Inhibitory effect of natural furanocoumarins on human microsomal cytochrome P450 3A activity

To investigate the possible drug interaction with herbal medicine, furanocoumarin derivatives isolated from several Umbelliferous crude drugs were examined for their inhibitory effects on a typical human drug metabolizing enzyme, cytochrome P450 3A (CYP3A). Most furanocoumarins tested at 0.1 mM reduced microsomal testosterone 6beta-hydroxylation as an index of CYP3A activity to less than 50% of the control. In particular, the dimer and trimer derivatives of furanocoumarins showed striking inhibition, whose potencies were similar to that of a typical CYP3A inhibitor, ketoconazole. Preincubation of dimer types of furanocoumarins increased suppression but not most of the monomer derivatives, suggesting that the inhibition on CYP3A activity was caused by at least plural mechanisms. These results raised the possibility that the furanocoumarin containing herbal medicines may alter pharmacokinetics of co-ingested drugs similar to the case with grapefruit juice.

Sulfotransferase catalyzing sulfation of heterocyclic amines

Cytosolic sulfation of arylamines to form sulfamates is found to be mediated by sulfotransferases of three gene families (SULT1 to 3). Among them, a SULT3 form (ST3A1) showed a high selectivity for N-sulfation of N-substituted aryl and alicyclic compounds. SULT1 (phenol) and SULT2 (hydroxysteroid) sulfotransferases showed N-sulfating activities of carcinogenic heterocyclic amines. For N-hydroxyarylamine O-sulfation, SULT1 forms showed high activity. In rats, ST1C1 mediated the metabolic activation of N-hydroxyarylamines. However, the related form (ST1C2) in humans showed the negligible activity. Instead, ST1A3 showed high metabolic activating abilities among human sulfotransferases.

Alteration of hepatic sulfation by endotoxin

Administration of an endotoxin, lipopolysaccharide (1 mg/kg body weight), caused decreases in hepatic sulfations of xenobiotics in male rats through changes in the amounts of at least three forms of sulfotransferases, ST1A1, ST1B1 and ST1C1. The time-dependent decrease was first observed with ST1B1, followed by ST1A1 and ST1C1; and the decrease was more profound in the levels of mRNAs than those of proteins. The decreases in their mRNAs were, however, prevented by the pretreatment with dexamethasone. These results suggest that the endotoxin suppresses the hepatic sulfation through a cytokine-mediated mechanism to change individually hepatic levels of ST1A1, ST1B1 and ST1C1.

Human CYP2C-mediated stereoselective phenytoin hydroxylation in Japanese: difference in chiral preference of CYP2C9 and CYP2C19

Regio- and stereoselective hydroxylation of phenytoin was determined in liver microsomes of nine extensive (EM) and three poor metabolizers (PM) of mephenytoin. Hydroxyphenytoins (HPPH) were isolated and quantified after separation into four regio- and stereoisomers. The total rates of microsomal phenytoin 4'- hydroxylation were approximately 3-fold higher than those of 3'-hydroxylation, and not significantly different in EM and PM. Formation of 4'-(R)-HPPH was 4.4-fold higher in EM than in PM, whereas no clear differences between EM and PM were detected in the formation of 4'-(S)-, 3'-(R)-, and 3'-(S)-HPPH. Cytochrome P450 (CYP)2C9, expressed in a fission yeast, Schizosaccharomyces pombe, catalyzed the formation of 4'-(R)- and 4'-(S)-HPPH stereoselectively, as observed with EM, in which predominantly 4'-(S)-HPPH was formed. Recombinant CYP2C19 was more stereoselective for 4'-(R)-HPPH formation. These results, in addition to inhibition experiments with anti-human CYP2C antibody, indicate that phenytoin hydroxylation is mainly catalyzed by CYP2C9. Furthermore, CYP2C19 showed limited contribution to phenytoin 4'-hydroxylation with a different chiral preference from CYP2C9.

