抄録
Possible existence of five different subunits, designated ST-20P, ST-21P, ST-40P, ST-41P, and ST-60P, of hydroxysteroid (H) sulfotransferases (ST's) in female Sprague-Dawley rat liver cytosol was demonstrated by cloning and sequencing of cDNA's isolated from both of two rat liver cDNA libraries. The HST subunits consisted of 284 amino acid residues (Mr 33, 084-33, 256) except ST-60P (Mr 33, 535) which had an additional Met as the second amino acid residue. They shared extremely strong or very strong homology (>83%) in amino acid sequence with one another, . The HST subunits, however, shared very weak (<40%) homology with those of rat liver phenol and estrogen ST's. Of the three rat liver cytosolic HST's, STa, STb, and STc, which were separable on an anion-exchange column by direct application of the cytosol and all catalyzed sulfation of dehydroepiandrosterone and bioactivation by sulfation of the potent carcinogen, 5-hydroxymethylchrysene (5-HCR), the major enzyme STa was strongly suggested by the study on expression of cDNA's encoding ST-40P and ST-41P in Escherichia coli to consist of these microheterogeneous subunits with only one amino acid substitution. The recombinant proteins showed enzyme activities toward all of the examined 20 hydroxysteroids, 13 bile acids, and 5-HCR at rates very similar to those by STa in the presence of Tween 20. The detergent that was found to be a powerful HST stabilizer completely prevented the hepatic and recombinant HST's from facile aggregation with marked loss of their enzyme activities. In the presence of Tween 20, the recombinant HST's and STa existed as dimers and were chromatographically and electrophoretically identical. STa, STb, and STc had an equal size of subunits (30.5 kDa) which were all immunostained with anti-STa-antibody, although HST's in the rat liver cytosol had been reported to have different sizes of subunits (28-60 kDa)without immuno-homology. The present study carried out using the recombinant enzymes provides the first direct evidence for the identity of ST's catalyzing the sulfation of hydroxysteroids and bile acids and proposes that the current nomenclature system used for distinguishing HST's from bile acid STs should be improved.
