A review of the effects of manipulation of the cysteine residues of rat aryl sulfotransferase IV

Abstract

Aryl sulfotransferase IV from rat liver has the broad substrate range that is characteristic of the enzymes of detoxication. With the standard assay substrates, 4-nitrophenol and 3′-phosphoadenosine 5′-phosphosulfate (PAPS), sulfation is optimum at pH 5.4 whereas the reaction is minimal in the physiological pH range. These properties preclude a physiological function for this cytosolic enzyme. Partial oxidation of the enzyme, however, results not only in an increase in the rate of sulfation but also in a shift of the pH optimum to the physiological pH range. The mechanism for this dependence on the redox environment involves oxidation at Cys 66, the cysteine residue that is conserved throughout the phenol sulfotransferase family. As documented by mass spectroscopic methods, oxidation by GSSG leads to the formation of an internal disulfide between Cys 66 and Cys 232; for mutants at Cys 232, the oxidation product is a mixed disulfide of Cys 66 and glutathione. Both of these disulfide species activate the enzyme and allow it to function at a pH optimum in the physiological range. The activated enzyme differs from the reduced form by a more circumscribed substrate spectrum. All five mutants, in which each of the cysteines of the sulfotransferase subunit have been changed to serine, are catalytically active. Only Cys 66 is required for the redox response.