Prevalence of human papillomavirus among oesophageal squamous cell carcinoma cases: systematic review and meta-analysis.

BACKGROUND: Oncogenic human papillomavirus (HPV) has been hypothesised as a risk factor for oesophageal squamous cell carcinoma (OSCC), but aetiological research has been limited by the varying methodology used for establishing HPV prevalence. The aims of this systematic review and meta-analysis were to estimate the prevalence of HPV DNA detected in OSCC tumours and the influence of study characteristics. METHODS: Study-level estimates of overall and type-specific HPV prevalence were meta-analysed to obtain random-effects summary estimates. RESULTS: This analysis included 124 studies with a total of 13 832 OSCC cases. The average HPV prevalence (95% confidence interval) among OSCC cases was 0.277 (0.234, 0.320) by polymerase chain reaction; 0.243 (0.159, 0.326) by in situ hybridisation; 0.304 (0.185, 0.423) by immunohistochemistry; 0.322 (0.154, 0.490) by L1 serology; and 0.176 (0.061, 0.292) by Southern/slot/dot blot. The highest HPV prevalence was found in Africa and Asia, notably among Chinese studies from provinces with high OSCC incidence rates. CONCLUSIONS: Future research should focus on quantifying HPV in OSCC cases using strict quality control measures, as well as determining the association between HPV and OSCC incidence by conducting large, population-based case-control studies. Such studies will provide a richer understanding of the role of HPV in OSCC aetiology.

Granulomatous typhlocolitis, lymphangitis, and lymphadenitis in a horse infected with Listeria monocytogenes, Salmonella Typhimurium, and cyathostomes.

A 15-year-old American Quarter horse mare was euthanized because of poor response to therapy for severe diarrhea. Significant gross findings were limited to the large intestines. The walls of the cecum and colon were thickened with widely scattered nodules in the mucosa and submucosa that extended into the enlarged colic lymph nodes. Microscopically, there was severe granulomatous typhlocolitis, lymphangitis, and lymphadenitis, with many intralesional Gram-positive, non-acid-fast coccobacilli and few cyathostomes. Intralesional bacteria were immunohistochemically and polymerase chain reaction (PCR) assay positive for Listeria monocytogenes. Concurrent infection with Salmonella enterica serovar Typhimurium was detected by PCR and culture. Infection with L. monocytogenes in horses is rare, and coinfection with Salmonella and small strongyles probably contributed to the development of granulomatous typhlocolitis.

Inhibition of microsomal 17 beta-hydroxysteroid oxidoreduction activities in rat liver by all-trans-, 9-cis- and 13-cis-retinoic acid.

Retinoids and steroid hormones mediate their biological effects through nuclear receptors. However, retinoids may alter the intracellular availability of ligands for steroid receptor activation by modulating the activity of biotransformation enzymes. This study investigated the modulation of NAD(H)-linked steroid oxidoreductases in rat hepatic microsomes by retinoids. 13-cis-Retinoic acid inhibited testosterone 17 beta-dehydrogenation (Ki 2.4 +/- 0.5 microM; Km/Ki ratio 0.34 +/- 0.06) but androstenedione reduction was less susceptible to inhibition (Ki 27 +/- 13 microM; Km/Ki ratio 0.045 +/- 0.12). All-trans-retinoic acid was less potent than the 13-cis-isomer and 9-cis-retinoic acid was of intermediate potency. In vivo administration of all-trans-retinoic acid (60 mg/kg i.p. for 7 days) decreased hepatic microsomal oxidoreduction activity, but exposure over shorter periods and 13-cis-retinoic acid were without effect. Thus, all-trans-retinoic acid elicits direct inhibition and may also alter the normal regulation of the oxidoreductase under certain conditions. Three geometric isomers of retinal were potent inhibitors of testosterone dehydrogenation (IC50s approximately 6 microM), but were ineffective against androstenedione reduction. These findings suggest that certain anti-hormonal effects of retinoids may be attributable in part to modulation of endobiotic biotransformation prior to receptor binding and activation.

The influence of amines on various platelet responses.

