Vitamin A (all-trans-retinol) must be taken up, either as carotenes from plants, or as retinyl esters from animal food. The most prominent carotenes are alpha-carotene, lycopene, lutein, beta-cryptoxanthine, and especially beta-carotene. After uptake they are mostly broken down to retinal. Retinyl esters are hydrolysed like other fats. In enterocytes, retinoids bind to retinol-binding protein (RBP). Transport from enterocytes to the liver happens via chylomicrons (Harrison & Hussain 2001, Harrison 2005).
View original pathway at:Reactome.
Episkopou V, Maeda S, Nishiguchi S, Shimada K, Gaitanaris GA, Gottesman ME, Robertson EJ.; ''Disruption of the transthyretin gene results in mice with depressed levels of plasma retinol and thyroid hormone.''; PubMedEurope PMCScholia
Fernández-Borja M, Bellido D, Vilella E, Olivecrona G, Vilaró S.; ''Lipoprotein lipase-mediated uptake of lipoprotein in human fibroblasts: evidence for an LDL receptor-independent internalization pathway.''; PubMedEurope PMCScholia
Blomhoff R, Helgerud P, Rasmussen M, Berg T, Norum KR.; ''In vivo uptake of chylomicron [3H]retinyl ester by rat liver: evidence for retinol transfer from parenchymal to nonparenchymal cells.''; PubMedEurope PMCScholia
Kontush A, Chapman MJ.; ''Functionally defective high-density lipoprotein: a new therapeutic target at the crossroads of dyslipidemia, inflammation, and atherosclerosis.''; PubMedEurope PMCScholia
Ruiz A, Winston A, Lim YH, Gilbert BA, Rando RR, Bok D.; ''Molecular and biochemical characterization of lecithin retinol acyltransferase.''; PubMedEurope PMCScholia
Ruiz FX, Gallego O, Ardèvol A, Moro A, Domínguez M, Alvarez S, Alvarez R, de Lera AR, Rovira C, Fita I, Parés X, Farrés J.; ''Aldo-keto reductases from the AKR1B subfamily: retinoid specificity and control of cellular retinoic acid levels.''; PubMedEurope PMCScholia
Harrison EH, Hussain MM.; ''Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A.''; PubMedEurope PMCScholia
Harrison EH, Gad MZ.; ''Hydrolysis of retinyl palmitate by enzymes of rat pancreas and liver. Differentiation of bile salt-dependent and bile salt-independent, neutral retinyl ester hydrolases in rat liver.''; PubMedEurope PMCScholia
Amengual J, Widjaja-Adhi MA, Rodriguez-Santiago S, Hessel S, Golczak M, Palczewski K, von Lintig J.; ''Two carotenoid oxygenases contribute to mammalian provitamin A metabolism.''; PubMedEurope PMCScholia
Rigtrup KM, McEwen LR, Said HM, Ong DE.; ''Retinyl ester hydrolytic activity associated with human intestinal brush border membranes.''; PubMedEurope PMCScholia
von Lintig J, Kiser PD, Golczak M, Palczewski K.; ''The biochemical and structural basis for trans-to-cis isomerization of retinoids in the chemistry of vision.''; PubMedEurope PMCScholia
During A, Harrison EH.; ''Mechanisms of provitamin A (carotenoid) and vitamin A (retinol) transport into and out of intestinal Caco-2 cells.''; PubMedEurope PMCScholia
Gad MZ, Harrison EH.; ''Neutral and acid retinyl ester hydrolases associated with rat liver microsomes: relationships to microsomal cholesteryl ester hydrolases.''; PubMedEurope PMCScholia
Folli C, Calderone V, Ottonello S, Bolchi A, Zanotti G, Stoppini M, Berni R.; ''Identification, retinoid binding, and x-ray analysis of a human retinol-binding protein.''; PubMedEurope PMCScholia
