Drug-metabolizing Enzymes In Rats Research Articles

Effect of bilberry extract (Vaccinium myrtillus L.) on drug-metabolizing enzymes in rats

Vaccinium myrtillus L. (bilberry) fruit is a blue-colored berry with a high content of anthocyanins. These bioactive secondary metabolites are considered to play a major role in the health-promoting properties of bilberries. Our in vivo study was designed to assess the possible influence of bilberry extract on drug-metabolizing enzymes (DMEs). Rats were exposed to bilberry extract in drinking water at two concentrations (0.15 and 1.5 g/L). Selected DMEs were determined (mRNA expression and enzymatic activity) after 29 and 58 days in rat liver. In addition, a panel of antioxidant, physiological, biochemical and hematological parameters was studied; these parameters did not demonstrate any impact of bilberry extract on the health status of rats. A significant increase in activity was observed in cytochrome P450 (CYP) 2C11 (131% of control) and CYP2E1 (122% of control) after a 29-day administration, while the consumption of a higher concentration for a longer time led to a mild activity decrease. Slight changes were observed in some other DMEs, but they remained insignificant from a physiological perspective. According to our results, we conclude that the consumption of bilberries as a food supplement should not pose a risk of interacting with co-administered drugs based on their metabolism.

The inhibitory effects of β-caryophyllene, β-caryophyllene oxide and α-humulene on the activities of the main drug-metabolizing enzymes in rat and human liver in vitro

Sesquiterpenes, the main components of plant essential oils, are often taken in the form of folk medicines and dietary supplements. Several sesquiterpenes possess interesting biological activities but they could interact with concurrently administered drugs via inhibition of drug-metabolizing enzymes. Therefore, the present study was designed to test the potential inhibitory effect of tree structurally relative sesquiterpenes β-caryophyllene (CAR), β-caryophyllene oxide (CAO) and α-humulene (HUM) on the activities of the main drug-metabolizing enzymes. For this purpose, rat and human hepatic subcellular fractions were incubated with CAR, CAO or HUM together with specific substrates for oxidation, reduction and conjugation enzymes and their coenzymes. HPLC, spectrophotometric and spectrofluorimetric analyses of product formations were used. All tested sesquiterpenes significantly inhibited cytochromes P4503A (CYP3A) activities in rats as well as in human hepatic microsomes, with CAO being the strongest inhibitor. A non-competitive type of inhibition was found. On the other hand, none of the tested sesquiterpenes significantly affected the activities of carbonyl-reducing enzymes (CBR1, AKRs, NQO1) or conjugation enzymes (UGTs, GSTs, SULTs, COMT). As CYP3A enzymes metabolize many drugs, their inhibition by CAO, CAR and HUM might affect the pharmacokinetics of concurrently administered drugs. Similar results obtained in rat and human hepatic microsomes indicate that rats could be used for further testing of possible drug-sesquiterpenes interactions in vivo.

Effect of single-walled carbon nanotubes on expression of phase II drug-metabolizing enzymes in rats

Effect of single-walled carbon nanotubes on expression of phase II drug-metabolizing enzymes in rats

Effects of the standardized extract of Centella asiatica ECa233 on hepatic Phase II drug-metabolizing enzymes in rats

Effects of the standardized extract of Centella asiatica ECa233 on hepatic Phase II drug-metabolizing enzymes in rats

Induction of Xenobiotic Receptors, Transporters, and Drug Metabolizing Enzymes by Oxycodone

Perturbations of the expression of transporters and drug-metabolizing enzymes (DMEs) by opioids can be the locus of deleterious drug-drug interactions (DDIs). Many transporters and DMEs are regulated by xenobiotic receptors [XRs; e.g., pregnane X receptor (PXR), constitutive androstane receptor (CAR), and Aryl hydrocarbon receptor (AhR)]; however, there is a paucity of information regarding the influence of opioids on XRs. The objective of this study was to determine the influence of oxycodone administration (15 mg/kg intraperitoneally twice daily for 8 days) on liver expression of XRs, transporters, and DMEs in rats. Microarray, quantitative real-time polymerase chain reaction and immunoblotting analyses were used to identify significantly regulated genes. Three XRs (e.g., PXR, CAR, and AhR), 27 transporters (e.g., ABCB1 and SLC22A8), and 19 DMEs (e.g., CYP2B2 and CYP3A1) were regulated (P < 0.05) with fold changes ranging from -46.3 to 17.1. Using MetaCore (computational platform), we identified a unique gene-network of transporters and DMEs assembled around PXR, CAR, and AhR. Therefore, a series of transactivation/translocation assays were conducted to determine whether the observed changes of transporters/DMEs are mediated by direct activation of PXR, CAR, or AhR by oxycodone or its major metabolites (noroxycodone and oxymorphone). Neither oxycodone nor its metabolites activated PXR, CAR, or AhR. Taken together, these findings identify a signature hepatic gene-network associated with repeated oxycodone administration in rats and demonstrate that oxycodone alters the expression of many transporters and DMEs (without direct activation of PXR, CAR, and AhR), which could lead to undesirable DDIs after coadministration of substrates of these transporters/DMEs with oxycodone.