Four amines, galactosamine, mannosamine, histamine and arginine were studied for their effects on platelet aggregation, platelet morphological changes, platelet protein phosphorylation and platelet secretion. Galactosamine inhibited platelet aggregation in response to arachidonic acid and ionophore A23187 but did not inhibit changes in platelet morphology, or in platelet protein phosphorylation in response to these agents and only partially inhibited platelet secretion. The results suggest that galactosamine can be used as a selective inhibitor of platelet-platelet attachment without having a significant effect on intracellular processes. Mannosamine was similar to galactosamine except that it partially suppressed phosphorylation of myosin light chain. Histamine was similar to mannosamine except that some platelet damage was seen in platelets exposed to histamine and arachidonic acid or ionophore A23187. Arginine was non-selective: it suppressed platelet aggregation, secretion and phosphorylation of myosin light chain and a 40 kDa protein (40P) in response to arachidonic acid and ionophore A23187. Arginine was also potent in suppressing platelet morphological changes. When the same four amines were evaluated for their effects on thrombin-induced aggregation; secretion was inhibited concomitantly with inhibition of aggregation. Inhibition of myosin light chain and 40P phosphorylation was evident with galactosamine, suggesting that when thrombin is used as the agonist, galactosamine is not a specific inhibitor of platelet-platelet attachment. These amines therefore have various effects on platelet responses. Under some conditions and with arachidonic acid or ionophore A23187 as agonist, one of them, galactosamine, can be used as a selective inhibitor of platelet-platelet attachment.

Differential regulation of endobiotic-oxidizing cytochromes P450 in vitamin A-deficient male rat liver.

1. The hepatic CYP4A-dependent omega-hydroxylation of arachidonic acid and CYP2C11-dependent 2alpha-/16alpha-hydroxylations of testosterone were decreased to 74 and 60% of respective control in microsomal fractions from vitamin A-deficient rats. Decreases in the rates of arachidonic acid omega-1-hydroxylation and testosterone 6beta-, 7alpha- and 17alpha-hydroxylations were less pronounced. 2. Corresponding decreases in microsomal CYP4A and CYP2C11 immunoreactive protein expression to 64 and 68% of respective control were observed in vitamin A-deficient rat liver. Expression of CYP3A proteins was unchanged from vitamin A-adequate control. 3. Northern analysis revealed a selective decrease in CYP4A2 mRNA expression in vitamin A-deficient rat liver to approximately 5% of control; expression of the related CYP4A1/4A3 mRNAs was not decreased. CYP2C11 mRNA expression was also decreased in vitamin A-deficient male rat liver to 39% of control levels. 4. Intake of the deficient diet containing all-trans-retinoic acid (ATRA) during the final week of the experiment restored CYP4A2 mRNA and CYP4A protein. Administration of exogenous androgen or episodic growth hormone was ineffective. In contrast, CYP2C11 expression was restored by ATRA and androgen, but not by growth hormone. 5. From these studies it emerges that CYP4A2, a fatty acid omega-hydroxylase in rat liver, is highly dependent on vitamin A for optimal expression, whereas CYP2C11 is indirectly down regulated by androgen deficiency resulting from vitamin A-deficiency. Altered CYP expression in vitamin A-deficiency provides insights into the relationship between dietary constituents and the intracellular formation of vasoactive eicosanoids as well as the clearance of androgenic steroids.

Pretranslational up-regulation of the hepatic microsomal delta4-3-oxosteroid 5alpha-oxidoreductase in male rat liver by all-trans-retinoic acid.

