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R A Lubet

Publications and source records attributed to R A Lubet.

At least 19 recordsLinked to original sources
PubMed1992

Induction of a pleiotropic response by phenobarbital and related compounds. Response in various inbred strains of rats, response in various species and the induction of aldehyde dehydrogenase in Copenhagen rats.

The ability of phenobarbital (PB) to induce a "pleiotropic response" which includes both cytochromes P450 (CYP) as well as other drug-metabolizing enzymes was investigated in mice, rabbits, hamsters, and various inbred strains of rats. PB induced similar drug-metabolizing enzymes (CYP2B, CYP3A, and epoxide hydrolase) in rats, mice, rabbits and hamsters. PB and two structural analogues (ethylphenylhydantoin and barbital) induced a variety of drug-metabolizing enzymes (CYP2B, CYP3A, CYP2A, epoxide hydrolase) in a series of inbred strains of rats. In contrast, levels of aldehyde dehydrogenase (ALDH) (propionaldehyde, NAD+) which were expressed constitutively in all strains of rats were induced by PB in only two of the eight strains (ACI, Copenhagen). Further investigations of ALDH induction by structurally diverse compounds in Copenhagen rats demonstrated a strong correlation between the induction of ALDH and other elements of the pleiotropic response (CYP2B, CYP3A, epoxide hydrolase). These results imply that induction of ALDH (propionaldehyde, NAD+) is associated with the PB pleiotropic response in Copenhagen rats.

Aldehyde Dehydrogenase↗
PubMed1992

Two human liver cDNAs encode UDP-glucuronosyltransferases with 2 log differences in activity toward parallel substrates including hyodeoxycholic acid and certain estrogen derivatives.

Two human liver UDP-glucuronosyltransferase cDNA clones, HLUG25 [Jackson, M. R., et al. (1987) Biochem. J. 242, 581-588] and UDPGTh-2 [Ritter, J. K., et al. (1990) J. Biol. Chem. 266, 7900-7906] have previously been shown to encode isozymes active in the glucuronidation of hyodeoxycholic acid (HDCA) and certain estrogen derivatives (estriols and 3,4-catechol estrogens), respectively. Here we report that the UDPGTh-2-encoded isoform (udpgth-2) and the HLUG25-encoded isoform (udpgth-1) have parallel aglycon specificities. Following expression in COS-1 cells, each isoform metabolized three types of dihydroxy- or trihydroxy-substituted ring structures, including the 3,4-catechol estrogen (4-hydroxyestrone), estriol and 17-epiestriol, and HDCA, but the udpgth-2 isozyme is 100-fold more efficient than udpgth-1. udpgth-1 and udpgth-2 are 86% identical overall (76 differences out of 528 amino acids), including 55 differences in the first 300 amino acids of the amino terminus, a domain which confers isoform substrate specificity. The data indicate that a high level of conservation in the amino terminus is not required for the preservation of substrate selectivity. Analysis of glucuronidation activity encoded by UDPGTh-1/UDPGTh-2 chimeric cDNAs constructed at their common restriction sites, SacI (codon 297), NcoI (codon 385), and HhaI (codon 469), showed that nine amino acids between residues 385 and 469 are important for catalytic efficiency, suggesting that this region represents a domain which is critical for catalysis but distinct from that responsible for aglycon selection.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗
PubMed1992

A pleiotropic response to phenobarbital-type enzyme inducers in the F344/NCr rat. Effects of chemicals of varied structure.