Effects of dietary inulin, statin, and their co-treatment on hyperlipidemia, hepatic steatosis and changes in drug-metabolizing enzymes in rats fed a high-fat and high-sucrose diet

BackgroundRats fed a high-fat and high-sucrose (HF) diet develop hepatic steatosis and hyperlipidemia. There are several reports that a change in nutritional status affects hepatic levels of drug-metabolizing enzymes. Synthetic inulin is a dietary component that completely evades glucide digestion. Supplementing a HF diet with inulin ameliorates hypertriglycemia and hepatic steatosis, but not hypercholesterolemia. This study aimed at distinguishing the effects of synthetic inulin and 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin), which inhibit cholesterol biosynthesis.MethodsWe examined effects of co-treatment with synthetic inulin (5%) and fluvastatin (0, 4, and 8 mg/kg, per os) on body weight, epidydimal white adipose tissue weight, serum and hepatic lipid profiles, and hepatic cytochrome P450 (CYP) mRNA and protein profiles in rats fed a standard diet or a HF diet for 3 weeks.ResultsTreatment with the synthetic inulin (5%) or fluvastatin at 4 mg/kg (lethal dose in rats fed the HF diet, 8 mg/kg) ameliorated the elevation in hepatic triacylglycerol and total cholesterol levels in rats fed the HF diet. Whereas co-treatment with the inulin (5%) and fluvastatin (4 mg/kg) had a tendency to more strongly suppress the elevation in serum levels of very low density lipoprotein triacylglycerol than either treatment alone, no additive or synergistic effect was found in decrease in hepatic lipid levels. Hepatic levels of CYP1A1/2 and CYP2E1 mRNA and protein and methoxyresorufin O-demethylase and ethoxyresorufin O-deethylase activities were reduced in rats fed the HF diet. The synthetic inulin alleviated the reduction in hepatic levels of CYP1A1/2 and CYP2E1 mRNA and protein more strongly than fluvastatin, and no synergistic effects were observed on co-treatment. Furthermore, hepatic levels of aryl hydrocarbon receptor mRNA were decreased in rats fed the HF diet and recovered to near normal values with the intake of dietary inulin, which correlated with change in CYP1A1/2.ConclusionsDietary inulin alone was effective to prevent the development of hepatic steatosis, ameliorate nutritional effects, and alleviate the hepatic change in the expression of CYP1A1/2 and CYP2E1, while co-treatment with statin did not have additive or synergistic effects and statin may cause adverse effects in rats fed the HF diet.

Molecular Mechanism of Tetrabromobisphenol A (TBBPA)-induced Target Organ Toxicity in Sprague-Dawley Male Rats

Brominated flame retardants (BFRs) are present in many consumer products ranging from fabrics to plastics and electronics. Wide use of flame retardants can pose an environmental hazard, which makes it important to determine the mechanism of their toxicity. In the present study, dose-dependent toxicity of tetrabromobisphenol A (TBBPA), a flame retardant, was examined in male prepubertal rats (postnatal day 18) treated orally with TBBPA at 0, 125, 250 or 500 mg/kg for 30 days. There were no differences in body weight gain between the control and TBBPA-treated groups. However, absolute and relative liver weights were significantly increased in high dose of TBBPA-treated groups. TBBPA treatment led to significant induction of CYP2B1 and constitutive androstane receptor (CAR) expression in the liver. In addition, serum thyroxin (T4) concentration was significantly reduced in the TBBPA treated group. These results indicate that repeated exposure to TBBPA induces drug-metabolising enzymes in rats through the CAR signaling pathway. In particular, TBBPA efficiently produced reactive oxygen species (ROS) through CYP2B1 induction in rats. We measured 8-hydroxy-2'-deoxyguanosine (8-OHdG), a biomarker of DNA oxidative damage, in the kidney, liver and testes of rats following TBBPA treatment. As expected, TBBPA strongly induced the production of 8-OHdG in the testis and kidney. These observations suggest that TBBPA-induced target organ toxicity may be due to ROS produced by metabolism of TBBPA in Sprague- Dawley rats.

