The in Vivo Toxicity of CS2 to Liver Microsomes: Binding of Labelled CS2 and Changes of the Microsomal Enzyme Activities

Abstract The binding of ³⁵S and ¹⁴C labelled CS₂ to liver microsomes was studied in control and phenobarbitone pretreated rats 3 and 6 hrs after an intraperitoneal injection. The level of hepatic cytochrome P-450, the activities of epoxide hydratase and UDP-glucuronosyltransferase were analyzed in the same animals. The binding of the sulphur label was considerably higher than that of carbon 3 hrs after the injection, the difference being less evident at 6 hrs. The measurable P-450 declined after the CS₂ injection. It was approximately 40 % in the phenobarbitone pretreated rats and 60 % in control rats of the values of animals which were not treated with CS₂. CS₂ did not affect microsomal epoxide hydratase activity, while it increased the measurable activity of UDP-glucuronosyltransferase. The increase was evident 3 hrs after the injection of CS, in the phenobarbitone pretreated rats. It could also be detected in the control animals 6 hrs after the injection. The present data suggest that the change in the measurable P-450 results from the binding of the metabolite(s) of CS₂ to the cytochrome, and its subsequent degradation. The increase in measurable UDP-glucuronosyltransferase activity results probably from the activated perturbation of the structure of microsomal membrane by the metabolites of CS₂ in vivo.     

On the Aromatic Hydroxylation of Amphetamine in Rat Liver Microsomes and Perfused Liver Preparations: Effects of Long-term Administration

Abstract The liver microsomal p-hydroxylation of amphetamine to parahydr-oxyamphetamine (pOHA) was dependent on NADP and inhibited by carbon monoxide indicating the involvement of cytochrome P-450. SKF 525-A, fenfluramine and desmethylimipramine were the most effective inhibitors of this pathway of amphetamine metabolism. Repeated administration of phenobarbital resulted in reduced p-hydroxylation of amphetamine in vitro. Chronic administration of amphetamine reduced the microsomal p-hydroxylation of amphetamine without apparent changes in the cytochrome P-450 levels or in the activity of NADPH-cytochrome c reductase. The aromatic hydroxylation of aniline and the demethylation of ethylmorphine was not affected by this treatment. However, the 455 nm complex formed during the microsomal metabolism of N-hydroxy-amphetamine was increased by the long-term administration of amphetamine. These results indicate some pecularities of the in vitro hydroxylation of amphetamine by rat liver microsomes. Amphetamine disappeared from the perfusate of the perfused liver at the same rate in rats given a single dose of amphetamine and in rats given amphetamine orally for four weeks. The excretion of pOHA and its conjugate increased at 60 and 90 min. and 30, 60 and 90 min. respectively in the perfusate of the same experiment as compared to the controls. The total excretion of radioactive amphetamine metabolites at the end of the perfusion was increased in the perfusate and reduced in the bile compared to the control experiment.     

Stimulation of calcium uptake of muscle microsomes by phenothiazines and barbiturates

The calcium pump of sacroplasmic reticulum of rabbit and frog is stimulated by phenothiazines in the concentration range 10–100 μM, and by barbiturates in the concentration range 0.5–4.0 mM at 25°C. When potassium, which is an intracellular activator of the calcium pump, is incorporated in the medium in 100 mM concentration the phenothiazines, and less so the barbiturates, reduced the rate of calcium uptake. Chlorpromazine sulphoxide, a pharmacologically inactive metabolite of chlorpromazine, did not activate in the absence of potassium nor inhibit in its presence. Barbituric acid also was without effect. When the “extra” AtPase was measured during chlorpromazine-stimulated calcium uptake no increase in efficiency of the system was detected.     