Kanai M, Raz A, Goodman DS.; ''Retinol-binding protein: the transport protein for vitamin A in human plasma.''; PubMedEurope PMCScholia
Korenbrot JI.; ''Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models.''; PubMedEurope PMCScholia
Schreiber R, Taschler U, Preiss-Landl K, Wongsiriroj N, Zimmermann R, Lass A.; ''Retinyl ester hydrolases and their roles in vitamin A homeostasis.''; PubMedEurope PMCScholia
Kefalov VJ.; ''Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches.''; PubMedEurope PMCScholia
Naylor HM, Newcomer ME.; ''The structure of human retinol-binding protein (RBP) with its carrier protein transthyretin reveals an interaction with the carboxy terminus of RBP.''; PubMedEurope PMCScholia
D'Ambrosio DN, Clugston RD, Blaner WS.; ''Vitamin A metabolism: an update.''; PubMedEurope PMCScholia
Yu KC, Jiang Y, Chen W, Cooper AD.; ''Rapid initial removal of chylomicron remnants by the mouse liver does not require hepatically localized apolipoprotein E.''; PubMedEurope PMCScholia
Seeliger MW, Biesalski HK, Wissinger B, Gollnick H, Gielen S, Frank J, Beck S, Zrenner E.; ''Phenotype in retinol deficiency due to a hereditary defect in retinol binding protein synthesis.''; PubMedEurope PMCScholia
Harrison EH.; ''Mechanisms of digestion and absorption of dietary vitamin A.''; PubMedEurope PMCScholia
Herbert J, Wilcox JN, Pham KT, Fremeau RT, Zeviani M, Dwork A, Soprano DR, Makover A, Goodman DS, Zimmerman EA.; ''Transthyretin: a choroid plexus-specific transport protein in human brain. The 1986 S. Weir Mitchell award.''; PubMedEurope PMCScholia
Schupp M, Lefterova MI, Janke J, Leitner K, Cristancho AG, Mullican SE, Qatanani M, Szwergold N, Steger DJ, Curtin JC, Kim RJ, Suh MJ, Albert MR, Engeli S, Gudas LJ, Lazar MA.; ''Retinol saturase promotes adipogenesis and is downregulated in obesity.''; PubMedEurope PMCScholia
van Bennekum AM, Fisher EA, Blaner WS, Harrison EH.; ''Hydrolysis of retinyl esters by pancreatic triglyceride lipase.''; PubMedEurope PMCScholia
Inagami S, Ong DE.; ''Purification and partial characterization of cellular retinol-binding protein, type two, from human small intestine.''; PubMedEurope PMCScholia
Mello T, Nakatsuka A, Fears S, Davis W, Tsukamoto H, Bosron WF, Sanghani SP.; ''Expression of carboxylesterase and lipase genes in rat liver cell-types.''; PubMedEurope PMCScholia
Nayak N, Harrison EH, Hussain MM.; ''Retinyl ester secretion by intestinal cells: a specific and regulated process dependent on assembly and secretion of chylomicrons.''; PubMedEurope PMCScholia
Brown WV, Baginsky ML.; ''Inhibition of lipoprotein lipase by an apoprotein of human very low density lipoprotein.''; PubMedEurope PMCScholia
Peterson J, Fujimoto WY, Brunzell JD.; ''Human lipoprotein lipase: relationship of activity, heparin affinity, and conformation as studied with monoclonal antibodies.''; PubMedEurope PMCScholia
Wolf G.; ''The visual cycle of the cone photoreceptors of the retina.''; PubMedEurope PMCScholia
Yamauchi Y, Deguchi N, Takagi C, Tanaka M, Dhanasekaran P, Nakano M, Handa T, Phillips MC, Lund-Katz S, Saito H.; ''Role of the N- and C-terminal domains in binding of apolipoprotein E isoforms to heparan sulfate and dermatan sulfate: a surface plasmon resonance study.''; PubMedEurope PMCScholia
