Biosynthesis of β-glucuronides of retinol and of retinoic acid in vivo and in vitro

Abstract

After the intraportal injection of retinol-6,7-14C to rats, the 0-ether derivative of retinol, retinyl β-glucosiduronate, appears in the bile. Both retinoyl β-glucuronide and retinyl β-glucosiduronate are also synthesized in vitro when washed rat liver microsomes are incubated with uridine diphosphoglucuronic acid (UDPGA) and either retinoic acid or retinol, respectively. The synthesis of retinoyl β-glucuronide was also demonstrated in microsomes of the kidney and in particulate fractions of the intestinal mucosa. The glucuronides were characterized by their UV absorption spectra, by their quenching of UV light or fluorescence under it, by their thinlayer chromatographic behavior in two solvent systems, and by the identification of products released during their hydrolysis by β-glucuronidase.With retinoic acid as the substrate, the UDP glucuronyl transferase of rat liver microsomes had a pH optimum of 7.0, a temperature optimum of 38°C, and a marked dependence on the concentrations of both retinoic acid and UDPGA, but was unaffected by a number of possible inhibitors, protective agents, and competitive substrates. The conversion of retinal to retinoic acid and the synthesis of retinoyl β-glucuronide from retinoic acid could not be detected in whole homogenates, cell fractions, or outer segments of the bovine retina.

Keywords

• retinol
• retinyl β-glucosiduronate
• retinoic acid
• retinoyl β-glucuronide
• rat liver microsomes
• glucuronyl transferase