PubMed Health⌕ Search

Biomedical subjects

O Pelkonen

Publications and source records attributed to O Pelkonen.

At least 55 records · Page 3Linked to original sources
PubMed1999

Metabolism of xenobiotics and chemical carcinogenesis.

In order to avoid the accumulation of harmful xenobiotics in cells, living organisms have developed ways for their elimination. Multiple xenobiotic metabolizing enzymes with variable but partially overlapping catalytic properties play a key role in the elimination process. These enzymes are encoded by superfamilies of genes which, during the course of evolution, have evolved in a way that has made it possible for the different species to survive and take advantage of different habitats and diet containing a variable composition of harmful xenobiotics. As a result of this evolutionary process, species have achieved capacities to metabolize xenobiotics which are appropriate for their survival but which may differ considerably from those of other species. This evolutionary process may also explain the interethnic and interindividual variability of drug metabolism in humans. Because many carcinogens are substrates of drug-metabolizing enzymes it is reasonable to assume that humans have a variable capacity to activate or inactivate carcinogens. This has been shown to be the case. It appears that most of the carcinogen-metabolizing enzymes are inducible by xenobiotics: they respond to environmental stimuli and therefore vary in their activity. Furthermore, many of the encoding genes are polymorphic and multiple allelic variants relevant for the phenotype may exist in human populations. Analysis of the genetic variability that affects the capacity to metabolize carcinogens in humans has shown that a few members of the cytochrome P450, glutathione S-transferase and N-acetyltransferase gene families may play an Important role in chemical carcinogenesis. Yet for several enzymes such a role has not been established until now, although their catalytic properties and expression in human tissues suggest that such a role should exist. More studies on the role of individual enzymes in chemical carcinogenesis are therefore warranted.

Arylamine N-Acetyltransferase↗
PubMed1999

Xenobiotic-metabolizing enzymes and cancer risk: correspondence between genotype and phenotype.

Metabolic activation and/or inactivation of a carcinogen is usually studied in appropriate in vitro systems but ultimately needs confirmation from in vivo studies, i.e. phenotype studies. It determines what initially happens to a carcinogen to which an organism is exposed. Consequently, it is of major importance to investigate the correspondence between any particular genotype of a carcinogen-metabolizing enzyme and its phenotypic expression, if any. The need to elucidate the relationship between genotype and phenotype is particularly important now, when methods for uncovering changes in genomic DNA are rather easy, even routine. There are several examples where the correspondence between a variant allele and an altered phenotype, measured by a probe drug or by some other means, has been elucidated (e.g. several alleles of CYP2D6). However, there are also cases where this correspondence has either not been studied (sometimes because of a lack of suitable probe substances) or has remained unclear (e.g. CYP1A1 or CYP2E1), despite case-control studies demonstrating an association between a variant allele and cancer risk. In the end one has to address the basic question as to how the genotype determines the phenotype and whether there is any biologically plausible link between the genotypic differences and cancer susceptibility. A knowledge of the complete sequence of events, from the gene to the outcome, would be helpful in unravelling the implications and possible preventive and treatment strategies to be employed in cases where clear associations between carcinogen-metabolizing enzymes and cancer susceptibility have been uncovered.

Alleles↗
PubMed1999

The CYP2A subfamily: function, expression and genetic polymorphism.

The CYP2A6 gene is one of the three members of the human CYP2A gene subfamily, the others being CYP2A7 and CYP2A13. The CYP2A6 enzyme catalyses the oxidation of several compounds that have clinical or toxicological interest, including pharmaceuticals, procarcinogens, and tobacco smoke constituents. CYP2A6 is expressed mainly in liver, and only trace amounts are found in extrahepatic tissues. Coumarin is a high-affinity substrate for CYP2A6, and a phenotyping test based on coumarin 7-hydroxylation has been developed. Two mutant alleles of the CYP2A6 gene have been found, i.e. CYP2A6*2 and CYP2A6*3. Homozygosity for both mutated alleles appears to confer a poor metabolizer (PM) phenotype, detectable by slow or non-existent 7-hydroxylation of coumarin. Very little is known about the inducibility and regulation of CYP2A6, but studies on the mouse orthologue, CYP2A5, have revealed novel pathways for induction. Since CYP2A6 polymorphism was found fairly recently, nothing is known presently about associations between variant CYP2A6 alleles and diseases or other adverse outcomes of exposure to toxins. Such studies, however, are clearly warranted, given the wide range of procarcinogens and other toxins metabolized by the CYP2A6 enzyme.