Fierce Y, de Morais Vieira M, Piantedosi R, Wyss A, Blaner WS, Paik J.; ''In vitro and in vivo characterization of retinoid synthesis from beta-carotene.''; PubMedEurope PMCScholia
Ji ZS, Brecht WJ, Miranda RD, Hussain MM, Innerarity TL, Mahley RW.; ''Role of heparan sulfate proteoglycans in the binding and uptake of apolipoprotein E-enriched remnant lipoproteins by cultured cells.''; PubMedEurope PMCScholia
Amengual J, Lobo GP, Golczak M, Li HN, Klimova T, Hoppel CL, Wyss A, Palczewski K, von Lintig J.; ''A mitochondrial enzyme degrades carotenoids and protects against oxidative stress.''; PubMedEurope PMCScholia
Marçais C, Verges B, Charrière S, Pruneta V, Merlin M, Billon S, Perrot L, Drai J, Sassolas A, Pennacchio LA, Fruchart-Najib J, Fruchart JC, Durlach V, Moulin P.; ''Apoa5 Q139X truncation predisposes to late-onset hyperchylomicronemia due to lipoprotein lipase impairment.''; PubMedEurope PMCScholia
Christoffersen C, Ahnström J, Axler O, Christensen EI, Dahlbäck B, Nielsen LB.; ''The signal peptide anchors apolipoprotein M in plasma lipoproteins and prevents rapid clearance of apolipoprotein M from plasma.''; PubMedEurope PMCScholia
Pugh EN, Lamb TD.; ''Amplification and kinetics of the activation steps in phototransduction.''; PubMedEurope PMCScholia
Blomhoff R, Holte K, Naess L, Berg T.; ''Newly administered [3H]retinol is transferred from hepatocytes to stellate cells in liver for storage.''; PubMedEurope PMCScholia
von Lintig J.; ''Metabolism of carotenoids and retinoids related to vision.''; PubMedEurope PMCScholia
Burns ME, Pugh EN.; ''Lessons from photoreceptors: turning off g-protein signaling in living cells.''; PubMedEurope PMCScholia
Hagen E, Myhre AM, Tjelle TE, Berg T, Norum KR.; ''Retinyl esters are hydrolyzed in early endosomes of J774 macrophages.''; PubMedEurope PMCScholia
Shirakami Y, Lee SA, Clugston RD, Blaner WS.; ''Hepatic metabolism of retinoids and disease associations.''; PubMedEurope PMCScholia
Havel RJ, Kane JP, Kashyap ML.; ''Interchange of apolipoproteins between chylomicrons and high density lipoproteins during alimentary lipemia in man.''; PubMedEurope PMCScholia
Ruiz FX, Porté S, Gallego O, Moro A, Ardèvol A, Del Río-Espínola A, Rovira C, Farrés J, Parés X.; ''Retinaldehyde is a substrate for human aldo-keto reductases of the 1C subfamily.''; PubMedEurope PMCScholia
Jackson RL, Tajima S, Yamamura T, Yokoyama S, Yamamoto A.; ''Comparison of apolipoprotein C-II-deficient triacylglycerol-rich lipoproteins and trioleoylglycerol/phosphatidylcholine-stabilized particles as substrates for lipoprotein lipase.''; PubMedEurope PMCScholia
Ahnström J, Faber K, Axler O, Dahlbäck B.; ''Hydrophobic ligand binding properties of the human lipocalin apolipoprotein M.''; PubMedEurope PMCScholia
Futamura M, Dhanasekaran P, Handa T, Phillips MC, Lund-Katz S, Saito H.; ''Two-step mechanism of binding of apolipoprotein E to heparin: implications for the kinetics of apolipoprotein E-heparan sulfate proteoglycan complex formation on cell surfaces.''; PubMedEurope PMCScholia
Moise AR, Kuksa V, Imanishi Y, Palczewski K.; ''Identification of all-trans-retinol:all-trans-13,14-dihydroretinol saturase.''; PubMedEurope PMCScholia
Gallego O, Ruiz FX, Ardèvol A, Domínguez M, Alvarez R, de Lera AR, Rovira C, Farrés J, Fita I, Parés X.; ''Structural basis for the high all-trans-retinaldehyde reductase activity of the tumor marker AKR1B10.''; PubMedEurope PMCScholia
