Hepatic Cytochrome P450 Protein Research Articles

Sex- and lifestyle-related factors are associated with altered hepatic CYP protein levels in people diagnosed with mental disorders.

In this study, we used human postmortem tissue to investigate hepatic protein expression levels of CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 by LC‒MS/MS in a population of people suffering from mental disorders (n=171). We report hepatic protein levels of these six CYP isoforms in 171 individuals in total, and define a focused population dataset of 116 individuals after excluding 55 samples due to low Microsomal Protein Per Gram of Liver (MPPGL) yield. Postmortem decay was most likely the reason for the low MPPGL yield in the 55 samples. In the focused population, we found women to have significantly higher protein levels of CYP3A4 than men in addition to decreased CYP3A4 protein levels among obese individuals. Furthermore, MPPGL was negatively correlated with body mass index (BMI). An increase in CYP1A2 protein levels was observed among smokers and increased CYP2E1 protein levels were observed among individuals with a history of alcohol abuse. Finally, individuals who received phenobarbital (CYP3A4 inducer) had significantly higher CYP3A4 levels. In conclusion, lifestyle-related factors prevalent among people suffering from mental disorders are associated with altered CYP protein levels which may alter drug metabolism and affect the efficacy of commonly prescribed drugs. Furthermore, this investigation demonstrates that postmortem hepatic tissue can be used to study how lifestyle and effectors affect hepatic CYP-levels in a large cohort of patients. Significance Statement Using a large number of postmortem hepatic tissue specimens (n=116) originating from the autopsy of individuals diagnosed with mental disorders, we were able to show that hepatic CYP-levels were affected by alcohol, smoking, BMI, and sex and that MPPGL was affected by BMI. Theses lifestyle-related changes may alter drug metabolism and affect the efficacy of commonly prescribed drugs. It is a novel approach to use a large postmortem cohort to investigate how lifestyle and effectors affect hepatic CYP-levels.

Kupffer cell inactivation by carbon monoxide bound to red blood cells preserves hepatic cytochrome P450 via anti-oxidant and anti-inflammatory effects exerted through the HMGB1/TLR-4 pathway during resuscitation from hemorrhagic shock

Red blood cell (RBC) transfusions for controlling hemorrhaging induce systemic ischemia reperfusion, resulting in a decrease in hepatic cytochrome P450 (CYP) levels. Carbon monoxide (CO), when bound to red blood cells (CO-RBC) has the potential to protect the hepatic CYP protein to produce a resuscitative effect in a hemorrhagic shock rat model. The aim of this study was to investigate the mechanism by which CO-RBC resuscitation from a massive hemorrhage protects against a decrease in hepatic CYP. In the early phase (∼1h) after a hemorrhage and RBC resuscitation, hepatic CYP protein levels were significantly decreased with increasing hepatic free heme levels, but were maintained by a pre-treatment of gadolinium chloride (GdCl3), a Kupffer cell inhibitor, and Trolox, an anti-oxidant agent, as well as CO-RBC resuscitation. Under these conditions, the production of reactive oxygen species (ROS) derived from activated Kupffer cells was increased, but this increase was suppressed by CO-RBC resuscitation. At a late phase (6∼24h), CYP mRNA levels decreased after hemorrhage and RBC resuscitation, but not in the case of CO-RBC resuscitation. The increases in plasma IL-6 and TNF-α levels were decreased by CO-RBC resuscitation via the suppression of the toll-like receptor-4 (TLR-4) and the expression of the high mobility group box-1 (HMGB-1). Hepatic CYP protection after a hemorrhage and CO-RBC resuscitation can be attributed to the inactivation of Kupffer cells, resulting in the suppression of ROS production in the early phase and the suppression of inflammatory cytokine production via the TLR-4/HMGB-1signal pathway in the late phase.

