Developmental changes in the isoelectric variants of rat hepatic hydroxysteroid sulphotransferase

Heterogeneous zonal distribution of sulfotransferase isoenzymes in rat liver

By employing dual-digitonin-pulse perfusion technique, the cytosolic fractions were prepared from periportal (PP) and perivenous (PV) hepatocytes of male and female rat livers. Sulfotransferase (ST) activities toward 2-naphthol (2NAP) (at pH 5.5 and 7.4), 4-nitrophenol, dehydroepiandrosterone (DHEA), and cortisol were measured in each fraction. DHEA-ST activity was mainly localized in PP fraction in males, while in females it was slightly higher in PP fraction than in PV samples. In contrast, phenol ST activities were higher in PV fraction in both sexes. With anti-HS-ST and anti-P-ST antisera, the levels of immunoreactive ST polypeptides were compatible with the levels of ST activities except that in female PP fraction the level of immunoreactive P-ST polypeptide was low in spite of comparatively high levels of P-ST activities. Chromatofocusing of PP and PV fractions separated P-ST activities into three major fractions (I-III), which have distinct catalytic and electrochemical properties. Fraction I was localized only in the PP samples in both sexes and revealed ST activities toward 2NAP at pH 5.5 and 7.4, while fraction II was localized in the PP and PV samples in both sexes with 2NAP-ST activity only at pH 7.4. Fraction III which had ST activities at pH 5.5 and 7.4 was present only in the PV samples in female rats, whereas in male rats it was present in both PP and PV samples. With anti-P-ST antiserum, the immunoreactive polypeptide was present in fraction III, but no immunoreactive band was detected in fractions I and II, suggesting the presence of immunochemically and electrochemically different P-ST(s) in these fractions.

Site-directed mutagenesis of rat hepatic hydroxysteroid sulfotransferases

Two cDNA clones of rat hepatic hydroxysteroid sulfotransferase (ST) (ST-40 and ST-20) were isolated and expressed in Escherichia coli cells. Several histidine residues in their coding regions are highly conserved in the ST superfamily, and histidine mutants were constructed by site-directed mutagenesis. The substitution of alanine or lysine for the histidine at position 98 in the ST-40 enzyme resulted in a loss of ST activities toward dehydroepiandrosterone (DHEA), androsterone (AD) and cortisol (CS). The mutation of histidine 98 into alanine abolished the specific binding to 3'-phosphoadenosine 5'-phosphate agarose, suggesting that the residue is located at a critical position in the 3'-phosphoadenosine 5'-phosphosulfate (PAPS) binding site. In the ST-20 enzyme, the replacement of histidine 98 with alanine also resulted in the loss of ST activity toward its preferential substrate, CS. In the ST-40 enzyme, the mutation at histidine 256 into alanine markedly reduced CS-ST activity, but DHEA-ST activity was not changed. Furthermore, selective decrease in CS-ST activity was also observed in the alanine mutant at lysine 254 or at asparagine 255 of the ST-40 enzyme. Kinetic analysis on the ST-40 and its mutant at asparagine 255 indicated that the Km value for CS was significantly increased in the mutant without any change in the Km values for 3'-phosphoadenosine 5'-phosphosulfate and DHEA. Inhibition studies demonstrated that DHEA-ST activity was competitively inhibited by AD, but not by CS in the ST-40 enzyme, whereas triethylamine, a noncompetitive inhibitor of hydroxysteroid ST, inhibited DHEA-ST activity in the ST-40 enzyme but did not inhibit CS-ST activity in either ST-40 or ST-20 enzymes. These data provide evidence that DHEA and CS bind to different sites, which probably function in a different manner in the ST-40 enzyme.

Effect of structurally diverse peroxisome proliferators on rat hepatic sulfotransferase

