Retinol Binding Protein

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Introduction: Vitamin A, in its various forms, is an essential component of mammalian health. In addition to its well-documented role in vision, Vitamin A contributes to several other important biological functions including nuclear transcription, skin cell differentiation, growth, and immunity. As animals are not capable of synthesis, vitamin A and its metabolites (collectively know as the retinoids) must be obtained through the diet (Goodman 1984). Two major forms of vitamin A are found in food: retinol and carotenoids. All of these fat-soluble vitamins contain two distinct structural features that contribute to all of their activity. The first is a β-ionone ring to which the second critical motif, an isoprenoid chain, is attached. Retinol, the major circulating form of vitamin A, is not biologically active. Rather, it serves as the metabolic precursor for the active retinoids (Chapman 2012). Oxidation at C-15 converts retinol to the visual pigment retinal, while subsequent oxidation of the aldehyde produces retinoic acid, which is involved in gene transcription (Figure 1). It is important for the body to maintain plasma retinol homeostasis to serve as a precursor reservoir for these active retinoids. Mobilization of Vitamin A from the liver and its circulation to peripheral tissues is a highly regulated process. Delivery without appreciable loss of retinol requires it be bound tightly to plasma retinol-binding protein (RBP). RBP, first described by Goodman and colleagues in 1968, is synthesized in the endoplasmic reticulum of hepatocytes and contains a single binding site for only all-trans-retinol (Goodman 1980). Upon secretion into the plasma, Retinol-RBP circulates as a complex with a larger protein, transthyretin (TT... ... middle of paper ... ... retinol release may function near the membrane surface at near-neutral bulk pH (Ptitsyn et.al. 1993). Conclusion Through its specific interaction with trans retinol, RBP allows this vital nutrient to circulate without appreciable loss. In addition to ligand-binding, RBP interacts with the carrier protein transthyretin (TTR). This protein complex prevents the loss of RBP through glomerular filtration and is also suggested as a vehicle for specific interactions with alleged cell-surface receptors that mediate retinol uptake. In conclusion, retinol-binding protein provides a comprehensive model for the role of proteins as carrier molecules. Structural features such as a characteristic up-and-down beta barrel, disulfide bonds, salt bridges and a hydrophobic core are essential in influencing the complex interactions that are essential to RBP function.

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