Administration of all-trans-retinoic acid (ATRA; 60 mg/kg daily for 3 days) to male rats increased the rate of 5alpha-dihydrotestosterone (5alpha-DHT) formation from testosterone in microsomal fractions in vitro. The formation of androstane-3alpha,17beta-diol from testosterone was also increased because of the higher concentration of 5alpha-DHT produced in microsomal incubations. Northern analysis confirmed that the increased rate of 5alpha-DHT formation was due to the pretranslational up-regulation in delta4-3-oxosteroid 5alpha-oxidoreductase (EC 1.3.99.5) mRNA expression in ATRA-treated male rat liver. Thus, ATRA elicited in male rat liver a partial feminization of the expression of this enzyme, which normally exhibits a female-selective distribution in the rat. Subsequent experiments evaluated whether the administration of human chorionic gonadotropin or thyroxine to ATRA-treated male rats decreases 5alpha-reductase activity to that observed in untreated male rat liver. Although these treatments did not decrease 5alpha-reductase to untreated male levels, it was found that administration of ATRA to gonadectomized male rats produced complete feminization of the enzyme. Again, up-regulation was confirmed at the mRNA level. The activity of the male-specific cytochrome P450 2C11 (as reflected by microsomal testosterone 16alpha-hydroxylation activity) was correspondingly decreased by treatments that increased steroid 5alpha-reductase activity. Thus, gonadectomy in combination with ATRA administration effected a more pronounced decrease in 16alpha-hydroxylation activity than either treatment alone. These findings suggest that ATRA is a novel positive regulator of the 5alpha-reductase that in combination with the removal of circulating androgen, which normally suppresses 5alpha-reductase levels, feminizes the expression of this enzyme in rat liver.

Comparative induction of CYP3A and CYP2B in rat liver by 3-benzoylpyridine and metyrapone.

3-Benzoylpyridine (3BP) is a major metabolite of HGG-12, and oxime that has been synthesized as a potential antidote to the toxic effects of soman and other anticholinesterases. Structural similarities exist between 3BP, the cytochrome P450 (CYP)-inducer metyrapone (MET) and other 3-substituted pyridines that interact with CYPs. The present study evaluated the regulatory effects of 3BP on CYP expression in rat liver. Both 3BP and MET (100 mg/kg) increased total hepatic microsomal holo-CYP content significantly 24 h after administration to male rats. Pronounced increases in activities mediated by CYP2B (androstenedione 16 beta-hydroxylation and 7-pentylresorufin O-depentylation) were produced by 3BP and MET, which correlated with respective 9- and 14-fold increases in CYP2B immunoreactive protein. In addition, both agents slightly increased rates of microsomal CYP3A-dependent steroid 6 beta-hydroxylation, troleandomycin metabolite complex formation and total CYP3A immunoreactive protein. Induction of the dexamethasone-inducible CYP3A23 mRNA to 4.5- and 2.5-fold of control was detected in liver of MET- and 3BP-induced rats; CYP3A2 mRNA levels were unchanged. Analogous in vitro studies revealed that MET was a preferential inhibitor of CYP3A-mediated steroid 6 beta-hydroxylation activity, but 3BP was inactive against constitutive steroid hydroxylase CYPs. These findings indicate that the structurally related 3BP and MET elicit similar induction effects on CYPs 2B and 3A23 in rat liver after in vivo administration, but differential inhibitory effects of the chemicals on CYP activity in vitro. Recent reports have implicated a microsomal binding site in the induction of CYP3A1/3A23 in rat liver. In light of the present findings, substituted pyridines like 3BP may be useful tools in structure-activity studies to evaluate the physicochemical requirements for binding to this protein.

Induction of cytochromes P450 2B and 2E1 in rat liver by isomeric picoline N-oxides.