The effects of a number of phenobarbital-type inducers on selected drug-metabolizing enzymes in male F344/NCr rats were determined by measuring specific catalytic activities and/or by measuring the levels of RNA which hybridize with specific probes for the corresponding genes. The effects on hepatic CYP2B1 were assessed by measuring the levels of CYP2B1-specific RNA and benzyloxyresorufin O-dealkylase and testosterone 16 beta-hydroxylase activities. Levels of CYP3A were monitored by measuring the rate of hydroxylation of testosterone at the 6 beta-position. Microsomal epoxide hydrolase activity was determined by measurement of cellular RNA specific for this form and by assaying the hydrolysis of benzo[a]pyrene-4,5-oxide. UDP-glucuronyltransferase activity was assayed by measuring the glucuronidation of 3-hydroxybenz[a]anthracene. Levels of glutathione S-transferase Ya/Yc were measured by quantifying total cellular RNA coding for the proteins. When male F344/NCr rats were administered various doses of phenobarbital or dichlorodiphenyltrichloroethane (DDT), strong correlations between the induction of CYP2B1 and the induction of epoxide hydrolase or UDP-glucuronyltransferase activities were observed. Treatment of rats with barbiturates, hydantoins, halogenated pesticides such as DDT or alpha-hexachlorocyclohexane, 2,4,5,2',4',5'-hexachlorobiphenyl, CYP2B1 inhibitors such as clotrimazole or clonazepam, or such structurally-diverse compounds as 2-hexanone or diallyl sulfide resulted in induction of CYP2B1-mediated enzyme activity and induction of certain other forms of cytochrome P450, microsomal epoxide hydrolase, at least one form of UDP-glucuronyltransferase, and multiple forms of glutathione S-transferase. This suggests that, as a class, compounds which induce CYP2B1 also induce a coordinate hepatic pleiotropic response which includes induction of these other phase I and phase II drug-metabolizing enzymes.

Animals↗
PubMed1992

A markedly diminished pleiotropic response to phenobarbital and structurally-related xenobiotics in Zucker rats in comparison with F344/NCr or DA rats.

Phenobarbital (PB) and certain structurally-related compounds induce a variety of hepatic drug-metabolizing enzymes in many strains of rats. Thus, following administration of PB (300, 500 ppm), barbital (BB, 1500 ppm) or 5-ethyl-5-phenylhydantoin (EPH, 500 ppm), CYP2B1-mediated benzyloxyresorufin O-dealkylase activity and epoxide hydrolase activity were profoundly induced in female DA and F344/NCr rats. In contrast, outbred female lean and obese Zucker rats showed markedly reduced CYP2B1 responses (less than 15% and less than 5% of those observed in the female DA or F344/NCr rat) to PB (doses less than or equal to 300 ppm), BB (1500 ppm) or EPH (500 ppm). In parallel studies, profound increases in RNA levels coding for CYP2B1, glutathione S-transferases Ya/Yc (alpha subclass), or epoxide hydrolase were detected in the female F344/NCr rat following treatment with PB (300 ppm), BB (1500 ppm) or EPH (500 ppm). In contrast, lean Zucker rats showed a strong response only to the highest dose of PB (500 ppm), implying that the diminished response in the Zucker rats may occur at some pretranslational level. Similar studies with lower doses of PB, EPH or BB in male lean Zucker rats showed a decreased response, relative to that in male F344/NCr rats. However, this insensitivity was not as profound as that observed in the female Zucker rats. In fact, the response to PB-type inducers in male or female Zucker rats is probably most clearly explained as a shift of the dose-response curve sharply to the right (decreased responsiveness, compared to F344/NCr or DA rats of the same sex). This decreased responsiveness of female lean Zucker rats to induction of CYP2B1, relative to that of F344/NCr rats, was also observed with the structurally-diverse PB-type inducers clonazepam, clotrimazole and 2-hexanone. In contrast, the female Zucker rat (obese or lean) displayed a pronounced response to induction of CYP1A-mediated ethoxyresorufin O-deethylase activity by beta-naphthoflavone, a prototype inducer of CYP1A1 and CYP1A2. The Zucker rat would thus appear to represent a potentially exploitable genetic model for examining the mechanism of enzyme induction by the myriad xenobiotics which induce a PB-type response.

Animals↗
PubMed1992

Induction of hepatic CYP1A activity as a biomarker for environmental exposure to Aroclor 1254 in feral rodents.