Subacute effects of the brominated flame retardants hexabromocyclododecane and tetrabromobisphenol A on hepatic cytochrome P450 levels in rats

The brominated flame retardants tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCD) are found in the environment, e.g., in sediments and organisms, in food items, human blood samples and mother's milk. In this study, the effects of both compounds on rat hepatic cytochrome P450 (CYP) levels and activities were investigated. Juvenile/young male and female Wistar rats were treated orally with various doses via the feed (TBBPA) or by gavage (HBCD). After 28 days of treatment the animals were sacrificed and hepatic mRNA and microsomes were isolated. HBCD treatment led to a significant induction of CYP2B1 mRNA, CYP2B1/2B2 protein and 7-pentoxyresorufin O-depentylase (PROD) activity suggesting a phenobarbital-type of induction. Furthermore, a significant increase in CYP3A1/3A3 mRNA, CYP3A1 protein, and luciferin benzylether debenzylase (LBD) activity was found, being more pronounced in females than in males. The effect on CYP3A1/3A3 mRNA was significant in female rats at a daily dose of 3.0 mg/kg body weight and above. HBCD exhibited no effects on CYP1A2 mRNA, CYP1A1/1A2 protein, or microsomal 7-ethoxyresorufin O-deethylase (EROD) activity suggesting lack of activation of the aryl hydrocarbon receptor. No significant effects on any of the parameters measured were obtained with TBBPA. Our findings suggest that oral exposure to HBCD induces drug-metabolising enzymes in rats probably via the CAR/PXR signalling pathway. Induction of CYPs and co-regulated enzymes of phase II of drug metabolism may affect homeostasis of endogenous substrates including steroid and thyroid hormones.

Effect of dietary phytic acid on hepatic activities of lipogenic and drug-metabolizing enzymes in rats fed 1,1,1-trichloro-2,2-bis (P-chlorophenyl) ethane (DDT)

This study was conducted to examine the influence of dietary sodium phytate (10.2g/kg diet) on hepatic activities of lipogenic and drug-metabolizing enzymes in rats fed 1,1,1-trichloro-2,2-bis (P-chlorophenyl) ethane (DDT) (0.7g/kg diet). Dietary DDT enhanced hepatic lipids, elevated hepatic activities of lipogenic enzymes, increased serum levels of cholesterol and phospholipid, and raised hepatic activities of drug-metabolizing enzymes. Dietary phytate significantly decreased the rises in hepatic lipids and the activities of lipogenic enzymes due to DDT feeding. The enhancement in hepatic activity of phase II drug-metabolizing enzyme, glutathione S-transferase, that had been caused by DDT feeding was significantly enhanced with dietary phytate. These results suggest that dietary phytate can protect DDT-fed animals against an accumulation of hepatic lipids, by depressing of hepatic lipogenesis. The present study also implies that dietary phytate might improve the function of drug-metabolizing enzymes exposed to DDT.

Ebselen protects against the reduction in levels of drug-metabolizing enzymes in livers of rats with deoxycholic acid-induced liver injury.

Ebselen is a seleno-organic compound that inhibits oxidative stress by lipid peroxidation through a glutathione peroxidase-like activity. We studied the effect of ebselen on the expression of hepatic drug-metabolizing enzymes in rats with deoxycholic acid-induced liver injury. Hydrophobic bile acids, such as deoxycholic acid, are known to cause cholestatic liver injury, and it was reported that expression of hepatic cytochrome P-450 (CYP) was reduced by deoxycholic acid administration in rats. Hydrophobic bile acids induce lipid peroxidation in the liver, and this may be one mechanism of the development of liver injury. In the present study, we investigated the effect of ebselen (30 mg/kg/day for 10 days) on rats ingesting deoxycholic acid (1% of diet for 10 days). Deoxycholic acid decreased levels of CYP1A1, 2B1, 2E1 and 3A2 to 34, 58, 62 and 37% of control values, respectively, and increased serum alkaline phosphatase (ALP) and alanine aminotransferase (ALT) activities to 1.8 and 8.6 times the levels of controls, respectively. Administration of ebselen with deoxycholic acid prevented the decreases in levels of CYP1A1 and 3A2 (86 and 65% of control, respectively) and the increases in serum ALP and ALT activities (1.4 and 1.9 times of control, respectively) caused by deoxycholic acid. These results indicate that ebselen may have a protective effect against hydrophobic bile acid-induced liver injury.

