Category: biosynthesis
Dietary retinyl esters / β-carotene → retinol (vitamin A) → retinaldehyde (RDH enzymes; reversible) → retinoic acid (RALDH; irreversible). Retinoic acid binds RAR (nuclear receptor) → controls hundreds of genes in differentiation, embryogenesis, vision, immune function. Photoreceptor cycle: 11-cis-retinal + opsin = rhodopsin → light → all-trans-retinal + isomerization back to 11-cis by RPE65 (target of LUXTURNA gene therapy for LCA). Pharmacology: tretinoin (all-trans retinoic acid) = APL (acute promyelocytic leukemia) treatment + topical for acne / photoaging; isotretinoin (13-cis) = severe nodulocystic acne (teratogenic; iPLEDGE program); acitretin = psoriasis. Vitamin A deficiency causes xerophthalmia (preventable blindness in low-resource settings) — major WHO supplementation target.