Biosynthesis of retinoyl-β-glucuronide, a biologically active metabolite of all-trans-retinoic acid

Abstract All- trans -retinoic acid is metabolized in vitro to a biologically active metabolite, retinoyl-β-glucuronide. We have studied the synthesis of this metabolite in vitro . The identity of the product was established by cochromatography on reverse-phase high-performance liquid chromatography, β- d -glucuronidase hydrolysis, and fast atom bombardment and collisionally activated decomposition/fast atom bombardment mass spectrometry. The formation of retinoyl-β-glucuronide is catalyzed by a UDP-glucuronosyltransferase with apparent K m 's of 54.7 μ m for all- trans -retinoic acid and 2.4 m m for UDP-glucuronic acid. The reaction requires enzyme, UDP-glucuronate, and no other factor. It is strongly inhibited by millimolar concentrations of coenzyme A. The specific activity of UDP-glucuronosyltransferase is greatest in the liver and least in the kidney of those tissues examined. The specific activity of the enzyme is increased by vitamin A deficiency. The increased specific activity observed in the vitamin A-deficient rat liver is uncharacteristic of retinoic acid inactivation enzymes; therefore, retinoyl-β-glucuronide may be of functional importance.