title: Redox control of aryl sulfotransferase specificity.
abstract: Aryl sulfotransferase IV from rat liver has the very broad substrate range that is characteristic of the enzymes of detoxication. With the conventional assay substrates, 4-nitrophenol and PAPS, sulfation was considered optimal at pH 5.5 whereas the enzyme in the physiological pH range was curiously ineffective. These properties would seem to preclude a physiological function for this cytosolic enzyme. Partial oxidation of the enzyme, however, results not only in a substantial increase in the rate of sulfation of 4-nitrophenol at physiological pH but also in a shift of the pH optimum to this range and radically altered overall substrate specificity. The mechanism for this dependence on redox environment involves oxidation at Cys66, a process previously shown to occur by formation of a mixed disulfide with glutathione or by the formation of an internal disulfide with Cys232. Oxidation at Cys66 acts only as a molecular redox switch and is not directly part of the catalytic mechanism. Underlying the activation process is a change in the nature of the ternary complex formed between enzyme, phenol, and the reaction product, adenosine 3',5'-bisphosphate. The reduced enzyme gives rise to an inhibitory, dead-end ternary complex, the stability of which is dictated by the ionization of the specific phenol substrate. Ternary complex formation impedes the binding of PAPS that is necessary to initiate a further round of the reaction and is manifest as profound, substrate-dependent inhibition. In contrast, the ternary complex formed when the enzyme is in the partially oxidized state allows binding of PAPS and the unhindered completion of the reaction cycle.
authors.affiliation: Laboratory of Biochemistry and Metabolism, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Marshall
authors.forename: P
authors.initials: P
authors.lastname: McPhie
authors.forename: W B
authors.initials: WB
authors.lastname: Jakoby
mesh_terms: D000818:Animals; D015239:Arylsulfotransferase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002855:Chromatography, Thin Layer; D003545:Cysteine / Q000737:chemistry; D004220:Disulfides; D004305:Dose-Response Relationship, Drug; D005978:Glutathione / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D009596:Nitrophenols / Q000494:pharmacology; D009711:Nucleotides / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D019800:Phenol / Q000378:metabolism; D010724:Phosphoadenosine Phosphosulfate / Q000494:pharmacology; D011485:Protein Binding; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013053:Spectrophotometry; D013379:Substrate Specificity; D013997:Time Factors
chemical_list: D004220:Disulfides; D009596:Nitrophenols; D009711:Nucleotides; D011994:Recombinant Proteins; D019800:Phenol; D010724:Phosphoadenosine Phosphosulfate; D015239:Arylsulfotransferase; D005978:Glutathione; D003545:Cysteine; D010100:Oxygen; C024836:4-nitrophenol