Pyridine derivatives are widely used solvents and precursors for the synthesis of chemicals of industrial importance. Oxidized metabolites have been implicated in the observed toxicity of pyridines and are known to induce drug-metabolizing enzymes in rat liver. In this study the three isomeric picoline (methylpyridine) N-oxides, as major oxidized metabolites of 2-, 3- and 4-picoline, were evaluated as inducers of cytochrome P450 (CYP) enzymes in rat liver. After a single dose of 100 mg/kg 24 h before sacrifice the 3- and 4-isomers were effective inducers of microsomal substrate oxidations associated with the phenobarbital-inducible CYPs 2B; upregulation of CYP2B protein was confirmed by immunoblotting. In contrast, the 2-isomer did not increase CYP2B protein or activity in rat liver but CYP2E1 protein expression was upregulated by the isomers to 160-200% of control. The three chemicals increased aniline 4-hydroxylation activity in rat liver, which is consistent with induction of CYPs 2B or 2E1 and 4-nitrophenol 2-hydroxylation activity was increased in microsomal fractions from 3- and 4-picoline N-oxide-treated rats. The activities of several other CYPs were also determined and CYP1A-dependent 7-ethylresorufin O-deethylation was increased (to approximately 6- and 2-fold of control) by the 3- and 4-isomer, respectively, whereas the activity of CYP3A-mediated androstenedione 6beta-hydroxylation was decreased by the agents--most notably by the 2-isomer. During NADPH-supported oxidation of CCl4, lipid peroxidation was increased in microsomes from 3- and 4-picoline N-oxide-pretreated rats and was modulated in vitro by the CYP2B inhibitor orphenadrine, but not by the CYP2E1 inhibitor 4-methylpyrazole. These findings establish that particular isomers of picoline N-oxide rapidly upregulate CYP2B or, to a lesser extent, CYP2E1 and implicate CYP2B in the enhanced lipid peroxidation observed in microsomes from rats treated with 3- and 4-picoline N-oxides. Such induction process may contribute to the hepatotoxicity of pyridines by enhancing the capacity for microsomal lipid peroxidation.

Alcohol consumption and cognitive functioning in college students.

Decreased cognitive performance was significantly correlated with increased quantity of alcohol per occasion and with total lifetime consumption in both women and men college students tested while sober. In men, however, increased performance on some tasks was also significantly correlated with increased frequency of drinking.

Social drinking and cognitive functioning in college students: a replication and reversibility study.

The purposes of this study were to replicate a previous study of the relationship between alcohol consumption and cognitive functioning in college students and to investigate the reversibility of negative effects of social drinking on cognitive functioning when randomly assigned subjects abstained from drinking for 2 weeks. The previous study was replicated by administering the same battery of neuropsychological tests to 170 subjects (103 women) during the first testing session. Like the original study, the present study demonstrated several significant predicted inverse relationships between drinking and cognitive performance, but specific relationships between various drinking and cognitive variables were not replicated. As in the original study, some significant nonpredicted relationships also occurred. At the end of the first testing session, subjects were randomly assigned either to abstain from drinking or to maintain their usual drinking patterns for 2 weeks. They were then administered a different neuropsychological battery designed to assess functions similar to the original battery. Consumption the previous 2 weeks was significantly lower in the abstain group than in the maintain group, but there were no significant differences in the predicted direction between groups on the cognitive variables. Several significant predicted inverse correlations between drinking variables and cognitive performance occurred, but some nonpredicted relationships occurred also. Problems and implications for research in social drinking are discussed.

Alcohol use and cognitive functioning in men and women college students.

Two studies of the relationship between alcohol consumption and cognitive functioning in men and women college students are presented. Study 1 showed several predicted relationships of decreased cognitive performance on various tests with increased quantity of alcohol per occasion and total lifetime consumption in both women and men. Study 2a was designed to replicate study 1, but the pattern of relationship of cognitive and consumption variables was quite different, e.g., increased cognitive performance was associated with increased quantity per occasion for several tests in males. Study 2b was designed to demonstrate reversibility of the negative effects of consumption on cognition by randomly assigning half of the subjects to abstain for two weeks. Reversibility was not demonstrated. Difficulties in studying these effects in college students are discussed.

Well-differentiated hepatocellular carcinoma in a ring-tailed lemur (Lemur catta).

A 16-year-old male ring-tailed lemur (Lemur catta) was presented with severe cachexia and an abdominal mass. The encapsulated, multilobular mass replaced the right medial lobe of the liver and compressed the adjacent gall bladder. Multiple haemorrhages and necrotic foci were found within the mass. Microscopically, neoplastic cells formed cords of moderately pleomorphic, polygonal cells with mild to moderate anaplasia. Immunohistochemical markers used for diagnosis of hepatocellular carcinomas in man were used to characterize the neoplastic cells, which expressed hepatocyte-specific antigen, but not glypican-3 or polyclonal carcinoembryonic antigen. Gross, microscopical and immunohistochemical features of the tumour were most consistent with a well-differentiated hepatocellular carcinoma. Although this tumour is common among prosimians, to the authors' knowledge this is the first documented case in a ring-tailed lemur. Hepatocellular carcinomas have been associated with hepatitis virus infections and excessive hepatic iron in man; however, no association was established between this tumour and viral infection or hepatic iron storage disease in the present case.

