SULFATION OF DICYCLIC AND TRICYCLIC PHENOLS BY RAT-LIVER ARYL SULFOTRANSFERASE ISOZYMES

Aryl sulfotransferases (ASTs) catalyze the sulfation of a variety of h ydroxyl-containing substrates, including phenols, aryl oximes, benzyli c alcohols, and arylhydroxamic acids. Sulfation of the latter class of substrates (e.g., N-hydroxy-2-acetamidofluorene) can yield highly uns table sulfuric acid esters capable of covalently binding to cellular n ucleophiles. Accordingly, these enzymes have been implicated in the bi oactivation of the arylhydroxamic acid (and precursor arylamine) class of hepatocarcinogens. Rat liver contains three well-characterized iso forms of AST. To understand better the factors which influence isozymi c substrate specificity, the present study focused on steric and regio chemical 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 as suming an absence of substrate-specific pH effects, the results sugges t that regiochemical and steric factors play an important role in subs trate specificity and provide insight into isozymic differences in act ive-site topology. For both AST I and AST II, the kinetic results were consistent with an active-site model in which the hydrophobic substra te binding pocket is wider, but less elongated, than that for AST IV. In addition, kinetic results for AST II with Q-phenylphenol were indic ative of negative cooperativity which was unique to this isozyme. In c ontrast to those for ASTs I and II, the kinetic results for AST IV sug gest an active-site model that is linearly extended. This elongated ac tive-site model accommodates lengthy substrates and appears to derive little catalytic benefit from additional aromatic rings which increase substrate width. (C) 1994 Academic Press, Inc.