Enzymatic characterization of human cytosolic sulfotransferases; identification of ST1B2 as a thyroid hormone sulfotransferase

Sulfotransferases (ST) are contained as multiple forms in human tissues with overlapping substrate specificities. To identify a form which contributes to the metabolism of 3, 3',5-triiodothyronine (T3), the functional properties of human STs were compared using recombinant STs, ST1A3, ST1A5, ST1B2, ST1E4 and ST2A3. ST1B2 showed a high affinity (Km 46.2 microM) for T3 sulfation, whereas ST1A3, ST1A5, ST1E4 and ST2A3 showed high affinities to p-nitrophenol (Km 0.4 microM), dopamine (Km 7.1 microM), beta-estradiol (Km 0.3 microM) and dehydroepiandrosterone (Km 3.3 microM), respectively. In Western blotting using antibodies raised against an individual ST, hepatic absolute amounts of these STs were determined. The content of ST1B2 in human liver correlated well with T3 sulfation activities in human liver (r=0.96). These results indicate that ST1B2 is biochemically distinct from other forms of ST, and is involved in the metabolism of T3 in human. In addition, studies of thermal stability and 2,6-dichloro-4-nitrophenol (DCNP) inhibition showed that ST1B2 was thermostable and more DCNP resistant than other forms of ST. Affinities for a co-factor, phosphoadenosine 5'-phosphosulfate, also differed 9-fold among 5 different forms of ST.

Bacterial expression and functional characterization of a rat thyroid hormone sulfotransferase, ST1B1

At least three forms of phenol sulfotransferase (ST) ST1B1, ST1A1 and ST1C1 are contained in rat livers. To identify the form contributing to the metabolism of 3,3',5-triiodothyronine (T3), functional characterization of these forms was performed by expression in Escherichia coli. ST1B1 and ST1C1 were shown to be active on sulfation towards T3 with high affinity (Km: 44.4 and 25.8 microM, respectively), whereas ST1A1 had low affinity. In Western blotting using antibodies raised against the individual ST, hepatic contents of each ST were quantitatively determined. ST1B1 showed no clear sex-difference, whereas the level of ST1C1 was higher in adult males than adult females. The content of ST1B1 was 1.4, 6.8 and 10 times higher than that of ST1C1 in adult males, adult females and both sexes of immature rats, respectively. The developmental pattern of ST1B1 was similar to that of ST1A1, but differed from that of ST1C1. These results indicate that ST1B1 and ST1C1 are involved in T3 metabolism in rats and ST1B1 is the constitutive form across sexes and ages.

N-sulphoconjugation of amines by human cytosolic hydroxysteroid sulphotransferase

1. N- and O-sulphoconjugation of various substrates were studied with human liver cytosol and purified cytosolic sulphotransferase in the presence of 3-phosphoadenosine 5-phosphosulphate. 2. Human liver cytosol catalysed N-sulphoconjugation of alicyclic and aryl-amines, and O-sulphoconjugation of hydroxysteroid and phenol. Activities of amine sulphoconjugation in the cytosol correlated well with those of hydroxysteroid but not with phenol. 3. Alicyclic amine sulphotransferase in human liver cytosol was purified to homogeneity by anion exchange, affinity and hydroxyapatite chromatography. Sulphoconjugating activities of alicyclic amine co-purified with those for hydroxysteroid conjugation. Subunit molecular weight of the purified sulphotransferase was 34 kDa. Contents of the purified enzyme correlated with the sulphoconjugating activities of hydroxysteroid and alicyclic amine. From these results, we concluded that the alicyclic amine sulphotransferase purified in this study was identical to hydroxysteroid sulphotransferse in human liver cytosol. 4. The results of this study indicate that hydroxysteroid sulphotransferase in human liver cytosol catalyses N-sulphoconjugation of alicyclic and aryl-amines. Hydroxysteroid sulphotransferase in the cytosol is reported to catalyse O-sulphoconjugations of various compounds including hydroxysteroids, bile acids, cholesterol, and aliphatic and benzylic alcohols. The present and previously reported results indicate that hydroxysteroid sulphotransferase in the cytosol catalyses both N- and O-sulphoconjugations of several substrates.

