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

Spodoptera frugiperda retinol dehydratase catalyzes the conversion of retinol to the retro-retinoid anhydroretinol. It shares sequence homology with the family of mammalian cytosolic sulfotransferases and provides the first link between sulfotransferases and retinol metabolism. In this study the enzymatic properties of retinol dehydratase were examined using bacterially expressed protein. We show that retinol dehydratase can catalyze the transfer of the sulfonate moiety to small phenolic compounds and exhibits many functional similarities to the mammalian cytosolic sulfotransferases. The bisubstrate reaction that it catalyzes between retinol and the universal sulfonate donor 3'-phosphoadenosine 5'-phosphosulfate seems to involve ternary complex formation and to proceed via a Random Bi Bi mechanism. In addition to the low nanomolar Km value for free retinol, retinol dehydratase is strongly inhibited by retinol metabolites, suggesting a preference for retinoids. Conversely, a number of tested mammalian cytosolic sulfotransferases do not utilize retinol, indicating that retinol is not a general substrate for sulfotransferases.