Fat soluble vitamin :Vitamin A

Vitamin A

1. FAT SOLUBLE VITAMINS-VITAMIN:A DR.KOPPUKONDA RAVI BABU B.Sc., MBBS ; MD BIOCHEMISTRY ASSISTANT PROFESSOR

2. VITAMIN A

3. Vitamin A Vitamin A serves an important role in vision, is required for gene expression , embryonic development, for immune and reproductive functions, and is an antioxidant .

4. VITAMIN –A : Chemistry Vitamin A is the nutritional term for the group of compounds with a 20-carbon structure containing a methyl-substituted cyclohexenyl ring (β-ionone ring) and an isoprenoid side chain . with a hydroxyl group (retinol), an aldehyde group (retinal), a carboxylic acid group (retinoic acid), or an ester group ( retinyl ester) at the terminal C15 .

5. Fig.Vitaminic Forms of A1, A2, and β-Carotene.

6. Fig. β-Carotene and the major vitamin A vitamers. Asterisk shows the site of symmetrical cleavage of β-carotene by carotene dioxygenase, to yield retinaldehyde.

7. VITAMIN –A : Chemistry Retinol, the principal vitamin A vitamer , can be oxidized reversibly to retinal —which shares all the biological activity of retinol—or further oxidized to retinoic acid, which shows some of its biological activity. The principal storage forms of vitamin A are retinyl esters, particularly palmitate . The term retinoids refers to retinol , its metabolites, and synthetic analogs with similar structure. Some dietary carotenoids (C40 polyisoprenoid compounds) are included in the vitamin A family and are classified as provitamin A because they are cleaved biologically to yield retinol .

8. VITAMIN –A : Chemistry β-carotene, α-carotene, and β- cryptoxanthin . Vitamin A compounds are yellowish oils or low-melting-point solids (depending on isomeric purity) that are practically insoluble in water but are soluble in organic solvents and mineral oil .

9. VITAMIN –A : Chemistry Vitamin A is sensitive to O2 and to ultraviolet light, which induces a greenish fluorescence with an absorbance peak at 325 nm. Provitamin A, β-carotene. This compound is an orange-to-purple , water-insoluble solid that is oxidized in air to inactive products. The other carotenes, cryptoxanthin and β- apocarotenals , are asymmetric with only one β-ionone ring and yield less vitamin A activity .

10. VITAMIN-A : Dietary Sources animal-derived foods, such as liver, offal, and fish oils . Other sources are full cream milk, butter, and fortified margarines . The provitamin A carotenoids are obtained from yellow to orange fruits and vegetables, and from green leafy vegetables . Good sources are pumpkin, carrots, tomatoes, apricots, grapefruit, lettuce, and most green vegetables. 25% - carotenoids and approximately 75%-preformed retinol .

11. VITAMIN – A :Absorption, Transport, Metabolism, and Excretion Preformed vitamin A, most often in the form of retinyl esters or carotenoids , is subject to emulsification and mixed micelle formation by the action of bile salts before they are transported into the intestinal cell. The retinyl esters are moved across the mucosal membrane and hydrolyzed to retinol within the cell to be then re- esterified by cellular RBP II And packaged into chylomicrons , which then enter the mesenteric lymphatic system and pass into the systemic circulation. A small amount of the ingested retinoid is converted into retinoic acid in the intestinal cell. The efficiency of absorption of preformed vitamin A is high (70%–90%).

12. Absorption, Transport, Metabolism, and Excretion continue…. Carotenoids , also in micellar form, are absorbed into the duodenal mucosal cells by passive diffusion . The efficiency of absorption of carotenoids is much lower than that for vitamin A (9% and 22%),and is subject to more variables, including the carotenoid type, the amount in the meal, matrix properties, nutrient status, and genetic factors. Once inside the mucosal cell, β-carotene is principally converted to retinal by the enzyme β-carotene-15,15′-dioxygenase. Retinal is converted by retinal reductase to retinol and esterified . Betacarotene can also be cleaved eccentrically to β- apocarotenals .