Specimens of the feral mouse species Reithrodontomys fulvescens trapped from a polychlorinated biphenyl (PCB)-contaminated field location had hepatic ethoxyresorufin (ETR) O-dealkylase activities and immunoreactive CYP1A protein contents which were two- to threefold higher than those measured in animals of the same species and sex collected from non PCB-contaminated reference sites. Specimens with hepatic ETR O-dealkylase activities differing by as little as 50% could readily be assigned as originating from the PCB or reference sites by the use of a specific chemical inhibitor of cytochrome P450IA (CYP1A). The relative levels of ETR O-dealkylase activity in R. fulvescens significantly correlated with hepatic PCB burdens (r = 0.819, P less than 0.01). When the magnitudes of the induced ETR O-dealkylase activities corresponding to given hepatic PCB burdens were compared between the feral animals, F344/NCr rats (Rattus norvegicus) or B6C3F1 mice (Mus musculus) exposed in the laboratory to dietary Aroclor 1254, the order of sensitivity to the inducing effects of PCBs were F344/NCr rat greater than B6C3F1 mouse greater than R. fulvescens.

Animals↗
PubMed1992

Effects of the oxazolidinedione anticonvulsants trimethadione and dimethadione and the barbiturate homolog 5,5-dimethylbarbituric acid on N-nitrosodiethylamine-initiated renal and hepatic carcinogenesis in the F344/NCr rat.

The oxazolidinedione anticonvulsant trimethadione (3,5,5-trimethyl-2,4-oxazolidinedione, TMO) as well as its major metabolite, dimethadione (5,5-dimethyl-2,4-oxazolidinedione, DMO), and a structural analog from the barbiturate series, 5,5-dimethylbarbituric acid (DMB), were fed to F344/NCr male rats previously given a single initiating injection of N-nitrosodiethylamine (NDEA). The known promoter, phenobarbital (5-ethyl-5-phenylbarbituric acid, PB), was employed in this study as a positive control. At dosage levels equimolar to 500 ppm PB, none of the three compounds promoted development of hepatocellular adenomas or carcinomas, in contrast to PB. The two oxazolidinedione analogs and DMB caused minimal or no induction of cytochrome P450 isozyme 2B1 (CYP2B1)-mediated alkoxyresorufin O-dealkylase activities following short-term (2 weeks) feeding to separate groups of 6-week-old male F344/NCr rats, in contrast to the dramatic induction caused by PB. Promotion of neither thyroid nor renal neoplasia was observed following prolonged feeding of any of the tested compounds, although a significantly higher frequency of premalignant renal cortical tubular lesions (dysplasias) was seen in rats exposed to TMO following NDEA initiation than in those treated with NDEA alone. These studies provide important additional data on structure/liver tumor promoting activity relationships, and yield further evidence that within this group of structurally related anticonvulsants, it is possible to separate anticonvulsant activity from tumor promoting activity in the rat liver.

Animals↗
PubMed1992

Induction of hepatic CYP1A in male F344/NCr rats by dietary exposure to Aroclor 1254: examination of immunochemical, RNA, catalytic, and pharmacokinetic endpoints.

Male F344/NCr rats were exposed to low dietary concentrations of Aroclor 1254 (0-33 ppm) for 7 days, following which the induction of selected hepatic drug metabolizing enzymes was monitored. CYP1A1, measured indirectly by assaying the O-dealkylation of ethoxyresorufin in 9000 g supernatants, was increased 1.5-, 3-, 8-, and 37-fold following 7 days of exposure to 1.0, 3.3, 10, and 33 ppm Aroclor, respectively. In contrast, the O-dealkylation of benzyloxyresorufin, an indirect measure of CYP2B1 activity, was increased approximately 4-fold following exposure to 33 ppm dietary Aroclor. Measurement of the non-P450-mediated activities epoxide hydrolase, DT-diaphorase, and aldehyde dehydrogenase (NADP+, benzaldehyde) revealed < 4-fold inductions following feeding of 33 ppm Aroclor. In view of the relatively high sensitivity of the CYP1A-specific catalytic endpoint as a biomarker for Aroclor exposure, alternative endpoints for detecting induction of this subfamily of P450 were also examined. The extent of in vivo CYP1A induction was assessed by measuring serum concentrations of zoxazolamine 150 min following an intraperitoneal dose of 100 mg/kg body wt. Slight decreases in serum zoxazolamine concentration were observed in rats exposed to as little as 1.0 ppm dietary Aroclor 1254, while profound decreases were seen in rats exposed to > or = to 10 ppm Aroclor. Immunodetection of CYP1A1 protein, with a monoclonal antibody directed against this cytochrome, revealed a 2.9-fold increase in rats exposed to as little as 1.0 ppm Aroclor, and approximately 10- and 44-fold increases following exposure to 3.3 and 10 ppm dietary Aroclor, respectively. Increases in total hepatocellular RNA coding for CYP1A1 and CYP1A2, quantified by hybridization to specific oligonucleotide probes, corresponded well to the increases in hepatic O-dealkylase activity for ethoxyresorufin (CYP1A1) and methoxyresorufin (CYP1A2), respectively. Thus, CYP1A induction, directly or indirectly measured with a variety of endpoints, represents a highly sensitive biomarker for exposure to relatively low doses of Aroclor 1254 in the rat.

