Retinol renal loss

RBP forms a 1 1 complex with the tetrameric thyroxine-binding prealbumin, transthyretin. This is important to prevent urinary loss of retinol bound to the relatively small RBP (Mr 21,000), which would be filtered by the glomerulus transthyretin has an Mr of 54,000 hence, the complex will not normally be filtered, ffowever, moderate renal damage, or the increased permeability of the glomerulus in infection, may result in considerable loss of vitamin A bound to RBP-transthyretin. 

A mild infection, such as measles, commonly triggers the development of xerophthalmia in children whose vitamin A status is marginal. In addition to functional deficiency as a result of impaired synthesis of RBP (Section 2.2.3) and transthyretin in response to infection, there may be a considerable urinary loss of vitamin A because of increased renal epithelial permeability and proteinuria, permitting loss of retinol bound to RBP-transthyretin. The American Academy of Pediatrics Committee on Infectious Diseases (1993) recommended vitamin A supplements for aU children who have been hospitalized with measles. 

As retinol is needed, stellate cell retinyl esters are hydrolyzed by one or more REHs and retinol is transferred back to hepatocytes for combination with newly synthesized RBP. The holo-RBP complex then passes through the Golgi secretory apparatus and binds noncovalently with a tetramer of TTR. The larger size of this transport complex ( 75 kDa) compared to holo-RBP alone ( 21 kDa) helps to prevent the rapid loss of retinol and RBP during renal glomerular filtration. 

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