Retinol-binding protein with transthyretin

Interactions of Retinol-Binding Protein with Transthyretin and Its Receptor... 

BJ Burri, MA Kutnink, TR Neidlinger. Assay of human transthyretin-bound holo-retinol-binding protein with reversed-phase high-performance liquid chromatography. J Chromatogr 567 369-380, 1991. 

Rosales FJ, Ross AC (1998) A low molar ratio of retinol binding protein to transthyretin indicates vitamin A deficiency during inflammation Studies in rats and a posteriori analysis of vitamin A-supplemented children with measles. J Nutr 128 1681-1687... 

Transthyretin (formerly prealbumin binds T4 and forms a complex with retinol-binding protein)... 

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). 

Free retinol is released from the liver as a 1 1 complex of retinol with the 21-kDa retinol-binding protein.k l This protein is normally almost saturated with retinol and is bound to another serum protein, the 127-residue transthyretin (prealburnin).m/n Some of the retinol is oxidized to retinoic acid. 

When vitamin A stores are adequate, the liver secretes retinol bound to retinol-binding protein (RBP) into the circulation to provide tissues with a constant supply of vitamin A. In the circulation, the retinol-RBP complex is found bound to another circulating protein of hepatic origin, transthyretin (TTR). TTR also binds thyroid hormone and consequently plays a role in the transport of both vitamin A and thyroid hormone. The molecular size of the retinol-RBP complex is quite small, and the formation of the... 

Thyroid hormones have a lengthy life-span in the bloodstream — several days — probably because they are bound to proteins in the circulation. More than of T4 and T3 is bound to plasma proteins. These prolerns arc thyroid hormone-binding protein, transthyretin, and albumin. Most of the hormone is carded by thyroid hormone-binding protein. Transthyretin (from thyroid and retinol) occurs in a Irl complex with retinol-binding protein In the bloodstream. This complex serves to prevent the loss of retinol-binding protein, which is a small protein, in the urine. Transthyretin has also been called prealbumin, it binds T4 and not TJ,... 

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. 

Transthyretin amyloidosis (also called familial amyloid polyneuropathy) is an autosomal dominant syndrome characterized by peripheral neuropathy. This disease results from one of five mutations identified thus far in the gene for transthyretin. Transthyretin is also called prealbumin (although it has no structural relationship to albumin) because it migrates ahead of albumin in standard electrophoresis at pH 8.6. Transthyretin is synthesized in the liver and is a normal plasma protein with a concentration of 20-40 mg/dL. It transports thyroxine and retinol binding protein (Chapter 38). The concentration of transthyretin is significantly decreased in malnutrition and plasma levels are diagnostic of disorders of malnutrition (Chapter 17). 

Measurements of the levels of semm proteins such as albumin, transthyretin (also known as prealbumin), transferrin and retinol-binding protein are used as biochemical parameters in the assessment of protein energy malnutrition (Table 17-1). An ideal protein marker should have rapid turnover and present in sufficiently high concentrations in semm to be measured accurately. Transthyretin has these properties it is a sensitive indicator of protein deficiency and is effective in assessing improvement with refeeding. 

Transthyretin (also known as prealbumin) Liver plasma circulating form is a tetramer composed of four identical monomers. M.W. 55,000 1-2 days N 20-40 mg/dl Mild 10-15 mg/dl Moderate 5-10 mg/dl Severe <5 mg/dl Circulates in plasma in a 1 1 complex with retinol-binding protein, transports thyroxine, has a small body pool, and has a short half-life. Sensitive indicator of protein deficiency and in the improvement with protein refeeding. 

Retinol-binding protein (RBP) Liver M.W. 21,000 10-12 hours N 3.5-9.0 mg/dl Circulates in plasma in 1 1 complex with transthyretin, transports retinol and thyroxine, plasma levels influenced by glomerular filtration rate, retinol and zinc status, considered to be too sensitive and therefore has limited value... 

