Metabolism Of Ethoxyresorufin Research Articles

Interference with xenobiotic metabolic activity by the commonly used vehicle solvents dimethylsulfoxide and methanol in zebrafish (Danio rerio) larvae but not Daphnia magna

Organic solvents, such as dimethylsulfoxide (DMSO) and methanol are widely used as vehicles to solubilise lipophilic test compounds in toxicity testing. However, the effects of such solvents upon innate detoxification processes in aquatic organisms are poorly understood. This study assessed the effect of solvent exposure upon cytochrome P450 (CYP)-mediated xenobiotic metabolism in Daphnia magna and zebrafish larvae (4d post fertilisation). Adult D. magna were demonstrated to have a low, but detectable, metabolism of ethoxyresorufin in vivo and this activity was not modulated by pre-exposure to DMSO or methanol (24h, up to 0.1% and 0.05% v/v, respectively). In contrast, the metabolism of ethoxyresorufin in zebrafish larvae was significantly reduced by both solvents (0.1% and 0.05% v/v, respectively) after 24h of exposure. In zebrafish, these observed decreases in activity towards ethoxyresorufin were accompanied by decreased expression of a variety of genes coding for drug metabolising enzymes (corresponding to CYP1, CYP2, CYP3 and UDP-glucuronyl transferase [UGT] family enzymes), measured by quantitative PCR. Reduction of gene expression and CYP1 enzyme activities by methanol (0.05% v/v) in zebrafish larvae was partially reversed by co-exposure with Aroclor 1254 (100μgL−1). Overall this study suggests that relatively low concentrations of organic solvents can impact upon the biotransformation of certain xenobiotics in zebrafish larvae, and that this warrants consideration when assessing compounds for metabolism and toxicity in this species.

Defining the in Vivo Role for Cytochrome b5 in Cytochrome P450 Function through the Conditional Hepatic Deletion of Microsomal Cytochrome b5

In vitro, cytochrome b5 modulates the rate of cytochrome P450-dependent mono-oxygenation reactions. However, the role of this enzyme in determining drug pharmacokinetics in vivo and the consequential effects on drug absorption distribution, metabolism, excretion, and toxicity are unclear. In order to resolve this issue, we have carried out the conditional deletion of microsomal cytochrome b5 in the liver to create the hepatic microsomal cytochrome b5 null mouse. These mice develop and breed normally and have no overt phenotype. In vitro studies using a range of substrates for different P450 enzymes showed that in hepatic microsomal cytochrome b5 null NADH-mediated metabolism was essentially abolished for most substrates, and the NADPH-dependent metabolism of many substrates was reduced by 50–90%. This reduction in metabolism was also reflected in the in vivo elimination profiles of several drugs, including midazolam, metoprolol, and tolbutamide. In the case of chlorzoxazone, elimination was essentially unchanged. For some drugs, the pharmacokinetics were also markedly altered; for example, when administered orally, the maximum plasma concentration for midazolam was increased by 2.5-fold, and the clearance decreased by 3.6-fold in hepatic microsomal cytochrome b5 null mice. These data indicate that microsomal cytochrome b5 can play a major role in the in vivo metabolism of certain drugs and chemicals but in a P450- and substrate-dependent manner.

Transient induction of cytochromes P450 1A1 and 1B1 in MCF-7 human breast cancer cells by indirubin

The aryl hydrocarbon receptor (AhR), when activated by exogenous ligands such as 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD), regulates expression of several phase I and phase II enzymes and is also involved in the regulation of cell proliferation. Several studies suggest that endogenous AhR ligand(s) may exist. One putative endogenous ligand is indirubin, which was recently identified in human urine and bovine serum. We determined the effect of indirubin in MCF-7 breast cancer cells on induction of the activities of cytochromes P450 (CYP) 1A1 and 1B1, as measured by estradiol and ethoxyresorufin metabolism, and on induction of the CYP1A1 and CYP1B1 mRNAs. With 4-hr exposure, the effects of indirubin and TCDD at 10 nM on CYP activity were comparable, but the effects of indirubin, unlike those of TCDD, were transitory. Indirubin-induced ethoxyresorufin- O-deethylase activity was maximal by 6–9 hr post-exposure and had disappeared by 24 hr, whereas TCDD-induced activities remained elevated for at least 72 hr. The effects of indirubin on CYP mRNA induction were maximal at 3 hr. Indirubin was metabolized by microsomes containing cDNA-expressed human CYP1A1 or CYP1B1. The potency of indirubin was comparable to that of TCDD in a CYP1B1-promoter-driven luciferase assay, when MCF-7 cells were co-exposed to the AhR ligands together with the CYP inhibitor, ellipticine. Thus, if indirubin is an endogenous AhR ligand, then AhR-mediated signaling by indirubin is likely to be transient and tightly controlled by the ability of indirubin to induce CYP1A1 and CYP1B1, and hence its own metabolism.