Animals↗
PubMed1998

Genotyping of human cytochrome P450 2A6 (CYP2A6), a nicotine C-oxidase.

Cytochrome P450 2A6 (CYP2A6) is a polymorphic enzyme responsible for the oxidation of certain precarcinogens and drugs and is the major nicotine C-oxidase. The role of CYP2A6 for nicotine elimination was emphasised recently by the finding that smokers carrying defective CYP2A6 alleles consumed fewer cigarettes [Pianezza et al. (1998) Nature 393, 750]. The method used for CYP2A6 genotyping has, however, been found to give erroneous results with respect to the coumarin hydroxylase phenotype, a probe reaction for the CYP2A6 enzyme. The present study describes an allele-specific PCR genotyping method that identifies the major defective CYP2A6 allele and accurately predicts the phenotype. An allele frequency of 1-3% was observed in Finnish, Spanish, and Swedish populations, much lower than described previously.

Aryl Hydrocarbon Hydroxylases↗
PubMed1998

Cytochrome P4502A6 (CYP2A6) expression in human hepatocellular carcinoma.

The hepatic cytochrome P4502A6 (CYP2A6) enzyme mediates the oxidative metabolism of several procarcinogens that have liver as their primary target. Mouse models indicate that liver tumors invariably overexpress CYP2A forms, and that inflammation and cirrhosis may regulate the CYP2A expression pattern. In this study, the distribution of the CYP2A6 protein was investigated in a series of 24 human hepatocellular carcinoma (HCC) samples by immunohistochemical analysis. A polyclonal antibody was raised in chicken against CYP2A5, the mouse orthologue of CYP2A6. The antibody was characterized and found to be specific for CYP2A members. In DBA/2 mouse liver, a strong increase of CYP2A5 protein amount, localized in the perivenous region, occurred in response to treatment with pyrazole. In human HCC samples, overexpression of CYP2A6 protein was associated with the presence of chronic inflammation and cirrhosis. CYP2A6 protein was observed in 9 of 16 (56%) of samples with non-neoplastic hepatocytes and in 10 of 24 (42%) HCC samples. The staining for CYP2A6 protein was very heterogeneous in tumor cells, suggesting that increased expression of CYP2A6 occurred in a distinct subpopulation of neoplastic cells. In Kaplan-Meyer survival analysis, there was a tendency toward a more favorable prognosis in patients with CYP2A6-positive tumors in comparison with patients with CYP2A6-negative tumors. These data suggest that, in human HCC, in contrast to mouse liver tumors, CYP2A6 overexpression is not an invariable phenotype.

Animals↗
PubMed1998

Expression of xenobiotic-metabolizing cytochrome P450s in human pulmonary tissues.

The purpose of the study was to obtain a comprehensive picture of the expression of cytochrome P450s (CYP) in the human lung, broncho-alveolar macrophages (BAM), and peripheral blood lymphocytes. The methods used were reverse transcriptase-polymerase chain reaction (RT-PCR) with gene-specific primers and immunohistochemistry with specific anti-peptide antibodies. In RT-PCR, CYPs 1A1, 2B6/7, 2E1, 2F1, 3A5 and 4B1 were detected in cDNA prepared from whole lung tissue. BAMs expressed CYPs 1B1, 2B6/7, 2C, 2E1, 2F1, 3A5 and 4B1. These tissues lacked CYPs 1A2, 2A6, 2D6, and 3A7. In peripheral blood lymphocytes, only CYP1B1 and CYP2E1 mRNAs were consistently detected. In immunohistochemistry with anti-CYP3A antibodies, epithelial staining of CYP3A5 was observed in 100% of individuals, while only about 20% exhibited CYP3A4 staining. CYP3A5 protein was localized in the bronchial wall, bronchial glands, bronchiolar epithelium, alveolar epithelium, vascular endothelium and alveolar macrophages. The results indicate that several different xenobiotic-metabolizing CYPs are present in the human lung, possibly contributing to in situ activation of pulmonary procarcinogens.

Cytochrome P-450 Enzyme System↗
PubMed1998

Induction of CYP2A5 by pyrazole and its derivatives in mouse primary hepatocytes.