Blomhoff R, Blomhoff HK.; ''Overview of retinoid metabolism and function.''; PubMedEurope PMCScholia
Visual phototransduction is the process by which photon absorption by visual pigment molecules in photoreceptor cells is converted to an electrical cellular response. The events in this process are photochemical, biochemical and electrophysiological and are highly conserved across many species. This process occurs in two types of photoreceptors in the retina, rods and cones. Each type consists of two parts, the outer segment which detects a photon signal and the inner segment which contains the necessary machinery for cell metabolism. Each type of cell functions differently. Rods are very light sensitive but their flash response is slow so they work best in twilight conditions but are not good at detecting objects moving quickly. Cones are less light-sensitive and have a fast flash response so they work best in daylight conditions and are better at detecting fast moving objects than rods.
The visual pigment consists of a chromophore (11-cis-retinal, 11cRAL, A1) covalently attached to a GPCR opsin family member. The linkage is via a Schiff base forming retinylidene protein. Upon photon absorption, 11cRAL isomerises to all-trans retinal (atRAL), changing the conformation of opsin to an activated form which can activate the regulatory G protein transducin (Gt). The alpha subunit of Gt activates phosphodiesterase which hydrolyses cGMP to 5'-GMP. As high level of cGMP keep cGMP-gated sodium channels open, the lowering of cGMP levels closes these channels which causes hyperpolarization of the cell and subsequently, closure of voltage-gated calcium channels. As calcium levels drop, the level of the neurotransmitter glutamate also drops causing depolarization of the cell. This effectively relays the light signal to postsynaptic neurons as electrical signal (Burns & Pugh 2010, Korenbrot 2012, Pugh & Lamb 1993).
11cRAL cannot be synthesised in vertebrates. Vitamin A from many dietary sources is the precursor for 11cRAL. It is taken from food in the form of esters such as retinyl acetate or palmitate or one of four caretenoids (alpha-carotene, beta-carotene, gamma-carotene and beta-cryptoxanthin). Retinoids are transported from the gut to be stored in liver, until required by target organs such as the eye (Harrison & Hussain 2001, Harrison 2005). In the eye, in the form 11cRAL, it is used in the retinoid (visual) cycle to initiate phototransduction and for visual pigment regeneration to ready the photoreceptor for the next phototransduction event (von Lintig 2012, Blomhoff & Blomhoff 2006, von Lintig et al. 2010, D'Ambrosio et al. 2011, Wang & Kefalov 2011, Kefalov 2012, Wolf 2004).
Nascent chylomicrons (CM) containing all-trans-retinyl esters (atREs) are secreted from intestinal cells and transported to the liver via the lymphatic system (Nayak et al. 2001, During & Harrison 2007).
Chylomicrons (CM) are large (75–450 nm), spherical lipoprotein particles secreted by intestinal cells postprandially and transport dietary fat and fat-soluble vitamins in the lymphatic system (Nayak et al. 2001, During & Harrison 2007). All-trans-retinyl esters (atREs) are packaged into nascent chylomicrons.
