Retinol binding protein

Retinol -binding proteins ( RBP) are transport proteins in blood plasma and in the cytosol of vertebrate cells. They bind to free vitamin A (retinol, retinal, retinoic acid ), and so prevent uncontrolled effects of these molecules. They also facilitate the transport of lipophilic molecules through the cell interior lipophobic and the blood. In humans there are at least four RBPs and two proteins that are specific for retinoic acid ( CRABP ). Mutations in the RBP4 gene can lead to RBP deficiency with symptoms of vitamin A hypovitaminosis.

RBPs all consist of about 135 amino acids, with CRBP I and II are identical in sequence to 56%. The four proteins have different functions and are therefore distributed differently in the different cells. RBPs are closely related to lipocalins.

RBP1 ( CRBP I)

The CRBP I promotes the conversion of retinol to Retinylaldehyd or to retinoic acid. It is present mainly in the pigment epithelium of the eye or the Kupffer cells of the liver.

RBP2 ( CRBP II)

CRBP II is required, and can only be found in the enterocytes of the small intestine for absorption of retinoids from the intestinal lumen.

CRABP

The exact function of retinsäurebindenen proteins CRABP I and CRABP II is not yet clear. It is clear that CRABP I to retinoic acid in the cell decreases, whereas CRABP II with retinoic acid to form a complex and this thus transported into the nucleus. The CRABP I is present in most cells of the body, the CRABP II, however, especially in skin cells.