In vitro metabolism of retinoic acid in hamster intestine and liver.

The metabolism of retinoic acid has been studied in subcellular fractions of hamster intestine and liver. This metabolism is induced in the tissues of vitamin Adeficient hamsters by pretreatment of the animals with oral doses of retinoic acid. The metabolic profile obtained after high pressure liquid chromatography reveals two major classes of metabolites, designated Peaks 4 and 5. Each of these peaks is more polar than retinoic acid and retains the carboxyl carbon atom. The metabolic profile observed in this system parallels closely that obtained after examining the metabolism of retinoic acid both in viva and in tracheal organ culture. The metabolic activity is localized in the 100,000 x g pellet, requires NADPH and oxygen, and is strongly inhibited by carbon monoxide. It is inducible by retinoids, but to only a minor extent by phenobarbital or 3-methylcholanthrene. The apparent K,,, values for the disappearance of retinoic acid are approximately 1 x lo-’ and 1 X 10m7 M for the liver and intestinal reactions, respectively. The metabolism is inhibited by carbon monoxide, by antioxidants, and by some P-450 inhibitors, but not by inhibitors of peroxidation. The biological activity of the Peak 4 and Peak 5 region metabolites has been assayed in tracheal organ culture. Peak 5 is more active than Peak 4 in the reversal of keratinization in culture but is approximately a log less active than the parent retinoic acid. Therefore, these metabolic products most likely represent the initial steps in the deactivation of the retinoic acid molecule.