A toxicological investigation of a celery seed extract having anti-inflammatory activity

An extract of the seed from celery (Apium graviolens) (CSE), and fractions thereof, have been found to possess anti-inflammatory activity, gastro-protective activity, and anti-Helicobacter pylori activity. In view of the potential for employing these extracts for therapeutic use, toxicological investigations were undertaken with an alcoholic extract (A-CSE) which has previously been shown to have the above pharmacological activities. A 28-day toxicity study was performed in rats according to Good Laboratory Practice (GLP) conditions. Eighteen adult male and 18 adult female rats were randomly assigned to 3 treatment groups of 6 rats/sex/group and were dosed orally with A-CSE of 0, 150 or 5,000mg/kg per day. Daily observations of vital signs and body weights were recorded and ophthalmological investigations were performed. At autopsy, the principal organs were weighed and sections collected for histological analysis. Serum and urine samples were collected at termination for routine clinical chemistry. Under non-GLP conditions alpha-2-μ-globulin immunohistochemistry was performed on kidney tissues and hepatic cytochrome P450 protein was determined, as well as, the enzymatic activities of the principal isoforms. All animals survived treatments with no visible or behavioral signs of toxicity being observed during the study. There were no statistically significant differences in body weight gains, body weight gains per day or cumulative absolute body weight gains, for either sex, in any treatment groups when compared with controls. Slightly increased liver weight and liver to body and brain weight ratios were observed in female rats and in liver to body weight ratios in male rats given high dose A-CSE which was a test article effect, but the absence of any microscopic correlates for the liver weight increases suggests that these were not toxicologically significant. Treatment related macroscopic changes were not observed at necropsy and microscopic findings were limited to minimal increases in gastric eosinophils in several male and female rats in the 5,000mg/kg per day treatment groups. Minimal focal degeneration of renal tubules was observed sporadically in both sexes assigned to all treatment groups including control and was consistent with early spontaneous nephropathy of laboratory rats and thus was not considered to represent a pathologic change associated with the test article. Increased serum globulin and phosphorus levels were observed in male rats given 5,000mg/kg per day A-CSE and decreased serum triglycerides levels in female animals given 150 or 5,000mg/kg per day A-CSE. The increase in serum globulin and phosphorus in male animals was small in magnitude and not considered toxicologically significant. The mechanism for the decrease in serum triglycerides in female rats was not apparent. Changes in urinalysis parameters were limited to small decreases in urine pH in female animals in the 150 and 5,000mg/kg per day groups and were not deemed toxicologically significant. Alpha-2-μ-globulin immunohistochemistry was performed on kidney tissues from all animals and found to be within normal physiologic limits. Minor corneal mineralization occurred in some animals from all treatment groups. Cataracts were observed in one in the control and one in an animal that had 5,000mg/kg per day but since the cataracts occurred in the metabolically inactive region of the lens, these were not considered indicative of test article related lesions. There were no changes in total hepatic microsomal protein or in total cytochrome P450 protein. Although male rats appeared to have to higher levels of total microsomal protein than female rats, there appeared to be no treatment effect in either male or female animals. As regards the activity of the various isoforms tested (CYP2B1/2, CYP1A1/2, CYP3A1/2), with the large range of activities detected for each P450 isoform, no clear change in activity or protein were observed, however, these data were not statistically analyzed. These results suggest that there are no toxicologically significant sub-chronic effects of oral A-CSE in rats. The no adverse effect level for systemic toxicity would appear to be 5,000mg/kg per day.

Bariatric Surgery-Induced Weight Loss Reduces Hepatic Lipid Peroxidation Levels and Affects Hepatic Cytochrome P-450 Protein Content

To evaluate the effects of surgical weight loss on hepatic lipid peroxidation levels and cytochrome P-450 protein expression in patients with nonalcoholic fatty liver disease (NAFLD). NAFLD and nonalcoholic steatohepatitis (NASH) affect hepatic cytochrome P-450 (CYP) protein expression and activity, and CYP2E1 may play a role in the pathogenesis of NAFLD and NASH through induction of oxidative stress and lipid peroxidation. NAFLD and NASH are associated with increased systemic lipid peroxidation levels and elevated hepatic CYP2E1 activity, but hepatic CYP3A4/5 activity is decreased. Liver biopsies from 20 patients with NAFLD who underwent bariatric surgery were obtained intraoperatively and at 15 +/- 7 months following surgery. Hepatic malondialdehyde (MDA) levels (a marker of lipid peroxidation), CYP2E1 and CYP3A4/5 protein expression, and steatosis, as a percent of total area, were measured by immunohistochemistry followed by digital image quantitation. Following weight loss, as reflected by reduced BMI (54 +/- 9 vs. 37 +/- 9 kg/m2; P < 0.001), features of the metabolic syndrome, grade and stage of liver disease, and liver histology were all significantly improved (P < 0.01). Hepatic MDA staining (35 +/- 18% vs. 23 +/- 14%; P = 0.02), CYP2E1 protein content (68 +/- 9% vs. 56 +/- 11%; P < 0.001), and steatosis (17 +/- 7% vs. 2 +/- 3%; P < 0.001) were significantly reduced following weight loss. CYP3A4/5 protein content was unchanged (57 +/- 13% vs. 55 +/- 13%; P = 0.433). The reduction in lipid peroxidation was independently associated with changes in CYP2E1 protein expression after bariatric surgery (r = 0.477; P = 0.033). Elevations in hepatic lipid peroxidation and CYP2E1 expression that are seen in NAFLD improve significantly with weight loss induced by bariatric surgery.