Relationship between polymorphisms of genes encoding microsomal epoxide hydrolase and glutathione S-transferase P1 and chronic obstructive pulmonary disease

Background Cigarette smoking is the major risk factor for chronic obstructive pulmonary disease (COPD). However, only 10% -20% of chronic heavy cigarette smokers develop symptomatic disease. COPD is most likely the result of complex interactions between environmental and genetic factors. Genetic susceptibility to COPD might depend on the variations in enzyme activities that detoxify cigarette smoke products, such as microsomal epoxide hydrolase (mEH) and glutathione Stransferase (GST). In this study, we investigated the relationship between polymorphisms in the genes encoding mEH and glutathione S-transferase P1 (GSTP1) and COPD in a Chinese population.Methods Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was performed to find mEH polymorphism in exon 3 (Tyr113→His), exon 4 (His139→Arg) and GSTP1 polymorphism in exon 5 (Ile105→Val) in 100 COPD patients and 100 age- and sex-matched healthy controls.Results The proportion of mEH exon 3 heterozygotes was significantly higher in patients with COPD than that in the control subjects (42% vs 32% ). The odds ratio (OR) adjusted by age, sex, body mass index (BMI) and cigarette years was 2.96 (95% CI 1.24 - 7. 09). There was no marked difference in very slow activity genotype versus other genotypes between COPD patients and the controls. When COPD patients were non-smokers, the OR of very slow activity genotype versus other genotypes was more than 1.00; and when COPD patients were smokers ( current smokers and exsmokers), the OR was less than 1.00. There was no significant difference in GSTP1 polymorphism adjusted by age, sex, BMI and smoking between COPD patients and the controls.Conclusions mEH exon 3 heterozygotes might be associated with susceptibility to COPD in China.The interaction might exist between mEH genotype and smoke. The gene polymorphism for GSTP1 might not be associated with susceptibility to COPD in the Chinese population.     

Evidence that unsaturated fatty acids are potent inhibitors of renal UDP-glucuronosyltransferases (UGT): kinetic studies using human kidney cortical microsomes and recombinant UGT1A9 and UGT2B7

Renal ischaemia is associated with accumulation of fatty acids (FA) and mobilisation of arachidonic acid (AA). Given the capacity of UDP-glucuronosyltransferase (UGT) isoforms to metabolise both drugs and FA, we hypothesised that FA would inhibit renal drug glucuronidation. The effect of FA (C2:0-C20:5) on 4-methylumbelliferone (4-MU) glucuronidation was investigated using human kidney cortical microsomes (HKCM) and recombinant UGT1A9 and UGT2B7 as the enzyme sources. 4-MU glucuronidation exhibited Michaelis-Menten kinetics with HKCM (apparent K(m) (K(m)(app)) 20.3 microM), weak substrate inhibition with UGT1A9 (K(m)(app) 10.2 microM, K(si) 289.6 microM), and sigmoid kinetics with UGT2B7 (S(50)(app)440.6 microM) Similarly, biphasic UDP-glucuronic acid (UDPGA) kinetics were observed with HKCM (S(50) 354.3 microM) and UGT1A9 (S(50) 88.2 microM). In contrast, the Michaelis-Menten kinetics for UDPGA observed with UGT2B7 (K(m)(app) 493.2 microM) suggested that kinetic interactions with UGTs were specific to the xenobiotic substrate and the co-substrate (UDPGA). FA (C16:1-C20:5) significantly inhibited (25-93%) HKCM, UGT1A9 or UGT2B7 catalysed 4-MU glucuronidation. Although linoleic acid (LA) and AA were both competitive inhibitors of 4-MU glucuronidation by HKCM (K(i)(app) 6.34 and 0.15 microM, respectively), only LA was a competitive inhibitor of UGT1A9 (K(i)(app) 4.06 microM). In contrast, inhibition of UGT1A9 by AA exhibited atypical kinetics. These data indicate that LA and AA are potent inhibitors of 4-MU glucuronidation catalysed by human kidney UGTs and recombinant UGT1A9 and UGT2B7. It is conceivable therefore that during periods of renal ischaemia FA may impair renal drug glucuronidation thus compromising the protective capacity of the kidney against drug-induced nephrotoxicity.     

Liver microsomal biotransformation of nitro-aryl drugs: mechanism for potential oxidative stress induction