The first lipocalin whose 3-D structure was solved and refined at high resolution was the human plasma retinol-binding protein (RBP) [22, 23]. RBP acts as a natural transporter of vitamin A (retinol) in the blood of vertebrates. Upon complexation in a hydrophobic cavity with complementary shape, the poorly soluble terpenoid alcohol becomes packaged by the protein and protected from oxidation or double-bond isomerization. RBP is synthesized in the liver and directly loaded with fhe hgand in fhe hepatocyte, where retinol is stored. Furthermore, the holo-RBP forms a structurally defined ternary complex with transthyretin [24], also known as prealbumin. After delivery of the retinol ligand to a target tissue, fhe complex decomposes and fhe monomeric apo-RBP becomes filtered out by fhe kidney and degraded. 

Vitamin A is transported m the plasma as retinol bound to a carrier protein, called retinol-binding protein (RBP), which itself forms a complex with the thyroxine-binding protein, known as transthyretin (TTR). This complex exists in equilibrium with free holo-RBP, which can then interact with a specific cell-surface receptor, thereby inducing the release of its retinol to the target cell. Thus, RBP possesses at least three molecular-recognition properties it binds retinol and it interacts with both TTR and the cell-surface receptor (for reviews, see refs. I and 2). 

To study the interaction of retinol-binding protein (RBP) with its plasma carrier, transthyretin (TTR), spectrofluorimetry, and circular dichroism have previously been used. Both these techniques require milligram quantities of the proteins and this sets limitations on the use of these techniques for the study of RBP-TTR interactions using recombinant proteins. The Escherichia coll expression system described in Chapter 11 does not readily produce milligram quantities of RBP for routine use. For this reason, we have developed a highly sensitive method which employs radioiodinated I-RBP (unpublished). The method requires only microgram quantities of protein. This chapter describes a method to radioiodinate RBP without loss of activity and protocols for its use in the study of its interaction with TTR. 

Most of the retinol that is delivered to the eye originates in the liver, and is released into the circulation (mainly in the all-trans configuration) bound to an M, 21,000 protein, serum retinol-binding protein (RBP). This protein, which contains no bound lipid or carbohydrate, circulates as a 1 1 complex with transthyretin (TTR, originally prealbumin) (see Chapter 8 for detailed discussion). 

Retinol is released from the liver bound to an a-globulin, retinol-binding protein (RBP) this serves to maintain the vitamin in aqueous solution, protects it against oxidation and also delivers the vitamin to target tissues. RBP is secreted from the liver as a 1 1 complex with the thyroxine-binding prealbumin, transthyretin. This is important to prevent urinary loss of retinol bound to the relatively small RBP, which would otherwise be filtered by the kidney, with a considerable loss of vitamin A from the body. 

Bemi R, Malpeli G, Folli C, Murrell JR, Liepnieks JJ, Benson MD (1994) The Ile-84- Ser amino acid substitution in transthyretin interferes with the interaction with plasma retinol-binding protein. J Biol Chem 269 23 395-23 398... 

Waits RP, Yamada T, Uemichi T, Benson MD (1995) Low plasma concentrations of retinol-binding protein in individuals with mutations affecting position 84 of the transthyretin molecule. Clin Chem 41 1288-1291... 

Figure 4 Model of atRA biogeneration in mammals. REH, retinyl ester hydrolase (e.g., ES4 and ES10) TTR, transthyretin RAR-RXR, the heterodimer of retinoic acid receptors with retinoid X receptors atRCHO, all-frans-relinal atROH, all-frans-retinol CRBP(I), cellular retinol binding protein, type I LRAT, lecithin retinol acyltransferase SRBP, semm retinol binding protein. CRBP(I), CRABP(I), and CRABP(II) have been placed in the same cell for simplicity. This does not necessarily occur in vivo.

Liver Storage and Release of Retinol Tissues can take up retinyl esters from chylomicrons, but most is left in the chylomicron remnants that are taken up into the liver by endocytosis. The retinyl esters are hydrolyzed at the hepatocyte cell membrane, and free retinol is transferred to the rough endoplasmic reticulum, where it binds to apo-RBP. Holo-RBP then migrates through the smooth endoplasmic reticulum to the Golgi and is secreted as a 1 1 complex with the thyroid hormone binding protein, transthyretin (Section 2.2.3). 

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