Lecture 42: Fat Soluble Vitamins 2 - Importance of Vitamin A PDF

Lecture 42 Fat Soluble Vitamins 2 Importance of Vitamin A DR- LAILA SAWALHA 1  Specific Objectives By the end of this lecture students can be able to: Understand the functions of vitamin A. Explain visual cycles. Discuss effects of dietary...

Lecture 42 Fat Soluble Vitamins 2 Importance of Vitamin A DR- LAILA SAWALHA 1  Specific Objectives By the end of this lecture students can be able to: Understand the functions of vitamin A. Explain visual cycles. Discuss effects of dietary deficiency of vitamin A 2  Fat soluble Vitamins A,D,E, and K  Vitamin A Food Source Liver, cream, butter, cheese and egg yolk are good sources of vitamin A Vegetable sources contain the yellow pigment beta carotene.( Carrot, papaya, mango, pumpkins) and green leafy vegetables ( spinash)  Active forms of vitamin A 1. Retinol: A primary alcohol 2. Retinal: This is the aldehyde derived from the oxidation of retinol. 3. Retinoic acid: This is the acid derived from the oxidation of retinal. 4. β-carotene: Plant foods contain β-carotene, which can be oxidatively cleaved in the intestine to yield two molecules of retinal.  B. Absorption and transport of vitamin A 1. Transport to the liver: Retinol ester that present in diet are hydrolyzed in the intestinal lumen releasing retinol and free fatty acids. Then retinol is re-esterifed to long chain fatty acids and then secreted as component of chylomicron which travel to liver and adipose tissue. retinyl ester is stored in the liver and adipose tissue. 7 2. