured consistently by several techniques. However, this model appears to be more complex than one involving nonspecific depletion of cytochrome P-450 alone, and may be influenced also by concomitant changes in the electron transport chain or other aspects of hepatic metabolism.

Quantitation of mRNAs specific for the mixed-function oxidase system in rat liver and extrahepatic tissues during development

Evaluation of ontogenetic expression of the cytochrome P450PCN and cytochrome P450b gene families as well as the NADPH-cytochrome P450 oxidoreductase and epoxide hydrolase genes in Holtzmann rats showed that basal levels of mRNAs encoding these enzymes could be detected in most tissues. Distinct developmental patterns of mRNA expression are evident for these four proteins in liver and extrahepatic tissues. Levels of cytochrome P450b-like mRNA were comparable in adult lung and liver, while cytochrome P450PCN-homologous mRNA exhibited low levels in lung and approximately 100-fold higher levels in liver. Cytochrome P450PCN-homologous mRNA also reached substantial levels in adult intestine, and was also present in placenta, where it increased approximately 4-fold 24 h before birth. Epoxide hydrolase mRNA was demonstrated to be highest in liver followed by kidney, lung, and intestine but was extremely low in brain. NADPH-cytochrome P450 oxidoreductase mRNA in kidney, lung, prostate, adrenal, and intestine exhibited levels comparable to that found in liver; however, the pattern of expression for oxidoreductase mRNA was unique in that levels declined at maturity in liver, kidney, and intestine but not in lung and brain. Development of mixed-function oxidase and epoxide hydrolase activities in liver was distinct from that in other tissues in that mRNAs for all four proteins rose dramatically after parturition. Testis from immature males demonstrated low levels of all the mRNAs assayed, which ranged from 20% (oxidoreductase) to less than 1% (cytochrome P450PCN and epoxide hydrolase) of the levels found in liver.

Haem synthesis during cytochrome P-450 induction in higher plants. 5-Aminolaevulinic acid synthesis through a five-carbon pathway in Helianthus tuberosus tuber tissues aged in the dark

Chlorophyll and haem synthesis in illuminated Jerusalem artichoke tuber tissues were very efficiently inhibited by gabaculine (3-amino-2,3-dihydrobenzoic acid). This inhibition seems to be due specifically to a blockade of the pathway for 5-aminolaevulinate biosynthesis which used glutamate as a substrate (the so-called C5 pathway) since we could not detect any inhibition of protein synthesis in the treated tissues and there was no effect of gabaculine on the glycine-dependent yeast 5-aminolaevulinate synthase used as a model. In dark-aged artichoke tissues, gabaculine also effectively blocked cytochrome P-450 induction, peroxidase activity and 5-aminolaevulinic acid synthesis, thus suggesting the involvement of a C5 pathway in cytoplasmic and microsomal haemoprotein synthesis in this higher plant. Allylglycine and (2-amino-ethyloxyvinyl)glycine, two olefinic glycine analogues which are potential suicide inhibitors of pyridoxal phosphate enzymes, were also demonstrated to be effective blockers of chlorophyll synthesis in artichoke tuber and Euglena cells exposed to light.

Effect of unbalanced diets on incorporation of delta-aminolevulinic acid into cytochrome P-450

The in vivo syntheses of two liver microsomal cytochromes P-450 PB3a, P-450 UT50 [(1987) Eur. J. Biochem., submitted] (Mr 50,000, 52,000) have been estimated by measuring the specific activity 2 h after i.p. administration of delta-[3H]aminolevulinic acid to male Sprague Dawley rats. The animals were fed either a standard rat chow (5% lard, 22% casein) or unbalanced diets (high lipid, 30% lard or low protein, 6% casein) with or without 50 ppm Phenoclor DP6. The high-lipid diet supported a more rapid body weight gain but had little impact on cytochrome P-450 content, expressed either per whole liver or per mg microsomal protein, and on the incorporation of the precursor into cytochrome P-450. The latter was determined by measuring the radioactivity incorporated into the cytochrome P-450 fraction, partially purified by affinity chromatography, as well as into two cytochrome P-450 isozymes (Mr 50,000 or 52,000) purified by DEAE-52 cellulose ion-exchange chromatography. The low-protein diet, on the other hand, severely depressed body weight gain and cytochrome P-450 content as well as incorporation of radioactivity, the lower-Mr cytochrome (Mr 50,000) being particularly affected. However, when a potent inducer, Phenoclor DP6, was added to the low-protein diet, cytochrome synthesis was restored indicating that the effect was reversible.