13. Absorption, Transport, Metabolism, and Excretion continue…. β- apocarotenals can be further degraded to retinal or retinoic acid. The newly synthesized retinyl esters , from both preformed vitamin A and carotenoids , along with exogenous lipids and nonhydrolyzed carotenoids , then pass with chylomicrons via the lymphatic system to the liver, where uptake by parenchymal cells again involves hydrolysis.

14. Absorption, Transport, Metabolism, and Excretion continue…. In the liver , retinol is bound with and transthyretin ( thyroxine -binding prealbumin in a 1 : 1 : 1 complex of sufficient size to prevent loss by glomerular filtration and is returned to the circulation, or stored as esters within the stellate cells . Delivery of retinol to the tissue is controlled by the availability of the vitamin A–protein complex in the circulation, although this control mechanism can be bypassed by large doses of retinol .

15. Absorption, Transport, Metabolism, and Excretion continue…. Retinoic acid from the intestinal mucosa is transported bound to serum albumin via the portal vein. Retinoic acid cannot be significantly reduced to retinal, but it is rapidly metabolized in tissue, such as liver, to yield more polar catabolites ( eg , 5,6-epoxyretinoic acid) and conjugates, such as retinoyl β- glucuronide , which are excreted .

16. Absorption, Transport, Metabolism, and Excretion continue…. A small amountof retinoic acid undergoes enterohepatic circulation after intestinal hydrolysis of the glucuronide , which is excreted in bile .

17. VITAMIN –A:Functions Retinal in vision is considered the most important physiological function of vitamin A. All- transretinol is the predominant circulating form of vitamin A. Cells of the retina isomerize this to the 11 -cis alcohol that is reversibly dehydrogenated to 11 -cis retinal . This sterically hindered geometrical isomer of the aldehyde combines as a lysyl -linked Schiff base with suitable proteins ( eg , opsin ) to generate photosensitive pigments, such as rhodopsin .

18. VITAMIN –A:Functions Illumination of such pigments causes photoisomerization and the release of all- trans-retinal and the protein, a process that couples the large conformational change with ion flux and optic nerve transmission. The all- trans-retinal is isomerized to the 11- cis isomer, which combines with the liberated protein to reconstitute the photo pigment in a visual cycle. The pyridine nucleotide–dependent dehydrogenase ( reductase ) can also reduce the all- trans-retinal to all -trans-retinol.

19. VITAMIN –A:Functions role in reproduction, growth, embryonic development, and immune function. Many of these functions are mediated through the binding of retinoic acid to specific nuclear receptors that regulate genomic expression . In normal growth, and in maintenance of the integrity of epithelial cells, retinoic acid acts through the activation of retinoic acid receptors and retinoid X receptors in the nucleus to regulate various genes that encode for structural proteins, enzymes, extracellula matrix proteins, RBPs, and receptors.

20. Summary of kinetics and mechanism of action of vitamin A

21. Wald’s Visual Cycle

22. Functions of vitamin A

23. E. Distribution of vitamin A

24. VITAMIN-A:Requirements and Reference Nutrient Intakes Retinol activity equivalent (RAE) 1 RAE = 1 μg retinol, 12 μg β-carotene, or 24 μg carotenoids . RDAs for vitamin A are the following: 900 μg RAE for men 19 years and older; 700 μg RAE for women 19 years and older, with up to 770 μg RAE/day in pregnancy and up to 1300 μg RAE/day in lactation; 300 to 900 μg RAE for children 1 to 18 years, dependent on age and gender; and an adequate intake (AI) of 400 μg RAE at 0 to 6 months and 500 μg RAE from 7 to 12 months for infants.

25. VITAMIN – A : Intravenous Supply TPN, is 1000 μg retinol . This is usually provided as retinol palmitate and may be supplied with other fat-soluble vitamins in a mixture dissolved in a fat emulsion for intravenous feeding, or may be designed to be compatible with a mixture of all vitamins suitable for addition to other water-soluble nutrients.

26. VITAMIN A : Deficiency Vitamin A deficiency primarily affects infants and children . Risk factors include poverty, low birth weight, poor sanitation, malnutrition, infection, and parasitism. Because hepatic accumulation of vitamin A occurs during the last trimester of pregnancy, preterm infants are relatively vitamin A deficient at birth.

