Retinoic Acid Regulates Retinol Metabolism via Feedback Inhibition of Retinol Oxidation and Stimulation of Retinol Esterification in Ferret Liver 1 2

When the plasma concentration of retinoic acid is increased, there is an accompanying reduction of circulating levels of retinol, suggesting that retinoic acid may have a regulatory effect on retinol metabolism in vivo. To determine which specific step(s) of retinol metabolism might be regulated by retinoic acid, retinol was incubated with ferret liver microsomes or cytosol with retinoic acid in vitro. Incubating the microsomal fraction with retinoic acid resulted in a dose-dependent (up to 0.5 μmol/L) decrease inw contrary, no retinoic acid inhibitory effect was observed on retinal synthesis in the cytosol incubation, or in the cytosol plus microsome incubation. However, when retinoic acid was added to the cytosolic incubation mixture in the presence of the retinal oxidative inhibitor, citral, a dose-dependent inhibition of retinal synthesis was observed. Furthermore, the effect of retinoic acid on retinyl ester metabolism in ferret liver was studied by using endogenous retinyl esters of ferret liver as the substrate. When retinoic acid was added to the incubation mixture of microsomes plus cytosol, small, nonsignificant increases in retinol and retinyl esters were observed. When retinoic acid was added in the presence of citral, both the inhibition of retinol oxidation and the stimulation of retinol esterification were dose dependent up to ~0.3 μmol/L and then remained the same up to 1.0 μmol/L. These data strongly suggest that retinoic acid has a regulatory effect on retinol metabolism in ferret liver, which may occur via feedback inhibition of retinol oxidation and stimulation of retinol esterification.