Retinoic Acid Metabolism

In the blood, serum retinol (Vitamin A) travels in association with RBP4 and enters the cells by membrane transport through STRA6 membrane transporter. Once in the cytosol, retinol molecules are sequestered by membrane systems and bind to RBP1, which plays a role in vitamin A cytoplasmic trafficking. Inside the cell, retinol undergoes the 2-steps oxidative pathway to generate all-trans retinoic acid (RA). In this case, the conversion of retinol into retinal is performed by the Retinol dehydrogenase (RDH) whereas the second oxidative step is the irreversible conversion of retinal into RA, catalyzed by Retinaldehyde dehydrogenase (RALDH). RA is finally transferred into the nucleus by RBP2 to act as a signaling molecule. RA intracellular levels must be tightly controlled to maintain the appropriate CYP26A1 degrades the excess of RA.