Retinol plasma transport

Spear PA, Higueret P, Garcin H. 1994. Effects of fasting and 3,3,4.4 -5,5 -hexabromobiphenyl on plasma transport of thyroxine and retinol Fasting reverses elevation of retinol. J Toxicol Environ Health 42(2) 173-183. 

In our studies, we have administered tritium-labeled vitamin A in one of its two physiological plasma transport vehicles (associated with either retinol-binding protein or chylomicrons) so that tracer data can be extrapolated to the vitamin A compounds of interest (retinol, retinyl esters, and metabolites). To prepare pH]retinol in its plasma transport complex (Green and Green, 1990b), vitamin A-depleted rats are used as donors to maximize hepatic secretion of the labeled vitamin on acciunulated liver apoRBP. pH]Retinol or pH]retinyl acetate in an emulsion with Tween 40 is administered intravenously to donor rats and blood is harvested 100 min later when plasma radioactivity is maximal. Plasma is isolated and stored under a nitrogen atmosphere at 4°C plasma is used for in vivo studies within 23 days. 

After uptake of the chylomicron retinyl esters, hydrolysis and reesterification occur in the liver. The resulting retinyl esters (predominantly retinyl palmitate) are stored in the liver and can be mobilized as needed in a highly regulated process. Vitamin A mobilization from hepatic retinyl ester stores takes place as the free alcohol retinol bound to a specific plasma transport protein retinolbinding protein (RBP). 

Hydrophobic compounds are transported in plasma bound to transport proteins (e.g. the plasma retinol binding protein section 11.2.2.2) or dissolved in the lipid core of plasma lipoproteins (section 5.6.2), and net intracellular accumulation to a higher concentration than in plasma depends on an intracellular binding protein that has a greater affinity for the ligand than does the plasma transport protein. 

The predominant retinoid in the fasting circulation is retinol, all of which is bound to its specific plasma transport protein, retinol-binding protein (RBP) [1,2]. Although retinol accounts for approximately 95 to 99% of all retinoid in the circulation, other retinoids also are present. Fasting human and rodent blood contains very low levels of both sAhtrans- and 13-cw-retinoic acid (approximately 0.2 to 0.7% of those of retinol) [3], as well as low levels of retinyl esters in lipoprotein fractions, particularly very low-density lipoproteins (VLDL) and low density lipoproteins (LDL) [4]. Soluble glucuronides of both retinol and retinoic acid are also detectable in the circulation of humans and rodents [5], as are provitamin A carotenoids like P-carotene... 

Plasma retinol is transported by a specific 21-kDa transport protein, retinal binding protein (RBP). Most RBP is produced in the liver, but some extra-hepatic organs also synthesize it. Each molecule of RBP binds one molecule of all-tra s-retinol nonco-valently. In plasma, the retinol-RBP complex (holo-RBP) forms a larger complex with a cotransport... 

The first example is the plasma-borne retinol-binding protein, RBP, which is a single polypeptide chain of 182 amino acid residues. This protein is responsible for transporting the lipid alcohol vitamin A (retinol) from its storage site in the liver to the various vitamin-A-dependent tissues. It is a disposable package in the sense that each RBP molecule transports only a single retinol molecule and is then degraded. 

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). 

K2. Kanai, M., Raz, A., and Goodman, D. S., Retinol-binding protein the transport protein for vitamin A in human plasma. J. Clin. Invest. 47, 2025-2044 (1968). 

In the body retinol can also be made from the vitamin precursor carotene. Vegetables like carrots, broccoli, spinach and sweet potatoes are rich sources of carotene. Conversion to retinol can take place in the intestine after which retinyl esters are formed by esterifying retinol to long chain fats. These are then absorbed into chylomicrons. Some of the absorbed vitamin A is transported by chylomicrons to extra-hepatic tissues but most goes to the liver where the vitamin is stored as retinyl palmitate in stellate cells. Vitamin A is released from the liver coupled to the retinol-binding protein in plasma. 

Pharmacokinetics Rapidly absorbed from the GI tract if bile salts, pancreatic lipase, protein, and dietary fat are present. Transported in blood to the liver, where it s metabolized stored in parenchymal hepatic cells, then transported in plasma as retinol, as needed. Excreted primarily in bile and, to a lesser extent, in urine. 

Release from the liver When needed, retinol is released from fie liver and transported to extrahepatic tissues by the plasma retax -binding protein (RBP). The retinol-RBP complex attaches to spe cific receptors on the surface of the cells of peripheral tissues, permitting retinol to enter. Many tissues contain a cellular letaiol-binding protein that carries retinol to sites in the nucleus where the vitamin acts in a manner analogous to steroid hormones. 

