Cholesterol and hydroxycholesterol sulfotransferases: Identification, distinction from dehydroepiandrosterone sulfotransferase, and differential tissue expression

In humans, the biotransformation of cholesterol and its hydroxylated metabo lites (oxysterols) by sulfonation is a fundamental process of great importa nce. Nevertheless, the sulfotransferase enzyme(s) that carries out this fun ction has never been clearly identified. Cholesterol is a relatively poor s ubstrate for the previously cloned hydroxysteroid sulfotransferase (HST), i .e. dehydroepiandrosterone (DHEA) sulfotransferase (HST1). Recently, clonin g of a single human gene that encodes for two proteins related to HST1 was reported. These newly cloned sulfotransferases (HST2a and HST2b), while exh ibiting sequence similarity to other members of the soluble sulfotransferas e superfamily, also contain unique structural features. This latter aspect prompted an examination of their substrate specificity for comparison with HST1. Thus, HST1, HST2a, and HST2b were overexpressed as fusion proteins an d purified. Furthermore, a novel procedure for the isolation of cholesterol and oxysterol sulfonates was developed that was used in association with H PLC to resolve specific sterol sulfonates. HST1 preferentially sulfonated D HEA and, to a lesser extent, oxysterols: whereas cholesterol was a negligib le substrate. The reverse, however, was the case for the HST2 isoforms, par ticularly HST2b, which preferentially sulfonated cholesterol and oxysterols , in contrast to DHEA, which served as a poor substrate for this enzyme. RT -PCR analysis revealed distinct patterns of HST1, HST2a, and HST2b expressi on. It was particularly notable that both HST2 isoforms, but not HST1, were expressed in skin, a tissue where cholesterol sulfonation plays an importa nt role in normal development of the skin barrier. In conclusion, substrate specificity and tissue distribution studies strongly suggest that HST2a an d HST2b, in contrast to HST1, represent normal human cholesterol and oxyste rol sulfotransferases. Furthermore, this study represents the first example of the sulfonation of oxysterols by a specific human HST.