T1014 Bariatric Surgery-Induced Weight Loss Is Associated with Changes in Hepatic Cytochrome P-450 Protein Content

T1014 Bariatric Surgery-Induced Weight Loss Is Associated with Changes in Hepatic Cytochrome P-450 Protein Content

Long-Term Feeding of Ginkgo biloba Extract Impairs Peripheral Circulation and Hepatic Function in Aged Spontaneously Hypertensive Rats

We previously demonstrated that Ginkgo biloba extract (GBE) produces an anti-hypertensive effect via enhanced vasodilation responses in young spontaneously hypertensive rats (SHR) and hepatic hypertrophy occurs with increased cytochrome P450 (CYP) mRNA expression in young rats. In the present study, to clarify whether these actions of GBE are observed in older rats, we investigated cardiovascular functions and hepatic CYP protein expressions in aged SHR fed a control diet or a diet containing 0.5% GBE for 4 weeks. In aged SHR, GBE feeding significantly increased liver weight per 100 g body weight without changing the body weight. Furthermore, significant increases in alanine aminotransferase level in serum and marked increase in CYP2B protein expression in the liver were observed in aged SHR fed GBE. On the other hand, GBE feeding did not affect blood pressure, but significantly reduced heart rate and blood flow velocity in tail arteries of aged SHR. Furthermore, GBE feeding did not affect contractile response to phenylephrine, relaxation responses to not only sodium nitroprusside but also acetylcholine, and protein levels of endothelium nitric oxide synthase and soluble guanylate cyclase in aortas of aged SHR. These results suggested that long-term GBE feeding impairs peripheral circulation due to bradycardia and hepatic function in aged SHR. Thus, in the elderly population with hypertension, the use of GBE may need to be assessed for effects on heart rate and liver function.

Impact of transgene expression on drug metabolism following systemic adenoviral vector administration

Systemic administration of a first-generation adenovirus expressing E. coli beta-galactosidase (AdlacZ) alters expression and function of two hepatic drug-metabolizing enzymes, cytochrome P450 (CYP) 3A2 and 2C11, for 14 days. The objective of these studies was to determine how the transgene cassette influences CYP expression and function. Sprague-Dawley rats were given 5.7 x 10(12) viral particles (vp)/kg of either: AdlacZ, Ad expressing murine erythropoietin (Epo), Ad without a transgene (Null), or phosphate-buffered saline (Vehicle). Hepatic CYP protein expression, activity, mRNA and alanine aminotransferase (ALT) levels were analyzed 0.25, 1, 4, and 14 days following a single intravenous injection. Administration of Epo did not alter CYP3A2 activity, but induced RNA levels by a factor of 2 at 4 and 14 days (P< or =0.01). This vector suppressed CYP2C11 activity levels by 45% at 1 day (P< or =0.05) and RNA levels throughout the study period (P< or =0.05). The Null vector suppressed CYP3A2 activity by 36, 63, 34, and 45% at 0.25, 1, 4 and 14 days, respectively (P< or =0.05). CYP2C11 activity was suppressed 1 day after administration (41%) and RNA levels were suppressed at 6 h (53%) and 1 day (36%, P< or =0.05). In contrast, AdlacZ suppressed both CYP3A2 and 2C11 at all time points. The immunogenic and biological nature of the transgene cassette can influence changes in CYP3A2, but not the 2C11 isoform. The shift in transcription and translation of protein for maintenance of physiologic homeostasis to production of viral proteins and transgene product and their associated toxicity during viral infection may explain our observations.

Down-regulation of hepatic CYP1A2 plays an important role in inflammatory responses in sepsis.

