SinEx DB 2.0 update 2020: database for eukaryotic single-exon coding sequences

Single-exon coding sequences (CDSs), also known as ‘single-exon genes’ (SEGs), are defined as nuclear, protein-coding genes that lack introns in their CDSs. They have been studied not only to determine their origin and evolution but also because their expression has been linked to several types of human cancers and neurological/developmental disorders, and many exhibit tissue-specific transcription. We developed SinEx DB that houses DNA and protein sequence information of SEGs from 10 mammalian genomes including human. SinEx DB includes their functional predictions (KOG (euKaryotic Orthologous Groups)) and the relative distribution of these functions within species. Here, we report SinEx 2.0, a major update of SinEx DB that includes information of the occurrence, distribution and functional prediction of SEGs from 60 completely sequenced eukaryotic genomes, representing animals, fungi, protists and plants. The information is stored in a relational database built with MySQL Server 5.7, and the complete dataset of SEG sequences and their GO (Gene Ontology) functional assignations are available for downloading. SinEx DB 2.0 was built with a novel pipeline that helps disambiguate single-exon isoforms from SEGs. SinEx DB 2.0 is the largest available database for SEGs and provides a rich source of information for advancing our understanding of the evolution, function of SEGs and their associations with disorders including cancers and neurological and developmental diseases.

Database URL:http://v2.sinex.cl/

Rescue from neonatal death in the murine model of hereditary tyrosinemia by glutathione monoethylester and vitamin C treatment

Hereditary tyrosinemia type 1 (HT1) is a recessive disease caused by a deficiency of the enzyme fumarylacetoacetate hydrolase (FAH) that catalyzes the conversion of fumarylacetoacetate (FAA) into fumarate and acetoacetate. In mice models of HT1, FAH deficiency causes death within the first 24h after birth. Administration of 2-(2-nitro-4-trifluoro-methylbenzoyl)-1,3 cyclohexanedione (NTBC) prevents neonatal death in HT1 mice, ameliorates the HT1 phenotype but does not prevent development of hepatocellular carcinoma later on. FAA has been shown to deplete cells of glutathione by forming adducts. We tested whether a combination of a cell membrane permeable derivative of glutathione, glutathione monoethylester (GSH-MEE) and vitamin C could provide an alternative effective treatment for HT1. GSH-MEE (10 mmol/kg/j)/vitamin C (0.5 mmol/kg/j) treatment was given orally to pregnant/nursing female mice. While FAH-/- pups died in absence of treatment, all FAH-/- pups survived the critical first 24h of life when the mothers were on the GSH-MEE/vitamin C treatment and showed normal growth until postnatal day 10 (P10). However, after P10, pups showed failure to thrive, lethargy and died around P17. Thus, GSH-MEE/vitamin C supplementation could rescue the mice model of HT1 from neonatal death but it did not prevent the appearance of a HT1 phenotype in the second week after birth.

Evaluation of dichloroacetate treatment in a murine model of hereditary tyrosinemia type 1

Hereditary tyrosinemia type 1 (HT1) is an autosomal recessive disease severely affecting liver and kidney and is caused by a deficiency in fumarylacetoacetate hydrolase (FAH). Administration of 2-(2-nitro-4-trifluoro-methylbenzyol)-1,3 cyclohexanedione (NTBC) improves the HT1 phenotype but some patients do not respond to NTBC therapy. The objective of the present study was to evaluate whether administration of dichloroacetate, an inhibitor of maleyl acetoacetate isomerase (MAAI) to FAH-knockout mice could prevent acute pathological injury caused by NTBC withdrawal. DCA (0.5 and 5g/L) was given in combination with a standard diet or with a tyrosine-restricted diet. With the low-tyrosine diet body weight loss and most of hepatic and renal injuries were prevented regardless the DCA dose. The administration of DCA with a standard diet did not prevent damage nor the oxidative stress response nor the AFP induction seen in FAH-knockout mice. DCA was shown to inhibit hepatic MAAI activity to 86% (0.5g/L) and 94% (5g/L) of untreated wild-type mice. Interestingly, FAH(-/-) mice deprived of NTBC (NTBC-OFF) and NTBC-treated FAH-knockout mice had similar low hepatic MAAI activity levels, corresponding to 10-20% of control. Thus the failure of DCA treatment in FAH(-/-) mice seems to be attributed to the residual MAAI activity, high enough to lead to FAA accumulation and HT1 phenotype.