Administration, Oral↗
PubMed1992

Tumor-promoting and hepatocarcinogenic effects of 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in DBA/2NCr and C57BL/6NCr mice and an apparent promoting effect on nasal cavity tumors but not on hepatocellular tumors in F344/NCr rats initiated with N-nitrosodiethylamine.

The tumor-promoting and carcinogenic effects of 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in the liver and in other organs were quantified and compared to those of phenobarbital (PB) in two inbred strains of mice (C57BL/6NCr, DBA/2NCr) and in F344/NCr rats initiated at 5 weeks of age with N-nitrosodiethylamine (NDEA; 90 mg/kg in mice, 75 mg/kg in rats). Two weeks later animals were placed on a regimen of TCPOBOP once every 2 weeks (administered i.p. or i.g.) or on a diet containing 500 p.p.m. PB as a positive control for the duration of the experiment. Mice were administered TCPOBOP (3.0 mg/kg/dose) for 30 weeks followed by control diet, while rats were given the TCPOBOP regimen (3.0 or 30 mg/kg/dose) for the full 78 weeks of the experiment. TCPOBOP was a complete carcinogen and an extremely potent promoter in both strains of mice, particularly the DBA strain in which NDEA followed by TCPOBOP (i.p.) resulted in death of all the animals within 30 weeks from multiple hepatocellular tumors. TCPOBOP alone induced 100% tumor incidence in DBA mice within 60 weeks. In addition, in both strains of mice, a high proportion of those animals with liver tumors had metastases to the lungs. In contrast, TCPOBOP was ineffective as a liver tumor promoter in F344 rats at even 10 times the dose administered to mice. Interestingly however, TCPOBOP, when given subsequent to NDEA, caused a significant increase in nasal cavity tumors in F344 rats. PB was an effective liver tumor promoter in male DBA mice and male F344 rats, but was relatively ineffective as a promoter in C57 mice. When tumor-promoting activity and induction of cytochrome P450 IIB1 were compared, good agreement between these two parameters was observed. PB was an effective inducer of P450 IIB1 in the rats and in both strains of mice and a potent liver tumor promoter in both DBA mice and F344 rats, whereas TCPOBOP was a potent inducer and tumor promoter in both strains of mice but was negligibly effective as either an inducer or a promoter in F344 rats at even 10-fold higher dosage.

Animals↗
PubMed1992

Induction of various cytochromes CYP2B, CYP2C and CYP3A by phenobarbitone in non-human primates.