Lipoprotein lipase dimers (LPL:LPL) are tethered to heparan sulfate proteoglycans (HSPG) at endothelial cell surfaces (Fernandez-Borja et al. 1996; Peterson et al. 1992). Both syndecan 1 (Rosenberg et al. 1997) and perlecan (Goldberg 1996) HSPG molecules are capable of tethering LPL. The LPL enzyme catalyzes the hydrolysis and release of triacylglycerols (TG) associated with circulating chylomicrons to leave a CM remnant (CR). This reaction is annotated here as causing the hydrolysis and release of 50 molecules of TG. In vivo, the number is much larger, and TG depletion probably occurs in the course of multiple encounters between a chylomicron and endothelial LPL. This reaction is strongly activated by chylomicron-associated apo C-II protein both in vivo and in vitro (Jackson et al. 1986). Chylomicron-associated apoC-II protein inhibits LPL activity in vitro (Brown and Baginsky 1972), and recent studies have indicated a positive regulatory role for apoA-5 protein, though its molecular mechanism of action remains unclear (Marcais et al. 2005; Merkel and Heeren 2005). CRs can then be taken up by liver parenchymal cells in two ways; 1) directly by the LDL receptor or 2) apoE/HSPG-directed uptake by LDL receptor-related proteins.
Nascent chylomicrons (CMs) acquire copies of apolipoproteins C-II, C-III, and E from circulating spherical (mature) high-density lipoprotein particles, becoming mature chylomicrons (Havel et al. 1973, Bisgaier & Glickman 1983). Here, this interaction is annotated to involve the transfer of a single copy of each lipoprotein, but a mature chylomicron in fact contains approximately 25 copies of apolipoprotein E and 180 copies of C apolipoproteins (Bhattacharya & Redgrave 1981).
When the low-density lipoprotein receptor (LDLR) is missing, saturated or inhibited, chylomicron remnants (CRs) containing all-trans-retinyl esters (atREs) can be cleared from circulation by interaction with cell-surface heparan sulfate proteoglycan (HSPG) and secreted apolipoprotein E (apoE). This complex is then presented to LDL receptor-related proteins (LRPs; reviews May et al. 2007, Li et al. 2001, Hussain 2001) for internalization (Ji et al. 1993).
Retinyl esters (REs) are stored in lipid droplets (LDs) in hepatic stellate cells (HSCs) until there is a demand for retinoid by the body. Mobilization of atREs stores require lipases with retinyl ester hydrolase (REH) activity. At present, the identity of the REH mediating atRE mobilization is unknown (see reviews Shirakami et al. 2012, Schreiber et al. 2012). In studies performed with rat livers, Mello et al. found that the carboxylesterases ES4 and ES10 possessed REH activity and were localised to HSCs (Mello et al. 2008) but it's not confirmed that these are the actual REHs involved in retinoid mobilization. The human orthologue to these rat enzymes is presently unknown.
In the bloodstream, circulating retinol binding protein 4 (RBP4, in complex with atROL), binds transthyretin (TTR, a 51 kDa protein) in a 1:1 molar complex (Naylor & Newcomer, 1999). The resultant TTR:RBP4:atROL complex is larger and therefore less susceptible to glomerular filtration, maintaining normal levels of retinoid and RBP4 in the circulation. In TTR-deficient mice, plasma levels of atROL and RBP4 were observed to be 5% of wild type levels, highlighting the importance of TTR binding to RBP4:atROL (Episkopou et al. 1993). TTR is also a transporter for thyroxine in the brain (not shown here) (Herbert et al. 1986).
At the stellate cell surface, all-trans-retinol (atROL) binds to retinol binding protein 4 (RBP4, holo-RBP) (Kanai et al. 1968). atROL is insoluble in aqueous conditions and it's thought RBP4 picks up atROL from the outer leaflet of the plasma membrane. RBP4 is a 21 kDa protein secreted into the bloodstream by the liver in an atROL-dependent manner. Defects in RBP4 cause retinol-binding protein deficiency (RBP deficiency, MIM:180250), causing night vision problems and progressive atrophy of the retinal pigment epithelium (RPE) (Seeliger et al. 1999).
