Retinoid-binding proteins in plasma and in cells.

Much has been learned during the past decade about the specific retinoid-binding proteins that exist in plasma, and in the intracellular compartment in a number of tissues. Vitamin A is mobilized from liver stores and transported in plasma in the form of the lipid alcohol retinol, bound to a specific transport protein, retinol-binding protein (RBP). A great deal is now known about the chemical structure, metabolism, and biological roles of RBP. Vitamin A mobilization from the liver is highly regulated by factors that control the rates of RBP production and secretion. Retinol deficiency specifically blocks the secretion of RBP, which can then be rapidly stimulated by intravenous retinol repletion. The cellular and molecular mechanisms that mediate these phenomena are under investigation. Delivery of retinol to peripheral tissues appears to involve specific cell surface receptors for RBP. The retinol so delivered enters the target cell, where it may become associated with the intracellular binding protein for retinol (CRBP). A number of tissues of rats, humans, and other species contain soluble proteins with binding specificity for retinol (CRBP) or for retinoic acid (CRABP). These proteins have been purified from several tissues and partly characterized. They differ in a number of ways from plasma RBP, and differ from each other in regard to binding specificity and immunoreactivity. It has been suggested that these intracellular proteins may play a direct role in the biological expression of vitamin A activity in the cell. Studies are in progress to explore this and other possibilities.