Adenosine-5'-phosphosulfate kinase (APSK) catalyzes the phosphorylation of adenosine-5'-phospho-sulfate (APS) to 3'-phospho-APS (PAPS). In plants, this enzymatic activity is biochemically regulated through an intersubunit disulfide bond between Cys86 and Cys119 in the N-terminal loop of APSK To examine if O(3P) generated by the photodeoxygenation of 2,8-dihydroxymethyldibenzothiophene S-oxide could specifically oxidize APSK at its regulatory site, APSK was irradiated in the presence of 2,8-dihydroxymethyldibenzothiophene S-oxide. Near-quantitative alteration of APSK from the enzymatically active monomeric form to the inhibited dimeric form was achieved. The photoinduced increase of dimeric APSK was strongly implicated to arise from the formation of the Cys86-Cys119 disulfide bond.