Aldehyde Oxidase Contributes to All--Retinoic Acid Biosynthesis in Human Liver.

All--retinoic acid (RA) is a critical endogenous signaling molecule. RA is predominantly synthesized from retinaldehyde by aldehyde dehydrogenase 1A1 (ALDH1A1), but aldehyde oxidase (AOX) may also contribute to RA biosynthesis. The goal of this study was to test the hypothesis that AOX contributes significantly to RA formation in human liver. Human recombinant AOX formed RA from retinaldehyde (K ∼1.5 ± 0.4 µM; k ∼3.6 ± 2.0 minute). In human liver S9 fractions (HLS9), RA formation was observed in the absence of NAD, suggesting AOX contribution to RA formation. In the presence of NAD, Eadie-Hofstee plots of RA formation in HLS9 indicated that two enzymes contributed to RA formation. The two enzymes were identified as AOX and ALDH1A1 based on inhibition of RA formation by AOX inhibitor hydralazine (20%-50% inhibition) and ALDH1A1 inhibitor WIN18,446 (50%-80%inhibition). The expression of AOX in HLS9 was 9.4-24 pmol mg S9 protein, whereas ALDH1A1 expression was 156-285 pmol mg S9 protein measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) quantification of signature peptides. The formation velocity of RA in the presence of NAD correlated significantly with the expression of ALDH1A1 and AOX protein. Taken together, the data show that both AOX and ALDH1A1 contribute to RA biosynthesis in the human liver, with ALDH1A1 being the high-affinity, low-capacity enzyme and AOX being the low-affinity, high-capacity enzyme. The results suggest that in the case of ALDH1A dysfunction or excess vitamin A, AOX may play an important role in regulating hepatic vitamin A homeostasis and that inhibition of AOX may alter RA biosynthesis and signaling. SIGNIFICANCE STATEMENT: This study provides direct evidence to show that human AOX converts retinaldehyde to RA and contributes to hepatic RA biosynthesis. The finding that AOX may be responsible for 20%-50% of overall hepatic RA formation suggests that alterations in AOX activity via drug-drug interactions, genetic polymorphisms, or disease states may impact hepatic RA concentrations and signaling and alter vitamin A homeostasis.