Hydroxysteroid sulfotransferase and a specific UDP-glucuronosyltransferase are involved in the metabolism of digitoxin in man

Effect of spironolactone on the expression of rat hepatic UDP-glucuronosyltransferase

Spironolactone (SL) increases the glucuronidation rate of several compounds. We analyzed the molecular basis of changes occurring in major rat liver UDP-glucuronosyltransferase (UGT) family 1 isoforms and in UGT2B1, a relevant isoform of family 2, in response to SL. UGT activity toward bilirubin, ethynylestradiol and p-nitrophenol was assayed in native and activated microsomes. Protein and mRNA levels were determined by Western and Northern blotting. The lipid composition and physicochemical properties of the microsomal membrane were also analyzed. Glucuronidation rates of bilirubin and ethynylestradiol (at both 3-OH and 17 beta-OH positions), determined in UDP-N-acetylglucosamine-activated membranes, were increased in SL group. Western blot analysis revealed increased levels of UGT1A1 and 1A5 (bilirubin and 3-OH ethynylestradiol conjugation), and 2B1 (17 beta-OH ethynylestradiol conjugation). Northern blot studies suggested transcriptional regulation by the steroid. Analysis of UGT activity in native vs. alamethicin-activated microsomes indicated increased latency, which was not associated to changes in physicochemical properties of the microsomal membrane. p-Nitrophenol glucuronidation rate and mRNA and protein levels of UGT1A6, the main isoform conjugating planar phenols, were not affected by the inducer. The data suggest transcriptional regulation of specific isoforms of hepatic UGT by SL, thus explaining previously reported increases in UGT activity toward selective substrates.

Roles of electrophilic sulfuric acid ester metabolites in mutagenesis and carcinogenesis by some polynuclear aromatic hydrocarbons

Hydroxylation of meso-methyl groups with subsequent formation of reactive benzylic esters bearing a good leaving group (e.g. sulfate) was proposed as a possible biochemical mechanism of activation and tumorigenicity of methyl-substituted polycyclic aromatic hydrocarbons (PAHs). In support of this postulation, recent studies have demonstrated the formation by rodent hepatic sulfotransferase activity of electrophilic, mutagenic, and carcinogenic sulfuric acid esters of several hydroxymethyl aromatic hydrocarbons including hydroxymethyl derivatives of benz[a]anthracene, 6-hydroxymethylbenzo[a]pyrene, 5-hydroxymethylchrysene, 9-hydroxymethyl-10-methylanthracene, and 1-hydroxymethylpyrene. Besides these hydroxymethyl PAHs containing a primary benzylic alcoholic group, some aromatic hydrocarbons with secondary benzylic hydroxyl functional group(s) are also metabolically activated through sulfuric acid esterification.

Human dehydroepiandrosterone sulfotransferase: molecular cloning of cDNA and genomic DNA

Human tissues contain at least three well-characterized cytoplasmic sulfotransferase (ST) enzymes, dehydroepiandrosterone (DHEA) ST and two of phenol ST (PST). DHEA ST catalyzes the sulfation of DHEA and other steroids. We cloned and expressed two cDNAs for human liver DHEA ST. The cloning strategy involved the design of PCR primers directed against two conserved domains in ST proteins. These primers were used to generate a specific PCR product that was then used successfully to clone cDNAs for DHEA ST from a human liver cDNA library. Two cDNAs were isolated that were approximately 1.1 and 1.8 kb in length. These two clones had identical open reading frames. Both cDNAs produced enzymatically active DHEA ST protein in a mammalian expression system. Northern blot analysis confirmed the presence of 1.1 and 1.8 kb transcripts in human liver. cDNAs for a number of eukaryotic enzymes have now been cloned, and they share significant sequence homology. These ST cDNAs appear to fall into distinct groups on the basis of amino acid sequences of the proteins that they encode, thus demonstrating that the enzymes comprise a gene superfamily. We have also isolated, a genomic clone for human DHEA ST that contains approximately 3 kb of 5'-flanking sequence, exon 1 and 1.7 kb of intron 1. Characterization of the structure and regulatory elements of this gene should help to elucidate mechanisms involved in the regulation of DHEA ST in humans.