Physiological role of the N-terminal processed P4501A1 targeted to mitochondria in erythromycin metabolism and reversal of erythromycin-mediated inhibition of mitochondrial protein synthesis.
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Recently, we showed that the major species of beta-naphthoflavone-inducible rat liver mitochondrial P450MT2 consists of N-terminal truncated microsomal P4501A1 (+33/1A1) and that the truncated enzyme exhibits different substrate specificity as compared with intact P4501A1. The results of the present study show that P450MT2 targeted to COS cell mitochondria by transient transfection of P4501A1 cDNA is localized inside the mitochondrial inner membrane in a membrane-extrinsic orientation. Co-expression with wild type P4501A1 and adrenodoxin (Adx) cDNAs resulted in 5-7-fold higher erythromycin N-demethylation (ERND) in the mitochondrial fraction but minimal changes in the microsomal fraction of transfected cells. Erythromycin, a potent inhibitor of bacterial and mitochondrial protein synthesis, caused 8-12-fold higher accumulation of CYP1A1 mRNA, preferential accumulation of P450MT2, and 5-6-fold higher ERND activity in the mitochondrial compartment of rat C6 glioma cells. Consistent with the increased mitochondrial ERND activity, co-expression with P4501A1 and Adx in COS cells rendered complete protection against erythromycin-mediated mitochondrial translation inhibition. Mutations that specifically affect the mitochondrial targeting of P4501A1 also abolished protection against mitochondrial translation inhibition. These results for the first time suggest a physiological function for the xenobiotic inducible cytochrome P4501A1 against drug-mediated mitochondrial toxicity. (+info)
7-ethoxycoumarin deethylation activity in perfused isolated rat brain.
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7-ethoxycoumarin (7-EC) deethylation activity was measured in the perfused rat brain in situ. Infusion of 7-EC into a brain through an internal carotid artery resulted in the formation of 7-hydroxycoumarin (7-HC) and its conjugates in the effluent perfusate collected from the superior vena cava. The rate of formation of products was 200 nmol/h/g when 130 microM 7-EC was infused. This value was much higher (more than 100 times) than that determined from the brain microsomal activity ( approximately 1 nmol/h/g), indicating that the activity determined with microsomes was an underestimate. This value was comparable to the activity in the perfused liver (30-50%), suggesting that drug metabolizing enzymes can play important roles within the brain. Pretreatment of rats with P-450 inducers such as phenobarbital and beta-naphthoflavone increased the deethylation activity in the perfused brain, as in the perfused liver. We conclude that the perfused brain is suitable for evaluating drug metabolizing activities under physiological conditions. (+info)
Effect of cryopreservation on cytochrome P-450 enzyme induction in cultured rat hepatocytes.
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In the present study, we evaluated the inducibility of cytochrome P-450 (CYP) CYP1A, CYP2B, CYP3A, and CYP4A by beta-naphthoflavone, phenobarbital, dexamethasone, and clofibric acid, respectively, in primary hepatocyte cultures prepared from both fresh and cryopreserved rat hepatocytes. Rat hepatocytes were successfully thawed and cultured after cryopreservation in liquid nitrogen for up to 1 month. Percentage of total recovery, viable cell recovery, and final viability of the cells were 68%, 72%, and 85%, respectively. Regardless of whether they were cryopreserved or not, cultured hepatocytes exhibited near-normal morphology. Treatment of cryopreserved hepatocytes with beta-naphthoflavone caused an 8-fold increase in 7-ethoxyresorufin O-dealkylase (CYP1A1/2) activity, with an EC50 of 1.5 microM; treatment with phenobarbital caused a 26-fold increase in 7-pentoxyresorufin O-dealkylase (CYP2B1/2) activity, with an EC50 of 10 microM; treatment with dexamethasone caused a 10-fold increase in testosterone 6beta-hydroxylase (CYP3A1/2) activity, with an EC50 of 1.3 microM, whereas treatment with clofibric acid caused a 3-fold increase in lauric acid 12-hydroxylase (CYP4A1-3) activity, with an EC50 of 170 microM. The induction of CYP1A, CYP2B, CYP3A, and CYP4A enzymes by these inducers was confirmed by Western immunoblotting. The patterns of P-450 induction in cryopreserved rat hepatocytes, in terms of concentration response, reproducibility, magnitude, and specificity of response, were similar to those observed in freshly isolated hepatocytes. Additionally, the magnitude and specificity of induction was similar to that observed in vivo in rats. In conclusion, under the conditions examined, cryopreserved rat hepatocytes appear to be a suitable in vitro system for evaluating xenobiotics as inducers of P-450 enzymes. (+info)
Kinetics of drug metabolism in rat liver slices: IV. Comparison of ethoxycoumarin clearance by liver slices, isolated hepatocytes, and hepatic microsomes from rats pretreated with known modifiers of cytochrome P-450 activity.
