Oxidation-dependent inactivation of aryl sulfotransferase IV by primary N-hydroxy arylamines during in vitro assays

The sulfation of primary N-hydroxy arylamines is a critical intermediate step in the bioactivation of many carcinogenic arylamines, arylamides and nitroaromatics. However, the study of this reaction in vitro is often complicated by the chemical instability of these molecules. We have examined the stability of two highly purified N-hydroxy arylamines, N-hydroxyaniline and N-hydroxy-2-aminofluorene, under different oxidative reaction conditions pertinent to the assay of sulfotransferases. Furthermore, these compounds, as well as the products of their oxidative degradation, were examined for their interactions with homogeneous aryl sulfotransferase (AST) IV. Under reaction conditions where oxidative degradation of the N-hydroxy arylamines occurred, N-hydroxyaniline and N-hydroxy-2-aminofluorene produced time-dependent and irreversible inhibition of AST IV. While this inhibition was not dependent upon the presence of 3'-phosphoadenosine 5'-phosphosulfate in the reaction mixture, analysis of the N-hydroxy arylamines by UV spectroscopy showed that the inhibition of AST IV did require non-enzymatic oxidation of the N-hydroxy arylamine. Under reaction conditions that prevented the oxidative degradation of N-hydroxyaniline, this N-hydroxy arylamine was a substrate for AST IV. Likewise, under similar conditions, 4-chloro-N-hydroxyaniline was also a substrate for the enzyme. In contrast, no AST IV catalyzed sulfation of N-hydroxy-2-aminofluorene was detected under conditions that prevented the oxidation of N-hydroxy-2-aminofluorene. Adequate protection of these N-hydroxy arylamines from oxidative degradation required the addition of L-ascorbic acid to reaction mixtures that had also been degassed and purged with argon. The irreversible inhibition of AST IV exhibited by these N-hydroxy arylamines, even in reaction mixtures where attempts were made to limit oxidative degradation by degassing and purging with argon, emphasized the importance of completely preventing such degradation when utilizing in vitro assays to assess the potential for an N-hydroxy arylamine to serve as a substrate for a specific sulfotransferase.