A REVIEW OF THE EFFECTS OF MANIPULATION OF THE CYSTEINE RESIDUES OF RAT ARYL SULFOTRANSFERASE-IV

Aryl sulfotransferase IV from rat liver has the broad substrate range that is characteristic of the enzymes of detoxication. With the standa rd assay substrates, 4-nitrophenol and 3'-phosphoadenosine 5'-phosphos ulfate (PAPS), sulfation is optimum at pH 5.4 whereas the reaction is minimal in the physiological pH range. These properties preclude a phy siological function for this cytosolic enzyme. Partial oxidation of th e enzyme, however, results not only in an increase in the rate of sulf ation but also in a shift of the pH optimum to the physiological pH ra nge. The mechanism for this dependence on the redox environment involv es oxidation at Cys(66), the cysteine residue that is conserved throug hout the phenol sulfotransferase family. As documented by mass spectro scopic methods, oxidation by GSSG leads to the formation of an interna l disulfide between Cys(66) and Cys(232); for mutants at Cys(232), the oxidation product is a mixed disulfide of Cys(66) and glutathione. Bo th of these disulfide species activate the enzyme and allow it to func tion at a pH optimum in the physiological range. The activated enzyme differs from the reduced form by a more circumscribed substrate spectr um. All hue mutants, in which each of the cysteines of the sulfotransf erase subunit have been changed to serine, are catalytically active. O nly Cys(66) is required for the redox response. (C) 1998 Published by Elsevier Science Ireland Ltd. All rights reserved.