[Cellular retinoid-binding proteins].

This paper will review the function of cellular retinoid-binding proteins and receptors (RAR), especially to address recent works on cellular uptake, intracellular transport, biosynthesis and metabolism of retinoids. Cellular retinol-binding protein (CRBP) binds retinol and retinal, cellular retinoic acid-binding protein (CRABP) binds retinoic acid, both of which have a molecular weight of 16 kDa, and cellular retinal-binding protein (CRALBP) binds retinal with a molecular weight of 36 kDa. CRBP (I) functions in cellular uptake of retinol from the plasma, solubilizes and renders retinol nontoxic in the aqueous system, and presents retinol to the appropriate enzymes to biosynthesize retinoic acid (an active form of retinoids) or retinyl esters (a storage form). CRBP (II) mainly distributes in the absorptive cell in the small intestine and functions in absorption of retinoids and carotenoids to biosynthesize retinyl esters present in the chylomicron. CRBP (III) is recognized as one of oncofetal proteins and binds both retinol and retinoic acid, the function of which still remains unclear. CRABP (I), previously recognized as a transport protein of retinoic acid to the nucleus, is now assumed to sequester retinoic acid from retinoic acid receptor(s) in the tissue where retinoic acid levels are required to be low. CRABP (I) also functions in the catabolism of retinoic acid to modulate its concentration in the cell. CRABP (II), expressed in the developmental stage of the fetus, is suggested to modulate the action of retinoic acid as a 'morphogen'.