Molecular requirements for inhibition of cytochrome p450 activities by roquefortine.

Roquefortine, a cyclopeptide derived from the diketopiperazine cyclo(Trp-dehydroHis), is a secondary metabolite produced by several Penicillium species. It has been reported to cause neurotoxic effect and to inhibit Gram-positive bacteria growth. The mechanisms responsible for its toxicity and metabolism are still unknown. In this study, we investigated the interaction of roquefortine with mammalian cytochromes P450. Roquefortine interaction with rat and human liver cytochromes P450 was monitored by difference UV-vis spectroscopy. It was found to interact with different forms of the cytochromes, giving rise to a type II difference spectrum, characteristic of the binding of an amino function to the heme iron. Roquefortine exhibited high affinity for microsomes from rats treated with various inducers, the K(s) values being in the range 0.2-8 microM. Similar results were observed with human P450 enzymes 1A1, 1A2, 2D6, and 3A4. Roquefortine had no effect on NAPDH cytochrome c reductase. Therefore, inhibition of NADPH consumption was observed using various rat liver microsomes alone or in the presence of 100 microM testosterone in the case of dexamethasone (DEX)-rat microsomes. Enzymatic inhibition was studied in terms of P450 3A activities, i.e., testosterone 6beta-hydroxylase (IC(50) around 10 microM) or bromocriptine metabolism (IC(50) > 50 microM) using DEX-rat liver microsomes or P450 3A4, benzphetamine N-demethylase using phenobarbital-rat liver microsomes (IC(50) > 30 microM), and ethoxyresorufin metabolism using 3-methylcholanthrene-rat liver microsomes (IC(50) 0.1 microM), P450 1A1, and 1A2. Roquefortine was compared with compounds of similar structure: cyclo(Phe-His), cyclo(Phe-dehydroHis), cyclo(Trp-His), phenylahistin. These studies indicate that the =N- imidazole moiety coordinates with the heme iron, and suggest that the dehydroHis moiety and the presence of a fused tetracycle play an important part in roquefortine inhibitory power.

Hepatic and pulmonary enzyme activities in horses.

To determine hepatic and pulmonary phase-I and phase-II enzyme activities in horses. Pulmonary and hepatic tissues from 22 horses that were 4 months to 32 years old. Pulmonary and hepatic tissues from horses were used to prepare cytosolic (glutathione S-transferase and soluble epoxide hydrolase) and microsomal (cytochrome P450 monooxygenases) enzymes. Rates of microsomal metabolism of ethoxyresorufin, pentoxyresorufin, and naphthalene were determined by high-performance liquid chromatography. Activities of glutathione S-transferase and soluble epoxide hydrolase were determined spectrophotometrically. Cytochrome P450 content was determined by carbon monoxide bound-difference spectrum of dithionite-reduced microsomes. Activity was expressed relative to total protein concentration. Microsomal protein and cytochromeP450 contents were detectable in all horses and did not vary with age. Hepatic ethoxyresorufin metabolism was detected in all horses; by comparison, pulmonary metabolism of ethoxyresorufin and hepatic and pulmonary metabolism of pentoxyresorufin were detected at lower rates. Rate of hepatic naphthalene metabolism remained constant with increasing age, whereas rate of pulmonary naphthalene metabolism was significantly lower in weanlings (ie, horses 4 to 6 months old), compared with adult horses. Hepatic glutathione S-transferase activity (cytosol) increased with age; however, these changes were not significant. Pulmonary glutathione S-transferase activity (cytosol) was significantly lower in weanlings than adult horses. Hepatic and pulmonary soluble epoxide hydrolase did not vary with age of horses. Activity of cytochrome P450 isoforms that metabolize naphthalene and glutathione S-transferases in lungs are significantly lower in weanlings than adult horses, which suggests reduced ability of young horses to metabolize xenobiotics by this organ.