Male and female patas (Erythrocebus patas) and cynomolgus (Macaca fascicularis) monkeys were treated with phenobarbitone (PB) and examined for the induction of various cytochrome P450 (P450)-mediated drug metabolizing enzymes. Hydroxylation of testosterone at the 6 beta, 2 beta, and 15 beta positions, metabolites normally associated with CYP3A P450s increased 2- to 5-fold in PB-treated animals. Induction of this P450 family was confirmed by the use of polyclonal antisera directed against the human CYP3A enzymes which inhibited both induced and constitutive 6 beta- and 15 beta-hydroxylase activities in both species of monkeys. The enzymatic activities testosterone 16 beta-hydroxylation, pentoxyresorufin O-dealkylation [PROD], and benzyloxyresorufin O-dealkylation [BZROD] typically associated with the rodent CYP2B subfamily in rodents were also examined. Testosterone 16 beta-hydroxylation activity was highly induced up to 15-fold in both species of monkeys to maximal levels similar to those induced in PB-treated male rats. BZROD was similarly induced up to 10-fold in both species of monkeys, but the maximal levels of BZROD achieved were substantially lower than those obtained in PB-treated rats. Finally, PROD yielded an idiosyncratic response showing substantial induction (> 30-fold) in certain patas monkeys (4 out of 8) but minimal (< 5-fold) induction in the other patas monkeys (4 out of 8) or any of the cynomolgus monkeys (0 out of 8). Immunodetection of various cytochromes using polyclonal antisera directed against rat cytochromes confirmed the induction of CYP3A proteins as well as the induction of protein(s) immunologically cross-reactive with rat CYP2B P450s. BZROD, PROD, or testosterone 16 beta-hydroxylase activities, were not inhibited by antibody to CYP3A P450s. However, high concentrations of polyclonal antiserum directed against rat CYP2B inhibited all three activities in PB-induced patas monkeys. In contrast, this antiserum failed to inhibit the hydroxylation of testosterone at the 6 beta or 2 beta positions. Induction of the CYP2C subfamily was observed by immunochemical detection. Interestingly, induction of this subfamily appeared to be more pronounced in cynomolgus than in patas monkeys. Finally, we failed to observe significant sex-dependent differences in P450-mediated enzymatic activities in either control or induced monkeys. These results confirm the induction of a similar spectrum of P450 proteins by PB in non-human primates to that which has previously been observed in rodents.

Animals↗
PubMed1991

Induction of cytochrome P450 and other drug metabolizing enzymes in rat liver following dietary exposure to Aroclor 1254.

Selected drug metabolizing activities were measured in female F344/NCr rats exposed to graded dietary concentrations of Aroclor 1254 (1 to 1000 ppm) for 7 days or to lower concentrations of Aroclor (1 to 10 ppm) for up to 28 days. Following the 7-day exposure, the hepatic O-dealkylation of ethoxyresorufin (ETR), mediated primarily by cytochrome P450IA, was increased 60-, 10-, and 4-fold by 33, 10, and 3 ppm Aroclor, respectively. In rats exposed to 10 and 3 ppm Aroclor for 28 days, this activity was increased approximately 30- and 10-fold, respectively. Hepatic ETR O-dealkylase activities correlated with Aroclor concentrations in the livers of exposed rats (r = 0.99, p less than 0.01). Although the O-dealkylation of benzyloxyresorufin was highly increased by 7-days dietary exposure to 1000 ppm Aroclor, the levels of Aroclor necessary for detection of induction were substantially higher than those required for detection of ETR O-dealkylase induction. Examination of the non-P450-mediated drug metabolizing activities, epoxide hydrolase and DT-diaphorase, similarly showed limited (approximately 10-fold) increases. In contrast, aldehyde dehydrogenase (benzaldehyde, NADP+) activity was highly increased (greater than 40-fold) at 1000 ppm, however this activity was increased to only a limited extent at lower Aroclor concentrations (e.g. approximately 3-fold at 33 ppm). These results support the potential use of cytochrome P450 activities as potential biomarkers for environmental exposure to PCBs and related compounds.

Aldehyde Dehydrogenase↗
PubMed1990

Expression of CYP1A1 gene in patients with lung cancer: evidence for cigarette smoke-induced gene expression in normal lung tissue and for altered gene regulation in primary pulmonary carcinomas.

