Few retinoids are appreciably soluble in water or aqueous body fluids. They gain solubility through association with specific proteins.
Plasma retinol is transported by a specific 21-kDa transport protein, retinal binding protein (RBP). Most RBP is produced in the liver, but some extra-hepatic organs also synthesize it. Each molecule of RBP binds one molecule of all-ira«s-retinol nonco-valently. In plasma, the retinol-RBP complex (holo-RBP) forms a larger complex with a cotransport protein, transthyretin (TTR), which also binds the hormone thyroxine.
Cellular retinoid-binding proteins (CRBP) are present in the cytoplasm of many types of cells. These proteins are similar in structure and size (~14.6 kDa), and each contains a single binding site that preferentially binds a particular form of retinoid (retinol, retinal, or retinoic acid), often preferring a specific isomer. Four cellular retinol-binding proteins (CRBP-I, -II, -III, and -IV) and two cellular retinoic acid-binding proteins (CRABP-I and -II) are expressed in many cells, yet each has a different tissue distribution. These proteins function as chaperones that confer aqueous solubility on otherwise insoluble retinoids while directing them to specific enzymes that then catalyze their further metabolism. These binding proteins may also play a role, although it is not yet well defined, in the delivery of retinoids (RA) to the nucleus for binding to nuclear retinoid receptors, which are discussed later.
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