Influence of oxidized glutathione on rat sulfotransferase 1A1 for interactions with hydroxylated polychlorinated biphenyls

Objective:The formation of disulfide bond within protein molecules is an outcome of oxidative stress.This study is to investigate the effects of partial oxidation of the sulfhydryl groups in the rat sulfotransferase(SULT) 1A1 protein on its specificity and enzymatic kinetics toward hydroxylated polychlorinated biphenyls(OH-PCBs).Methods:Rat liver cytosol was used as the native SULTs,and rat SULT1A1 was extracted and purified from recombinant E.coli cells expressing this enzyme;the enzyme activity was assayed by the formation of 3',5'-diphosphoadenosine,the reaction product of the co-factor of SULTs;the redox status of the sulfhydryl groups in the enzyme protein was modeled by pretreatment of the enzyme with oxidized/reduced glutathione(GSSG/GSH).Results:6'-OH PCB 35,4'-OH PCB 9 and 4'-OH PCB 12 could be conjugated with sulfate group in the presence of rat liver cytosol;the sulfation of 4'-OH PCB 9 and 2-naphthol by GSSG-pretreated rat SULT1A1 demonstrated decreased reaction rates and blockage of substrate inhibition.4'-OH PCB 6 was converted from an inhibitor to a substrate after pretreatment of rat SULT1A1 with GSSG,and the reaction rate increased along with the increase in the ratio of GSSG/GSH present in the pretreatment;for 4-OH PCB 14,pretreatment of the enzyme with pure GSSG was essential for the conversion of an inhibitor to a substrate.Conclusion:After oxidation of the intramolecular sulfhydryl groups,rat SULT1A1-catalyzed sulfation can apparently reduce the maximal velocity(Vmax) of its substrates,such as OH-PCBs,and eliminate substrate inhibition.The oxidation of the enzyme can result in the conversion of some inhibitory OH-PCBs to substrates,and the reaction rate is relevant to the ratio of GSSG/GSH employed for modulating the enzyme.