Although hepatic cytochrome P-450 protein concentrations are altered following endotoxin shock, changes in P-450 isoforms in sepsis have not been fully investigated. The aim of this study was to determine whether the major P-450 isoform in rat liver (i.e., CYP1A2) is down-regulated during the progression of sepsis and, if so, whether reduction of P-450 enzyme system plays an important role in the inflammatory response. Prospective, controlled, and randomized animal study. A university/institute research laboratory. Male adult Sprague-Dawley rats were subjected either to polymicrobial sepsis by cecal ligation and puncture (CLP) or to sham operation followed by the administration of normal saline solution (i.e., fluid resuscitation). P-450 isoforms in the liver (i.e., CYP1A2 and 4A1) were determined using reverse transcription polymerase chain reaction and Western blot analysis at various time points after CLP. The results indicate that CYP1A2 messenger RNA expression decreased significantly at 10 and 20 hrs whereas its protein concentrations decreased at 20 hrs after the induction of sepsis. In contrast, CYP4A1 messenger RNA and protein concentrations were not altered even at 20 hrs after CLP. In an additional experiment, all P-450 isoforms were inhibited by pretreatment with 1-aminobenzotriazole to determine the effect of cytochrome P-450 blockade on inflammatory responses by assessing proinflammatory cytokines. The results show further increases in serum concentrations of tumor necrosis factor-alpha, interleukin-1beta, and interleukin-6 in aminobenzotriazole-treated animals at 10 hrs after CLP, which was associated with elevated concentrations of circulating lactate and severe morphologic alterations in the liver. These results suggest that the integrity of the cytochrome P-450 enzyme system plays an important role in septic inflammatory response. The major hepatic P-450 isoform CYP1A2 is down-regulated and inhibition of P-450 enzyme system is associated with an exacerbated inflammatory response in sepsis. Treatment with pharmaceutical agents that regulate or are metabolized by P-450 enzymes might be approached cautiously in the septic patient if this holds true in a clinical setting.

Differences in the hepatic P450-dependent metabolism of estrogen and tamoxifen in response to treatment of rats with 3,3′-diindolylmethane and its parent compound indole-3-carbinol

Indole-3-carbinol (I3C), present in cruciferous vegetables, and its major in vivo product 3,3′-diindolylmethane (DIM), have been reported to suppress estrogen-responsive cancers. This effect may be mediated through the modification of cytochrome P450 (CYP) complement and activities leading to estrogen detoxification. We examined the effects of a 4-day treatment of female Sprague–Dawley rats with DIM at 8.4 and 42 mg/kg body weight (bwt), on the hepatic CYP protein level, CYP1A1, 1A2, 2B1/2 and 3A1/2 probe activities and CYP-dependent metabolism of 17β-estradiol (E2) and estrone (E1). At 42 mg/kg bwt, DIM effected a small increase (2.8-fold) in CYP1A1 activity, and at both dose levels it reduced CYP3A1/2 activity by ∼40%. At the higher dose level, DIM decreased the rates of oxidation of E2 to 4-OH-E2, 4-OH-E1, 6α-OH-E2 and 6(α+β)-OH-E1 by 39, 44, 71 and 60%, respectively, and E1 to 6(α+β)-OH-E1 by 39%. These effects were considerably different from those of I3C reported by us previously. We also examined the effects of DIM and I3C on the hepatic microsomal metabolism of tamoxifen (TAM). Whereas metabolism of TAM was unaffected by DIM, formation of N-desmethyl-TAM (and its presumed derivative) was increased ∼3-fold by I3C at 250 mg/kg bwt. Since N-desmethyl-TAM is transformed to a genotoxic metabolite, dietary exposure to I3C may enhance hepatic carcinogenicity of TAM in the rat. The differences between I3C and DIM in CYP-mediated activities and metabolism indicate that DIM is not a proximate intermediate in the mechanism of action of I3C.

150. Effect of Viral Capsid Proteins on Hepatic Cytochrome P450 Protein Expression and Activity

150. Effect of Viral Capsid Proteins on Hepatic Cytochrome P450 Protein Expression and Activity

Different alterations of cytochrome P450 3A4 isoform and its gene expression in livers of patients with chronic liver diseases.

