RBP4retinol

RBP4retinol refers to the holo-retinol-binding protein 4 complex, in which retinol (vitamin A alcohol) is bound by retinol-binding protein 4 (RBP4) in circulation. RBP4 carries retinol from the liver to extrahepatic tissues and serves as a key transport form of vitamin A in the bloodstream.

In plasma, RBP4 binds retinol in a tight pocket, forming the holo-RBP4 complex. This complex often associates with transthyretin (TTR), a tetrameric protein, which stabilizes the complex and helps prevent renal filtration and loss of retinol in urine. The RBP4–retinol system thus maintains a circulating reservoir of vitamin A and delivers it to cells that express the appropriate receptors.

Uptake of retinol from holo-RBP4 by target cells is primarily mediated by the cell-surface receptor STRA6. Once inside the cell, retinol can be bound by cellular retinol-binding proteins and then processed by retinol dehydrogenases and aldehyde dehydrogenases to produce retinaldehyde and retinoic acid, or it can be re-esterified for storage. Retinoic acid acts as a key regulator of gene expression, while retinol supports cellular metabolism and vision-related processes.

Clinical and physiological relevance centers on vitamin A status and metabolic regulation. Plasma RBP4 levels and the proportion of retinol bound to RBP4 reflect vitamin A status and liver–kidney–adipose tissue signaling. Alterations in RBP4retinol transport have been studied in relation to metabolic syndrome, insulin resistance, and ocular health, though interpretations vary across contexts.

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