The major polycyclic aromatic hydrocarbon inducible-cytochrome P4501A1 gene (CYP1A1) is presumed to be important in pulmonary carcinogenesis and toxicology because its product, the cytochrome P4501A1-dependent (CYP1A1-dependent) monooxygenase, transforms selected xenobiotics (including polycyclic aromatic hydrocarbon procarcinogens in cigarette smoke) to potent carcinogenic metabolites. CYP1A1 messenger RNA (mRNA) expression has not, however, been previously demonstrated in human pulmonary tissue. This report defines CYP1A1 gene expression in normal lung tissue and primary pulmonary carcinoma tissue obtained at thoracotomy from 56 patients with lung cancer. When Northern blot hybridization analyses were performed, 17 of 19 (89%) and zero of five (0%) samples of normal lung tissue from active cigarette smokers and nonsmokers, respectively, expressed the normal 2.8-kilobase CYP1A1 mRNA. In addition, a time-dependent decrease in expression of the CYP1A1 gene was noted in normal lung tissue from individuals who were former smokers, with a decrease in expression occurring as early as 2 weeks following cessation of cigarette smoking. Expression became undetectable in all patients who had stopped smoking more than 6 weeks prior to study. When CYP1A1 gene expression was evaluated in lung cancers, mRNA levels were detectable in one of four (25%) tumors from nonsmokers; two of 24 (8%) tumors from former smokers; and seven of 15 (47%) tumors from cigarette smokers. In addition, an approximately 10-kilobase CYP1A1 RNA species, which was not detectable in normal lung tissue, was observed in five of ten (50%) of the lung cancers that expressed the CYP1A1 gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome P-450 CYP1A1↗
PubMed1990

Induction of mutagenesis and transformation in BALB/c-3T3 clone A31-1 cells by diverse chemical carcinogens.

BALB/c-3T3 cells were employed to examine the genotoxic potential of a variety of known chemical carcinogens. BALB/c-3T3 cells displayed a dose-dependent transformation response to a variety of carcinogens (polycyclic hydrocarbons, methylating agents, ethylating agents, aflatoxin B1 [AFB1], and 4-nitroquinoline-N-oxide [4-NQO]). When the ability of these compounds to induce mutagenesis to resistance to the cardiac glycoside ouabain (OUAR) was examined, we found the short chain alkylating agents to be particularly effective mutagens, causing biologic effects at doses below those necessary to induce a transformation response. In contrast, the polycyclic hydrocarbons which were potent transforming agents were weaker, albeit significant, mutagens for the OUAR locus in this system, while AFB1 was quite weak. Further studies were performed with 5-azacytidine (5-AZA) and the nongenotoxic carcinogen cinnamyl anthranilate (ClN). 5-AZA was a potent transforming agent, but failed to cause mutagenesis. ClN similarly caused in vitro transformation. When a series of eight structurally diverse compounds were examined in both the BALB/c-3T3 and C3H10T1/2 mouse fibroblast transformation systems, the BALB/c-3T3 system was shown to be sensitive to a wide variety of potential carcinogens, whereas the C3H10T1/2 system proved routinely sensitive only to the polycyclic hydrocarbons.

Alkylating Agents↗
PubMed1990

Morphological transformation of BALB/3T3 cells by various procarcinogens in the presence of a rat liver S-9 activation system.

An Aroclor-induced rat hepatic S-9 metabolic activation system was incorporated into the BALB/3T3 cell transformation assay to increase its sensitivity to a wide range of procarcinogens. S-9 was prepared from Aroclor 1254-induced (500 mg/kg) Fischer 344 rats. Cyclophosphamide, dimethylnitrosamine, 2-aminofluorene, and 2-naphthylamine were metabolized to reactive forms capable of inducing both dose-dependent toxicity and morphological transformation of BALB/3T3 cells. Treatments without an exogenous metabolic activation system were nontoxic and nontransforming. Adaptation of this commonly used exogenous metabolic activation system to BALB/3T3 cells will allow detection of the transforming potential of procarcinogens which test negative in a standard assay.

2-Naphthylamine↗
PubMed1990

Induction of hepatic cytochrome P-450 mediated alkoxyresorufin O-dealkylase activities in different species by prototype P-450 inducers.

