Sult1a1 Phenolsulfating Phenol Sult1

SULT1A3 migrates with a mass of approximately 34,000 Da (Heroux et al., 1989; Wilborn et al., 1993). Analysis of the cDNAs encoding SULT1A1 indicates the use of multiple transcriptional start sites and incorporation of different nontranslated first exons (Weinshilboum et al., 1997). A systematic evaluation of the use of different promoters for the expression of SULT1A1 or the possible involvement of these promoters in the levels or tissue selective expression of SULT1A1 has not been reported.

Expressed liver SULT1A1 is capable of sulfating a wide variety of small phenolic compounds as well as B-estradiol (Falany et al., 1994), iodothyronines (Kester et al., 1999b), and minoxidil (Falany et al., 1990). SULT1A1 activity is generally measured with low micromolar concentrations of PNP although other phenol SULTs are also capable of catalyzing PNP sulfation. The broad substrate reactivity of SULT1A1 also leads to the sulfation of buffer components and contaminants as well as small endogenous compounds such as tyrosine in tissue preparations and small alcohols in reaction mixtures. When (35S)-PAPS is used in the assays, product identification is generally required to ensure proper kinetic analysis. SULT1A1 is recognized as the major drug/xenobiotic SULT in human tissues due to its high activity in the liver and gastrointestinal tract, its broad tissue distribution, and its broad substrate reactivity. As observed with all human SULTs, SULT1A1 is capable of sulfating and activating promutagens such as 1-hydroxymethylpyrene and N-hydroxyacetylaminofluorene to reactive electrophiles (Glatt, 1997; Glatt et al., 1994, 1995).

SULT1A1 has been expressed in COS and V79 cells as well as in E. coli, either as native enzyme or with a His tag to facilitate purification (Glatt et al., 1995; Lewis et al., 1996; Veronese et al., 1994b; Wilborn et al., 1993). The cloned enzyme is easily expressed and stable and possesses kinetic properties similar to those of the purified liver enzyme (Falany et al., 1994; Wilborn et al., 1993). The active enzyme is a dimer and apparently does not undergo posttranslational modification. Many of the human SULTs demonstrate substrate inhibition with increasing substrate concentrations. Substrate inhibition is observed with SULT1A1, particularly with high affinity substrates such as PNP. Substrate inhibition is due in part to the formation of nonproductive SULT enzyme-PAP complexes although allosteric modulation by substrate may be involved with some of the SULTs (Zhang et al., 1998). Gamage et al. (2003) have reported that two molecules of PNP are capable of binding the active site of SULT1A1. The active site may be flexible, permitting conjugation of a wide range of compounds by SULT1A1.

Initial studies on the variability and thermostability of PPST activity in human platelets strongly indicated that humans expressed multiple allelic variants of SULT1A1 (Weinshilboum, 1990). At least 17 allelic variants of human SULT1A1 have been reported encoding 6 different amino acid changes (Glatt et al., 2001). Two of these alleles are highly represented in the human population. The wild-type allele (SULT1A1*1) has a frequency of 0.674 in Caucasians in the U.S., whereas the Arg213His allele (SULT1A1*2) has a frequency of 0.313 (Raftogianis et al., 1997, 1999). The expressed Arg213His allele shows decreased thermostability compared to the wild-type allele and is apparently responsible for the thermostability differences described in platelet PPST activities (Raftogianis et al., 1999). The pharmacogenetic basis for a significant fraction of the phenol SULT activity in human tissues led to the investigation of the possible association of SULT1A1 expression with cancer development in several tissues (Bamber et al., 2001; Wang et al., 2002; Zheng et al., 2001).

The structural gene for SULT1A1 is located on chromosome 16q12.1-11.2 and is a member of the cluster of SULT1A genes present at this locus (Aksoy et al., 1994a; Dooley et al., 1993; Dooley and Huang, 1996). This locus contains the genes for SULT1A1, SULT1A2, and SULT 1 A3 that are greater than 92% identical in sequence, indicating that these genes arose by duplication of a single ancestral gene. The proximity of the structural genes is also responsible for the linkage in the expression of allelic variants of the SULT1A genes (Engelke et al., 2000). Structural analysis of the SULT1A genes demonstrates considerable conservation of exonic structure between the human SULT genes and the rodent SULT genes (Weinshilboum et al., 1997). A functional role for the conservation of the exonic structure has not been described.

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