Mouse liver CYP2A5 is induced by several structurally unrelated compounds. In intact mouse liver, pyrazole (PYR) and 4-hydroxypyrazole (4-OH) induce selectively the expression of CYP2A5 while expression of other CYPs is decreased. In this study we exposed mouse primary hepatocytes to PYR, 4-OH, 4-methylpyrazole (4Me; 0.1-20 mM) and 4-iodopyrazole (4-I; 0.1-5.0 mM). PYR and its derivatives increased coumarin 7-hydroxylase activity, with 4-1 and 4-OH being the strongest inducers, by 114-fold and 41-fold, respectively. However, only 4-1 treatment increased markedly the CYP2A5 protein content. CYP2B9/10-mediated pentoxyresorufin O-deethylase activity (PROD) was decreased by 80% by 4-Me and 4-1, and by 50% by 4-OH while PYR had no marked effect. PYR and 4-Me increased 2- to 3-fold the CYPA1/2-mediated ethoxyresorufin O-deethylase activity (EROD) while 4-OH and 4-1 had no marked effect on this enzyme. The time of exposure markedly affected the inducibility of 4-OH such that induction was 7-fold stronger when it was added to the incubation medium 24 h after the isolation of hepatocytes compared to exposure 3 h after their isolation. Cimetidine prevented the induction of coumarin 7-hydroxylase activity by PYR and 4-OH by 46 and 74%, respectively indicating that their effects on the expression of CYP2A5 are, at least partly, mediated via their metabolites. The data demonstrate that the regulation of CYP2A5 is different from other monooxygenases and that the effects of pyrazole and its derivatives are different in vivo and in vitro. Also, the timing of exposure markedly affects the inducibility of 4-OH in hepatocytes.

Animals↗
PubMed1998

Intracranial arterial dissection.

We review the angiographic and CT findings, precipitating factors and clinical features in nine patients with ten intracranial arterial dissections. The internal carotid artery was involved in five cases, the vertebral artery in four and the posterior inferior cerebellar artery in one. Angiography revealed irregular stenosis in four cases, irregular stenosis and a pseudoaneurysm in two, irregular stenosis and irregular dilatation in one, arterial occlusion in two and a pseudoaneurysm in one. CT demonstrated an infarct in four cases, a dense middle cerebral artery in two and subarachnoid haemorrhage in one. A possible precipitating factor was identified in five cases. Six patients recovered well, while three had persisting neurological deficits.

Adult↗
PubMed1998

CYP2D6 polymorphism is not crucial for the disposition of selegiline.

OBJECTIVE: To determine the possible impact of CYP2D6 polymorphism on the pharmacokinetics and pharmacodynamics of selegiline. METHODS: Five poor metabolizers and 8 extensive metabolizers of debrisoquin (INN, debrisoquine) were given 10 mg selegiline hydrochloride. The concentrations of selegiline and its main metabolites in serum were determined for 4 days. The pharmacodynamics were quantitated by measuring platelet monoamine oxidase type B activity for 3 weeks. In addition, the effect of selegiline and its main metabolites on the CYP2D6-catalyzed dextromethorphan O-demethylase activity and the effect of quinidine on the metabolism of selegiline were studied in human liver microsomes. RESULTS: Peak serum concentrations of selegiline were reached rapidly and ranged from 1 to 32 nmol/L. The metabolite concentrations were considerably higher and remained so for a longer period. There were no significant differences in the pharmacokinetic parameters of selegiline, desmethylselegiline, and l-amphetamine between poor metabolizers and extensive metabolizers. However, the area under the serum concentration-time curve (AUC) values of l-methamphetamine were, on average, 46% higher (P = .01) in poor metabolizers than in extensive metabolizers. No significant correlations were found between debrisoquin metabolic ratio and AUC values of selegiline or its metabolites, except for l-methamphetamine (rs = 0.90; P < .001). The maximum monoamine oxidase type B inhibition was 97% in both groups. The inhibitory potency of selegiline, desmethylselegiline, and l-methamphetamine toward dextromethorphan O-demethylase was very low (50% inhibitory concentration values from 160 to 580 mumol/L). Quinidine (< or = 100 mumol/L) did not inhibit the formation of desmethylselegiline or l-methamphetamine from selegiline. CONCLUSIONS: CYP2D6 is not important in the primary elimination of selegiline, and the biological effect of selegiline seems to be similar in poor metabolizers and extensive metabolizers of debrisoquin. The inhibitory effect of selegiline and its main metabolites on CYP2D6 activity seems to be negligible.