Relationship between polychlorinated biphenyl 126 treatment and cytochrome p4501a activity in chickens, as measured by in vivo caffeine and ex vivo ethoxyresorufin metabolism

Cytochrome P4501A (CYP1A) activity is often used as a biomarker of exposure of wildlife to polyhalogenated diaromatic hydrocarbons (PHDHs) and is usually measured ex vivo in liver tissue. A caffeine breath test with radiolabelled substrate (14C-CBT) has been developed to measure in vivo avian CYP1A activity. Research goals were to develop stable isotope methods (13C-CBT), determine dose-response relationships between caffeine N-demethylation (CNDM) and PHDH exposure, and assess the relative utility of the CBT and ex vivo ethoxyresorufin-O-deethylase (EROD) assay. The 13C-CBT methods were developed with 20 chickens (Gallus domesticus). Chickens received three intraperitoneal injections of 0, 1, 5, or 50 μg 3,3′,4,4′,5-pentachloro-biphenyl (PCB 126)/kg body weight, and CNDM was quantified by measurement of 13CO2/12CO2 in expired breath. The 13C-CBT was not as sensitive or specific as the EROD assay as an indicator of PHDH exposure and effect in birds. Constitutive CNDM of great interindividual variability was observed, and the magnitude of induction was greater for EROD activity than for CNDM (approximately 1,000- and 2-fold, respectively). Variability associated with baseline 13CO2/12CO2 ratios in expired breath reduced the sensitivity of the 13C-CBT method.

Human CYP2B6

Human cytochrome (CYP)2B6 cDNA was cloned and expressed in bacteria and in yeast. Its expression in Saccharomyces cerevisiae enabled us to obtain, at a high level, an active yeast-expressed CYP2B6 protein, so as to assess its role in the metabolism of ethoxyresorufin, pentoxyresorufin, benzyloxyresorufin, ethoxycoumarin, testosterone and cyclophosphamide. Kinetic analysis showed that human CYP2B6 preferentially metabolized benzyloxyresorufin and pentoxyresorufin, although other CYPs also metabolized these substrates in human liver microsomes. CYP2B6 also manifested a strong 4-hydroxycyclophosphamide activity. Its expression in Escherichia coli enabled us to produce a very specific anti-human CYP2B6 antibody. No cross reactivity of this antibody was observed with CYPs1A1, 1A2, 3A4, 3A5, 2C8, 2C9, 2C18, 2C19, 2D6 or 2E1. This antibody enabled us to study the hepatic and extrahepatic expression of CYP2B6 in man, as well as its expression and inducibility in primary cultured human hepatocytes and in different human cell lines. Immunoblot analysis revealed that the CYP2B6 protein was expressed in 43 of the 48 human liver samples tested, with levels ranging from 0.4 to 8 pmol/mg of microsomal protein with a mean of 1.7 pmol/mg protein. CYP2B was also expressed in human brain, intestine and kidney, and at a lower level in the lung. CYP2B mRNA was detected in human liver, kidney, lung, trachea and intestine. We also found that CYP2B6 is induced at protein and mRNA levels by phenobarbital (2 mM) and cyclophosphamide (1 mM), an anticancer drug known to be metabolized by CYP2B6. No expression or inducibility of CYP2B6 was observed in any of the human cell lines tested.

Mixed-Function Oxidase Activity in Sepsis

Hepatic dysfunction is a major contributor to death in multiple organ system failure. To evaluate whether this dysfunction increases with the length of sepsis, we studied the effect of fulminant CLP peritonitis with hyperoxia on mixed-function oxidase-MFO (cytochrome P450 content and activity) and lipid peroxidation in rat livers. Livers were harvested at 18, 21, 24, and 27 hr, homogenized, and microsomal fractions prepared. Cytochrome P450 concentration was determined by assay and P450 activity was determined by the metabolism of ethoxyresorufin and ethoxycoumarin. Lipid peroxidation was estimated by measuring malondialdehyde content. Septic rats showed decreases in P450 levels and activity, which worsened with duration of sepsis. These decreases were partially lessened by hyperoxia. Although there was a trend toward increased lipid peroxidation, this effect was not statistically significant. This study suggests that while MFO content and activity decrease with sepsis, these decreases do not appear to be related to the production of oxygen-derived free radicals. Furthermore, hyperoxia actually appears to have a protective role in this instance.