Involvement of endoplasmic reticulum stress in hereditary tyrosinemia type I

Hereditary tyrosinemia type I (HTI) is the most severe disease of the tyrosine degradation pathway. HTI is caused by a deficiency of fumarylacetoacetate hydrolase (FAH), the enzyme responsible for the hydrolysis of fumarylacetoacetate (FAA). As a result, there is an accumulation of metabolites such as maleylacetoacetate, succinylacetone, and FAA. The latter was shown to display mutagenic, cytostatic, and apoptogenic activities and to cause chromosomal instability. Herein, we demonstrate that FAA also causes a cellular insult leading to the endoplasmic reticulum (ER) stress signaling. Treatment of V79 Chinese hamster lung cells with an apoptogenic dose of FAA (100 mum) causes an early induction of the ER resident chaperone GRP78/BiP and a simultaneous phosphorylation of the eIF2alpha. FAA treatment also causes a subsequent induction of the proapoptotic CHOP (CEBP homologous protein) transcription factor as well as a late activation of caspase-12. Data obtained from fah(-/-) mice taken off the therapeutic 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3 cyclohexanedione drug are similar. However, in this mouse model, there is also an increase in proteasome activity indicative of ER-associated degradation. This difference observed between the two models may be due to the fact that the murine model measures the effects of all metabolites accumulating in hereditary tyrosinemia type I as opposed to the cellular model that only measures the effects of exogenous FAA.

La tyrosinémie héréditaire : une maladie du stress du réticulum endoplasmique ?

Hereditary tyrosinemia type 1 (HT1) is the most severe metabolic disease associated with tyrosine catabolism. An accumulation of toxic metabolites seems responsible for the pathology of HT1. The metabolite fumarylacetoacetate, accumulating due to a deficiency in fumarylacetoacetate hydrolase, displays apoptogenic, mutagenic, aneugenic and mitogenic activities. These effects may underlie the tumorigenic phenomenon observed in HT1. Fumarylacetoacetate in addition to causing disturbances in Ca2+ homeostasis, may induce endoplasmic reticulum stress.

Fumarylacetoacetate, the metabolite accumulating in hereditary tyrosinemia, activates the ERK pathway and induces mitotic abnormalities and genomic instability

Patients suffering from the metabolic disease hereditary tyrosinemia type I (HT1), caused by fumarylacetoacetate hydrolase deficiency, have a high risk of developing liver cancer. We report that a sub-apoptogenic dose of fumarylacetoacetate (FAA), the mutagenic metabolite accumulating in HT1, induces spindle disturbances and segregational defects in both rodent and human cells. Mitotic abnormalities, such as distorted spindles, lagging chromosomes, anaphase/telophase chromatin bridges, aberrant karyokinesis/cytokinesis and multinucleation were observed. Some mitotic asters displayed a large pericentriolar material cloud and/or altered distribution of the spindle pole-associated protein NuMA. FAA-treated cells developed micronuclei which were predominantly CREST-positive, suggesting chromosomal instability. The Golgi complex was rapidly disrupted by FAA, without evident microtubules/tubulin alterations, and a sustained activation of the extracellular signal-regulated protein kinase (ERK) was also observed. Primary skin fibroblasts derived from HT1 patients, not exogenously treated with FAA, showed similar mitotic-derived alterations and ERK activation. Biochemical data suggest that FAA causes ERK activation through a thiol-regulated and tyrosine kinase-dependent, but growth factor receptor- and protein kinase C-independent pathway. Pre-treatment with the MEK inhibitor PD98059 and the Ras farnesylation inhibitor B581 decreased the formation of CREST-positive micronuclei by approximately 75%, confirming the partial contribution of the Ras/ERK effector pathway to the induction of chromosomal instability by FAA. Replenishment of intracellular glutathione (GSH) with GSH monoethylester abolished ERK activation and reduced the chromosomal instability induced by FAA by 80%. Together these results confirm and extend the previously reported genetic instability occurring in cells from HT1 patients and allow us to speculate that this tumorigenic-related phenomenon may rely on the biochemical/cellular effects of FAA as a thiol-reacting and organelle/mitotic spindle-disturbing agent.

Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis

Hereditary tyrosinemia type I is the most severe metabolic disease of the tyrosine catabolic pathway mainly affecting the liver. It is caused by deficiency of fumarylacetoacetate hydrolase, which prevents degradation of the toxic metabolite fumarylacetoacetate (FAA). We report here that FAA induces common effects (i.e., cell cycle arrest and apoptosis) in both human (HepG2) and rodent (Chinese hamster V79) cells, effects that seem to be temporally related. Both the antiproliferative and apoptosis-inducing activities of FAA are dose dependent and enhanced by glutathione (GSH) depletion with L-buthionine-(S,R)-sulfoximine (BSO). Short treatment (2 h) with 35 microM FAA/+BSO or 100 microM FAA/-BSO induced a transient cell cycle arrest at the G2/M transition (20% and 37%, respectively) 24 h post-treatment. In cells treated with 100 microM FAA/-BSO, an inactivation, followed by a rapid over-induction of cyclin B-dependent kinase occurred, which peaked 24 h post-treatment. Maximum levels of caspase-1 and caspase-3 activation were detected at 3 h and 32 h, respectively, whereas release of mitochondrial cytochrome c was maximal at 24-32 h post-treatment. The G2/M peak declined 24 h later, concomitantly with the appearance of a sub-G1, apoptotic population showing typical nucleosomal-sized DNA fragmentation and reduced mitochondrial transmembrane potential (Deltapsi(m)). These events were prevented by the general caspase inhibitor z-VAD-fmk, whereas G2/M arrest and subsequent apoptosis were abolished by GSH-monoethylester or N-acetylcysteine. Other tyrosine metabolites, maleylacetoacetate and succinylacetone, had no antiproliferative effects and induced only very low levels of apoptosis. These results suggest a modulator role of GSH in FAA-induced cell cycle disturbance and apoptosis where activation of cyclin B-dependent kinase and caspase-1 are early events preceding mitochondrial cytochrome c release, caspase-3 activation, and Deltapsi(m) loss. -Jorquera, R., Tanguay, R. M. Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis.

The mutagenicity of the tyrosine metabolite, fumarylacetoacetate, is enhanced by glutathione depletion

The toxicity of tyrosine metabolites has been suggested, but not proven, to play a role in the ethiopathogenesis of hepatic alterations observed in hereditary tyrosinemia type I (HT I), a metabolic disease caused by a deficiency of the last enzyme in the tyrosine catabolic pathway, fumarylacetoacetate hydrolase. One of these metabolites, fumarylacetoacetate (FAA), is mutagenic in Chinese hamster V79 cells. We report here that FAA is a powerful glutathione depletor in this cell system. Moreover, the mutagenicity of FAA (100 microM) is potentiated by depletion of cellular glutathione (12% of control levels) by pretreatment with L-buthionine-(S,R)-sulphoximine. In this case, the mutation frequency induced by FAA is 10 times higher than in untreated, control cells. This enhancement is abolished by a partial replenishment of intracellular glutathione (32% of control levels) prior to FAA treatment. Reactive oxygen species are not generated during FAA treatment of glutathione-depleted or undepleted cells. Although the mechanism(s) underlying the mutagenic activity of FAA remains to be identified, these results show that the glutathione depletion activity of FAA may play an important role in the manifestation of its mutagenicity which likely contributes to the HT I-associated liver pathologies.

Tyrosine and its catabolites: from disease to cancer

Hereditary tyrosinemia type I (HT I, McKusick 276,700) is a metabolic disease with a pattern of autosomal recessive inheritance. The disease is caused by a deficiency of the enzyme involved in the last step in the degradation of the amino acid tyrosine, fumarylacetoacetate hydrolase (FAH). The result of this block is the accumulation of catabolites some of which have been proposed to be highly toxic due to their alkylating potential. In humans, hereditary tyrosinemia is often associated with the development of hepatocellular carcinoma in young patients. The reasons for the high incidence of hepatocellular carcinoma are unknown but it has been suggested that it may be caused by accumulated metabolites such as fumarylacetoacetate (FAA) and maleylacetoacetate (MAA). The various mutational defects in the FAH gene are reviewed. The use of two mouse models of this disease to study the molecular basis of the pathologies associated with HT I are discussed. Finally, some preliminary data on the mutagenic potential of FAA and MAA in a gene reversal assay are presented.

