Interaction with retinol

Malpeli, G, Stoppini, M., Zappom, M.C, Folk, C, and Berm, R (1995) Interactions with retinol and retinoids of bovine cellular retinol-binding protein. Eur. J Biochem, 229,486-493... 

Bemi R, Clerici M, Malpeli G, Cleris L, Formelli F (1993) Retinoids in vitro interaction with retinol-binding protein and influence on plasma retinol. FASEB Jl 179-1184... 

The other major class of extracellular LBPs of mammals is the lipocalins (Flower, 1996). These are approximately 20 kDa, P-sheet-rich proteins, performing functions such as the transport of retinol in plasma or milk, the capture of odorants in olfaction, invertebrate coloration, dispersal of pheromones, and solubilizing the lipids in tears (Flower, 1996). The retinol-binding protein (RBP) of human plasma is found in association with a larger protein, transthyretin, the complex being larger than the kidney threshold and thus not excreted, although the RBP itself may dissociate from the complex to interact with cell surface receptors in the delivery of retinol (Papiz et al., 1986 Sundaram et al., 1998). 

Perez, M.D. and Calvo, M. 1995. Interaction of (3-lactoglobulin with retinol and fatty acids and its role as a possible biological function for this protein a review. J. Dairy Sci. 78, 978-988. 

Antioxidant compounds are an important defense for immediate detoxication of highly reactive intermediates. They act as competing nucleophiles and can bind to the intermediate, forming a less reactive species. Glutathione is one antioxidant molecule that can directly interact with free radicals. Other chemical antioxidants include vitamins A (retinol), C (ascorbic acid), and E. Ascorbic acid, for example, may react directly with reactive intermediates by hydrogen abstraction, resulting in the formation of dehydroascorbic acid. 

In contrast to retinoids, carotenoids were considered non-toxic, even when taken chronically in large amounts, until recently, when it was found that ethanol interacts with carotenoids, interfering with their conversion to retinol. In baboons, the consumption of ethanol together with beta-carotene resulted in more striking hepatic injury than consumption of either compound alone (98). This interaction occurred at a total dose of 7.2-10.8 mg of beta-carotene per Joule of diet. This dose is common in people who take supplements and is the same order of magnitude used in the Beta-Carotene and Retinol Efficacy Trial (CARET) (30 mg/day) (99) and in another study (20 mg/day for 12 weeks) (100). The amount of alcohol given to the baboons was equivalent to that taken by an average alcohohc. The well-known toxicity... 

The other interesting polar interaction involves Lys-40. The positive charge on the side chain of Lys-40 appears to interact with the isoprene tail of the retinol, with the NZ atom located above the plane of the conjugated 7r-electron field. The /3-ionone ring is located in a hydrophobic environment formed by the two helices and /3C-j8D and /3E-j8F turns. Eight water molecules were also identified in the cavity. All of them make hydrogen bonds to internal polar side chains or water molecules. 

Twenty two amino acids come within 5.1 A of the bound retinol molecule. Five of them are within 3.6 A—Lys-40, Thr-53, Arg-58, Trp-104, and Gin-106. In fact, the polar side chains of these residues are close to the bound retinol and again it seems that at least part of the lipid-protein interactions occur through polar atoms. Of particular interest are Lys-40 and Gln-106. As in CRBP, Lys-40 is located above the isoprene tail of the retinol and could interact with the ir-electron field of the ligand. Gln-106, on the other hand, makes a hydrogen bond with the only polar group, the hydroxyl group of the retinol molecule. 

The overall folding of /3-LG is remarkably similar to that of human plasma retinol-binding protein (Papiz et al., 1986). The core is made up of an eight-stranded antiparallel /3 sheet, which forms a /3 barrel. There is also a short a helix at the C terminus. As described by Monaco et al. (1987), the interior of the /3 barrel is essentially hydrophobic. The conformational difference between the A form and B form of the protein is not significant. The most interesting observation is that the bound retinol molecule interacts with the protein in a way completely different from that of serum retinol-binding protein. The calculated difference map showed that the molecule is not located in the central j3 barrel but binds to the a-heIix//3-barrel interface delimited by about 15 residues. Most of the... 

Berni, R., M. Stoppini and M.C. Zapponi. The piscine plasma retinol-binding protein. Purification, partial amino acid sequence and interaction with mammalian transthyretin. Eur. J. Biochem. 204 99-106, 1992. 

Sivaprasadarao, A. and J.B. Findlay. The interaction of retinol-binding protein with its plasma-membrane receptor. Biochem. J. 255 561-569, 1988. 

The examples discussed in sections 13.2.1 -13.2.4 show that exclusive reliance on the crystal structures of a ligand itself may not be an infallible indicator of a biologically active conformation. It may be justified for non-polar and rigid ligands, like retinol, where the crystal environment is composed of many isotropically distributed at-om/atom potential contributions. In such cases, it can be assumed that the adopted conformation is mainly determined by intramolecular forces and that no pronounced perturbations arise from strong directional interactions with the crystal or active site environment. This assumption is also valid for hydrophobic portions of a ligand skeleton. Conformations obtained by computational methods on the isolated molecule have some relevance for the biologically active conformation only for hydrophobic molecules. 

Retinoids (derivatives of retinol) act like steroid hormones and interact with specific receptor proteins in the cell nucleus. The ligand-receptor complexes bind to specific DNA sequences, where they control the transcription of particular genes. 

To mobilize liver stores, our model predicts that retinyl esters in PC or SC are hydrolyzed and the resulting retinol is transferred to the slower turning-over retinol pool (compartment 5 in SC and 3 in PC), presumably bound to CRBP. It is then transferred to an exocytosis compartment. Our kinetic data indicate that this retinol does not need to go back to PC before secretion into plasma. Maybe apoRBP can interact with RBP receptors and equilibrate retinol between intraceUular CRBP and plasma RBP. If so, this is an excellent example of homeostatic control since cellular retinol pools are in equilibrium with plasma retinol. If an exchange of retinol between apoC P and apoRBP is shovm to be mediated by a specific membrane protein, the protein should perhaps be named a retinol transporter, rather than an RBP receptor (Blomhoff et aL, 1991). 

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