All-trans-retinyl esters (atREs) are the main storage form of retinol in hepatic stellate cells (HSCs). Lecithin retinol acyltransferase (LRAT) mediates the esterification of all-trans-retinol (atROL) with a variety of fatty acyl groups (FACYLs) to form REs which are stored in lipid droplets in the cytosol. Cellular retinol-binding protein 1 (RBP1) is an effective donor of atROL for LRAT (Ruiz et al. 1999).
Once hydrolysed from all-trans-retinyl esters (atREs), all-trans-retinol (atROL) can either be esterified for storage or secreted into the bloodstream for transport to target tissues/organs (Kanai et al. 1968). The mechanism of secretion/efflux is currently unknown.
Once inside liver parenchymal cells, all-trans-retinyl esters (atREs), are hydrolysed to all-trans-retinol (atROL) and fatty acids (FAs) by a neutral, all-trans-retinyl ester hydrolase (NREH). No NREH has been characterised yet although both acidic and neutral REH activity has been shown to be associated with endosomes and plasma membrane preparations from rodent livers (Harrison & Gad 1989, Gad & Harrison 1991, Hagen et al. 1999). As the acidity increases, early endosomes change to late endosomes and further hydrolysis of atREs is mediated by acid retinyl ester hydrolase (AREH). Like NREH, AREH has not yet been characterised (see refs above). The translocation mechanism of atROL to cytosol is unknown.
All-trans-retinol (atROL) is stored in hepatic stellate cells (HSCs) in its esterified form. Before esterification takes place, atROL binds to cellular retinol-binding protein 1 (RBP1) (Folli et al. 2001). The resultant complex (RBP1:atROL) serves as a substrate for lecithin retinol acyltransferase (LRAT), the main enzyme responsible for the esterification of atROL. It is thought the binding of RBP1 serves to translocate atROL to the endoplasmic reticulum where LRAT is located.
When the body is in a retinoid-sufficient state, all-trans-retinol (atROL) is transferred to hepatic stellate cells (HSCs) for storage. The transfer was established in rat experiments (Blomhoff et al. 1982, Blomhoff et al. 1984). The mechanism of transport is as yet unknown.
When the low-density lipoprotein receptor (LDLR) is missing, saturated or inhibited, chylomicron remnants (CRs) containing all-trans-retinyl esters (atREs) bind apolioprotein E (apoE). ApoE, secreted by hepatocytes, acts as a high-affinity ligand for the LDL-related receptor protein (LRP) family. CR:atREs:apoE then binds to cell-surface heparan sulfate proteoglycan (HSPG), abundant in the space of Disse. HSPG/apoE binding plays a critical role in the capture of CR:atREs, ready for internalization via LRPs (Futamura et al. 2005, Yamauchi et al. 2008).
Chylomicron remnants (CRs) are "sieved" when they arrive at the liver by size, the appropriate sized remnants passing through the space of Disse. Once inside, CRs containing all-trans-retinyl esters (atREs) can be directly and rapidly taken up by liver parenchymal cells via the low-density lipoprotein receptor (LDLR) using apolipoprotein E (apoE) as a ligand. Internalization of remnants occur via endocytosis (see review D'Ambrosio et al. 2011). This reaction is inferred from uptake studies in mice (Yu et al. 2000). Defects in LDLR cause familial hypercholesterolemia (FH, MIM:143890), a common autosomal disease that affects about 1 in 500 people in most countries. Abnormal LDLR doesn't remove LDL from circulation resulting in high levels of LDL in blood, leading to early cardiovascular disease via atherosclerosis. The defect was first described by Brown and Goldstein (1974).
Once inside liver parenchymal cells, all-trans-retinyl esters (atREs), are hydrolysed to all-trans-retinol (atROL) and fatty acids (FAs) by a neutral, all-trans-retinyl ester hydrolase (NREH). No NREH has been characterised yet although both acidic and neutral REH activity has been shown to be associated with endosomes and plasma membrane preparations from rodent livers (Harrison & Gad 1989, Gad & Harrison 1991, Hagen et al. 1999). As the acidity increases, early endosomes change to late endosomes and further hydrolysis of atREs is mediated by acid retinyl ester hydrolase (AREH). Like NREH, AREH has not yet been characterised (see refs above). The translocation mechanism of atROL to cytosol is unknown.