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To evaluate the theory that within precision-cut liver slices intercellular transport occurs in parallel with cellular metabolism and to illustrate the constraints this places on clearance predictions, the kinetics of ethoxycoumarin O-deethylation have been determined under varying conditions of hepatic cytochrome P-450 activity. Liver slices, isolated hepatocytes, and microsomes were obtained from rats treated with the inducers phenobarbital (PB) and beta-naphthoflavone (betaNF) and the inhibitor aminobenzotriazole (ABT). In hepatocytes and microsomes, a two-site kinetic model with a high-affinity, low-capacity site and an unsaturated low-affinity, high-capacity site described the hydroxycoumarin formation data. There were marked increases in Vmax (2- to 5-fold and 50- to 70-fold for PB and betaNF, respectively) in both systems and in CLint (3- and 9-fold for PB and betaNF, respectively) in hepatocytes and substantial decreases in both parameters (3-8 and 12-23% of control, respectively) in ABT hepatocytes and microsomes. A qualitatively similar response was evident in slices obtained from livers of rats treated with phenobarbital and ABT, but although slices from betaNF livers produced high metabolic rates (comparable to slices obtained from livers of rats treated with phenobarbital), these showed a linear increase with substrate concentration without indication of a high-affinity site. The intrinsic clearance parameters were scaled to full liver capacity using hepatocellularities and microsomal recovery indices to allow direct comparison of these responses. The slice system consistently underestimated the effects of the modifiers. When compared with hepatocytes, estimates of 30, 15, and 1% for ABT, PB, and betaNF, respectively, were observed and the degree of underestimation was dependent on the magnitude of intrinsic clearance and was consistent with the above theory. (+info)
In vitro sulfoxidation of aldicarb by hepatic microsomes of channel catfish, Ictalurus punctatus.
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The carbamate pesticide, aldicarb, demonstrates significant acute toxicity in mammals, birds, and fish, and is readily biotransformed by most organisms studied. Metabolic products of aldicarb include the more toxic sulfoxide and the less toxic sulfone as two of the major products. Both the cytochrome P450 (CYP) and the flavin monooxygenase systems (FMO) are involved in this process. This study examined the capacities of liver microsomes of male channel catfish (Ictalurus punctatus), which lack FMO, to biotransform aldicarb in vitro. In addition, the acetylcholinesterase inhibitory potencies of aldicarb and its sulfoxide and sulfone derivatives were determined. For metabolism studies, incubations of [14C]-aldicarb (0.1mM) were carried out for up to 15-90 min using 1.0 mg/mL of hepatic microsomal protein. Total NADPH- dependent biotransformation was low (< 3.0% conversion to polar metabolites), and was inhibited by carbon monoxide. The only metabolite detected was aldicarb sulfoxide (Kmapp = 53.8 +/- 25.3 microM; Vmaxapp = 0.040 +/- 0.007 nmol/min/mg). Treatment of fish with the CYP modulators beta-naphthoflavone (BNF, 50 mg/kg) and ethanol (EtOH, 1.0% aqueous) had no effect on sulfoxide production. No correlation existed between CYP isoform expression (determined by western blot) and aldicarb sulfoxidation rates, suggesting the involvement of an unmeasured CYP isoform or involvement of several isoforms with low specificity. This study indicates that a low rate of bioactivation of aldicarb to aldicarb sulfoxide may be responsible for the resistance of channel catfish to aldicarb toxicity relative to that of other piscine species. (+info)
Alternatives in the induction and preparation of phenobarbital/naphthoflavone-induced S9 and their activation profiles.