Common genetic variation near the connexin-43 gene is associated with resting heart rate in African Americans: a genome-wide association study of 13,372 participants.

BACKGROUND: Genome-wide association studies have identified several genetic loci associated with variation in resting heart rate in European and Asian populations. No study has evaluated genetic variants associated with heart rate in African Americans. OBJECTIVE: To identify novel genetic variants associated with resting heart rate in African Americans. METHODS: Ten cohort studies participating in the Candidate-gene Association Resource and Continental Origins and Genetic Epidemiology Network consortia performed genome-wide genotyping of single nucleotide polymorphisms (SNPs) and imputed 2,954,965 SNPs using HapMap YRI and CEU panels in 13,372 participants of African ancestry. Each study measured the RR interval (ms) from 10-second resting 12-lead electrocardiograms and estimated RR-SNP associations using covariate-adjusted linear regression. Random-effects meta-analysis was used to combine cohort-specific measures of association and identify genome-wide significant loci (P≤2.5×10(-8)). RESULTS: Fourteen SNPs on chromosome 6q22 exceeded the genome-wide significance threshold. The most significant association was for rs9320841 (+13 ms per minor allele; P = 4.98×10(-15)). This SNP was approximately 350 kb downstream of GJA1, a locus previously identified as harboring SNPs associated with heart rate in Europeans. Adjustment for rs9320841 also attenuated the association between the remaining 13 SNPs in this region and heart rate. In addition, SNPs in MYH6, which have been identified in European genome-wide association study, were associated with similar changes in the resting heart rate as this population of African Americans. CONCLUSIONS: An intergenic region downstream of GJA1 (the gene encoding connexin 43, the major protein of the human myocardial gap junction) and an intragenic region within MYH6 are associated with variation in resting heart rate in African Americans as well as in populations of European and Asian origin.

Midwives.

PIP: The American College of Obstetrics and Gynecology, the Nurses Association of the College, and the American College of Nurse Midwives believe that improved maternity care and health services for women in the U.S. can be achieved by the cooperative efforts of teams of physicians, nurse-midwives, and obstetric registered nurses. Yet the Massachusetts Medical Society opposed House Bills 848 and 2200 authorizing the practice of midwifery in licensed facilities by a certifed nurse-midwife who functions as a member of a team that includes an obstetrician-gynecologist. The opposition was on the grounds that the use of midwives will dilute quality medical care. Does this mean the Society cannot recognize the deficits in availability and quality of maternity care in the U.S. and does not believe the reports of lower infant and maternal mortality rates in European countries using midwifery than in the U.S.? Or does it reflect the tendency of the physician to consider pregnancy and labor as illness? The American Medical Association and the American Academy of Pediatrics have endorsed the use of pediatric nurse associates and physician assistants in health care. The Massachusetts Medical Society should endorse the use of midwives in health care with a physician being called in to provide diagnosis and therapy where a condition incompatible with health is detected.^ieng

Inhibition and inactivation of constitutive cytochromes P450 in rat liver by parathion.