Exposure to perfluorocarboxylic acids, pthalate esters, and some hypolipidemic agents results in the proliferation of peroxisomes in the rodent liver. The structural diversity of these compounds suggests mechanistic diversity in their toxicity as well. To establish reliable biomarkers of peroxisome proliferation (PP) in compounds with distinct chemical toxicities, this study investigated the effect of in vivo exposure to perfluoro-n-octanoic acid, perfluoro-n-decanoic acid, di(2-ethylhexyl)phthalate (DEHP) and clofibrate on two-dimensional electrophoretic protein patterns of rat hepatic sulfotransferases, ST1A1, ST1C1 and ST2A1. After exposure to peroxisome proliferative doses, both ST1A1 and ST1C1 abundance in whole liver homogenates was significantly reduced, but only as a result of perfluorocarboxylic and exposure. The well-established PPs, DEHP and clofibrate had no effect on sulfotransferase expression whatsoever. The observed down-regulation of these STs is significant with respect to their normal detoxication activities and its potential correlation to carcinogenesis warrants further study. The present investigation supports previous studies that demonstrate the unique features of perfluorocarboxylic acid toxicity, relative to classic peroxisome proliferators and endorses the continued use of 2D protein-mapping of Sts and other proteins as biomarkers of chemical toxicity.

Isoelectric focusing as a tool for the investigation of post-translational processing and chemical modifications of proteins

It has been demonstrated that good agreement may be observed between computed and experimental isoelectric point (pI) values when proteins of known sequence are focused under denaturing conditions on immobilized pH gradient IPG slabs, at least in the pH range 4-7.5. Hence, discrepancies between expected and found in this experimental set-up may be reliably ascribed to some kind of post-transcriptional processing, or chemical modification, having taken place in the sample. This evaluation is made easier when the comparison is set between the pI of a parent molecule and that (or those) of one to several of its derivatives as resolved in a single experiment (for instance, as a spot row in two-dimensional maps); no previous knowledge is required in these cases about the amino acid composition of the primary structure. The effects on protein surface charge are discussed in this review mainly for two biologically relevant processes, glycosylation and phosphorylation. Then, the pI shifts are analysed for some protein modifications that may occur naturally but can also be artefactually elicited, such as NH2 terminus blocking, deamidation and thiol redox reactions. Finally, carboxymethylation and carbamylation are used to exemplify chemical treatments often applied in connection with electrophoretic techniques and involving charged residues. Procedures to be applied in order to verify whether a given modification has occurred, and often relying on the focusing of a treated specimen, are detailed in each section. Numerical examples on model proteins are also discussed. As an important field of application of the above concepts may be genetic engineering, an exhaustive bibliographic list dealing with pI evaluation and structural assessment on recombinant proteins is included.

Biochemistry and molecular biology of drug-metabolizing sulfotransferase

Sulfation is an important conjugation reaction in the metabolism of various xenobiotics and endogenous compounds and is catalyzed by sulfotransferase (ST) present in cytosols. The cloning studies on STs have provided the basis for the understanding of the ST multigene family. STs are classified into hydroxysteroid (or alcohol), aryl (or phenol), estrogen, flavonol and polysaccharide STs and recent developments in the molecular characterization of these isoforms are reviewed. Regulation and localization of ST isoforms in various tissues are characterized at the molecular level by virtue of the specific antibodies and the corresponding cDNA probes. The recent developments are summarized. ST inhibitors are potent tools for the study on ST multiplicity and for the characterization of the enzyme structure. It also appears to be important to understand exogenous and endogenous ST inhibitors in clinical environment. The recent developments are reviewed.

Studies on rat hepatic hydroxysteroid sulfotransferase--immunochemistry, development and pI variants

Rat hepatic hydroxysteroid sulfotransferase with sulfoconjugates androsterone (androsterone-sulfating sulfotransferase) is an oligomer consisting of several subunits with distinct pI values but with the same molecular mass (pI variants). N-terminal amino acid sequences of the pI variants are all identical. The enzyme is exclusively present in the liver, in which its lobular localization is sex-dependent. The localization of the enzyme is markedly different from that of an isoenzyme of phenol sulfotransferase. In weanling and adult female rats, the relative abundance of the pI variants is different. During development from weanling stage to adulthood, the amounts of acidic variants increase, whereas the relative levels of alkaline variants remain constant.

Inhibition of rat liver hydroxysteroid sulfotransferase activity by alkylamines

Triethylamine, which was used as an elution solvent for column chromatography to purify chemically synthesized 3'-phosphoadenosine 5'-phosphosulfate (PAPS), was a potent inhibitor of rat liver sulfotransferase (ST) activities toward androsterone and dehydroepiandrosterone, but not ST activities toward cortisol and 2-naphthol. Examination of fourteen primary, secondary and tertiary amines revealed that a secondary amine, di-n-butylamine, and three tertiary amines, triethylamine, tri-n-propylamine and tri-n-butylamine, specifically inhibited ST activities toward androgen.