il\VOLVEMENT OF RETII\OIC ACID II{ - MSpace at the University of ...

proteins are found to be involved in the regulation of retinoid metabolism, transportation
and signaling processes. They achieve their function by protecting easily oxidized and
isomerized retinoids from reacting with other molecules. Retinoid binding proteins are
also involved in the optimization of the enzymatic catabolism as they prevent cells from
treating retinoids as extemal and foreign compounds, thus providing a control of their
metabolism.
Two major families of retinoid-binding proteins are identified according to their
location; extracellular retinoic binding proteins and intracellular retinoid binding proteins
(Chytil and Ong 1987; Newcomer 1995) . Extracellular retinoid binding proteins, also
known as lipocalins, are involved in the mobilization and transportation of retinol from
its storage sights to the target organs (Napoli 1996). The most prominent members of this
family are the androgen dependent epidimal RA binding proteins, inter-photo and cellular
retinal binding proteins (Ho et al. 1989; Newcomer 1995; Saari et al. 1994). These
proteins are involved in the transportation of retinoids in the plasma, epididmal and
retinal tissues by preventing the oxidation of retinoids and preserving the isomeric
structure o f transported molecules.
The second group consists of the intracellular cytosolic, lipid binding proteins
which bind fatty acids, steroids and specific retinoid isomers (Banaszak et al. 1994). The
most recognized members of this family are cellular retinol binding proteins type 1 and
type 2 (CRBP I and CRBP II), as well as cellular retinoic acid binding proteins type 1 and
2 (CRABP I and CRABP II) (Bashor et al. 1973; Ong and Chytil 1975; Sani and Hill
1974). Their structure enables them to form beta barrel binding pocket which fully
engulfs retinoids and makes them inaccessible to a number of oxidizing agents (Kleywegt
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