Retinol-binding protein 3-carotene

FIGURE 3.2.2 Metabolic pathways of carotenoids such as p-carotene. CM = chylomicrons. VLDL = very low-density lipoproteins. LDL = low-density lipoproteins. HDL = high-density lipoproteins. BCO = p-carotene 15,15 -oxygenase. BCO2 = p-carotene 9, 10 -oxygenase. LPL = lipoprotein lipase. RBP = retinol binding protein. SR-BI = scavenger receptor class B, type I. 

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. 

Retinol (vitamin A) is found in foods of mammalian origin in the form of retinyl ester, or in fruits and vegetables as carotenoids with provitamin A activity, especially P-carotene (provitamin A). In enterocytes, retinol binds to cellular retinol-binding protein type II (CRBPII), which directs the esterification by the enzyme lecithin retinol acyltransferase (LRAT). 

Retinyl esters and the P-carotene are incorporated into chylomicrons and taken up mainly by hepatocytes. In the liver retinol may be stored in stellate cells as retinyl esters, oxidized to retinoic acid or liberated into cells bound to retinol-binding proteins (RBP). All E retinoic acid and its 9Z isomer have an affinity for nuclear receptors. They activate the transcription and bind as dimers to specific nucleotide sequences, present in promoters of target genes. 

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. 

Vitamin A is readily absorbed from the intestine as retinyl esters. Peak serum levels are reached 4 h after ingestion of a therapeutic dose. The vitamin is distributed to the general circulation via the lymph and thoracic ducts. Ninety percent of vitamin A is stored in the liver, from which it is mobilized as the free alcohol, retinol. Ninety-five percent is carried bound to plasma proteins, the retinol-binding protein. Vitamin A undergoes hepatic metabolism as a first-order process. Vitamin A is excreted via the feces and urine. Beta carotene is converted to retinol in the wall of the small intestine. Retinol can be converted into retinoic acid and excreted into the bile and feces. The elimination half-life is 9 h. 

Fig. 2. Tissue distribution and metabolism of retinoids in fish. Dietary carotenoids (e.g. /3-carotene (/3C)) and retinyl esters (e.g. retinyl palmitate (RP)) are converted into retinol (Rol) in the lumen of the gut. Retinol is then re-esterified and packaged into chylomicrons and transported to the portal circulation. When required elsewhere, stored retinyl esters (e.g. RP) in the liver are hydrolyzed to retinol and transported in the blood bound to the retinol-binding protein (RBP). Retinol is converted in target tissues to RA, RP or retinal (Ral). RA may exert its effects locally, or be returned to the circulation and transported throughout the body bound to albumin. RA can then be sequestered in other tissues.

During absorption some /3-carotene is also converted to retinoid (Dimitrov et al, 1988 Olson, 1989 van Vliet et al, 1992 Scita et al, 1993) and transferred via a plasma chylomicron (renmant) retinyl ester compartment to a liver retinyl ester compartment. From here it is released in a plasma retinol-binding protein-retinol (RBP-ROH) compartment for transfer to target tissues. Eventually it is lost irreversibly from the RBP-ROH compart-... 

The predicted chylomicron retinyl-d4 ester curve is characterized by a sharp peak similar to that of the /3-carotene-dg, again in accord with the expected rapid clearance of chylomicrons foUowing a meal. The predicted plasma retinol-binding protein-retinol-d4 curve shows an initial rise and fall, then a sustained level of retinol-d4 after the /3-carotene-dg dose. This is very similar in shape to that seen when the same subject ingested retinyl-d4 acetate in a previous experiment (Song et oL, 1995). The sustained level of retinol-d4 is a result of the slow release of retinoid into the plasma from... 

Figure L Effects of dietary fats on the levels of -carotene dioxygenase activity and cellular retinol binding protein type II in rat intestinal mucosa. (Adapted with permission from reference 22. Copyright 1998 The American Society for...

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... 

P-carotene and retinyl esters are the two major precursors of retinol in our diet. Once ingested, they are converted to retinol in the intestines, and stored in the liver after they are reconverted to retinyl esters. Retinol is mobilized from the liver when it is generated by hydrolysis of retinyl esters it is then transported to the circulatory system where it complexes to plasma retinol binding protein. Once delivered to skin cells, retinol apparently is taken up by passive... 

Separation and Assay. Procedures for the separation, purification, and assay of carotenoids and retinoids by h.p.l.c., g.c., and g.c.-m.s. are given in an extensive article." Another, general, review includes information on the h.p.l.c. separation of retinoids.A particularly useful method has been developed for resolution and analysis of some carotenoid optical isomers.For example, (3R,3 R)-, (3S,3 S)-, and (3/ ,3 5)-astaxanthin were converted into the diastereomeric (-)-camphanic acid diesters, which were separated by h.p.l.c. This procedure has been used to analyse the isomeric composition of a natural astaxanthin sample. An h.p.l.c. procedure for separation of a-, P-, and y-carotenes (173)—(175) and lycopene (176) has been described." Several papers describe methods for the h.p.l.c. separation and purification of various retinal and retinol isomers and derivatives.A procedure for the preparative t.l.c. of oxidation products of retinyl acetate has been described,and a competitive protein-binding radioassay for retinoic has been reported. 

The daily requirement of vitamin A (Table 6.3) is provided to an extent of 75% by retinol intake (as fatty acid esters, primarily retinyl palmitate), while the remaining 25% is through P-carotene and other provitaminactive carotenoids. Due to the limited extent of carotenoid cleavage, at least 6 g of P-carotene are required to yield 1 g retinol. Vitamin A absorption and its storage in the fiver occur essentially in the form of fatty acid esters. Its content in fiver is 250pg/g fresh tissue, i.e. a total of about 240-540 mg is stored. The fiver supplies the blood with free retinol, which then binds to proteins in blood. The plasma concentration of retinol averages 1.78 pmol/1 in women and 2.04 lamol/l in men. 

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