A new nomenclature system for the cytosolic SULTs has recently been devised (Blanchard et al., 2004)*, and the nomenclature used here follows this new system.

Like many drug-metabolizing enzymes, cytosolic SULTs are derived from a large superfamily of genes. Full-length cDNAs encoding more than 50 mammalian and avian cytosolic SULTs have now been cloned and sequenced and many of the expressed proteins characterized. The majority of these enzymes sulfate small molecule xenobiotics or endogenous hormones and neurotransmitters and, based on amino acid sequence analysis, can be subdivided into six families (Figure 6.1). Sequence databases for other model organisms, for example the zebrafish (Danio rerio) and amphibian (Xenopus laevis, Silurana tropicalis), contain numerous cytosolic SULT sequences with orthologs in mammalian species although the Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae databanks contain few such sequences.

At present, the human SULT enzyme family comprises 11 isoforms, which can be subdivided based on amino acid sequence identity and enzymatic function into the SULT1 (SULTs 1A1, 1A2, 1A3, 1B1, 1C2, 1C4, and 1E1), SULT2 (SULTs 2A1, 2B1a, and 2B1b), and SULT4 (SULT4A1) families. These enzymes are produced from ten genes (SULTs 2B1a and 2B1b are generated by alternate splicing of the first exon of SULT2B1) that share many common structural features (reviewed in Weinshilboum et al., 1997). The SULT1 and SULT2 families are the largest and, it is assumed, the most important for xenobiotic and endobiotic metabolism. The properties of the various human SULTs are summarized in Table 6.1.

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