To determine whether parenchymal cells or hepatic cytochrome P450 protein was changed in chronic liver diseases, and to compare the difference of CYP3A4 enzyme and its gene expression between patients with hepatic cirrhosis and obstructive jaundice, and to investigate the pharmacologic significance behind this difference. Liver samples were obtained from patients undergoing hepatic surgery with hepatic cirrhosis (n=6) and obstructive jaundice (n=6) and hepatic angeioma (controls, n=6). CYP3A4 activity and protein were determined by Nash and western blotting using specific polychonal antibody, respectively. Total hepatic RNA was extracted and CYP3A4cDNA probe was prepared according the method of random primer marking, and difference of cyp3a4 expression was compared among those patients by Northern blotting. Compared to control group, the CYP3A4 activity and protein in liver tissue among patients with cirrhosis were evidently reduced. (P<0.01) Northern blot showed the same change in its mRNA levels. In contrast, the isoenzyme and its gene expression were not changed among patients with obstructive jaundice. Hepatic levels of P450s and its CYP3A4 isoform activity were selectively changed in different chronic liver diseases. CYP3A4 isoenzyme and its activity declined among patients with hepatic cirrhosis as expression of cyp3a4 gene was significantly reduced. Liver's ability to eliminate many clinical therapeutic drug substrates would decline consequently. These findings may have practical implications for the use of drugs in patients with cirrhosis and emphasize the need to understand the metabolic fate of therapeutic compounds. Elucidation of the reasons for these different changes in hepatic CYP3A4 may provide insight into more fundamental aspects and mechanisms of imparied liver function.

Rapid Quantification of CYP3A4 Expression in Human Leukocytes by Real-Time Reverse Transcription-PCR

Cytochrome P450 3A4 (CYP3A4) contributes to the metabolism of a wide variety of drugs and endogenous substrates, such as steroid hormones (1)(2). Variations in the catalytic activity of CYP3A4 are predominantly caused by enzyme induction mediated by transcriptional activation or by competitive substrate inhibition. Such variation may strongly influence the bioavailability of drugs and may modulate drug interactions. CYP3A4 is one of the predominant CYPs in the human liver, accounting for ∼30% of the total hepatic cytochrome P450 protein (2)(3). Relatively high CYP3A4 concentrations have been found in the small intestinal epithelium (70% of total CYP protein) and in the kidney (2). There are conflicting results concerning the amount of CYP3A4 in human peripheral blood lymphocytes. Several authors could not detect any CYP3A4 mRNA or protein, whereas some studies reported poor CYP3A4 expression in the white cell fraction (4)(5)(6). Thus, we assumed that CYP3A4 is expressed in lymphocytes in very small amounts and that only a very sensitive method could detect them. We developed a sensitive quantitative real-time reverse transcription-PCR (RT-PCR) method that allows rapid and correct determination of CYP3A4 mRNA expression in leukocytes. We investigated CYP3A4 mRNA expression in 31 human blood samples from healthy volunteers (20 males and 11 females; mean age, 29 years; range, 20–64 years) and in three human liver samples obtained from the International Institute for the Advancement of Medicine (Exton, PA). Before blood collection, all volunteers signed informed consents that were accepted by the Ethical Committee of the Charite. Leukocytes were separated from 8 mL of whole blood in a Vacutainer® cpt cell preparation tube system (Becton Dickinson). Small liver fragments were disrupted with a homogenizer (Potter; Braun). Samples were stored at −80 °C. Total cellular RNA was extracted by the TRIzol® LS method …

Temporal Analysis of Hepatocyte Differentiation by Small Hepatocyte-Like Progenitor Cells during Liver Regeneration in Retrorsine-Exposed Rats

Liver regeneration after two-thirds surgical partial hepatectomy (PH) in rats treated with the pyrrolizidine alkaloid retrorsine is accomplished through the activation, expansion, and differentiation of a population of small hepatocyte-like progenitor cells (SHPCs). We have examined expression of the major liver-enriched transcription factors, cytochrome P450 (CYP) enzymes, and other markers of hepatocytic differentiation in SHPCs during the protracted period of liver regeneration after PH in retrorsine-exposed rats. Early-appearing SHPCs (at 3-7 days after PH) express mRNAs for all of the major liver-enriched transcription factors at varying levels compared to fully differentiated hepatocytes. In addition, SHPCs lack (or have significantly reduced) expression of mRNA for hepatocyte markers tyrosine aminotransferase and alpha-1 antitrypsin, but their expression levels of mRNA and/or protein for WT1 and alpha-fetoprotein (AFP) are increased. With the exception of AFP expression, SHPCs resembled fully differentiated hepatocytes by 14 days after PH. Expression of AFP was maintained by most SHPCs through 14 days after PH, gradually declined through 23 days after PH, and was essentially absent from SHPC progeny by 30 days after PH. Furthermore, early appearing SHPCs lack (or have reduced expression) of hepatic CYP proteins known to be induced in rat livers after retrorsine exposure. The resistance of SHPCs to the mitoinhibitory effects of retrorsine may be directly related to a lack of CYP enzymes required to metabolize retrorsine to its toxic derivatives. These results suggest that SHPCs represent a unique parenchymal (less differentiated) progenitor cell population of adult rodent liver that is phenotypically distinct from fully differentiated hepatocytes, biliary epithelial cells, and (ductular) oval cells.

