Retinol is a lipid alcohol in which the hydrophilic hydroxyl tail is attached to its otherwise all hydrophobic hydrocarbon chain. The precursor of retinol is vitamin A. A deficiency of vitamin A in vertebrate leads to light blindness and eventually to the deterioration of the outer segments of rods, the photoreceptor cells on retina of eyes.

Two classes of retinol-binding proteins have been identified. The retinol-binding protein found in tissue has a molecular weight of 14,000 and carries retinol as a non-covalently-bound ligand; the retinol-binding protein found in plasma has an alpha-1 mobility on electrophoresis and a molecular weight of 21,000-22,000. The protein has one binding site for retinol and is responsible for the transport of vitamin A from its storage site in the liver to various vitamin A dependent tissue.

Each RBP molecule can only transport a single retinol molecule. RBP is synthesized in the hepatocytes, where it picks up one molecule of retinol in the endoplasmic reticulum. The synthesis and the secretion of RBP from the hepatocytes to the plasma is regulated by retinol. The retinol-protein complex circulates in plasma in the form of a protein-protein complex with prealbumin. In plasma, the prealbumin bound to the RBP-retinol complex stabilize the complex and prevents its loss via the kidney. A cell surface receptor recognize the complex and causes RBP ro release the retinol and to cause a conformational change of the complex which reduces its affinity for prealbumin. The free RBP molecule is then excreted through the kidney glomerus, reabsorbed in the proximal tubule cells, and degraded.

RBP is a single polypeptide chian of 182 amino acid residues which fold into an up-and-down beta barrel structure(Branden & Tooze,1991).

show topology of the up-and-down beta barrel.

The eight beta strands are all antiparallel to each other and are connected by hairpin loops. The eight antiparallel beta strands twist and curl to hold the retinol molecule in the middle. One end of the barrel is open to the solvent while the other end is closed by tight side-chain packing. The hydrophilic hydroxyl tail of the retinol molecule is at the open surface of the RBP molecule. The hydrophobic retinol molecule is packed against hydrophobic side chains from the beta strands in the barrel's core. The removal of the retinol molecule would leave big space in the barrel's core.

The RBP belongs to a superfamily of protein structure. The superfamily include biliverdin-binding protein and beta-lactoglobulin. All three proteins have polypeptide chains of approximately the same lengths that are wrapped into very similar up-and-down eight-stranded antiparallel beta barrels. They all tightly bind hydrophobic ligands inside the barrel.