SULT1C3, an orphan sequence of the human genome, encodes an enzyme activating various promutagens

The human genome contains a sequence that is homologous to genes encoding soluble sulphotransferases (SULTs) based on the nucleotide sequence and possible intron/exon splice sites. The putative coding sequence (termed SULT1C3) was synthesized and integrated into a bacterial expression vector. We used the cDNA-expressed protein for raising an antiserum and studying enzyme activities. No activity was detected with 4-nitrophenol and 1-naphthol, known substrates of all other members of the human SULT1 subfamily. The activity was also negligible with paracetamol, ethanol, 5-hydroxymethylfurfural, 2-hydroxymethylpyrene, 2-(alpha-hydroxy)ethylpyrene, and corticosterone, compounds for which we have developed sensitive enzyme assays with direct determination of the product by HPLC-UV, HPLC-fluorescence or HPLC-MS/MS. Since diverse sulpho conjugates are chemically reactive - often short-lived and mutagenic - we expressed SULT1C3 in Ames'Salmonella typhimurium strains TA1538 and TA100, as we had done with many other SULTs previously. The expression level of SULT1C3 protein amounted to 2% of the total cytosolic proteins, which is in the middle range of other SULTs expressed in this model. Using recombinant bacterial tester strains in mutagenicity assays, we observed SULT1C3-mediated activation of several large benzylic alcohols derived from alkylated polycyclic hydrocarbons: 1-hydroxymethylpyrene, both enantiomers of 1-(alpha-hydroxy)ethylpyrene, 6-hydroxymethylbenzo[a]pyrene and 6-hydroxymethylanthanthrene. 1'-Hydroxysafrole was the smallest molecule activated by SULT1C3 up to date. Our study demonstrates that SULT1C3 has sulphotransferase activity and that it prefers relatively large substrates. The substrates detected were activated to mutagens, which cannot be the regular function of the enzyme. The physiological substrates remain to be identified. Probably, they are relatively large, endogenous or common exogenous, molecules.