The induction of alkoxyresorufin metabolism: A potential indicator of environmental contamination

Methods of biochemical monitoring of individual animals for exposure to environmental contaminants are of great potential use. The hepatic metabolism of various alkoxyresorufins, which are highly specific substrates for certain forms of cytochrome(s) P450, is highly induced by a variety of environmental contaminants. Thus, the O-dealkylation of pentoxy- or benzyloxyresorufin was induced greater than 20-fold in the rat by alpha-hexachlorocyclohexane, 2,4,5,2',4',5'-hexabromobiphenyl, DDT and Aroclor-1254, while the metabolism of ethoxyresorufin was highly induced by 5,6-benzoflavone, 3,4,5,3',4',5'-hexabromobiphenyl and Aroclor-1254. Additionally, rats exposed to diets containing as little as 12 ppm DDT displayed greater than five-fold increases in the rate of hepatic O-dealkylation of benzyloxyresorufin. Induction of the hepatic metabolism of these resorufin ethers in 9000 xg supernatant fractions taken from rats exposed to potential environmental contaminants may constitute a valuable diagnostic indicator of the presence of a variety of pollutants, including polycyclic aromatic hydrocarbons, organochlorine pesticides, polyhalogenated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin. These results suggest the potential applicability of these substrates in detecting chemical contamination in the environment.

Substrate specificity of the mouse skin mixedfunction oxidase system

The metabolism of nine model substrates for the mixed-function oxidase system was studied in skin and liver microsomes from Balb/C mice. Rates of skin metabolism per mg microsomal protein ranged from 0.5-15% of liver rates depending on the substrate. Relative to liver, mouse skin preferentially metabolized ethoxyresorufin, benzo[alpha]pyrene and diphenyloxazole over aldrin, coumarin and the C1-C4 7-alkyl umbelliferone ethers. NADPH-cytochrome P-450-dependent metabolism of aldrin and ethoxyresorufin was differentially inhibited in skin microsomes by metyrapone and alpha-naphthoflavone, respectively. Biphasic Eadie-Hofstee plots were obtained in both skin and liver microsomes for the metabolism of aldrin, whereas metabolism of ethoxyresorufin in both systems was described by linear kinetics. It is concluded that mouse skin contains multiple forms of cytochrome P-450, and that forms functionally analogous to those induced by polycyclic hydrocarbons in rodent liver are present in untreated mouse skin.

Induction of hepatic cytochrome p-450 in fish and the indication of environmental induction in scup ( Stenotomus chrysops)

The responses of liver microsomal monooxygenases in three teleost species, croaker ( Micropogon undulatus), rainbow trout ( Salmo gairdneri) and scup ( Stenotomus chrysops) to polynuclear aromatic hydrocarbon (PAH) induccrs were compared. In all three species PAH induced higher raies of ethoxyresorufin and/or benzo[ a]pyrene (B[ a]P) metabolism and levels of cytochrome P-450, but the species differed in some other responses. The induction of P-450 appears to be associated with the synthesis of new (or additional) microsomal protein at 57000 daltons in trout and 54000 daltons in scup, identifiable by SDS-polyacrylamide gel electrophoresis. Further da.a suggest that ethoxyresorufin metabolism in scup may be catalyzed by a minor form of P-450 distinct from the major induced protein. All scur examined thus far from natural populations exhibit properties of hepatic microsomes characteristically associated with the induced state. B[ a]P is metabolized by control and untreated scup with a high regiospecificity normally associated with the production of reactive and mutagenic metabolites. which may have implications for animals in the field.

The effect of albumin on the metabolism of ethoxyresorufin through O-deethylation and sulphate-conjugation using isolated rat hepatocytes

Ethoxyresorufin was metabolised by suspensions of isolated rat hepatocytes through O-deethylation to resorufin, followed by sulphate-conjugation of the resorufin. The deethylation but not the conjugation was greatly induced by 3-methylcholanthrene pretreatment in vivo. Induction altered the apparent deethylation V max but not the apparent K m value. With control hepatocytes there was a 3-fold difference between the apparent V max values for deethylation (0.07) and conjugation (0.22 nmole/min/10 6 hepatocytes) but none between the apparent K m values (1.3 μM, deethylation and 1.0 μM, conjugation). After induction the deethylation V max (10 nmole/min/10 6 hepatocytes) was almost 13-fold higher than the conjugation V max (0.81 nmole/min/10 6 hepatocytes), but again there was no difference in the K m values for the two reactions (2 μM). A significant proportion of the deethylation product, resorufin, passed out from the hepatocytes and then re-entered them in order to undergo conjugation. Extracellular bovine serum albumin inhibited the conjugation by binding resorufin that had left the hepatocytes. Albumin greatly increased the total resorufin formed from ethoxyresorufin, despite inhibiting very slightly the initial rate of deethylation.