Phosphorothioate pesticides, such as parathion (O,O-diethyl-O-4-nitrophenyl phosphorothioate), undergo enzymic oxidation to the active insecticidal agents that are the analogous organophosphorus compounds. In hepatic microsomal fractions, the NADPH-mediated conversion of parathion to paraoxon occurs with concomitant loss of cytochrome P450 (P450) and associated activities. In this study, the capacity of parathion to inactivate specific P450 enzymes was studied in rat hepatic microsomes. Parathion was a potent inhibitor of P450 3A2- and 2C11-mediated androst-4-ene-3,17-dione (androstenedione) 6 beta- and 16 alpha-hydroxylation (Ki values of 13 +/- 2 and 2.3 +/- 0.1 microM, respectively, and Km/Ki ratios of 1.4 +/- 0.2 and 11 +/- 1, respectively). After a 10-min preincubation between parathion and NADPH-supplemented microsomes, to inactivate P450 before androstenedione hydroxylation was carried out, the corresponding Km/Ki ratios were increased to 3.5 +/- 0.4 and 35 +/- 6, reflecting 2.5- and 3.2-fold enhancement of inhibition of P450 3A2- and 2C11-dependent activities. In contrast to these findings, P450 2A1/2-mediated androstenedione 7 alpha-hydroxylation was refractory to inhibition and P450 2C6-mediated progesterone 21-hydroxylation was inhibited but not inactivated by the pesticide. Further studies established that androstenedione 6 beta- and 16 alpha-hydroxylation pathways were inactivated with maximal half-times of 2.59 min and 1.72 min, respectively. Although the incubation of parathion (50 microM) with rat liver microsomes for 10 min led to a 16% decrease in P450 estimated spectrophotometrically, immunoblot analysis revealed no change in the microsomal content of P450 2C11 apoprotein. Finally, NADPH-mediated metabolism of parathion to paraoxon (by desulfuration) and 4-nitrophenol (by oxidative cleavage of the phosphorothioate ester) occurred efficiently in microsomes (4.32 and 4.35 nmol/min/mg of protein, respectively). P450 loss was estimated under the same incubation conditions and, thus, 210 parathion molecules were oxidized for each molecule of holo-P450 lost. These findings establish that parathion is a potent inhibitor and inactivator of the principal constitutive P450s, 3A2 and 2C11, in rat liver, whereas the P450s 2A1 and 2A2 are refractory to either inhibition or inactivation. Another major constitutive enzyme, P450 2C6, is inhibited effectively by parathion but does not appear to be subject to inactivation.

Hepatic biotransformation of parathion: role of cytochrome P450 in NADPH- and NADH-mediated microsomal oxidation in vitro.

In vitro studies have established that cytochrome P450 (P450) is deactivated by the electrophilic sulfur atom released during the enzymic oxidation of parathion to paraoxon. However, in vivo studies in rats have been unable to demonstrate significant P450 loss. This study evaluated the possibility that there may be alternate pathways of parathion biotransformation in liver, other than those mediated by P450 and supported by NADPH. Initial experiments confirmed that parathion administration did not decrease microsomal P450 or testosterone hydroxylation activities. Subsequent in vitro experiments identified an NADH-dependent pathway of parathion biotransformation, and MS was used to confirm that paraoxon and 4-nitrophenol were the products of both the NADH- and NADPH-dependent reactions. The Michaelis constants of the NADH-dependent formation of paraoxon and 4-nitrophenol (26 +/- 6 microM and 53 +/- 10 microM, respectively) were approximately 3-fold greater than those for the NADPH-supported reactions (9 +/- 1 microM and 18 +/- 3 microM, respectively). Induction of male rats with phenobarbital and dexamethasone, but not beta-naphthoflavone, produced similar increases in the rates of NADH- and NADPH-mediated parathion metabolism. Rates of NADH- and NADPH-dependent metabolism were highly correlated in linear relationships. An anti-NADPH-cytochrome P450 reductase (NADPH-P450 reductase) antibody partially inhibited microsomal parathion oxidation mediated by either cofactor, and the P450 inhibitor clotrimazole was similarly effective against the NADH- and NADPH-supported oxidation of parathion. Finally, a reconstituted system containing P450 2B1, NADPH-P450 reductase, and phospholipid supported parathion oxidation mediated by NADH.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced inhibition of microsomal cytochrome P450 3A2 in rat liver during diltiazem biotransformation.