Transplacental carcinogenicity of low doses of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone administered subcutaneously or intratracheally to hamsters

Our previous studies have demonstrated that doses of 300-50 mg/kg 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) injected subcutaneously into pregnant hamsters cause a 44% incidence of respiratory-tract tumors in the offspring. In this study, we have extended the assay of the carcinogenic potency of NNK to doses ranging from 50 mg to 0.05 mg/kg body weight and to a second route of administration, intratracheal instillation, which is more relevant to inhalation of tobacco smoke by pregnant women. Among the offspring whose mothers had been injected subcutaneously with NNK (20-1 mg/kg), the total tumor incidence (57.2%-16.7%) decreased with decreasing dose levels. After intratracheal instillation of 50-0.05 mg/kg NNK the overall incidence varied from 28.6% to 50% but no dose response was observed. The main target organs were the adrenal glands (Phaechromocytomas) and nasal cavities (adenocarcinomas of the olfactory region). A low incidence of ductular adenomas of the pancreas was observed with low doses of NNK instilled intratracheally. These results demonstrate that NNK, at doses that are comparable to the cumulative exposure during a 9-month period in women, is a potent transplacental carcinogen in hamsters.

DNA single-strand breaks and toxicity induced by 4-(methyl-nitrosamino)-1-(3- pyridyl)-1-butanone or N-nitrosodimethylamine in hamster and rat liver

Syrian golden hamsters and F344 rats display contrasting susceptibilities to hepatocarcinogenesis induced by the tobacco-specific nitrosamine 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosodimethylamine (NDMA). In this study, the time courses of DNA single-strand breaks (SSB) and toxicity induced by NNK and NDMA in hamster and rat liver were compared. DNA SSB reached a maximum 12 h after carcinogen treatment, partially correlating with previous reports on time courses of DNA methylation. The persistence of DNA SSB up to 2-3 weeks after NNK or NDMA treatment reflects a deficient repair of some DNA lesions. No significant species differences in the kinetics of DNA SSB induction and repair were observed following NNK or NDMA (0.39 mmol/kg) treatment. However, NNK induced slightly more DNA SSB than NDMA in both species. This could reflect the formation of intermediates with more DNA-damaging capacity or inhibiting DNA processes. A significant hepatotoxic effect of NNK, evaluated by plasma markers of liver injury, was observed in rats and hamsters 12-24 h post-treatment. In contrast, NDMA induced earlier (< 12 h) enzyme elevations. Maximum hepatotoxic effects were observed 24 h (NNK-treated hamsters and rats, NDMA-treated rats) or 2 weeks (NDMA-treated hamsters) after carcinogen administration. Three weeks after treatment, hepatotoxicity of NNK and NDMA was still detected in hamsters, but not in rats. These results suggest that the toxic effects of NNK and NDMA initiate a regenerative process that occurs faster in rat than in hamster liver. Since hepatocarcinogenesis occurs in NNK- but not in NDMA-treated rats, promutagenic lesions generated from NNK might be fixed preferentially during cell proliferation.

Effects of age and ethanol on DNA single-strand breaks and toxicity induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone or n-nitrosodimethylamine in hamster and rat liver

The effects of age and ethanol exposure on liver DNA single-strand breaks (SSB) and liver cell injury induced in hamsters and rats by a single equimolar dose (0.39 mmol/kg) of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) or N-nitrosodimethylamine (NDMA) were investigated. NNK induced more DNA SSB (by 10-50%) than NDMA in the liver of adult hamsters and rats, but similar differences were not observed in newborn animals. Nitrosamine-induced hepatic DNA damages was compared in newborn and adult animals. While newborn hamsters were less sensitive to NNK-induced DNA damage than adult hamsters, newborn rats were more sensitive to NDMA-induced DNA damage than adult rats. In utero ethanol exposure did not alter significantly the induction of hepatic DNA SSB by NNK or NDMA compared to newborn hamsters and rats. Interestingly, species differences in the extents of NNK-induced hepatic DNA SSB and toxicity were observed in ethanol-consuming adult hamsters and rats. Ethanol treatment of hamsters caused a significant reduction (by 35%) of the frequency of hepatic DNA SSB and a 3.5-fold enhancement of hepatotoxicity induced by NNK.

