Investigation of the Inhibition of UDP-Glucuronosyltransferases (UGTs) by Retinoic Acid and Retinaldehyde

Retinoic acid and retinaldehyde are important metabolites of vitamin A, and have been reported to show important function in controlling biological processes. This study aims to determine the inhibition of retinoic acid and retinaldehyde on the activities of important drug-metabolizing enzymes (DMEs) UDP-glucuronosyltransferases (UGTs). In vitro incubation system for the glucuronidation of 4-methylumbelliferone (4-MU) glucuronidation was used. For majority of UGT isoforms, retinaldehyde showed stronger inhibition than retinoic acid towards majority of UGT isoforms. Selecting UGT1A6 and UGT2B7 as the representative UGT1A isoform and UGT2B isoform, we furtherly determined the inhibition kinetics of retinaldehyde on UGT1A6 and UGT2B7. Concentration-dependent inhibition of retinaldehyde on the activity of UGT1A6 and UGT2B7 was demonstrated. Lineweaver-Burk plot showed that the intersection was located in the vertical axis and horizontal axis for UGT1A6 and UGT2B7, indicating the noncompetitive inhibition and competitive inhibition of retinaldehyde on UGT1A6 and UGT2B7. Furthermore, the second plot was drawn using the slopes of lines in the Lineweaver-Burk plot versus the concentration of retinaldehyde, and the linear fitting equation was y = 0.3x + 2.67 and y = 12.5x + 58.4. Based on these two equations, the inhibition kinetic parameters were 8.9 mu M and 4.7 mu M for UGT1A6 and UGT2B7, respectively. Taken together, we can suggest that the plasma concentration of endogenous substances or drugs undergoing UGT1A6- and UGT2B7-catalyzed glucuronidation might increase if the plasma concentration exceed the threshold value of retinaldehyde.