Pharmacokinetic drug interactions involving the calcium channel blocker diltiazem (DTZ) have been attributed to inhibition of microsomal cytochrome P450 (P450)-mediated drug oxidation. Accumulation of certain DTZ metabolites during dosage with the drug, as well as dose-related differences in DTZ pharmacokinetics, suggests that DTZ metabolites may also participate in P450 inhibition. The present study evaluated a series of putative DTZ metabolites as inhibitors of major constitutive P450s in rat liver in vitro, in relation to DTZ biotransformation. The principal finding to emerge was that the N-demethylated metabolite of DTZ was a more potent competitive inhibitor than DTZ of CYP3A2-dependent testosterone 6 beta-hydroxylation. This P450 appeared to be the preferred target for inhibition, because the observed K/K(m) ratio for inhibition of CYP3A2-dependent steroid hydroxylation was approximately 4- and 100-fold lower than those for CYP2C11 and CYP2A1-dependent pathways, respectively. It was also established that N-desmethyl-DTZ was a major metabolite formed during microsomal DTZ biotransformation in rat liver in vitro. The other primary metabolites, desacetyl-DTZ and O-desmethyl-DTZ, were ineffective inhibitors of any pathways of steroid oxidation by P450s, but several other potential metabolites, which were not detected in microsomal incubations, also inhibited P450 activity. Consistent with previous reports, there was no evidence of P450 inactivation or complexation by DTZ, but the drug and its N-desmethyl metabolite generated binding interactions with ferric P450 in rat hepatic microsomes. Considered together, the findings of the present study establish that N-desmethyl-DTZ is a preferential inhibitor of CYP3A2 in rat hepatic microsomes, with greater potency than the parent drug. This is consistent with clinical reports in which this metabolite accumulates during multiple-dose therapy with DTZ. The competitive nature of the inhibitory interaction suggests that the eventual elimination of N-desmethyl-DTZ should restore normal hepatic oxidation capacity.

Biotransformation of parathion in human liver: participation of CYP3A4 and its inactivation during microsomal parathion oxidation.

Studies in rat liver have shown that cytochrome P450 (CYP) enzymes mediate the oxidative biotransformation of the phosphorothioate pesticide parathion to paraoxon and 4-nitrophenol. Transfer of the phosphorothioate thionosulfur atom to the CYP apoprotein results in amino acid modification and enzyme inactivation. Our study investigated the role of human hepatic CYP in parathion oxidation and their relative susceptibilities to inhibition and inactivation. Rates of parathion oxidation varied about 10-fold in microsomes from 23 individual livers (1.72-18.33 nmol total metabolites/mg protein/min). Linear regression of rates of parathion oxidation with those of other microsomal CYP reactions implicated CYP3A4 in the reaction. Thus, parathion oxidation was correlated strongly with testosterone 6beta-hydroxylation (r2 = 0.95, n = 11), but not with activities mediated by CYP 1A2, 2C9 or 2E1. CYP 3A4 expressed in lymphoblastoid cell lines was an efficient catalyst of parathion oxidation, although CYP 1A2 and 2B6 also catalyzed the activity. The CYP3A4 inhibitors ketoconazole and triacetyloleandomycin decreased the observed rate of microsomal parathion oxidation, but chemicals known to interact preferentially with other human CYP were essentially noninhibitory. P450 was lost during parathion biotransformation in human hepatic microsomes. Thus, incubation (10 min) of parathion (25 microM) with NADPH-supplemented microsomes led to an apparent 19 +/- 4% decrease in holo-P450 content. Several CYP-specific oxidation reactions were inhibited and inactivated by parathion. Testosterone 6beta-hydroxylation (mediated by CYP3A4), 7-ethylresorufin O-deethylation (CYP1A2) and tolbutamide methyl hydroxylation (CYP2C9/10), but not aniline 4-hydroxylation (CYP2E1), were inhibited effectively by parathion. Preincubation of microsomes with parathion and NADPH intensified the extent of inhibition (i.e., elicited inactivation) of reactions mediated by 3A4 and 1A2 and, to a lesser extent, 2C9. In summary, these findings strongly implicate CYP 3A4 as the principal catalyst of parathion oxidation in human liver, although other CYP may play a lesser role. During parathion oxidation CYP3A4 undergoes significant inactivation. In view of the role of this enzyme in the oxidation of many therapeutic agents, exposure to phosphorothioate pesticides may adversely affect drug elimination in humans.