Apolipoprotein A-I gene expression is upregulated by polychlorinated biphenyls in rat liver

Xenobiotics such as polychlorinated biphenyls (PCB) increase serum cholesterol level (especially high density lipoprotein cholesterol) and apolipoprotein A-I (apo A-I) level in rats. The effect of PCB on serum apo A-I and hepatic apo A-I gene expression and the relationship between apo A-I and drug-metabolizing enzymes in rats were investigated. Serum levels of cholesterol and apo A-I were increased by dietary addition of PCB in a dose-dependent manner (0–500 mg/kg diet). Hepatic apo A-I mRNA level was also elevated by PCB in a similar fashion. Serum level of cholesterol gradually increased during feeding period of PCB (200 mg/kg diet, 105 days) and reached a two-fold higher level in PCB group than in controls. The levels of serum apo A-I and hepatic apo A-I mRNA linearly elevated during feeding period of PCB and were increased 3- or 4-fold, respectively, compared to controls. Although acute administration (16 hr) of PCB, 3-methylcholanthrene, and phenobarbital induced cytochrome P-450 gene expression in the liver, hepatic apo A-I gene expression was not increased by these xenobiotics. These results indicated that the serum levels of cholesterol and apo A-I had positive correlation with hepatic level of apo A-I mRNA in rats fed PCB, and that hepatic apo A-I gene expression was dependent upon intake of PCB but was not directly related to the induction of drug-metabolizing enzymes. This study demonstrated that xenobiotic-induced hyper-alpha-cholesterolemia would be caused by the increased apo A-I gene expression and cholesterol synthesis in the liver, coordinately.

Modification of hepatic drug-metabolizing enzymes in rats treated with alkyl sulfides

Natural compounds which elevate detoxification enzymes and/or reduce activating enzymes could be considered as good candidates to protect against cancer. In this work, we studied the modulation of hepatic drug-metabolizing enzymes in rats treated with dimethyl sulfide (DMS), dimethyl disulfide (DMDS), methylpropyl disulfide (MPDS), dipropyl sulfide (DPS), dipropyl disulfide (DPDS) and diallyl disulfide (DADS) issued from Allium species. Compounds containing methyl groups had little or no effect. Compounds with two propyl groups or two allyl groups provoked a pleiotropic response on drug-metabolizing enzymes. DPS, DPDS and DADS induced ethoxyresorufin O-deethylase, methoxyresorufin O-demethylase and mostly pentoxyresorufin O-depentylase and decreased nitrosodimethylamine N-demethylase and erythromycin N-demethylase. These modifications of enzyme activities were accompanied by an increase of CYP 2B1,2 and a decrease of CYP 2E1, evidenced by immunoblotting. The same treatments stimulated some phase II enzyme activities such as glutathione transferase and UDP-glucuronyl transferases. This pattern of induction and/or inhibition of drug metabolizing enzymes was qualitatively similar to that elicited by the enzyme inducer, phenobarbital. The magnitude of the effects produced by DPDS was smaller than those produced by DADS and DPS. Our results suggest a possible protective effect of alkyl sulfides as well as diallyl disulfide, on the first step of carcinogenesis via the modulation of enzymes involved in carcinogen metabolism.

Changes in drug-metabolizing enzymes of rats in ciprofibrate-induced hepatic nodules

1. Premalignant rat liver nodules produced in the resistant hepatocyte model, by exposure to carcinogenic chemicals (diethyl nitrosamine and 2-acetamidofluorene), and partial hepatectomy, exhibit decreased xenobiotic hydroxylase activities and increased conjugase activities, which are considered responsible for increased resistance to xenobiotic toxicity. 2. However, premalignant rat liver nodules generated by feeding the hypolipidaemic, peroxisomal proliferating drug, ciprofibrate, in a hypolipidaemic model, exhibit decreased hydroxylase activities but decreased conjugase activities also. 3. It is considered that reactive oxygen species (ROS) are generated in both the resistant hepatocyte model and in the hypolipidaemic model, resulting in lipid peroxidation, loss of haem, cytochromes and hydroxlase activities. 4. However, whereas there is a rebounding compensation of conjugase enzymes in the resistant hepatocyte model, this does not occur with the hypolipidaemic model, as peroxidation is probably persistent and the conjugases are continuously destroyed.

