Cellular retinol binding proteins CRBP

Within the enterocyte, retinol is bound to cellular retinol binding protein (CRBP 11) and is esterified by lecithin retinol acyltransferase (LRAT), which uses phosphatidylcholine as the fatty acid donor, mainly yielding retinyl palmitate, although small amounts of stearate and oleate are also formed. At unphysiologically high levels of retinol, when CRBP 11 is saturated, acyl coenzyme A (CoA) retinol acyltransferase (ARAT) esterifies the free retinol that accumulates in intracellular membranes. Then the retinyl esters enter the lymphatic circulation and then the bloodstream (in chylomicrons), together with dietary lipid and carotenoids (Norum et al., 1986 Olson, 1986 Blomhoff et al., 1991 Green et al., 1993 Harrison and Hussain, 2001). 

Within extra hepatic tissues retinol is bound to cellular retinol binding protein (CRBP). 

Cellular retinol-binding protein CRBP 2.1 Cowan et al. (1993)... 

Wang, B., Merz, K.M. Validation of the binding site structure of the cellular retinol-binding protein (CRBP) by ligand NMR chemical shift perturbations. J. Am. Chem. Soc. 2005,127, 5310-1. 

Retinaldehyde, when bound to retinol binding protein II (CRBPII), serves as a substrate for retinal reductase resulting in the production of retinol (14), which then binds to cellular retinol binding protein (CRBP) forming holo-CRBP. Holo-CRBP seems to be the preferred substrate for an esterification reaction (Fig. 7.6) mediated by lecithin retinol acyl transferase (LRAT), a microsomal enzyme that uses acyl groups donated from phosphatidylcholine (14).In cells not expressing CRBP, retinol esterification is carried out by a different enzyme, acyl CoArretinol acyl transferase (ARAT). 

Immediately internal to this, the visceral yolk sac endoderm is rich in transcripts of cellular retinol binding protein (CRBP I) (6). The mechanism of transfer involves receptor-mediated uptake of retinol from maternal RBP-retinol by the visceral yolk sac endoderm, where it binds to CRBP I and interacts with the enzymes mediating RA synthesis (7). The assumption that retinol is retained specifically in CRBP I-expressing embryonic tissues has been verified by using 14C-labeled retinyl acetate (delivered intravenously to the pregnant dam) as a source of retinol (8). 

Retinoids present within a given cell are bound by intracellular-binding proteins, such as cellular retinol-binding proteins (CRBPs) or cellular retinoic acid-binding proteins (CRABPs) (Figure 1.2), all of which exhibit extremely high affinities for their substrates (Noy 2000). CRBP-I, for example, protects retinol from oxidation... 

The existence of two classes of intracellular retmoid-binding proteins, the cellular retinol-binding proteins (CRBP I and II) and cellular retinoic acid-binding proteins (CRABPI and II), have been known for a long time (1). Yet, the precise roles of these proteins in retinoid physiology remains poorly defined. One of the major reasons for this has been the difficulties of generating high-quality antibodies to facilitate their detection in tissues and cells by immunohistochemical techniques. 

G. The Role of Cellular Retinol-Binding Protein (CRBP). 26... 

Other factors that may affect the availability of retinol for complex formation with apo-RBP are (1) the manner in which retinol is transported within the cell from the site(s) of retinyl ester hydrolysis to a molecule of apo-RBP and (2) the manner in which the retinol molecule is presented to the membrane-bound molecule of apo-RBP. In this regard, the possible roles of cellular retinol-binding protein (CRBP) and of the so-called cytosol retinyl ester lipoprotein complex in delivering retinol to apo-RBP need exploration (Chen et al., 1981 Sklan et al., 1982). More information is also needed about the movement of retinol between hepatocytes and fat-storing cells (lipocytes) under normal and abnormal conditions. These various topics are also reviewed in Chapter 7. 

Comparison of the Amino Terminal Amino Acid Sequences of Cellular Retinoic Acid-Binding Proteins (CRABP) from Rat and Cow with Cellular Retinol-Binding Protein (CRBP) from Rat"... 

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