Structure and expression of the rat CYP3A1 gene: isolation of the gene (P450/6betaB) and characterization of the recombinant protein

A P450 gene (P450/6betaB) of the CYP3A subfamily was isolated from a rat genomic library. Nucleotide sequencing of the exons revealed a high similarity with P450PCN1 cDNA (Gonzalez et al. (1985), J. Biol. Chem. 260, 7345-7441), but differed in 41 nucleotides, resulting in 11 changes and 2 deletions of amino acid residues. The P450/6betaB spanned about 30 kbp and consisted of 13 exons, and was in exon number and size identical with CYP3A2 gene except in the 6th exon, which was shorter than that of CYP3A2. 6beta-B mRNA, which may be transcribed from P450/6betaB, was detected on Northern blotting and by reverse transcription-polymerase chain reaction (RT-PCR). Profiles of the developmental change and induction by a treatment with several chemicals were very similar to those of P450PCN1 mRNA reported previously. P450PCN1 mRNA and gene, however, were not detected by PCR in rats. To determine whether P450/6betaB encodes an active protein, a cDNA was isolated and expressed. Expression of 6beta-B cDNA in COS-1 cells was carried out and revealed that the recombinant protein comigrated with purified P4506beta-4 previously identified as CYP3A1. The recombinant 6beta-B protein showed similar turnover rate and regioselectivity for testosterone with purified P4506beta-4 by the simultaneous addition of NADPH-cytochrome P450 reductase and cytochrome b5. These data suggest that P450/6betaB encodes an active P450 form corresponding to CYP3A1 and P450PCN1 reported previously does not exist in rats.

Purification and characterization of two amine N-sulfotransferases, AST-RB1 (ST3A1) and AST-RB2 (ST2A8), from liver cytosols of male rabbits

Two sulfotransferases (STs), designated as AST-RB1 (ST3A1) and AST-RB2 (ST2A8), with high a amine N-sulfonating activity, were purified from male rabbit liver cytosols. AST-RB1 and AST-RB2 were purified to homogeneity by the anion-exchange, affinity, and hydroxyapatite chromatography. The N-terminus of both enzymes were blocked. The subunit molecular mass of both enzymes was estimated to be 34 kDa on SDS-PAGE. AST-RB1 efficiently catalyzed N-sulfonation of alicyclic, alkyl, and arylamines such as 4-phenyl-1,2,3, 6-tetrahydropyridine, 1-[(5-chloro-2-oxo-3(2H)-benzothiazolyl)acetyl]-piperazine, desipramine, and aniline, whereas its catalytic activities toward 2-naphthol and dehydroepiandrosterone (DHEA) were very low. On the other hand, AST-RB2 efficiently catalyzed sulfonation of desipramine and DHEA, but had no activity toward 2-naphthol. Amino acid sequences of peptide fragments derived from the purified AST-RB1 showed no significant homology with previously reported STs, but those from the purified AST-RB2 shared a high similarity with those of the ST2 family. Both enzymes were expressed specifically in the liver. The present results strongly suggest that the purified AST-RB1 is a novel enzyme in terms of structure and catalytic properties showing high selectivity for amine substrates, and AST-RB2 is a quite unique from among ST2A enzymes of other species in its substrate specificity.

Hepatocyte nuclear factor 4-mediated activation of rat CYP3A1 gene and its modes of modulation by apolipoprotein AI regulatory protein I and v-ErbA-related protein 3

CYP3A1 gene (P450/6betaB) encodes testosterone 6beta-hydroxylase (EC 1.14.14.1) in rats. The promoter region of CYP3A1 gene contains three binding sites for nuclear factors: 6betaB-A (-105 to -86), 6betaB-B (-139 to -118), and 6betaB-C (-164 to -145). The 6betaB-A site shows a high degree of similarity to a consensus sequence of the binding site of hepatocyte nuclear factor 4 (HNF-4) and also to the 6betaA-A site on the rat CYP3A2 gene promoter region. Our previous study suggested an involvement of the 6betaA-A site in the basal transactivation of CYP3A2 gene using HepG2 cells. In the present study, transactivation through the 6betaB-A and 6betaA-A sites of CYP3A1 and CYP3A2 genes has directly been shown by coexpression of HNF-4 and CYP3A1 or CYP3A2 promoter-reporter fused genes. Similar experiments further showed that nuclear factor binding at the 6betaB-B site hampered HNF-4-mediated transactivation of CYP3A1 gene. Recombinant apolipoprotein AI regulatory protein I (ARP-1) and v-ErbA-related protein 3 (EAR-3) are shown to suppress HNF-4-mediated activation at the 6betaB-B site without competition of HNF-4.

