Glucuronidation of all-trans-retinoic acid in liposomal membranes

Retinoyl beta-D-glucuronide is a biologically active metabolite of retinoic acid. The kinetics of UDP-glucuronosyltransferase-catalyzed biosynthesis of retinoyl beta-D-glucuronide was examined in rat liver and intestinal native microsomes incubated with [3H retinoic acid incorporated into liposomes. The product was identified by cochromatography with authentic all-trans retinoyl beta-D-glucuronide, by hydrolysis with beta-D-glucuronidase, and by mass spectrometry. In vitamin A-sufficient rats the apparent Km values for all-trans-retinoic acid were 173 microM and 125 microM, and the apparent Vmax, 62 and 41 pmol/min per mg, for small intestinal and liver microsomes, respectively. In vitamin A-deficient rats repleted with all-trans-retinyl acetate, the apparent Km (91 microM) and Vmax (53 pmol/min per mg) for intestinal microsomes were in range of those of vitamin A-sufficient rats. The similarities in the kinetic parameters for UDP-glucuronosyltransferase in small intestinal mucosa and liver suggest that the reactions are catalyzed by the same enzyme. In vitamin A-deficient rats given a large amount all-trans-retinoic acid (1.2 mmol/day for 3 days) the apparent Km was 105 microM and Vmax, 127 pmol/min per mg of intestinal microsomal protein. We conclude that the kinetics of intestinal retinoic acid glucuronidation are not characteristic of simple detoxification reactions. Retinoyl glucuronide may be important in mediating retinoic acid metabolism and function.