Figure 1.

Biosynthesis and metabolism of RA in vertebrates

Retinol obtained from diet is converted to retinaldehyde by retinal dehydrogenase enzymes. Retinaldehyde can also be synthesized from β-carotene. Oxidation of retinaldehyde to RA is carried out by RALDH enzymes. RA is a diffusible morphogen with autocrine as well as paracrine actions. RA is the ligand for heteromeric RAR and RXR receptors, which bind to RARE in promoters of target genes. In non-target tissues, RA is degraded by CYP26 enzymes.