In the intestinal mucosal cells, /3-carotene is cleaved via an oxygenase (an enzyme that introduces molecular 02 into organic compounds) to frans-retinal (aldehyde form of trans-retinol, as shown in Table 6.2), which in turn is reduced to frans-retinol, vitamin Av Retinol is then esterified with a fatty acid, becomes incorporated into chylomicrons, and eventually enters the liver, where it is stored in the ester form until it is required elsewhere in the organism. The ester is then hydrolyzed, and vitamin Ax is transported to its target tissue bound to retinol-binding protein (RBP). Since RBP has a molecular weight of only 20,000 and would be easily cleared by the kidneys, it is associated in the bloodstream with another plasma protein, prealbumin. 

The thyroidogenic effects and corresponding biochemical mechanisms of PCBs and other OHS were recently reviewed by Brouwer et al. [44]. The selective retention of certain OH-PCB congeners in blood (Sect. 5.2.2 and 5.3.2) is concomitant with effects observed on the plasma levels of thyroid hormones. Thyroxine is transported in plasma by a protein complex consisting of TTR and retinol binding protein (RBP). Rats administered CB-77 were shown to have reduced plasma levels of both thyroxine and retinol [196]. A major metabolite of CB-77, 4-OH-3,3, 4, 5-tetrachlorobiphenyl, was identified as the active compound [40]. The same hydroxy-PCB metabolite was found to be retained in mouse fetal soft tissue [191,197]. 

Transferrin an iron-transport protein in the plasma, has a half-life of about 8 days thyroxine-binding protein, 2 days and retinol-binding protein, 0,4 days-Because of the relatively long half-life of albumin, the serum albumin level is not a particularly sensitive indicator of dietary protein status. Therefore, retinol-binding protein and thyroxine-binding protein have been used to assess malnutrition in poorly nourished populatiorrs. Note that these two proteins occur together as a complex in the bloodstream. 

The pigment epithelium of the retina receives all- tram-retinol from plasma RBP. It is then isomerized to ll-ct5-retinol, which may either be stored as 11 -ct5-retinyl esters or oxidized to 11-cts-retinaldehyde, which is transported to the photoreceptor cells bound to an interphotoreceptor retinoid binding protein. 

Palozza, P. 1998. Prooxidant actions of carotenoids in biologic systems. Nutr. Rev. 56(9) 257-265. Parker, R.S. 1996. Absorption, metabolism and transport of carotenoids. FASEB J. 10 542-551. Peng, Y.M., Peng, Y.S., Childers, J.M., Hatch, K.D., Roe, D.J., Lin, Y. and Lin, P. 1998. Concentrations of carotenoids, tocopherols and retinol in paired plasma and cervical tissue of patients with cervical cancer, precancer and noncancerous diseases. Cancer Epidemiol. Biomark. Prev. 7 347-350. 

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). 

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... 

Lipids, by virtue of their immiscibility with aqueous solutions, depend on protein carriers for transport in the bloodstream and extracellular fluids. Fat-soluble vitamins and free fatty acids are transported as noncovalent complexes. Vitamin A is carried by retinol-binding protein and free fatty acids on plasma albumin. However, the bulk of the body s lipid transport occurs in elaborate molecular complexes called lipoproteins. 

Oxidation of retinol produces retinoic acid tretinoin). The reaction is irreversible. Retinoic acid enters the portal blood, is transported bound to albumin, and is not stored to any great extent. The concentration of retinoic acid in plasma is normally 3-4 ng/mL. A biologically active metabolite, 5,6-epoxyretinoic acid, has been isolated from the intestinal mucosa of vitamin A-deficient rats following administration of H-retinoic acid. Several tissues have specific cellular retinoic acid-binding proteins (CRABPs). 

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. 

An instractive example of the effect of cysteine modification on amyloid formation is provided by the TTR protein. This plasma protein is a tetramer in solution, with four identical 127 amino acid sub-units and is responsible for transport of thyroxine and the retinol-binding protein-vitamin A complex (Hamilton and Benson 2001). Deposition of wtTTR occurs in senile systemic amyloidosis (SSA) (Table 1). This is a non-hereditary disorder that affects roughly 25% of individuals over the age of 80. The amyloid fibrils formed consist mainly of wfTTR and its fragments, and they build up in the heart. 

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). 

Sample preparation should liberate the vitamin from the matrix, e.g., tissue or plasma, where it often occurs chemically or physically bound. Many specific transport binding proteins are known, e.g., for retinol. Chemical bonding can include the incorporation of a vitamin into coenzymes, e.g., niacin and pantothenate in NAD and coenzyme A, respectively. Liberating vitamins from industrial product forms (formulations) is also an important issue. Here, vitamins are often encapsulated in small beadlets, e.g., from gelatin, which protects them from oxygen and makes them easier to add during processing. 

Muto, Y., Goodman, D-W.S. Vitamin A transport in rat plasma isolation and characterization of retinol-binding protein. J. biol. Chem. 247, 2533-2541 (1972)... 

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