Cytochrome P-450 cholesterol 7 alpha-hydroxylase: inhibition of enzyme deactivation by structurally diverse calmodulin antagonists and phosphatase inhibitors

Cytochrome P-450 cholesterol 7 alpha-hydroxylase (P-450Ch7 alpha) catalyzes the first and rate-limiting step in the conversion of cholesterol to bile acids. Incubation of rat liver microsomes in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer resulted in a time-dependent deactivation of P-450Ch7 alpha which was markedly accelerated by the nonionic detergent Tween 80. Microsomal NADPH-cytochrome P-450 reductase and cytochrome P-450-dependent 7-ethoxycoumarin O-deethylase activities were unaffected under these conditions, evidencing the selectivity of the deactivation process for P-450Ch7 alpha. The rate (t 1/2 = 15-19 min at 37 degrees C) and maximal extent of P-450Ch7 alpha deactivation (greater than or equal to 90%) were both unaffected by the presence of cytosolic proteins and were also not dependent on the initial enzyme level, as shown using liver microsomes isolated from untreated, cholestyramine-fed, and xenobiotic-induced rats exhibiting an eight-fold range in P-450Ch7 alpha activity. Scavengers for reduced oxygen species were also without effect. P-450Ch7 alpha was stabilized some six- to sevenfold (t 1/2 = 94-143 min) by the phosphatase inhibitor NaF. Of a series of other phosphatase inhibitors examined, including, among others, EDTA, vanadate, and molybdate, only phosphate-containing compounds and the calmodulin antagonist trifluoperazine, and inhibitor of the Ca2+-calmodulin-dependent phosphatase calcineurin, effectively stabilized P-450Ch7 alpha. Modulation of P-450Ch7 alpha deactivation by these inhibitors generally paralleled their effects on isolated calcineurin. A variety of structurally diverse calmodulin antagonists examined were also found to effectively protect P-450Ch7 alpha from deactivation; these include calmidazolium and tamoxifen (IC50 = 25 to 50 microM), chlorpromazine, thioridazine, amitriptyline, imipramine, and the naphthalene sulfonamide compound W-7 (IC50 = 50 to 300 microM). Structure-activity analysis of several phenothiazines and their derivatives indicated that although little activity was exhibited by the sulfoxides, some protection was provided by the corresponding sulfones. On the basis of these observations, various models for the molecular basis of enzyme deactivation are considered, including the hypothesis that a calcineurin-like microsomal phosphatase mediates deactivation of this cytochrome P-450 enzyme.

Cytochrome P-450 and NADPH-cytochrome P-450 reductase are degraded in the autolysosomes in rat liver

We have investigated the degradation in rat liver of two typical endoplasmic reticulum (ER) membrane proteins, phenobarbital (PB)-inducible cytochrome P-450 (P-450[PB]) and NADPH-cytochrome P-450 reductase (FP2). Autolysosomes, almost completely free from contamination by the other organelles such as ER, were prepared from leupeptin-treated rat livers according to the method of Furuno et al. (Furuno, K., T. Ishikawa, and K. Kato, 1982, J. Biochem., 91:1943-1950). Quantitative immunoblot analysis showed that these two proteins were found in large amounts in the autolysosomes regardless of PB treatment. The specific content of P-450 (PB) in the autolysosomes changed along with that in the microsomes during and after PB treatment, whereas hardly any P-450(PB) was detected in the cytosol fraction throughout the experiment. We also found a marked increase in the autolysosomal proteins 3 d after cessation of PB treatment when microsomal proteins are degraded most rapidly. Ferritin immunoelectron microscopy revealed directly that when the limiting membranes of the premature autolysosomes were partially broken the smooth vesicles segregated within the autolysosomes were heavily stained with ferritin anti-P-450(PB) conjugates. Thus, for the first time, we could present convincing evidence that P-450(PB) and FP2 are segregated to be degraded in the autolysosomes.