Studies on the Metabolic Fate of Cabergoline. (III): Identification of Liver Metabolites, Distribution of Brain, and Effect on Hepatic Drug-Metabolizing Enzymes in Rats.

The metabolites of cabergoline in liver were investigated in rats after a single oral administration. The identification was performed by HPLC and mass spectrometry equipped with an APCI interface. In addition to cabergoline, the identified metabolites are FCE-27395 which is the demethylated derivative of cabergoline and, possibly, one more demethylated of FCE-27395. Analysis of the metabolites in liver after a single oral administration of [14C]-cabergoline (0.5 mg/kg) showed a difference in the content of the metabolites between male and female rats. This sexual difference was due to the much higher content in FCE-27395 in male rat liver compared to female rat liver. For both sexes the major metabolite was FCE-27395, and other metabolites were found in minor quantities. The consecutive administration of cabergoline did not cause any induction of hepatic drug metabolizing enzymes, such as cytochrome P450, cytochrome b5, aniline hydroxylase, and aminopyrine demethylase. The distribution in the brain after a single intravenous administration of [3H]-cabergoline (0.5 mg/kg) was also investigated. The collected tissues were cerebral cortex, striatum, and hypophysis. The highest radioactivity was in the hypophysis, in which tissue the radioactivity tended to remain until 24 hour after dosing. Most of the radioactivity in these tissues could be attributed to cabergoline, as there were only traces of its metabolites.

Effect of fluvastatin, an inhibitor of 3-hydroxy-3-methyl glutaryl CoA reductase, on drug-metabolizing enzymes in rats

Fluvastatin (FV), a new cholesterol-lowering agent, has been studied for its effects on hepatic microsomal drug-metabolizing enzymes in male rats. FV was orally administered in dosages of 1, 5, and 30 mg/kg/day for 7 consecutive days. 3-Hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase activities were markedly decreased at all dose levels. The amount of microsomal protein and the contents of cytochromes P450 and b5 did not change. No induction of aniline hydroxylase, aminopyrine N-demethylase, testosterone hydroxylases (15 alpha-, 7 alpha-, 6 beta-, 16 alpha-, and 16 beta-), and UDP-glucuronosyltransferase were found. On the other hand, 7-ethoxycoumarin o-deethylase activity was slightly increased and lauric acid omega-1-hydroxylase activity tended to be decreased after treatment with FV.

Metabolism of gemcitabine in rat and dog.

1. The metabolism of 14C-gemcitabine in the male rat has been studied after intravenous administration of a single dose (10 mg/kg) or five doses (1 mg/kg/day) of 14C-gemcitabine. The metabolism in male dog has been studied after only single dosing. The effects of gemcitabine on hepatic drug-metabolizing enzymes in rat has also been studied. 2. The concentration of gemcitabine in the plasma was 11.84 micrograms/ml at 5 min, and then rapidly decreased after a single administration to rat. A deaminated uracil analogue of gemcitabine progressively increased with time. Gemcitabine and the uracil metabolite accounted for 80.0 and 11.8% of the radioactive dose in the 0-24-h urine samples respectively. Gemcitabine was the major component identified in lung, liver and kidney at 5 min after administration. 3. After repeated administration to rat, metabolites in the plasma and tissues were not remarkably different from those found after a single administration. 4. After a single administration to dog, the plasma concentration of gemcitabine was 12.39 micrograms/ml at 5 min. Gemcitabine and the uracil metabolite accounted for 8.3 and 71.8% of the dose in the 0-24-h urine samples respectively. 5. No differences were observed in enzymatic activities per whole liver between the gemcitabine-treated and control rat.

Effects of Methylsulfonyl Metabolites of PCBs Accumulated in Tissues of Mammals on Activities of Hepatic Microsomal Drug-Metabolizing Enzymes in Rats (Proceedings of the 19th Symposium on Toxicology and Environmental Health)

Effects of Methylsulfonyl Metabolites of PCBs Accumulated in Tissues of Mammals on Activities of Hepatic Microsomal Drug-Metabolizing Enzymes in Rats (Proceedings of the 19th Symposium on Toxicology and Environmental Health)

P-622 - Effects of Kampo medicine (Keigai-rengyo-to and Senkyu-chacho-san) on hepatic drug-metabolizing enzymes in rats.