RDH11 is the best-characterised enzyme that reduces all-trans-retinal (atRAL) to all-trans-retinol (atROL). In addition, several aldo-keto reductase (AKR) enzymes from subfamilies 1B and 1C also show all-trans-retinal (atRAL) reductase activity. AKR1B10 shows high reductase activity towards atRAL (Gallego et al. 2007, Ruiz et al. 2009) whereas AKR1C1, 1C3 and 1C4 all show much lower reductase activity towards arRAL (Ruiz et al. 2011).
All-trans-retinol (atROL) binds to cellular retinol-binding protein 1 (RBP1) (Folli et al. 2001). RBP1 is thought to be required for intracellular transport of atROL in the liver.
Beta,beta-carotene 9',10'-oxygenase (BCO2) is able to eccentrically cleave carotenoids to produce long chain (>C20) apocarotenoids (Amengual et al. 2011). This is in contrast to the other provitamin A-converting enzyme, BCMO1 which is able to symmetrically cleave carotenoids to produce apocarotenoids of C20 length, such as all-trans-retinal (atRAL). BCMO1 is the main enzyme involved in retinoid homeostasis and resides in the cytosol whereas BCO2 resides in the mitchondrion, has broad substrate activity and is proposed to provide an alternative, minor route for retinoid production. How apocarotenoids produced by BCO2 cleavage are utilised is the subject of further investigation (Amengual et al. 2013). Being in the mitochondrion, BCO2 is able to degrade carotenoids which, if otherwise allowed to accumulate, are implicated in oxidative damage to the cell (Amengual et al. 2011). In this example, beta-carotene (betaC) is cleaved by BCO2 to produce beta-apo-10'-carotenal (APO10al) and beta-ionone (bION) in an enterocyte cell. Carotenoids, such as betaC, can also be metabolised in many other cell types including hepatocytes and stellate cells of the liver.
Apoliprotein M (APOM) is a plasma protein usually associated with HDLs and to a lesser extent, with LDLs. APOM could be classed as a lipocalin (LCN) because it shares the structurally conserved beta-barrel, which in many LCNs, binds hydrophobic ligands. Mature APOM retains its signal peptide, which serves as a lipid anchor to attach it to lipoproteins, thereby keeping it in the circulation (Christoffersen et al. 2008). APOM is able to bind retinoids such as retinol, all-trans-retinoate and 9-cis-retinoate with low affinity although they may not be the natural ligands (Ahnstrom et al. 2007, Dahlback & Nielsen 2009). APOM does not bind cholesterol, vitamin K or arachidonate (Ahnstrom et al. 2007).
All-trans-retinol 13,14-reductase (RETSAT) is an ER membrane-associated protein that mediates the saturation of the 13-14 double bond of all-trans-retinol (atROL) to produce all-trans-13,14-dihydroretinol (at-13,14-dhROL). The product formed is a metabolite of unknown biological function. The human activity of RETSAT is inferred from mouse Retsat enzyme assays (Moise et al. 2004). In human and mouse, RETSAT is induced during adipogenesis and is directly regulated by the transcription factor peroxisome proliferator activated receptor gamma (PPAR-gamma). Ablation of RETSAT inhibits adipogenesis but this block was not overcome by the product of RETSAT enzymatic activity. In adipose tissue, RETSAT is expressed in adipocytes but is downregulated in obesity. RETSAT could be a novel target for therapeutic intervention in metabolic disease (Schupp et al. 2009).