Dissociation of cytochrome P-450 inactivation and induction

Polycyclic aromatic hydrocarbons bind to a cytosolic receptor that translocates into the nucleus and induces the synthesis of mRNA for a subset of cytochrome P-450 isozymes. The failure to detect a "phenobarbital" receptor, however, suggests that the induction of P-450b/e, the isozymes induced by phenobarbital, is mediated by a less direct mechanism. One attractive alternative is that a receptor exists for an endogenous substance that is specifically turned over by the trace amounts of P-450b/e present in uninduced liver. Changes in the concentration of this substance caused by agents that inhibit P-450b/e would then trigger the induction response. We report here that suppressing the catalytic activities of P-450b/e for prolonged periods with 1-aminobenzotriazole, a mechanism-based irreversible inhibitor, neither induces P-450b/e nor interferes with induction of these isozymes at the level of transcription by phenobarbital. The results argue against mechanisms for the induction of P-450b/e keyed to the effective catalytic availability of these isozymes.

Depression of cytochrome P-450 and alterations of protein metabolism in mice treated with the interferon inducer polyriboinosinic acid X polyribocytidylic acid

Treatment of mice with the interferon inducer polyriboinosinic acid X polyribocytidylic acid [poly(IC)] results in the depression of several hepatic proteins. In this study we examined synthesis and degradation of the proteins of liver cell organelles in mice treated with poly(IC). Effects on synthesis were determined by using [14C]- and L-[3H]leucine incorporation into control and poly(IC)-treated mice, respectively. At selected times after poly(IC) treatment the 3H/14C ratio was established for preparations of nuclei, mitochondria, lysosomes, smooth endoplasmic reticulum, rough endoplasmic reticulum, and 105,000g supernatant (cytosol). Time-dependent alterations in de novo protein synthesis were greatest in lysosomal and rough endoplasmic reticular fractions; both were depressed 9 h after treatment. The effects of poly(IC) on protein degradation were determined with [14C]bicarbonate. Poly(IC) treatment decreased the time required for disappearance of 50% of 14C-labeled protein (t1/2) of smooth and rough endoplasmic reticula. Examination of endoplasmic reticulum marker enzymes showed depression of cytochromes P-450 and b5 from 9 h onward after poly(IC) administration. Tyrosine aminotransferase activity was elevated 6 h after treatment with poly(IC), and then depressed after 9 h. The other organelle marker enzymes were not affected significantly. We conclude that poly(IC) decreases the content of proteins of the hepatic endoplasmic reticulum, including certain cytochrome P-450 isozymes, by decreasing rates of protein synthesis and increasing rates of protein degradation.

Inactivation of cytochrome P-450 by 2-isopropyl-4-pentenamide and other xenobiotics leads to heme-derived protein adducts

When cytochrome P-450 in phenobarbital-induced rat liver microsomes was destroyed by 2-isopropyl-4-pentenamide (AIA) in vitro, 50% of the degraded heme was recovered as heme-derived products irreversibly bound to microsomal proteins. In contrast, less than 50% of the degraded heme was accounted for as N-alkylated porphyrins. Furthermore, 64% of the irreversibly bound products was bound specifically to a 54-kD form of cytochrome P-450. Several other compounds which have been reported to destroy cytochrome P-450 by forming N-alkylated porphyrins also produced heme-derived protein adducts. These findings indicate that the formation of heme-derived protein adducts may represent an important pathway for the irreversible degradation of cytochrome P-450 by many xenobiotics.