The induction of cytochrome P-450-mediated alkoxyresorufin O-dealkylase activities by various xenobiotics was examined in liver from a variety of animal species in order to gain insights into the substrate specificities of the induced P-450s. We found that forms of cytochrome P-450 capable of mediating the O-dealkylation of the short-chain phenoxazone ethers methoxy-, ethoxy- and propoxyresorufin were highly induced by 3-methylcholanthrene-type inducers and by Aroclor-1254 in all species tested, although there were species differences in the relative turnover rates for the various substrates. For example, in hamster liver the turnover rates for the short-chain resorufin ethers decreased in the following order: methoxy greater than ethoxy much greater than propoxy, while in the rat liver almost the exact opposite order was observed: ethoxy = propoxy much greater than methoxy. In contrast, the degree of induction by phenobarbital-type inducers of isozymes catalyzing the O-dealkylation of pentoxy- or benzyloxyresorufin was highly species-dependent. Thus, F344/NCr rats, B6C3F1 mice and NZB rabbits showed the greatest (greater than 20-fold) induction of these activities, either by phenobarbital or Aroclor-1254, while Mongolian gerbils showed intermediate levels of induction and Syrian golden hamsters exhibited very low induction. In the Japanese quail, phenobarbital- or DDT-treatment resulted in minimal induction of pentoxy- or benzyloxyresorufin O-dealkylase activity, although significant induction of the latter activity occurred following treatment with 5,6-benzoflavone or with Aroclor-1254. Since substrate specificities of most enzymes can be rationalized based upon differences in the steric requirements at the enzyme active site, we employed molecular modeling techniques to calculate the molecular dimensions of the alkoxyresorufins. Surprisingly, the minimal energy conformations in vacuo of each of the resorufin ethers examined are essentially planar. However, alternative configurations, especially for the pentoxy- and benxyloxy-ethers, having greater three-dimensional bulk are also energetically possible.

Animals↗
PubMed1990

Liver tumor promoters and other mouse liver carcinogens.

Mouse liver tumors are important end points for safety assessment of chemicals in rodent carcinogenicity assays. The mechanisms of induction of these tumors, whether by chemicals with significant genotoxic activities or by chemicals without such activities, remain unknown. If test mice have spontaneous tumors of high or low background incidence, the promotion of these tumors or of spontaneously initiated or susceptible cells may provide a basis for carcinogenesis induced by these compounds. Likewise, chronic toxicity and its associated target cell hyperplasia may also lead to promotion of these tumors. Experimental systems have been developed to study the mechanisms of tumor promotion in mouse liver. These models should significantly advance research into mechanisms of hepatocarcinogenesis and tumor promotion in mouse liver and may subsequently be used for human risk assessment.

Age Factors↗
PubMed1989

Preferential activation of 6-aminochrysene and 2-aminoanthracene to mutagenic moieties by different forms of cytochrome P450 in hepatic 9000 X g supernatants from the rat.

6-Aminochrysene and 2-aminoanthracene were activated to metabolites which were mutagenic to Salmonella typhimurium TA98 by hepatocytes or hepatic 9000 X g supernatants (S9s) from control or xenobiotic-treated rats. Hepatocytes from Aroclor-1254-treated rats were more efficient than hepatocytes from untreated rats at activating these aromatic amines. When plate-incorporation and liquid-incubation bacterial mutagenesis assays were performed in the presence of limiting amounts of rat hepatic S9, 2-aminoanthracene was activated to a greater extent in both cases, as judged by his+ revertant formation, by 3-methylcholanthrene-induced hepatic S9 than by phenobarbital-induced or control S9s. In contrast, 6-aminochrysene was activated more efficiently by phenobarbital-induced S9 than by 3-methylcholanthrene-induced or control S9s. This unexpected finding was confirmed employing polyclonal antibodies directed against specific forms of rat cytochrome P450. Thus, when employing Aroclor-1254-induced S9 as a source of metabolic activation, antibody directed against cytochrome P450IA1 inhibited the activation of 2-aminoanthracene but not of 6-aminochrysene. In contrast, antibody directed against cytochrome P450IIB1 inhibited the activation of 6-aminochrysene but not of 2-aminoanthracene. These results suggest that under conditions in which the amounts of S9 added are rate-limiting, the two aromatic amines are preferentially activated by different induced forms of cytochrome P-450.

Animals↗