Molecular characterization of ST1C1-related human sulfotransferase

Carcinogenic arylamines such as N-hydroxy-2-acetylaminofluorene (N-OH-AAF) are metabolically activated by mammalian sulfotransferases to form N-hydroxyarylamine O-sulfates. We previously showed that rat ST1C1 efficiently mediate these activations. These reactions occur in liver cytosols of humans as well as rats. However, the enzyme responsible for N-OH-AAF activation has not been identified in humans. In the present study, a human cDNA (ST1C2) encoding a sulfotransferase showing a high similarity with ST1C1, has been isolated from a human fetal liver cDNA library and expressed using a bacterial expression system. A clear difference was observed in the pH optima for p-nitrophenol sulfation between ST1C2 and ST1C1 expressed in Escherichia coli. In addition, ST1C2 did not mediate 3'-phosphoadenosine-5'-phosphosulfate-dependent DNA binding of N-OH-AAF. These results suggest that human ST1C2 has a clear different substrate specificity, in spite of the structural similarity, with rat ST1C1.

cDNA cloning and expression of a novel CYP3A from the Syrian hamster, CYP3A31

A clone, encoding a cytochrome P450 protein consisting of 501 amino acids, was isolated from a cDNA library constructed from mRNA of Syrian hamster liver. The deduced amino acid sequence of this clone showed a high homology (65 to 81%) with other mammalian CYP3As and hence, this novel isozyme was named CYP3A31. By Northern blotting, using an oligonucleotide specific to CYP3A31, the mRNA for this isozyme was shown to be expressed constitutively in liver and induced by treatment with phenobarbital but repressed by 3-methylcholanthrene or dexamethasone treatments. The increase in mRNA expression by phenobarbital and decrease by dexamethasone corresponded to changes in CYP3A protein as analysed by Western blotting. These indicate that CYP3A31 might constitute one of the major CYP3A isozymes in the hamster.

Molecular cloning, expression, and enzymatic characterization of rabbit hydroxysteroid sulfotransferase AST-RB2

Cytosolic sulfotransferases, which consist of at least three gene families, play a major role in activation and detoxification of both endogenous and exogenous chemicals. We recently purified a rabbit sulfotransferase, AST-RB2, showing high activities to both hydroxysteroids and amines. To characterize this enzyme, a rabbit cDNA library was screened using anti-AST-RB2 antibodies. The isolated cDNA was judged to encode AST-RB2 (ST2A8) based on the amino acid sequences of peptide fragments obtained from purified AST-RB2. The cDNA showed high similarity to other mammalian hydroxysteroid sulfotransferases (ST2) at the amino acid level (58-68%), but low similarity to aryl sulfotransferases (ST1) (less than 37%). The protein expressed in Escherichia coli catalyzed sulfation of typical ST2 substrates. Therefore, ST2A8 was judged to belong to the ST2 family from both its primary structure and substrate specificity. The ST2A8 protein expressed in E. coli clearly differed from rat ST2A1 and ST2A2 on its localization (cytosol/insoluble fraction ratio). ST2A8 had no activity to lithocholate, but showed the highest catalysis on dehydroepiandrosterone and testosterone among the four forms (ST2A1, ST2A2, ST2A3, and ST2A8), indicating a clear difference between ST2A forms in substrate specificity to endogenous chemicals.

2-Methoxy-4-nitroaniline and its isomers induce cytochrome P4501A (CYP1A) enzymes with different selectivities in the rat liver

