Substrate specificity and kinetic mechanism of the insect sulfotransferase, retinol dehydratase

Spodoptera frugiperda retinol dehydratase catalyzes the conversion of retin ol to the retro-retinoid anhydroretinol. It shares sequence homology with t he family of mammalian cytosolic sulfotransferases and provides the first l ink between sulfotransferases and retinol metabolism. In this study the enz ymatic properties of retinol dehydratase were examined using bacterially ex pressed protein. We show that retinol dehydratase can catalyze the transfer of the sulfonate moiety to small phenolic compounds and exhibits many func tional similarities to the mammalian cytosolic sulfotransferases. The bisub strate reaction that it catalyzes between retinol and the universal sulfona te donor 3'-phosphoadenosine 5'-phosphosulfate seems to involve ternary com plex formation and to proceed via a Random Bi Bi mechanism. In addition to the low nanomolar K-m value for free retinol, retinol dehydratase is strong ly inhibited by retinol metabolites, suggesting a preference for retinoids. Conversely, a number of tested mammalian cytosolic sulfotransferases do no t utilize retinol, indicating that retinol is not a general substrate for s ulfotransferases.