P-622 - Effects of Kampo medicine (Keigai-rengyo-to and Senkyu-chacho-san) on hepatic drug-metabolizing enzymes in rats.

Modification of hepatic drug-metabolizing enzymes in rat fed naturally occurring allyl sulphides.

1. The effects of feeding allyl sulphides to rat (2000 ppm of the diet for 15 days) were investigated on various microsomal hepatic drug-metabolizing enzymes by their immunochemical detection and catalytic activity. 2. Allyl sulphides provoked a temporary dietary restriction, which enhanced the microsomal level of P450 and the activities of NADH-cytochrome c reductase and p-hydroxybiphenyl UDP-glucuronyltransferase (UDPGT 2), and lowered the activities of p-nitrophenol hydroxylase (PNPH), N-nitrosodimethylamine demethylase (NDMAD), laurate omega-hydroxylase (LAH) and glutathione S-transferase (GST). Therefore, pair-fed animals were used as a more relevant control for the dietary effects of allyl sulphides. 3. Diallyl sulphide (DAS) as well as diallyl disulphide (DADS) produced an enhancement of the microsomal level of P4501A2, 2B1/2 and 3A1/2, and epoxide hydrolase (EH) proteins, with an increase in the enzymatic activities they catalyse: ethoxyresorufin O-deethylase (EROD), aryl hydrocarbon hydroxylase (AHH), methoxyresorufin O-demethylase (MROD), ethoxycoumarin O-deethylase (ECOD), pentoxyresorufin O-depentylase (PROD), benzoxyresorufin O-debenzylase (BROD) and EH. Although P4502E1 proteins were lowered on treatment, NDMAD activity was not modified, and PNPH activity was even enhanced by allyl sulphides. Only DAS treatment raised erythromycin N-demethylase (ERDM) activity. 4. Both DAS and DADS increased the activity of GST and p-nitrophenol UDP-glucuronyltransferase (UDPGT 1), whereas UDPGT 2 activity was enhanced only by DAS.

Effect of red palm oil on some hepatic drug-metabolizing enzymes in rats

Red palm oil (RPO) from Elaeis guineensis is being considered for use as an edible oil in India as it is one of the richest natural sources of carotenoids. The effect of RPO on the host detoxification system, which is a vital mechanism in cancer prevention, was studied in three separate batches of Wistar/NIN inbred albino rats, and compared with controls, groundnut oil (GNO) and refined bleached deodorized palmolein oil (RBDPO). The first batch of 36 rats (12 from each group) comprised the adult males (26 wk old) of the third generation (F 2b) from a multigeneration reproduction study in which three groups were fed 10% GNO or RPO or RBDPO for three generations continuously. Phase II glutathione- S-transferase (GSH-T) activity was measured in the liver cytosol of these rats after they had twice completed the process of mating, gestation, lactation and weaning, because GSH-T is one of the principal detoxifying enzymes involved in conjugating reactions of phase II metabolism. The fourth generation (F 3b) weanling rats of the three groups, receiving GNO, RPO or RBDPO, were continued on the 10% oil diet for 9 wk, after which cytosolic GSH-T activity was measured. In the second experiment, eight male weanling Wistar/NIN inbred albino rats, 5 wk old, weighing 100–120 g, were fed 10% GNO, RPO or RBDPO for 4 wk in a 20% protein synthetic diet. Liver cytosolic GSH-T, reduced glutathione, microsomal total cytochrome P − 450, aminopyrine N-demethylase and ethoxyresorufin- O-deethylase activity were measured to elucidate the effect of RPO on some phase I and phase II reactions. Significantly higher levels of GSH-T were observed in F 2b and F 3b rats given RPO than in those given GNO or RBDPO. In the second experiment. GSH-T induction was also noted, together with increased levels of reduced GSH. Phase I enzymes and total cytochrome P − 450 levels were comparable between groups, indicating that no induction attributable to RPO had occurred. Thus, enhancement of one of the detoxifying phase II enzymes, in conjunction with the lack of induction of those activating phase I enzymes that are known to metabolize phenobarbitone and polycyclic aromatic hydrocarbons, suggests that RPO affords protection against chemical carcinogens, probably because of its carotenoid content.