We reported previously that 2-methoxy-4-nitroaniline (2-MeO-4-NA) is a selective inducer of cytochrome P4501A2 (CYP1A2) in the rat liver, and its molecular size is the smallest among known CYP1A2-selective inducers. In the present study, a structure-activity relationship on the CYP1A2-selective induction has been investigated using isomeric nitroanisidines and their related chemicals. Western blot analyses revealed that the chemicals removed a substituent (amino, methoxyl or nitro group) from a 2-MeO-4-NA molecule had no capacity for inducing CYP1A enzymes in rat livers. On the other hand, isomeric nitroanisidines such as 2-MeO-4-NA, 2-MeO-5-NA and 4-MeO-2-NA induced both CYP1A2 and CYP1A1 enzymes with different selectivities. As judged from the induced levels of CYP1A proteins, 2-MeO-4-NA (CYP1A2/CYP1A1 ratio; 9.5) and 4-MeO-2-NA (0.3) were the most selective inducers of CYP1A2 and CYP1A1, respectively, among the isomeric nitroanisidines (0.44 mmol/kg) used. The induced level of CYP1A2 protein was in the order 2-MeO-4-NA > 2-MeO-5-NA > 4-MeO-2-NA, although no significant difference was observed on their CYP1A2 mRNA level. On the contrary, increases in the levels of CYP1A1 mRNA and protein were in the order 4-MeO-2-NA > 2-MeO-5-NA > 2-MeO-4-NA. The present findings indicate that all three substituents (amino, methoxyl and nitro groups) are necessary components of nitroanisidines for induction of CYP1A enzymes, and also show that regio-isomeric positions of these substituents determine the selectivity in the induction of CYP1A enzymes.

Molecular cloning and expression of an amine sulfotransferase cDNA: a new gene family of cytosolic sulfotransferases in mammals

A cDNA of amine sulfotransferase-RB1 (AST-RB1), which efficiently catalyzes 4-phenyl-1,2,3,6-tetrahydropyridine (PTHP) sulfation, has been isolated by immunoscreening of a rabbit liver cDNA library. The cDNA consisted of 1,117 base pairs and encoded a protein of 301 amino acids with a molecular weight of 35,876. The deduced amino acid sequence matched at six positions those of peptide fragments obtained from purified AST-RB1 protein. The sequence had less than 38% identity at the amino acid level with cytosolic sulfotransferases in mammals, although high degrees of similarity were observed with regions conserved throughout mammalian sulfotransferases. These results indicate that AST-RB1, arbitrarily named sulfotransferase 3A1 (ST3A1), constitutes a new and third gene family of cytosolic sulfotransferases in mammals. ST3A1 expressed in Escherichia coli as a fused protein catalyzed sulfation of amines such as PTHP, aniline, 4-chloroaniline, 2-naphthylamine, and desipramine, but barely O-sulfation of typical aryl and hydroxysteroid sulfotransferase substrates. These data unequivocally demonstrate the existence of a cytosolic sulfotransferase showing a high selectivity for amine substrates, and indicate that multiple forms of sulfotransferase mediate sulfation of xenobiotics in mammalian livers.

Genetic polymorphisms in human liver phenol sulfotransferases involved in the bioactivation of N-hydroxy derivatives of carcinogenic arylamines and heterocyclic amines

Three related forms of phenol sulfotransferase (PSULT), thermostable ST1A2 (SULT1A2hum) and ST1A3 (SULT1A1hum) and a thermolabile TL-PST (SULT1A3hum), are known to exist in human livers. Thermostable forms, whose activities are polymorphically distributed, have been shown to mediate the bioactivation of carcinogenic N-hydroxy arylamines and heterocyclic amines. To clarify the nature of the sulfation polymorphism, the study compared the expressed levels of ST1A2, ST1A3 and TL-PST mRNAs in human livers by the method of reverse-transcriptase polymerase chain reaction (RT-PCR), utilizing HindIII, BamHI and SnaBI sites which were unique to the above PSULT cDNAs, respectively. Of the PCR products derived from human liver (n = 26), 43-89, < 1-29 and < 1-21% showed the restriction pattern characteristic for ST1A3, ST1A2 and TL-PST cDNAs, respectively, thus indicating that ST1A3 mRNA is the major transcript. Hepatic p-nitrophenol and dopamine sulfation rates ranged from 440-2670 and < 5-460 pmol/min per mg protein in the 26 individuals, respectively. The observed differences in the ST1A3 and TL-PST mRNA levels were consistent with the differences in p-nitrophenol and dopamine sulfations. Relative levels of hepatic ST1A3 mRNA were non-normally distributed and correlated significantly with p-nitrophenol sulfation. In addition, variant forms of ST1A3 mRNA encoding Arg213His and Met223Val were detected in human livers. With regard to Arg213His, 28 individuals who had homozygous 213Arg alleles, 15 individuals who were heterozygotes and nine homozygous 213His individuals were found by a newly established genotyping method among 52 human liver samples. Frequency of 223Val allele was apparently lower than that of 213His allele, as no homozygous 223Val individual and only three individuals who were heterozygotes (223Met/Val) were observed among 52 individuals. These results suggest that regulation of p-nitrophenol sulfation occurs at the level of gene transcription of ST1A3 which is the major transcript of the three PSULT mRNAs and that a polygenic basis for the apparent genetic polymorphism of sulfation was likely because of the existence of ST1A3 variants.

