EXPRESSION AND CHARACTERIZATION OF A NOVEL THYROID HORMONE-SULFATING FORM OF CYTOSOLIC SULFOTRANSFERASE FROM HUMAN LIVER

Sulfation is an important conjugation reaction for a wide range of end ogenous and exogenous compounds in humans, including steroids, bile ac ids, catecholamine neurotransmitters and thyroid hormones. The cDNA fo r a distinct human cytosolic sulfotransferase (ST), hST1B2, has been i solated from a human liver lambda Zap cDNA library. The hST182 cDNA co nsists of 1144 bp and contains the coding region for a novel human cyt osolic ST that has been termed hST1B2 on the basis of its sequence sim ilarity to a rat sulfotransferase, STIB1. The hST1B2 cDNA contains an 888-bp open reading frame that encodes a 296-amino acid protein with a calculated molecular mass of 34,897 Da. The hST1B2 cDNA also has a 12 7-bp 5' untranslated region (UTR) and a 129-bp 3'-UTR, including a 22- bp poly(A)(+) tract. The amino acid sequence of hST1B2 is 74%, 53%, 53 %, 52%, 56%, and 34% identical to the amino acid sequences of rat ST1B 1 and human P-PST-1, P-PST-2, M-PST, EST, and DHEA-ST, respectively. E nzymatically active hST1B2 was expressed in the bacterial expression v ector pKK233-2 for kinetic characterization and in the bacterial expre ssion vector pQE-31, which generates a histidine-tagged fusion protein for the generation of antibodies. Expressed hST1B2 sulfates small phe nols such as l-naphthol and p-nitrophenol and thyroid hormones, includ ing 3,3'-diiodothyronine, triiodothyronine, reverse triiodothyronine, and thyroxine. No activity was detected when several steroids or dopam ine were tested as substrates. High levels of hST1B2 message were dete cted by Northern blot analysis in RNA isolated from human liver, colon : small intestine, and blood leukocytes. Immunoblot analysis detected a protein with the same mass as expressed hST1B2 in several human tiss ues that also possessed hST1B2 message. These results indicate that a novel cytosolic ST is present in human tissues, which may have an impo rtant role in thyroid hormone and xenobiotic metabolism.