Toxic effects of several nitro-aryl drugs are attributed to the nitro-reduction that may be suffered in vivo, a reaction that may be catalysed by different reductases. One of these enzymes is NADPH-cytochrome P450 reductase, which belongs to the cytochrome P450 oxidative system mainly localized in the endoplasmic reticulum of the hepatic cell. This system is responsible for the biotransformation of oxidative lipophilic compounds, so that oxidative and reductive metabolic pathways of lipophilic nitro-aryl drugs can take place simultaneously. Because of the affinity of nitro-aryl drugs (xenobiotics) for the endoplasmic reticulum, we propose this subcellular organelle as a good biological system for investigating the toxicity induced by the biotransformation of these or another compounds.In this work we used rat liver microsomes to assess the oxidative stress induced by nitro-aryl drug biotransformation. Incubation of microsomes of rat liver with nifurtimox and nitrofurantoin in the presence of NADPH induced lipoperoxidation, UDP-glucuronyltransferase activation and an increase in the basal microsomal oxygen consumption. Nitro-aryl-1,4-dihydropyridines did not elicit these prooxidant effects; furthermore, they inhibited lipoperoxidation and oxygen consumption induced by Fe3+/ascorbate. Nifurtimox and nitrofurantoin modified the maximum absorption of cytochrome P450 oxidase and inhibited p-nitroanisole O-demethylation, an oxidative reaction catalysed by the cytochrome P450 system, signifying that oxidation may proceed in a similar way to that described for nitro-aryl-1,4-dihydropyridines. Thus the balance between lipophilic nitro-aryl drug oxidation and reduction may be involved in the potential oxidative stress induced by biotransformation.     

Identification of cytochrome P450 isoforms involved in the metabolism of loperamide in human liver microsomes

Objective: The purpose of the present study was to elucidate the cytochrome P450 (P450) isoform(s) involved in the metabolism of loperamide (LOP) to N-demethylated LOP (DLOP) in human liver microsomes. Methods: Three established approaches were used to identify the P450 isoforms responsible for LOP N-demethylation using human liver microsomes and cDNA-expressed P450 isoforms: (1) correlation of LOP N-demethylation activity with marker P450 activities in a panel of human liver microsomes, (2) inhibition of enzyme activity by P450-selective inhibitors, and (3) measurement of DLOP formation by cDNA-expressed P450 isoforms. The relative contribution of P450 isoforms involved in LOP N-demethylation in human liver microsomes were estimated by applying relative activity factor (RAF) values. Results: The formation rate of DLOP showed biphasic kinetics, suggesting the involvement of multiple P450 isoforms. Apparent K_m and V_max values were 21.1 M and 122.3 pmol/min per milligram of protein for the high-affinity component and 83.9 M and 412.0 pmol/min per milligram of protein for the low-affinity component, respectively. Of the cDNA-expressed P450 s tested, CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyzed LOP N-demethylation. LOP N-demethylation was significantly inhibited when coincubated with quercetin (a CYP2C8 inhibitor) and ketoconazole (a CYP3A4 inhibitor) by 40 and 90%, respectively, but other chemical inhibitors tested showed weak or no significant inhibition. DLOP formation was highly correlated with CYP3A4-catalyzed midazolam 1-hydroxylation (r_s=0.829; P<0.01), CYP2B6-catalzyed 7-ethoxy-4-trifluoromethylcoumarin O-deethylation (r_s=0.691; P<0.05), and CYP2C8-catalyzed paclitaxel 6-hydroxylation (r_s=0.797; P<0.05). Conclusion: CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyze LOP N-demethylation in human liver microsomes, and among them, CYP2C8 and CYP3A4 may play a crucial role in LOP metabolism at the therapeutic concentrations of LOP. Coadministration of these P450 inhibitors may cause drug interactions with LOP. However, the clinical significance of potential interaction of LOP metabolism by CYP2C8 and CYP3A4 inhibitors should be studied further.     

Paclitaxel metabolism in rat and human liver microsomes is inhibited by phenolic antioxidants

Paclitaxel is an important, recently introduced anti-neoplastic drug. Paclitaxel metabolites are virtually inactive in comparison with the parent drug. The study investigated whether phenolic antioxidants could inhibit metabolic inactivation sufficiently to increase paclitaxel effects. Cytochrome P450 (CYP)-catalysed metabolism of paclitaxel was investigated in rat and human liver microsomes. In rat microsomes, paclitaxel was metabolised mainly to C3'-hydroxypaclitaxel (C3'-OHP), less to C2-hydroxypaclitaxel (C2-OHP), di-hydroxypaclitaxel (di-OHP) and another monohydroxylated paclitaxel. In human liver microsomes, 6-hydroxypaclitaxel (6-OHP), formed by CYP2C8, was the main metabolite, while C3'-OHP, C2-OHP and another product different from di-OHP were minor metabolites, formed by CYP3A4. In individual human livers 6-OHP was formed at 1.8-fold to 13-fold higher rates than C3'-OHP. Kinetic parameters (K_m and V_max) of production of various metabolites in rat and human liver microsomes revealed differences between species as well as human individual differences. Nine phenolic antioxidants ((+)-catechin, (-)-epicatechin, fisetin, gallic acid, morin, myricetin, naringenin, quercetin and resveratrol) were tested for inhibition of paclitaxel metabolism. In rat microsomes, resveratrol was more inhibitory than fisetin; the other phenolic antioxidants were without effect. In human microsomes, the inhibiting potency decreased in the order fisetin >quercetin >morin >resveratrol, while the other phenolic antioxidants were not inhibitory; the formation of 6-OHP (CYP2C8) was generally more inhibited than that of C3'-OHP. The inhibition was mostly mixed-type. The results suggest that oral administration of some phenolic substances might increase paclitaxel blood concentrations during chemotherapy.     