27. VITAMIN A : Deficiency RDA of 400 μg RAE is sufficient. Infants with birth weights of less than 1500 g (t hose at <30 weeks’ gestation ) have virtually no hepatic vitamin A stores and are at risk of vitamin A deficiency.

28. VITAMIN A : Deficiency Various researchers have observed that: (1) bronchopulmonary dysplasia is common in premature infants; Rx: intramuscula r injections of 630 μg RAE every 2 days. (3) blood concentrations of vitamin A decline during TPN, often reaching concentrations of 10 to 15 μg / dL (normal: 20 to 65 μg / dL ) unless adequate supplements are given. (4) vitamin A (retinol) delivered in TPN solutions may be absorbed into the inner walls of plastic administration sets; however, this loss can be minimized by using ethylene vinyl acetate rather than polyvinyl chloride.

29. VITAMIN A : Deficiency Fat malabsorption , particularly caused by celiac disease or chronic pancreatitis, and protein-energy malnutrition predispose to vitamin A deficiency. Liver disease diminishes RBP synthesis , and ethanol abuse leads to both hepatic injury and competition with retinol for alcohol dehydrogenase , which is necessary for the oxidation of retinol to retinal and retinoic acid. Degenerative changes in the eyes and skin and poor dark adaptation or night blindness ( nyctalopia ),followed by degenerative changes in the retina.

30. VITAMIN A : Deficiency Xerophthalmia occurs when the conjunctiva becomes dry with small gray plaques with foamy surfaces ( Bitot spots ). These lesions are reversible with vitamin A administration. More serious effects of deficiency are known as keratomalacia and cause ulceration and necrosis of the cornea, which lead to perforation, prolapse , endophthalmitis , and blindness. skin changes include dryness, roughness, papular eruptions, and follicular hyperkeratosis. Atrophy of certain specialized epithelia, followed by metaplastic hyperkeratinization .

31. VITAMIN – A : Toxicity Ingestion of excess vitamin or as a side effect of inappropriate therapy. Hypervitaminosis A occurs after liver storage of retinol, and its esters exceeds 3000 μg /g tissue, with ingestion of more than 30,000 μg /day for months or years, or if plasma vitamin A concentrations exceed 140 μg / dL (4.9 μmol /L) . Older adults are more susceptible to vitamin A toxicity at lower doses because exposure to retinyl esters is longer because of delayed postprandial clearance of lipoproteins .

32. VITAMIN – A : Toxicity Acute toxicity present as abdominal pain, nausea, vomiting, severe headaches, dizziness, sluggishness, and irritability, followed within a few days by desquamation of the skin and recovery. Long-term toxicity : is characterized by Bone and joint pain, hair loss, dryness and fissures of the lips, anorexia, benign intracranial hypertension, weight loss, and hepatomegaly .

33. VITAMIN – A : Toxicity Threefold the RDA for several years resulted in classic histological changes of hepatotoxicity . Osteoporosis and hip fracture are associated with vitamin A intakes that are only twice the RDA . Infants given excess vitamin A over months to years can develop intracranial features, typically bulging fontanelle , and skeletal abnormalities at doses of 5500 to 6750 μg /day.

34. VITAMIN – A : Toxicity High vitamin A intake in humans, acting via 13- cis-retinoic acid, is teratogenic . The critical period of susceptibility is the first trimester of pregnancy , and primary abnormalities derive from the cranial neural crest cells .

35. VITAMIN – A : Toxicity hypercalcemia , especially in chronic kidney disease. Carotenemia - infants and children . yellowing of the skin is observed, is benign because the excess carotene is deposited rather than converted to vitamin A. There is a role for the measurement of β-carotene in the differential diagnosis of specific cases of jaundice in children . impaired activity of the enzyme β-carotene-15,15′-dioxygenase in children that leads to accumulation of β-carotene, especially when consuming carotene-rich foods, but it is a benign condition

36. VITAMIN – A : Toxicity Carotenemia has also been linked to amenorrhea ?. Increased concentrations have also been found in hypothyroid patients, in whom conversion to vitamin A is decreased, and in patients with hyperlipemia that is associated with diabetes mellitus .

