Human SULT1A SULTs

Abstract

Conjugation with sulfonate (SO31−) is an important pathway in the biotransformation of numerous xeno- and endobiotics such as drugs, chemical carcinogens, hormones, bile acids, and neurotransmitters. This reaction is often incorrectly referred to in the literature as sulfation (Banoglu, 2000; Klaassen et al., 1998; Li and Anderson, 1999; Zhu et al., 1993a, 1993b). Xenobiotics and endobiotics may undergo sulfonation directly (phase II metabolism: paracetamol, minoxidil) or following phase I metabolism after gaining a functional hydroxyl group (e.g., N-hydroxy arylamines). The sulfonate donor for these reactions is 3′-phosphoadenosine 5′-phosphosulfate (PAPS), and the transfer of this functional group is catalyzed by a supergene family of enzymes called sulfotransferases (SULTs). In the case of the above substrates, the SULTs that catalyze this reaction are localized in the cytosolic fraction of the cell. Another class of membrane-bound SULTs, identified in the Golgi apparatus, is involved in the sulfonation of proteins, peptides, lipids, and glycosaminoglycans that affect both their structural and functional characteristics (Habuchi, 2000; Negishi et al., 2001; Niehrs et al., 1994). The focus of this review is on the emerging cytosolic forms of SULTs, for which at least 56 cDNAs and 18 gene sequences have been cloned and characterized in organisms ranging from microbes to humans (Nagata and Yamazoe, 2000; Raftogianis, 2003; Rikke and Roy, 1996). To date, five distinct gene families of SULTs have been identified in mammals: SULT1, SULT2, SULT3, SULT4, and SULT5 (Raftogianis, 2003). The SULT3 family has only been found in mouse and rabbit and has been shown to primarily sulfonate amino groups (Raftogianis, 2003). SULT5 has only been isolated from mice, and limited informa-tion is available on this family (Nagata and Yamazoe, 2000). In relation to humans, three families have been identified, consisting of ten distinct members: SULT1 —A1, A2, A3, B1, C1, C2, E1; SULT2 — A1 and B1 (SULT2B1a andSULT2B1b); and SULT4 — A1 (Raftogianis, 2003). In this review, we will concentrate on the SULT1A subfamily of enzymes that have been extensively studied and shown to be capable of metabolizing a broad range of drug, xenobiotic, and endobiotic substrates (Table 10.1). The SULT1A members have mainly been described based on their metabolic preferences as aryl, phenol, and monoamine SULTs. Although the term aryl is frequently used to collectively describe these enzymes (Bernier et al., 1994a; Duanmu et al., 2000; Nagata et al., 1997; Zhu et al., 1993a), it is technically incorrect, as it refers to the group obtained by dropping a hydrogen atom from the nucleus of an aromatic ring (e.g., phenyl). Thus in line with the new nomenclature system enunciated by Raftogianis et al. (2003), we will refer to these as members of the SULT1A subfamily.