在大鼠肝微粒体中反式曲马朵代谢及反式氧去甲基曲马朵生成的立体选择性(英文)

目的:研究反式曲马朵[(±)一trans-T]代谢及反式氧去甲基曲马朵(M1)生成的立体选择性,方法:(±)-trans-T及其对映体分别与大鼠肝微粒体孵育,高效毛细管电泳法测定孵育液中(±)-trans-T及M1对映体的浓度。结果:以(±)-trans-T单一对映体为底物孵育时,(+)-trans-T的代谢速率较低,(+)-M1生成有较低的V_(max)和CL_(int).以(±)-trans-T消旋体为底物孵育时,(±)-trans-T对映体的代谢速率及(±)-M1对映体的生成速率不同程度地减慢。右美沙芬、普罗帕酮和氟西汀既能抑制(±)-trans-T的代谢,又能抑制M1的生成;普罗帕酮和氟西汀能增强(±)-trans-T代谢及M1生成的立体选择性,右美沙芬仅使M1生成的立体选择性增强。结论:在大鼠肝微粒体中,(±)-trans-T代谢及M1生成有立体选择;(±)-trans-T对映体间存在相互作用。右美沙芬、普罗帕酮及氟西汀对它们的立体选择性产生不同的影响。     

Effects of anticancer drugs on the metabolism of the anticancer drug 5,6-dimethylxanthenone-4-acetic (DMXAA) by human liver microsomes

AIMS: To investigate the effects of various anticancer drugs on the major metabolic pathways (glucuronidation and 6-methylhydroxylation) of DMXAA in human liver microsomes. METHODS: The effects of various anticancer drugs at 100 and 500 microM on the formation of DMXAA acyl glucuronide (DMXAA-G) and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid (6-OH-MXAA) in human liver microsomes were determined by high performance liquid chromatography (h.p.l.c.). For those anticancer drugs showing significant inhibition of DMXAA metabolism, the inhibition constants (Ki) were determined. The resulting in vitro data were extrapolated to predict in vivo changes in DMXAA pharmacokinetics. RESULTS: Vinblastine, vincristine and amsacrine at 500 microM significantly (P < 0.05) inhibited DMXAA glucuronidation (Ki = 319, 350 and 230 microM, respectively), but not 6-methylhydroxylation in human liver microsomes. Daunorubicin and N-[2-(dimethylamino)-ethyl]acridine-4-carboxamide (DACA) at 100 and 500 microM showed significant (P < 0.05) inhibition of DMXAA 6-methylhydroxylation (Ki = 131 and 0.59 microM, respectively), but not glucuronidation. Other drugs such as 5-fluoroucacil, paclitaxel, tirapazamine and methotrexate exhibited little or negligible inhibition of the metabolism of DMXAA. Pre-incubation of microsomes with the anticancer drugs (100 and 500 microM) did not enhance their inhibitory effects on DMXAA metabolism. Prediction of DMXAA-drug interactions in vivo based on these in vitro data indicated that all the anticancer drugs investigated except DACA appear unlikely to alter the pharmacokinetics of DMXAA, whereas DACA may increase the plasma AUC of DMXAA by 6%. CONCLUSIONS: These results indicate that alteration of the pharmacokinetics of DMXAA appears unlikely when used in combination with other common anticancer drugs. However, this does not rule out the possibility of pharmacokinetic interactions with other drugs used concurrently with this combination of anticancer drugs.