INTRACELLULAR BINDING PROTEINS FOR RETINOL AND RETINOIC ACID

Retinol binds to cellular retinol binding protein (CRBP) inside cells. Retinol is converted to retinoic acid (RA), and cellular binding proteins also exist for RA (called CRABP). These proteins protect and direct their bound molecules. RA can be liberated from its CRABP and bind a retinoic acid receptor (RAR and RXR) involved in gene expression (see later).

Cellular retinol binding protein CRBPI og CRBPII
Cellular retinoic acid binding protein CRABPI and CRABPII

Metabolism
Retinol is oxidized to retinal (rate limiting and reversibly). Retinal is oxidized to retinoic acid, which again is oxidized to e.g. 4-oxo-retinoic acid, an unstable product that is rapidly broken down. Some examples of enzymes mediating these reactions are shown below. Some are cytosolic while some are associated with microsomes (intracellular membranes like the endoplasmic reticulum).