Transplacental induction of pancreas tumors in hamsters by ethanol and the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

Epidemiological studies suggest that smoking during pregnancy and passive exposure of children to cigarette smoke may increase the cancer risk in children and young adults. We have previously shown that the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is an active transplacental carcinogen in Syrian golden hamsters when administered by s.c. injections to pregnant females. The majority of tumors in the offspring developed in the respiratory tract. Since in smoking women the respiratory tract is the portal of entry of tobacco-related carcinogens, including NNK, we have investigated the transplacental effects of NNK given by intratracheal instillation to pregnant hamsters. The modulating effect of ethanol on the transplacental carcinogenicity of NNK in this system was also investigated because smoking and consumption of alcoholic beverages are observed in pregnant women. Our data show that exposure to NNK via the maternal respiratory tract causes a similar tumor incidence in the offspring as the s.c. route of administration. Ethanol greatly enhanced the carcinogenic response to NNK, and up to 60% of the offspring exposed in utero to ethanol and NNK developed tumors of the exocrine pancreas.

Effect of tobacco smoke condensate on the metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone by adult and fetal hamster microsomes

The tobacco-specific N-nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent carcinogen in Syrian golden hamsters exposed pre- or postnatally to NNK. NNK requires metabolic activation, mainly by the cytochrome P-450 monooxygenase system, to exert its carcinogenic activity. Along with carcinogens, tobacco smoke contains other biologically active substances such as enzyme inducers and inhibitors. In this study, we have investigated the effects of tobacco smoke condensate (TSC) on microsomal metabolism of NNK in hamsters. TSC was instilled intratracheally to non-pregnant and pregnant hamsters on days 12, 13, and 14 of gestation. Following euthanasia on day 15 of gestation, liver and lung microsomes from adult and fetal hamsters were prepared, and the metabolism of NNK was analyzed by HPLC. Although TSC tended to increase the formation of some alpha-carbon hydroxylation metabolites with liver microsomes from adult hamsters, none of the metabolic pathways of NNK showed a statistically significant increase or decrease caused by TSC exposure. Similarly, no significant alterations of NNK metabolism were observed with lung microsomes from TSC-treated adult hamsters, as well as with liver or lung microsomes from fetal hamsters exposed in utero to TSC. As shown by Western blotting analyses, the protein levels of the P-450 enzymes most likely involved in NNK metabolism (i.e. P-450IIB1 and P-450IIE1) remained almost unchanged in liver or lung microsomes from TSC-exposed hamsters. Interestingly, the P-450IIB1 protein content was increased in lung microsomes from TSC-treated pregnant hamsters, an effect likely related to the altered hormonal status of these animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the cytochrome P4502E and 2B gene families in the lungs and livers of nonpregnant, pregnant, and fetal hamsters

Members of the cytochrome P4502E and 2B gene families have been implicated in the activation of nitrosamines to reactive species capable of binding to cellular macromolecules and initiating tumor formation in various rodent species. This study was initiated to determine the relative prevalence of these isozymes and their response to ethanol during pregnancy and late gestation. Nonpregnant and pregnant hamsters were given a 10% ethanol solution in their drinking water for 10 days (gestation days 5-15) prior to being killed. RNA blot analysis of liver and lung tissue from nonpregnant, pregnant, and fetal hamsters demonstrated tissue-specific expression of CYP2E and 2B in adult and fetal animals. The levels of RNA expression of both P450s in fetal hamsters were less than 30% of nonpregnant adult values. In pregnant hamsters, the hepatic levels of CYP2E and 2B RNAs were decreased compared to nonpregnant animals. In contrast, the pulmonary levels of CYP2B RNA were increased in pregnant versus non-pregnant hamsters, while no effect of pregnancy on the levels of CYP2E RNA was seen. Although rats contain a single CYP2E1 gene transcript. Northern analysis demonstrated the presence of 1.8 and 2.8 kb bands in both liver and lung tissue of the hamster. Pretreatment with ethanol had little effect on the levels of either P450 RNA species in the lungs or livers of nonpregnant, pregnant, and fetal hamsters. These results demonstrate differences in the levels of expression of members of the CYP2E and 2B gene families during pregnancy and late gestation compared to nonpregnant adult hamsters. Fetal animals, like the adults, apparently respond to ethanol treatment by altering the levels of these P450 isozymes at the post-transcriptional level.

