Fig. 1

Schematic of mechanisms for systemic retinoid transport. a Vitamin A (all-trans retinol) is secreted by hepatocytes bound to retinol binding protein 4 (RBP4). Hepatic retinol can also be esterified (retinyl ester) and transported linked to glucuronic acid (C₆H₁₀O₇) via a glycosidic bond – forming a retinyl-β-glucuronide. Further, all-trans retinoic acid is also found in serum, including bound to albumin (ALB) protein of hepatic origin and as a β-glucuronide complex. b Transthyretin (TTR) complexes with RBP4 bound retinol for systemic transport. The surface receptor protein stimulated by retinoic acid 6 (STRA6) is postulated to interact with the TTR:RBP4:Retinol complex, facilitating intracellular influx of retinol