37. VITAMIN – A : LABORATORY ASSESSMENT The plasma concentration of vitamin A- ( 20 μ g/g liver ). assessed by the measurement of retinol concentration. Retinol circulates in plasma as a 1 : 1 : 1 complex with RBP and transthyretin (TTR), forming a complex that prevents glomerular filtration. The circulating concentration of RBP is determined by dietary protein and Zn , which are necessary for RBP synthesis . Thus, protein malnutrition, liver disease, and Zn deficiency resulting in RBP deficiency will lead to hypovitaminosis A .

38. VITAMIN – A : LABORATORY ASSESSMENT Renal failure resulting in decreased excretion of RBP has been reported to result in hypervitaminosis A . Both RBP and TTR are negative acute-phase proteins ; thus inflammatory changes will result in transient falls in both proteins and plasma retinol . To distinguish inflammatory from nutritional causes of reduced plasma retinol concentrations, it may be necessary to measure CRP.

39. VITAMIN – A : LABORATORY ASSESSMENT Early chemical methods that are rarely used include the Carr-Price photometric method , which uses antimony trichloride in chloroform as the reagent, and the later Neeld -Pearson method , which uses trifluoroacetic acid to produce a blue pigment with the conjugated double bonds of vitamin A (and the carotenoids ). To improve specificity and sensitivity, later methods used high-pressure liquid chromatography (HPLC) after solvent extraction and other separation techniques, with fluorometric or spectrophotometric

40. VITAMIN – A : LABORATORY ASSESSMENT Both normal and reverse-phase HPLC have been used. Reverse-phase HPLC is preferable for acid-sensitive compounds ( eg , 5,6-epoxyretinoic acid). Photometric, electrochemical, and mass spectrophotometric detectors have all been used. serum is deproteinized with ethanol that contains internal standards, centrifuged, and extracted with hexane. This is followed by evaporation to dryness, and the residue is redissolved in tetrahydrofuran .

41. VITAMIN – A : LABORATORY ASSESSMENT HPLC-mass spectrometry methods. Recent advances : the use of supported liquid extraction methodology for sample preparation using modified diatomaceous earth (natural fossilized biominerals containing high silica content) packed into columns or 96-well plates.

42. VITAMIN – A :PREANALYTICAL VARIABLES Plasma, serum, or whole blood specimens are all suitable for retinol measurements. Fasting samples are recommended, especially if a patient is taking oral or parenteral vitamin A supplementation. A sample should be taken at least 8 hours after supplementation if fasting is not possible. Vitamin A samples are light sensitive and should be protected from light as much as possible by wrapping in foil. Vitamin A showed good stability in whole blood collected into tubes containing lithium (Li) heparin for up to 48 hours at room temperature and without light protection . Another study reported that vitamin A was stable for up to 72 hours in whole blood samples kept at 32°C and up to 14 days in serum stored at 11°C .

43. VITAMIN – A :Reference Intervals 20 to 40 μg / dL (0.70–1.40 μmol /L) for 1- to 6-year-old children, 26 to 49 μg / dL (0.91–1.71 μmol /L) for 7- to 12-year-old children, 26 to 72 μg / dL (0.91–2.51 μmol /L) for 13- to 19-year-old adolescents, And 30 to 80 μg / dL (1.05–2.80 μmol /L) for adults. Values more than 30 μg / dL (1.05 μmol /L) are associated with appreciable reserves in the liver and correlate well with vitamin A intake. Within the reference interval, values for men are generally approximately 20% higher than those for women.

44. VITAMIN – A :Reference Intervals By HPLC, with ultraviolet detection, the reference interval for serum α- carotene is 14 to 60 μ g/L (26–112 nmol /L), β-carotene is 90 to 310 μg /L (167–577 nmol /L), lutein is 80 to 200 μg /L (140–352 nmol /L), and lycopene 100 to 300 μg /L (186–559 nmol /L).