Enzymic synthesis of steroid sulphates. XI. Study of the oestrogen binding site of oestrogen sulphotransferase by affinity labelling with 4-mercuri-17beta-oestradiol

Enzymic synthesis of steroid sulphates. XV. Structural domains of oestrogen sulphotransferase

Pure preparations of oestrogen sulphotransferase (3'-phosphoadenylylsulphate:oestrone sulphotransferase, EC 2.8.2.4), exhibiting the normal four-isoenzyme pattern on gel electrophoresis, revealed limited proteolytic splits in the protein chain when examined by SDS-polyacrylamide gel electrophoresis. In addition to the normal 74000 molecular weight (74 kDa) protein band, an additional major band was seen at 36 kDa, often accompanied by band at 24kDa and 12kDa. Such preparations, either alone, or after reduction and S-carboxymethylation, showed an extremely strong resistance to dissociation, a concentration of 2% SDS being required for dissociation on Sephadex G-100 column chromatography. These lower molecular weight fragments, isolated by several techniques employing dissociative conditions, all showed a remarkable ability to reassociate to a species of approx. Mr 70000 when examined by SDS-polyacrylamide gel electrophoresis. In addition, the 12kDa fragment yielded dimeric, trimeric, tetrameric, pentameric and hexameric forms. Results of amino acid analyses and tryptic digestion fingerprints of the 36kDa, 24kDa and 12kDa fragments, in conjunction with N-terminal amino acid determinations, suggested in initial protease cleavage at a susceptible region midway in the chain. One, or both, of the resultant 36kDa lobes was then further attacked to yield the 12kDa and 24kDa species - the latter again being capable of cleavage to two 12kDa species. Tryptic maps indicated that the 12kDa, 36kDa and 72kDa species were discrete polypeptide chains and not composed of subunits of the 12kDa species. These data suggest that the enzyme contains a number of domains and that strong interaction occurs between them. If these domains possess oestrogen-binding properties this would explain the most unusual wave-like kinetics exhibited by the enzyme consistent with a rate equation of degree greater than 4. Such properties also provide evidence further to that previously reported which suggests that the enzyme may be genetically related to serum albumin.

Enzymic synthesis of steroid sulphates. XIV. Properties of human adrenal steroid alcohol sulphotransferase

Pure steroid alcohol sulphotransferase (EC 2.8.2.-) has the property of sulphurylating hydroxyl groups on different positions of the steroid ring. It has now been established that although only monosulphates are formed from substrates such as 3,17-diols, the position of the sulphate group depends on the relative configuration of the hydroxyl groups. Androst-5-ene-3 beta,17 beta-diol, for example, is sulphurylated mainly at the 17-position. In addition, compounds such as epitestosterone and 17 alpha-estradiol are sulphurylated at much higher rates than their respective 17 beta-epimers. It is believed that the steroid can approach the sulphurylation site via (i) ring A with the beta-side upwards, and in this mode a 3 beta-hydroxyl is sulphurylated at a higher rate than a 3 alpha-hydroxyl, or (ii) ring D with the beta-side downwards, and in this mode a 17 alpha-hydroxyl group is oriented in an analogous fashion to the 3 beta-hydroxyl in (i). The enzyme exhibits non-Michaelis-Menten kinetics within physiological concentrations (0-2 micro M) of the substrate dehydroepiandrosterone and evidence was obtained for the presence of multiple interacting steroid-binding sites. A regulatory role for the enzyme in the secretion of dehydroepiandrosterone from the human adrenal gland is proposed.