ABSTRACT
II. RETINOL-BINDING PROTEIN Plasma RBP occurs as a monomeric, 183-residue, nonglycosylated polypeptide with a calculated molecular mass of 21,062. It has a single binding site for retinol (Kn = 1.9 x w-1 M). Although RBP can bind retinoic acid (Kn = 2.1 x w-1 M) and retinaldehyde with almost equal affinity, under physiological conditions the protein specifically transports retinol. In serum, RBP occurs predominantly as a 76-kDa complex with transthyretin (ITR, previously known as prealbumin, molecular weight 55,000). This complex formation prevents the glomerular :filtration and subsequent renal catabolism of RBP. The protein binds to specific receptors on target cell membranes and releases retinol to the cell's interior. Within the cell, retinol is metabolized to active intermediates (e.g., retinaldehyde, retinoic acid, 14-hydroxyl-4,14-retroretinol) that bind to appropriate effector proteins (e.g., rhodopsin, retinoic acid receptors), which in tum influence the physiological and metabolic events of the cell. Available evidence now suggests that some of the retinol internalized by extrahepatic tissues is recycled by returning to plasma, a process which probably also involves RBP (8).