Sulfotransferase-mediated activation of mutagens studied using heterologous expression systems

Sulfation is a common final step in the biotransformation of xenobiotics and is traditionally associated with inactivation. However, the sulfate group is electron-withdrawing and may be cleaved off heterolytically in some molecules leading to electrophilic cations which may form adducts with DNA and other important cellular structures. Since endogenous sulfotransferases do not appear to be expressed in indicator cells of standard mutagenicity tests, rat and human sulfotransferases have been stably expressed in his- Salmonella typhimurium strain TA1538 and Chinese hamster V79 cells. Using these recombinant indicator cells, sulfotransferase-dependent genotoxic activities were detected with N-hydroxy-2-acetylaminofluorene, 2-acetylaminofluorene (in the presence of co-expressed rat cytochrome P450 1A2), hycanthone, 1'-hydroxysafrole, alpha-hydroxytamoxifen and various benzylic alcohols derived from polycyclic aromatic hydrocarbons. In several cases, it was critical that the reactive sulfuric acid conjugates were formed directly within the indicator cells, owing to the inefficient penetration of cell membranes. In other cases, spontaneous benzylic substitution reactions with medium components, such as halogenide ions or amino acids, led to secondary, membrane-penetrating reactive species. Different sulfotransferases, including related forms from rat and human, substantially differed in their substrate specificity towards the investigated promutagens. It is known that some sulfotransferases are expressed with high tissue and cell type specificities. This site-dependent expression together with the limitations in the distribution of reactive sulfuric acid conjugates may explain organotropic effects of compounds activated by this metabolic pathway.

Molecular cloning and characterization of rat ST1B1 and human ST1B2 cDNAs, encoding thyroid hormone sulfotransferases

Human and rat cDNAs encoding thyroid hormone sulfotransferases have been isolated from their liver cDNA libraries. The isolated sulfotransferases, termed rat ST1B1 and human ST1B2, share 77 and 74% homologies at nucleotide and deduced amino acid levels. These forms showed less than 36 and 56% homologies to hydroxysteroid and aryl sulfotransferases, indicating that they constitute a new gene subfamily of aryl sulfotransferase. Expression of ST1B1 and ST1B2 in COS-1 cells resulted in the appearance of 33.0 and 32.5 kDa proteins, respectively, whose mobilities were identical with proteins detected in rat and human livers in Western blots using antibodies raised against ST1B1 and ST1B2 produced in Escherichia coli. The recombinant forms catalyzed sulfation of p-nitrophenol, 3,3',5-triiodothyronine (T3) and dopamine, but not of beta-estradiol and dehydroepiandrosterone. ST1B1 and ST1B2 showed higher affinities for formation of T3 sulfate (apparent Km 40.2 and 63.5 microM, respectively) than did thermostable phenol sulfotransferase ST1A3 (apparent Km 413 microM) or thermolabile phenol sulfotransferase ST1A5 (apparent Km 180 microM). These data indicate that the newly characterized sulfotransferases constitute a distinct ST1 subfamily of enzymes catalyzing the sulfation of T3 as a typical endogenous substrate in rats and humans.

Specific CYP3A4 inhibitors in grapefruit juice: furocoumarin dimers as components of drug interaction