Characterization of antipyrine autoinduction in the rat utilizing a new microsampling technique for serial blood sample collections

A new microsampling technique is described and applied to the collection of a large number of blood samples from individual rats receiving a chronic intravenous antipyrine infusion. From these samples, a detailed description of the time-course of antipyrine autoinduction of clearance in the rat was provided. Antipyrine clearance was increased 4-5-fold over 8 d of antipyrine administration, with the increase in clearance following a monoexponential process in all animals. The assumption that the time-course of induction should reflect the degradation rate of induced enzymes was examined by comparing the half-life of antipyrine autoinduction with in vitro estimates for the half-life of induced cytochrome P-450 enzymes. The results of this comparison are discussed with respect to utilizing this kinetic model to quantitate the turnover of drug metabolism enzymes in vivo.

Carbon tetrachloride and 2-isopropyl-4-pentenamide-induced inactivation of cytochrome P-450 leads to heme-derived protein adducts

When CCl4 was incubated with rat liver microsomes from phenobarbital-treated rats in an aerobic or anaerobic atmosphere, over 69% of the heme moiety of cytochrome P-450 was destroyed. At least 45% of the degraded heme under both reaction conditions was accounted for as heme-derived products irreversibly bound to microsomal proteins. Furthermore, 33% of the irreversibly bound products were bound specifically to a 54-kDa form of cytochrome P-450. A structurally different compound, 2-isopropyl-4-pentenamide, also destroyed the heme moiety of cytochrome P-450 and produced heme-derived adducts of microsomal proteins that accounted for 28% of the destroyed heme. These results represent a novel mechanism for the destruction of cytochromes P-450 by xenobiotics.

Effects of inhibitors of RNA and protein synthesis on hepatic microsomal cytochrome P-450 concentration in fasted and fed rats

A number of sex-related differences in the metabolism of drugs, steroids and xenobiotics have been reported in studies on rats. Generally, male rats tend to metabolize these compounds more efficiently than females. In the studies presented here, male and female rats were fasted for 24-48 hr, and the effects of fasting on total hepatic microsomal cytochrome P-450 were examined. Hepatic cytochrome P-450, as determined by CO difference spectra, was increased significantly as a percentage of control in microsomes from fasted female rats when compared to fasted male rats. Cytochrome P-450 concentration increased from 0.57 +/- 0.07 nmole/mg protein to 0.99 +/- 0.08 nmole/mg protein following a 24-hr fast. In male rats, cytochrome P-450 levels were essentially unaffected by the 24-hr fast. Cytochrome b5 concentration was not altered by fasting. When female rats were fasted for 24 hr and refed, cytochrome P-450 levels were not significantly different from cytochrome P-450 levels in continuously fed animals. Treatment of fasted female rats with the protein synthesis inhibitor ethionine, or the RNA synthesis inhibitor actinomycin D, prevented the induction of cytochrome P-450 in the fasting animal. Cytochrome P-450 concentration in fed animals was not affected significantly by either inhibitor. Induction of cytochrome P-450 by phenobarbital (PB) and 3-methylcholanthrene (MC) under fed and fasted conditions was also investigated in male and female rats. Xenobiotic-induced cytochrome P-450 concentration was significantly higher in fasted female hepatic microsomes when compared to microsomes from fed female rats. Fasting did not significantly affect xenobiotic-induced cytochrome P-450 in male rats. Our results suggest that fasting in female rats results in an increase in cytochrome P-450 which is dependent upon synthesis of RNA and protein.

Turnover of messenger RNA, apoprotein and haem of cytochrome P-450b + e induced by phenobarbitone in rat liver

A single injection of phenobarbitone elicits asynchronous behaviour in the pattern of induction of mRNA, apoprotein and haem of cytochrome P-450b + e. The mRNA content reaches a maximum around 16h, the apoprotein content reaches a maximum around 30-36h, and the holoprotein content shows biphasic behaviour, with maxima around 16h and 30-36h. With the use of CoCl2 to block fresh transcription of cytochrome P-450b + e mRNA, the half-life of the preinduced mRNA was found to be 3h. The apoprotein and haem moieties of cytochrome P-450b + e turn over with half-lives of 16h and 8h respectively. The pattern of induction of delta-aminolaevulinate synthase shows biphasic behaviour, with maxima around 16h and 30-36h. The biphasic behaviour of the holoprotein content is thus due to differences in the extent of saturation of the apoprotein with haem, the process being regulated by the activity of the rate-limiting delta-aminolaevulinate synthase and the independent faster turnover rate of haem with respect to the apoprotein. Massive degradation of the haem moiety of preformed cytochrome P-450b + e results in the subsequent degradation of the apoprotein.