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With the aim of optimizing the efficiency of S9 fractions used in in vitro mutagenicity assays, different schemes for the induction of liver enzymes in rats were tried and the amount of S9 fraction required was assessed. The activity of 2-anthramine (2AA), 2-acetylaminofluorene (2AAF), 3-methylcholanthrene (3MTCL) and benzo[a]pyrene in bacterial mutagenicity tests was compared with the enzymatic activity in S9 fractions obtained from rats treated with either phenobarbital (NaPB), beta-naphthoflavone (betaNF) or combinations of both. Three pool systems prepared with different amounts of NaPB-induced S9 and betaNF-induced S9 were also analyzed for their activation capacities. Profiles of standard plate incorporation assays with Salmonella typhimurium TA98 increased with the amount of S9 fraction added for all drugs tested, except for 2AA, which showed a maximun of activity at low protein concentrations. According to these profiles, an optimal S9 protein content of 700-1000 microg/plate was estimated. For 2AAF and 3MTCL an S9 fraction obtained following a simultaneous treatment with NaPB (i.p.) and betaNF (oral gavage) (NaPB + betaNF) yielded the greatest response. This preparation was the only one which produced positive activation with 3MTCL as test drug. With the other test drugs all the S9 fractions were very active, including the NAPB + betaNF-induced S9. Both Phase I and Phase II cytochrome P450 enzymatic activities were enhanced in this S9 fraction. These results suggest that the simultaneous treatment (NaPB + betaNF) would be an adequate inducer for in vitro activation when used at 700-1000 microg protein/plate. (+info)
An expressional system of human cytochrome P-450 CYP1A1 gene transcription.
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AIM: To explore an expressional system of human cytochrome P-450 CYP1A1 (CYP1A1) gene transcription. METHODS: The plasmid pMC 6.3 K containing human CYP1A1 promoter was transiently transfected into Hep G2 cells. The expression of chloramphenical acetyltransferase (CAT) reporter gene was detected by ELISA. RESULTS: Both the CAT expression and CYP1A1 activity increased with the concentrations of beta-naphthoflavone from 2.5 to 10 mumol.L-1. At 10 mumol.L-1 of beta-naphthoflavone, the levels of CAT and CYP1A1 were 94-fold and 2.8-fold those of the corresponding control, respectively. Using this method, the study of 8 glucosinolates with various side chains on the induction of CYP1A1 gene transcription showed that none of the parent glucosinolates increased CAT expression, whereas the breakdown products of indol-3-yl-methyl glucosinolate (glucobrassicin), rather than indole-3-carbinol, increased the CAT expression. CONCLUSION: The CYP1A1 gene transcriptional system was more reliable and sensitive. (+info)
Cigarette smoke induces direct DNA damage in the human B-lymphoid cell line Raji.
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Human lymphoid cells (Raji) were exposed to water-soluble compounds from cigarette smoke (CS) generated in a smoking machine. DNA damage, as detected by alkaline single-cell microelectrophoresis (COMET assay), was induced in a time- and concentration-dependent manner in the cells. Most of the rapidly induced DNA damage was attributable to direct-acting compounds since cytochrome P450-related metabolic activities (ethoxy- and pentoxyresorufin-O-deethylases and coumarin-7-hydroxylase) were absent or very low. In addition, induction of DNA damage could be inhibited only slightly by beta-naphthoflavone and coumarin. Vitamin C enhanced DNA damage in Raji cells probably by redox cycling of catechol and hydroquinone present in CS implicating reactive oxygen intermediates as another source of DNA damage. N-acetylcysteine, a radical scavenger and glutathione precursor, reduced DNA damage in Raji cells when exposure to CS was followed by 2 h post-incubation in culture medium. Unrepaired DNA damage caused by CS persisted longer than gamma-irradiation-induced DNA damage. Among the CS constituents, acrolein, but not formaldehyde and acetaldehyde, induced DNA damage although less intensely than CS itself. At 50 and 100 microM concentrations, acrolein also inhibited repair of gamma- irradiation-induced DNA damage in the COMET assay. Inhibition of DNA synthesis by acrolein at 50 microM was demonstrated by an immunochemical assay for bromo-deoxyuridine incorporation; however, inhibition of a representative repair enzyme, 8-oxoguanosine hydrolase, by either CS or acrolein was not observed. The present results further confirm the presence of direct-acting genotoxic components and inhibitors of DNA repair in the gas phase of tobacco smoke, that may contribute to DNA damage and smoking-associated cancers of the upper aerodigestive tract. (+info)