Effects of pregnancy and ethanol treatment on the metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone by hamster liver and lung microsomes

The tobacco-specific N-nitrosamino 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent carcinogen in adult Syrian golden hamsters and causes a high incidence of tumors in the offspring of hamsters after in utero exposure. We have investigated how pregnancy and/or ethanol treatment modulates the microsomal metabolism of NNK. Pregnancy decreased the alpha-carbon hydroxylation (activation) of NNK, whereas it increased both the pyridine N-oxidation and carbonyl reduction of NNK in liver microsomes, but not in the lung. Ethanol treatment of nonpregnant hamsters induced both the hepatic microsomal alpha-carbon hydroxylation and pyridine N-oxidation of NNK, but it increased only the formation of NNAL, the N-nitroso alcohol NNAL, in the lung. Ethanol-consuming pregnant hamsters showed no changes in the hepatic or pulmonary metabolism of NNK. In contrast, fetal hamsters exposed in utero to ethanol showed a general increase in the rate of metabolism of NNK. Immunoblot analyses demonstrated a reduction in the P-450IIE1 and total P450IIB1/IIB2 protein levels in the liver of pregnant hamsters, whereas a moderate increase of P-450IIB1 was observed in the lung. Moreover, ethanol treatment increased the amount of immunodetectable P-450IIE1 and total P-450IIB1/IIB2 in the liver of nonpregnant hamsters, but only the hepatic P-450IIE1 was induced by ethanol in pregnant hamsters. The P-450IIB1 protein levels were not affected by ethanol treatment in the lung of nonpregnant, pregnant, or fetal hamsters. In contrast, the fetal hepatic P-450IIE1 and P-450IIB1/IIB2 protein levels were increased by transplacental ethanol exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Placental transfer of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone instilled intratracheally in Syrian golden hamsters

The tobacco-specific N-nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone (NNK) is a potent tumorigen in adult Syrian golden hamsters and an active transplacental carcinogen in this species. In this study, we have investigated the biodistribution and metabolism of NNK in maternal and fetal hamster tissues as a function of the dose and the time after NNK treatment. Hamsters on day 15 of gestation were instilled intratracheally with single doses (0.05-100 mg/kg) of [5-3H]NNK and sacrificed 30 min later or treated with a single dose (25 mg/kg) of [5-3H]NNK and sacrificed at various times (5-360 min) after treatment. Total radioactivity was quantified in maternal tissues (liver, lung, kidney, placenta, and stomach), in whole fetus and in fetal tissues (liver and lung). NNK and its metabolites were extracted from selected tissues (maternal plasma, amniotic fluid, fetal liver, and lung) and assayed by high-performance liquid chromatography-scintigraphy. Thirty min after treatment, radioactivity associated with NNK and its metabolites showed similar widespread tissue distribution patterns at all doses, with a linear dose relationship observed in whole fetus and fetal tissues. NNK levels detected in maternal plasma, amniotic fluid, fetal liver, and lung were also related linearly to dose. At high doses (25 mg/kg or more) of NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol was the major metabolite detected in maternal plasma. Pyridine N-oxidation of NNK predominated at the lowest doses (0.05 and 0.5 mg NNK/kg). The toxicokinetics of NNK demonstrated that this carcinogen is rapidly absorbed from the maternal lung (less than 5 min), metabolized mainly to 4-(methylnitrosamino)-1- (3-pyridyl)-1-butanol, and quickly distributed into the maternal-fetal compartment. Both NNK and its main metabolite 4-(methylnitrosamino)-1-(3- pyridyl)-1-butanol were eliminated slowly from the amniotic fluid, with levels still detectable up to 6 h after NNK treatment. These results demonstrated that NNK instilled intratracheally in pregnant hamsters crossed the placental barrier even at low doses. Moreover, NNK quickly reached fetal tissues and amniotic fluid and was eliminated slowly from these tissues, resulting in an extended exposure of the fetus to this tobacco-specific carcinogen.