Figure 37.

Key proteins involved in the transport of retinol from the liver to target tissues. Retinol travels in the circulation bound to RBP. In turn, RBP complexes with a transthyretin (TTR) tetramer, which prevents filtration of RBP across the glomeruli of the kidney. Holo-RBP can dissociate from the TTR tetramer and bind to the retinol membrane transporter, STRA6. all-trans-Retinol is picked up on the cytoplasmic side of STRA6 by CRBP, which shuttles the retinoid to the ER of the RPE. There it is esterified by LRAT to form all-trans-retinyl-esters, which are either used as substrates for visual chromophore production or stored in lipid bodies known as retinosomes: atROL, all-trans-retinol; atRE, all-trans-retinyl ester.