CONTROL OF ACTIVITY THROUGH OXIDATIVE MODIFICATION AT THE CONSERVED RESIDUE CYS(66) OF ARYL SULFOTRANSFERASE-IV

Oxidation at Cys(66) of rat liver aryl sulfotransferase IV alters the enzyme's catalytic activity, pH optima and substrate specificity. Alth ough this is a cytosolic detoxication enzyme, the pH optimum for the s tandard assay substrate 4-nitrophenol is at pH 5.5; upon oxidation, th e optimum changes to the physiological pH range. The principal effect of the change in pH optimum is activation, which is manifest by an inc rease in K'(cat) without any major influence on substrate binding. In contrast, with tyrosine methyl ester as a substrate, the enzyme's opti mum activity occurs at pH 8.0; upon oxidation, it ceases to be a subst rate at any pH. The presence of Cys(66) was essential for activation t o occur, thereby providing a putative reason underlying the conserved nature of this cysteine throughout the phenol sulfotransferase family. Mapping of disulfides by mass spectrometry showed the critical event to be the oxidation of Cys(66) to form a disulfide with either Cys(232 ) or glutathione, either one is effective. These results point to a me chanism for regulating the activity of a key enzyme in xenobiotic deto xication during cellular oxidative stress.