Sex differences in the diabetes-induced modulation of rat hepatic cytochrome p450 proteins

Sex differences in the diabetes-induced changes in hepatic cytochrome P450 proteins were investigated in rats treated with streptozotocin. Changes in specific cytochrome P450 proteins were monitored using diagnostic substrates and immunologically utilizing specific polyclonal antibodies. When expressed in terms of nmoles of total cytochrome P450, ethoxyresorufin O-deethylase activity was increased by treatment with streptozotocin, the extent of induction being the same in the two sexes. In contrast, lauric acid hydroxylase and ethylmorphine N-demethylase activities were induced only in the male rat. Finally, p-nitrophenol hydroxylase and pentoxyresorufin O-dealkylase were enhanced by the same treatment in both sexes, the effect being more pronounced in the male. These findings indicate that sex-specific changes in certain cytochrome P450 proteins exist in response to insulin-dependent diabetes but these cannot, however, be ascribed to sex differences in the severity of diabetes induced by streptozotocin since the degrees of hyperketonaemia and hyperglycaemia were the same in the two sexes. These are likely to reflect sex-specific differences in growth hormone and triglyceride levels in the diabetic animals.

Induction of rat hepatic mixed-function oxidases by acetone and other physiological ketones: their role in diabetes-induced changes in cytochrome P450 proteins.

1. To evaluate the role of ketone bodies in diabetes-induced changes in hepatic cytochrome P450 composition, rats were treated with acetone, 3-hydroxybutyrate or 1,3-butanediol. 2. Treatment with acetone enhanced the rat hepatic O-dealkylations of ethoxyresorufin and methoxyresorufin, and the hydroxylation of p-nitrophenol, but had no effect on lauric acid hydroxylation and ethylmorphine N-demethylation. Neither 3-hydroxybutyrate nor 1,3-butanediol modulated the metabolism of the above substrates. 3. Immunoblot analysis of hepatic microsomal proteins revealed that treatment with acetone increased the apoprotein levels of P4501A2, P4502B1/2 and P4502E1. 4. It is concluded that acetone is responsible, at least partly, for the diabetes-induced increase in hepatic microsomal P4501A2, P4502B1/2 and P4502E1 proteins but does not mediate the increases in the P4503A1 and P4504A1 proteins. On the basis of work from our own and other laboratories a mechanism for the diabetes-induced changes in hepatic cytochrome P450 proteins is proposed.

Bioactivation of N-nitrosopiperidine to mutagens: role of hepatic cytochrome P-450 proteins and contribution of cytosolic fraction.

In the present study the role of two families of cytochrome P-450 proteins and the contribution of the cytosolic fraction in the activation of N-nitrosopiperidine to mutagens in the Ames test were investigated. The bioactivation of this nitrosamine was preferentially catalysed by the phenobarbitone-induced cytochromes P-450, in contrast to the 3-methylcholanthrene-induced cytochromes P-448. The mutagenicity of nitrosopiperidine catalysed by microsomes, in the absence of cytosol, was lower when compared with that observed with S9 fractions. Cytosol itself could not activate nitrosopiperidine but potentiated the microsome-mediated mutagenicity of the carcinogen. The cytosolic potentiation was still evident when microsomal metabolism was terminated, indicating that cytosolic enzyme(s) can further convert the microsome-generated metabolites to more potent mutagens. The cytosolic enzyme(s) was inducible by prior treatment of the rats with phenobarbitone or Arochlor 1254 but not 3-methylcholanthrene. The microsome-mediated activation of nitrosopiperidine could be supported by NADH in the absence of NADPH. It is therefore concluded that the activation of nitrosopiperidine to mutagen(s) involves, in addition to NADH- and NADPH-dependent microsomal enzymes, cytosolic proteins.