Adrenergic Agents↗
PubMed1998

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity.

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

Cytochrome P-450 Enzyme System↗
PubMed1998

Metabolism of dexfenfluramine in human liver microsomes and by recombinant enzymes: role of CYP2D6 and 1A2.

Dexfenfluramine has been widely used as an appetite suppressant in the treatment of obesity. It was recently shown that the apparent non-renal clearance of dexfenfluramine was significantly lower in poor metabolizers than in extensive metabolisers of debrisoquine which suggested the involvement of the polymorphically expressed enzyme, CYP2D6, in dexfenfluramine metabolism. In this study, human liver microsomes and yeast-expressed recombinant enzymes were used to examine dexfenfluramine metabolism in vitro. In human liver microsomes, the major product of dexfenfluramine was nordexfenfluramine with lesser amounts of a novel metabolite, N-hydroxynordexfenfluramine, and ketone and alcohol derivatives being formed. Eadie-Hofstee plots (v against v/[s]) of nordexfenfluramine formation between 1 and 1000 microM substrate concentration were biphasic in three of four liver microsome samples examined, with mean Km values of 3 and 569 microM for the high and low affinity enzymes, respectively. At a substrate concentration (0.5 microM) around the known therapeutic plasma concentration, there was negligible inhibition of microsomal dexfenfluramine N-dealkylation by sulphaphenazole and ketoconazole, but between 33 and 100% inhibition by quinidine, and 0-58% inhibition by 7,8-naphthoflavone in seven liver samples. In human liver microsomes, there was also a significant correlation (rs= 0.79, n = 10, P < 0.01) between dextromethorphan O-demethylation and dexfenfluramine (at 1 microM) N-dealkylation activities. Dexfenfluramine was a specific inhibitor (IC50 46 microM) of CYP2D6-mediated dextromethorphan O-demethylation in human liver microsomes but did not appreciably inhibit six other cytochrome P450 isoform-selective activities for CYP1A2, 2A6, 2C9, 2C19, 2E1 and 3A activities in human liver microsomes. Yeast-expressed recombinant human CYP2D6 metabolized dexfenfluramine with high affinity (Km 1.6 microM, Vmax 0.18 nmol min(-1) nmol P450(-1)) to nordexfenfluramine which was the sole product observed. Recombinant CYP1A2 was a lower affinity enzyme (Km 301 microM, Vmax 1.12 nmol min(-1) nmol P450(-1)) and produced nordexfenfluramine with small amounts of N-hydroxynordexfenfluramine. This is the first detailed study to examine the in-vitro metabolism of dexfenfluramine in human liver microsomes and by recombinant human P450s. We were able to identify CYP2D6 (high affinity) and CYP1A2 (low affinity) as the major enzymes catalysing the N-dealkylation of dexfenfluramine in human liver microsomes.

Cytochrome P-450 CYP1A2↗
PubMed1998

Expression of cytochrome P450 genes encoding enzymes active in the metabolism of tamoxifen in human uterine endometrium.

Long-term tamoxifen therapy is associated with increased risk of uterine endometrial cancer and benign alterations. Tamoxifen is metabolized to reactive intermediates by endometrial tissue, and tamoxifen therapy-induced DNA adducts have been found in human endometrium. Since metabolic activation is often catalyzed by cytochrome P450 (CYP) enzymes, the expression profile of individual xenobiotic-metabolizing CYP genes was studied in human uterine endometrium by reverse transcriptase-polymerase chain reaction. The following CYP mRNAs were detected: CYP2B6, CYP2C, CYP2E1, CYP3A4, CYP3A5, CYP4B1, and CYP11A. Amplification of CYP1A1, CYP1A2, CYP2A6, CYP2D6, CYP2F1, CYP3A7, and CYP19 was not found. CYP3A5 and CYP4B1 transcripts were found only in samples from premenopausal women. These data suggest that the human endometrial epithelium has the potential of producing CYP enzymes known to generate genotoxic intermediates from tamoxifen and metabolites that affect oestrogen receptors.