Four components were isolated from grapefruit juice that inhibit human CYP3A-mediated drug oxidation. The structures of these compounds were identified as furocoumarin derivatives by absorption spectra, APCI-liquid chromatography/tandem mass spectrometry and nuclear magnetic resonance after their purification by reversed-phase high performance liquid chromatography. They include two new furocoumarins, 4-[[6-hydroxy-7-[[1-[(1-hydroxy-1-methyl)ethyl]-4-methyl-6- (7-oxo-7H-furo[3,2-g][1]benzopyran-4-yl)-4-hexenyl]oxy]-3,7-dimeth yl- 2-octenyl] oxy]-7H-furo[3,2-g][1]benzopyran-7-one (GF-I-1) and 4-[[6-hydroxy-7-[[4-methyl-I- (1-methylethenyl)-6-(7-oxo-7H-furo[3,2-g][1]benzopyran-4-yl)-4- hexenyl] oxy]-3,7-dimethyl-2-octenyl]oxy]-7H-furo[3,2-g][1]benzopyran-7-one (GF-I-4). These furocoumarins are strong candidates for causative agents of grapefruit juice-mediated drug interaction, because of an inhibition potential that is equal to or stronger than the prototypical CYP3A4 inhibitor, ketoconazole, on liver microsomal testosterone 6 beta-hydroxylation.

Distinct regulation of two hydroxysteroid sulfotransferases, ST2A1 and ST2A2, by growth hormone: a unique type of growth hormone regulation in rats

In the present study, changes in the mRNAs of two major forms of hydroxysteroid sulfotransferases (STs), ST2A1 and ST2A2, have been determined in different growth hormone (GH) states. Hepatic ST2A1 mRNA was detected in both sexes of mature Sprague-Dawley rats. The level was 5 times higher in the females than in the males. ST2A1 mRNA was undetectable in GH-deficient animals, such as hypophysectomized rats and spontaneous dwarf rats. Continuous infusion of GH (mimicking the female secretory pattern) increased hepatic levels of ST2A1 mRNA in both GH-deficient animals. ST2A2 mRNA was detected only in the livers of mature female rats and in both sexes of GH-deficient animals. Intermittent injection of GH (mimicking male secretory pattern) strongly suppressed hepatic levels of ST2A2 mRNA in both GH-deficient animals. These results indicate that pituitary GH independently regulates both ST2A1 and ST2A2 at the pretranslational levels. These differences in GH responses between ST2A1 and ST2A2 are in good agreement with their female-dominant and female-specific modes of expression in normal rats. Furthermore, the present study demonstrates a unique response of ST2A2 to the secretory pattern of GH among the drug-metabolizing enzymes in rat livers, in which ST2A2 mRNA levels are suppressed by the male secretory pattern but not by the female secretory pattern of GH.

Arylamine activating sulfotransferase in liver

Carcinogenic N-hydroxy-arylamines and -arylamides undergo metabolic activation by several enzymes in mammals to cause the DNA damage. Cytosolic sulfotransferases in rat and human livers, which belong to the ST1 (SULT1) family, have been studied to assess their properties to mediate the metabolic activation. A human orthologue of rat ST1C1 from, which catalyzes sulfation of N-hydroxy-2-acetylaminofluorene, was screened in a EMBL genomic library with ST1C1 cDNA [Nagata, K., S. Ozawa, M. Miyata, M. Shimada, D.-W. Gong, Y. Yamazoe and R. Kato (1993) J. Biol. Chem., 268, 24720-24725]. Sequencing of the hybridized clones indicate that at least 3'-terminal region of human ST1C1 orthologue contains sequence highly homologus to rat ST1C1 at both nucleotide and deduced amino acid levels. The experiments using anti-rat ST1C1 antibody and nucleotide probes for human ST1C1 showed no detectable band in Western blots and an mRNA-detecting method with polymerase chain reaction, respectively, in human liver samples.

Grapefruit component interacting with rat and human P450 CYP3A: possible involvement of non-flavonoid components in drug interaction

Active components in grapefruit juice, which modulate a cytochrome P450 (CYP3A) activity, were investigated. CYP3A-catalyzed 6beta-hydroxylation of testosterone in livers of rat and human was inhibited by the addition of an ethyl acetate-extract of grapefruit juice. Several components of grapefruit juice, including naringin, naringenin, limonin and obacunone, also showed inhibitory effects in human liver microsomes. However, the amounts of these components in grapefruit juice are too low to account for the inhibition by the ethyl acetate-extracts. Analyses with HPLC indicate the existence of inhibitory components in the extract, which are distinct from these known compounds and are specific to grapefruit juice. These results suggest that hydrophobic components other than flavonoids, probably coumarin derivatives, are responsible for the inhibitory effect of grapefruit juice.