Purification and characterization of pentobarbital-induced cytochrome P-450BM-1 from Bacillus megaterium ATCC 14581

When Bacillus megaterium ATCC 14581 is grown in the presence of barbiturates, a cytochrome P-450-dependent fatty acid monooxygenase (Mr 120000) is induced (Kim, B.-H. and Fulco, A.J. (1983) Biochem. Biophys. Res. Commun. 116, 843-850). Gel filtration chromatography of a crude monooxygenase preparation from pentobarbital-induced B. megaterium indicated that not all of the induced cytochrome P-450 present in the extract was accounted for by this high-molecular-weight component. Further purification revealed the presence of two additional but smaller cytochrome P-450 species. The minor component, designated cytochrome P-450BM-2, had a molecular mass of about 46 kDa, but has not yet been completely purified or further characterized. The major component, designated cytochrome P-450BM-1, was obtained in pure form, exhibited fatty acid monooxygenase activity in the presence of iodosylbenzenediacetate, and has been extensively characterized. Its Mr of 38000 makes it the smallest cytochrome P-450 yet purified to homogeneity. Although it is a soluble protein, a complete amino acid analysis indicated that it contains 42% hydrophobic residues. By the dansyl chloride procedure the NH2-terminal amino acid is proline; the penultimate NH2-terminal residue is alanine. The absolute absorption spectra of cytochrome P-450BM-1 show maxima in the same general regions as do P-450 cytochromes from mammalian or other bacterial sources, but they differ in detail. The oxidized form of P-450BM-1 has absorption maxima at 414, 533 and 567 nm, while the reduced form has peaks at 410 and 540 nm. The absorption maxima for the CO-reduced form of P-450BM-1 are found at 415, 448 and 550 nm. Antisera from rabbits immunized with pure P-450BM-1 strongly reacted with and precipitated this P-450, but showed no detectable affinity for either the 46 kDa P-450 or the 120 kDa fatty acid monooxygenase.

Enzymatic activation of chemicals to toxic metabolites

A variety of enzymes function in the oxygenation, oxidation-reduction, conjugation, and hydrolysis of drugs and other foreign chemicals. Often these enzymes detoxicate chemicals to prevent detrimental effects. In this review we will, however, concentrate on cases in which metabolism activates chemicals to reactive species which cause cellular damage. Particular attention will be given to mixed-function oxidases, which carry out a variety of oxygenations, as well as other reactions. (We will focus on cellular toxicity as opposed to initiation of tumorigenesis in this review.) In many cases, considerable circumstantial evidence exists linking these enzymes to enhanced toxicity of chemicals, although causal relationships have seldom been demonstrated. Further, in very few cases is the explicit cause of toxicity known. Modification of critical protein residues is suspected, although oxidative stress may also be involved in some cases. We discuss general aspects of mechanisms of toxic action, briefly list all cases in which metabolism is suspected to play a role in enhancing toxicity, and review a few examples in detail where substantial chemical and enzymatic information is available. The latter instances would involve knowledge of the enzymes involved, chemical evidence on the structures of the reactive metabolites, identification of adducts, and some inference into the biological processes which are effected to elicit toxicity. We consider, in this regard, vinyl halides (which have been a focus in our own laboratory), acetaminophen, pyrrolizidine alkaloids, and fluoroxene.