Part of nutritional vitamin A is in the form of retinyl esters (REs). The main fatty acids which can form esters with retinol are palmitate, oleate, stearate and linoleate. REs are digested together with other lipids, and by the same enzymes. Pancreatic lipase catalyses the hydrolysis of RE to all-trans-retinol (atROL) and fatty acid which are then both taken up by enterocytic cell membranes (Bennekum et al. 2000).
Part of retinol ester hydrolase activity in the small intestine is associated with the brush border membrane but the protein having it is not identified. It is thought to be phospholipase B (Rigtrup et al. 1994).
Transfer of fatty acyl residues (FACYLs) from lecithin is the main way to esterify all-trans-retinol (atROL). Lecithin is a generic name for the yellowy-brown fatty substances in animals and tissues. It can be composed of phosphatidylcholines, phosphatidylethanolamines, and phosphatidylinositols. Fatty acyl transfer is catalyzed by Lecithin retinol acyltransferase (LRAT) and takes place near the endoplasmic reticulum membrane. The main fatty acyl moieties that are substrates for LRAT are palmitoyl, oleoyl, stearoyl and linoleoyl groups present in the A1 position of membrane phosphatidylcholine molecules. LRAT esterifies atROL with these acyl groups to form all-trans-retinyl esters (atREs). The aim is not storage but transport via chylomicrons (Ruiz et al. 1999).
Although the enzyme catalysing retinal reduction in human enterocytes is not identified, the best candidate is retinol dehydrogenase 11 (RDH11, RalR1). It is expressed in the intestine, has a basic pH optimum, and localises to the ER membrane where LRAT catalyses the next step in the pathway. However, RDH11 catalyses retinal reduction to retinol in vitro and uses NADPH as cofactor (Fierce et al. 2008).
In enterocytes, the dominant retinol-binding protein is RBP2 (CRBPII) which is abundant and binds retinol faster than the cell membrane. So, even though lipophilic retinol can easily enter the cell membrane of bowel enterocytes, it is collected by the abundancy of RBP2 into the enterocyte cytosol where it is further processed (Inagami & Ong 1997).
As long as vitamin A is needed, beta-carotene-monooxygenase (BCMO1) catalyses the cleavage of carotenes, resulting mainly in retinal (Fierce et al. 2008).
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The visual pigment consists of a chromophore (11-cis-retinal, 11cRAL, A1) covalently attached to a GPCR opsin family member. The linkage is via a Schiff base forming retinylidene protein. Upon photon absorption, 11cRAL isomerises to all-trans retinal (atRAL), changing the conformation of opsin to an activated form which can activate the regulatory G protein transducin (Gt). The alpha subunit of Gt activates phosphodiesterase which hydrolyses cGMP to 5'-GMP. As high level of cGMP keep cGMP-gated sodium channels open, the lowering of cGMP levels closes these channels which causes hyperpolarization of the cell and subsequently, closure of voltage-gated calcium channels. As calcium levels drop, the level of the neurotransmitter glutamate also drops causing depolarization of the cell. This effectively relays the light signal to postsynaptic neurons as electrical signal (Burns & Pugh 2010, Korenbrot 2012, Pugh & Lamb 1993).
11cRAL cannot be synthesised in vertebrates. Vitamin A from many dietary sources is the precursor for 11cRAL. It is taken from food in the form of esters such as retinyl acetate or palmitate or one of four caretenoids (alpha-carotene, beta-carotene, gamma-carotene and beta-cryptoxanthin). Retinoids are transported from the gut to be stored in liver, until required by target organs such as the eye (Harrison & Hussain 2001, Harrison 2005). In the eye, in the form 11cRAL, it is used in the retinoid (visual) cycle to initiate phototransduction and for visual pigment regeneration to ready the photoreceptor for the next phototransduction event (von Lintig 2012, Blomhoff & Blomhoff 2006, von Lintig et al. 2010, D'Ambrosio et al. 2011, Wang & Kefalov 2011, Kefalov 2012, Wolf 2004).
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