title: Site-directed mutagenesis of rat hepatic hydroxysteroid sulfotransferases.
abstract: 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.
authors.affiliation: Kyoritsu College of Pharmacy, Tokyo, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Homma
authors.forename: K
authors.initials: K
authors.lastname: Ogawa
authors.forename: K
authors.initials: K
authors.lastname: Hirono
authors.forename: Y
authors.initials: Y
authors.lastname: Morioka
authors.forename: M
authors.initials: M
authors.lastname: Hirota
authors.forename: I
authors.initials: I
authors.lastname: Tanahashi
authors.forename: M
authors.initials: M
authors.lastname: Matsui
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D018076:DNA, Complementary / Q000235:genetics; D003687:Dehydroepiandrosterone / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D007527:Isoenzymes / Q000235:genetics*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000235:genetics; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D015238:Sulfotransferases / Q000235:genetics*
chemical_list: D018076:DNA, Complementary; D007527:Isoenzymes; D011993:Recombinant Fusion Proteins; D003687:Dehydroepiandrosterone; D015238:Sulfotransferases; C028152:alcohol sulfotransferase