Turnover of membrane proteins: kinetics of induction and degradation of seven forms of rat liver microsomal cytochrome P-450, NADPH-cytochrome P-450 reductase, and epoxide hydrolase

The in vivo turnover of several rat liver microsomal proteins was studied using techniques designed to maximize antibody recognition specificity and minimize reutilization of radioactive labels. The kinetics of degradation of seven cytochrome P-450 isozymes, NADPH-cytochrome P-450 reductase, and epoxide hydrolase were determined in untreated rats and rats treated with phenobarbital or beta-naphthoflavone. In the cases where induction of these enzymes occurred with the above chemicals, rates of synthesis of the proteins were also estimated. In general, the degradation rates of the different proteins were rather similar to each other, and the effects of phenobarbital and beta-naphthoflavone on these rates were not very great. However, in the case of cytochromes P-450, a general trend was observed in which the heme moiety was degraded more rapidly than the apoprotein. Changes in the rates of synthesis of the individual proteins appear to contribute more to the altered steady-state levels which are expressed than do the rates of degradation, and profiles of steady-state enzyme concentrations predicted by the kinetic constants approximate those observed in vivo.

Turnover of newly synthesized cytochromes P-450scc and P-45011 beta and adrenodoxin in bovine adrenocortical cells in monolayer culture: effect of adrenocorticotropin

The turnover of newly synthesized cytochromes P-450scc and P-45011 beta, and adrenodoxin was investigated in bovine adrenocortical cells in primary monolayer cultures. Cells were pulse-radiolabeled with [35S]methionine, and specific newly synthesized enzymes were immunoisolated at various times following labeling and quantitated. Adrenocorticotropin (ACTH) treatment did not alter the average turnover rate of total cellular proteins or that of total mitochondrial proteins. The half-life of total cellular proteins of control and ACTH-treated cells was determined to be 20.5 and 23 h, respectively. The half-life of mitochondrial proteins of control and ACTH-treated cells was determined to be 42.5 and 44 h, respectively. The turnover rate of newly synthesized cytochrome P-450scc was approximately the same as total mitochondrial protein (t1/2 = 38 h), and was unchanged by ACTH treatment (t1/2 = 42 h). ACTH treatment did not greatly alter the turnover rate of adrenodoxin. The half-life of adrenodoxin from control and ACTH-treated cells was determined to be 20 and 17 h, respectively. However, ACTH treatment appeared to increase the half-life of cytochrome P-45011 beta from 16 h in control cells to 24 h in treated cells. The differential rate of turnover of mitochondrial proteins studied here supports the contention that mitochondria are subject to heterogeneous degradation. It appears that chronic treatment of bovine adrenocortical cells in culture with ACTH leads to increased steroidogenic capacity, primarily as a result of increased synthesis of steroidogenic enzymes, although, as shown for cytochrome P-45011 beta, ACTH action might also increase steroidogenic capacity by increasing the half-life of this steroid hydroxylase.

At least six forms of extremely homologous cytochromes P-450 in rat liver are encoded at two closely linked genetic loci

A subpopulation of phenobarbital-induced cytochrome P-450 in rat liver has been shown to consist of four closely related forms of the enzyme that appeared to be strain-related. In the present study, polypeptides composing this family were analyzed by two-dimensional electrophoresis of hepatic microsomes from 64 individual phenobarbital-treated rats. The animals surveyed included both sexes from four inbred and five outbred strains/colonies and F1 progenies from 10 crosses. Two new members of this polypeptide family were identified on the basis of their unique electrophoretic behavior and peptide maps. Eight phenotypes were observed that consisted of two to four member polypeptides. The six closely related cytochromes P-450 were found to be encoded at two genetic loci with at least four alleles at the P-450b locus and at least two alleles at the P-450e locus. Most colonies of outbred strains were characterized by polymorphism at one or both of these loci, and in no case did they contain unique alleles. Analyses of parents and their F1 progenies indicated that the P-450b and P-450e loci are closely linked on the same autosome and are expressed codominantly. Furthermore, the products of these loci appear to be coordinately regulated. The extreme homology between P-450b and P-450e genes, their high degree of polymorphism, and their close linkage suggest that they are subject to the same genetic mechanisms that maintain these features in other multigene families.

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.