Adult↗
PubMed1998

Cytochrome P450 specificity of metabolism and interactions of oxybutynin in human liver microsomes.

Oxybutynin has an extensive first pass metabolism after oral administration, the main active metabolite being N-desethyloxybutynin. The purpose of this study was to investigate the CYP isoform specificity of oxybutynin N-deethylation and possible interactions. Oxybutynin N-deethylation in human liver microsomes in vitro was potently inhibited by ketoconazole (IC50 4.5 microM), less and variably by itraconazole and not by quinidine or several other reference inhibitors, suggesting that CYP3A enzymes are predominant catalysts of the reaction. Recombinant CYP3A5 enzyme had higher activity in oxybutynin N-deethylation than recombinant CYP3A4. Ketoconazole inhibited oxybutynin N-deethylation by the recombinant CYP3A4 and CYP3A5 almost completely, whereas itraconazole inhibited the activity of CYP3A4 more potently than that of CYP3A5. Oxybutynin inhibited CYP3A4- and CYP2D6- associated activities (testosterone 6 beta-hydroxylase and dextromethorphan O- demethylase, respectively) in human liver microsomes. CYP1A1/2-, CYP2A6-, CYP2C9- and CYP2E1-associated activities were inhibited less potently or not at all by oxybutynin when compared with reference inhibitors. Although the reasons for the weak and variable inhibition by itraconazole remain to be studied, it seems that oxybutynin is predominantly metabolized by CYP3A4 and CYP3A5 but not by CYP2D6. However, it seems to have some affinity also to the latter enzyme.

Antifungal Agents↗
PubMed1998

Developmental expression of cytochrome P450 enzymes in human liver.

Drug-metabolizing cytochrome P450 enzymes, the major phase I enzymes, are active in human liver already at very early stages of intrauterine development, although presumably at fairly low concentrations and in low numbers. During maturation, these enzymes go through various developmental programmes towards adulthood. The major increase both in abundance as well as in number of different enzymes takes place after birth, probably during the first year of life. Detailed information concerning these developmental changes is still limited. The major drug-metabolizing P450 enzymes appear to be primarily members of the CYP3A subfamily in all stages of development. The balance between different members of this subfamily, however, undergoes significant switches from the foetal predominant CYP3A7 to the major adult form CYP3A4. The ontogeny of the other cytochrome P450 enzymes is less well characterized, but the major switch-on appears to occur mainly after birth. Developmental expression of P450 enzymes is one of the key factors determining the pharmacokinetic status of developing individuals both pre- and postnatally.

Cytochrome P-450 Enzyme System↗
PubMed1997

Hepatitis A impairs the function of human hepatic CYP2A6 in vivo.

Hepatitis virus A (HVA) is a worldwide sporadic disease but its effects on pharmacokinetics and individual drug responses have not been studied. In this study, the 7-hydroxycoumarin (7OHC) excretion test used in vivo as a bioindex of hepatic CYP2A6 activity was performed in 20, previously healthy, acute jaundice HVA patients. Volunteers with an acute HVA were treated with one p.o. administration of 5 mg coumarin (Venalot). Among the patients, 11 were children (6-10 years; two girls and nine boys), the rest (15-40 years old) consisted of two men and seven women. Urinary excretion of 7OHC was measured after overnight fasting in four fractions: 0 h before any medication (to detect if any basal 7OHC excretion exits), and after a 5-mg coumarin capsule p.o., 0-2, 2-4 and 4-8 h fractions were collected and urine volumes were recorded. Urinary excretion of 7-hydroxycoumarin occurred to a similar extent in healthy adults and children. The first 2-h 7OHC excretion was decreased by 26% (P < 0.05) and total (0-8 h) 7OHC excretion was decreased by 37% (P<0.01) among HVA-positive adults (age range 15-40 years) compared with the values obtained from healthy volunteers. In 11 HVA-positive children (age 6-10 years), the first 2-h 7OHC excretion was only 20% (P < 0.0001) and the total 7OHC excretion 28% (P < 0.0001) of the value observed in healthy controls. These results suggest that (i) an acute HVA decreases the metabolic clearance of drugs such as coumarin which are rapidly metabolised by CYP2A6 and (ii) this decrease is even more prominent in children. Such metabolic responses may be of clinical importance and may also interfere with other drug therapy in these patients.

Adolescent↗