Retinol Delivery RBP Receptors

RBP is responsible for the delivery of retinol from the liver to the extrahepatic sites of action of the vitamin. Evidence is available that this delivery process may involve cell-surface receptors for RBP. Thus, studies have been reported that suggest that there are specific cell-surface receptors for RBP on monkey small intestine mucosal cells, on bovine pigment epithelial cells from the eye, and on chicken testicular cell membranes. 

Direct binding of I-labeled RBP to isolated pigment epithelial cells of bovine retinas was demonstrated by Heller (1975a) using a much lower concentration of labeled RBP [than that used by Rask and Peterson (1976)] of high specific radioactivity. Nonspecific binding was corrected for by carrying out 

In a subsequent study, Chen and Heller (1977) showed that bovine pigment epithelial cells were capable of taking up retinol and retinoic acid when these retinoids were presented to the cells as complexes with serum RBP. Neither ftee retinol nor retinol or retinoic acid presented as serum albumin complexes were taken up by the pigment epithelial cells. The uptake of the retinoids was accompanied by dissociation of the retinoid-RBP complex, with the retinoid being taken up by the cells while the protein remained outside the cell. 

Autoradiographic studies with l-labeled holo-RBP have provided data that have been interpreted as showing the presence of binding sites for RBP on the surface of pigment epithelial cells. These studies were carried out both in vivo (injecting rats with holo-RBP Bok and Heller, 1976) and in vitro 

In summary, although the data available are limited, these several studies are consistent in suggesting that RBP delivers retinol to target tissues at specific cell-surface sites that recognize RBP, and that retinol is released from RBP at these locations. After its delivery, the retinol enters the cell for subsequent metabolism and action. The apo-RBP does not appear to enter the cell but rather dissociates from the receptor and returns to the circulation, where it shows a reduced affinity for TTR and is selectively filtered by the renal glomeruli. Much more information is needed about the putative RBP receptors on target cells, in order to understand more fully the phenomena involved in the retinol delivery process and their regulation. 

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