Sulfation of Di- and Tricyclic Phenols by Rat Liver Aryl Sulfotransferase Isozymes

Aryl sulfotransferases (ASTs) catalyze the sulfation of a variety of hydroxyl-containing substrates, including phenols, aryl oximes, benzylic alcohols, and arylhydroxamic acids. Sulfation of the latter class of substrates (e.g., N-hydroxy-2-acetamidofluorene) can yield highly unstable sulfuric acid esters capable of covalently binding to cellular nucleophiles. Accordingly, these enzymes have been implicated in the bioactivation of the arylhydroxamic acid (and precursor arylamine) class of hepatocarcinogens. Rat liver contains three well-characterized isoforms of AST. To understand better the factors which influence isozymic substrate specificity, the present study focused on steric and regiochemical factors with the sulfation of polyaromatic phenols as a model system. Seven di- and tricyclic phenols were tested as substrates for ASTs I, II, and IV. Based on a comparison of kinetic constants and assuming an absence of substrate-specific pH effects, the results suggest that regiochemical and steric factors play an important role in substrate specificity and provide insight into isozymic differences in active-site topology. For both AST I and AST II, the kinetic results were consistent with an active-site model in which the hydrophobic substrate binding pocket is wider, but less elongated, than that for AST IV. In addition, kinetic results for AST II with Q-phenylphenol were indicative of negative cooperativity which was unique to this isozyme. In contrast to those for ASTs I and II, the kinetic results for AST IV suggest an active-site model that is linearly extended. This elongated active-site model accommodates lengthy substrates and appears to derive little catalytic benefit from additional aromatic rings which increase substrate width.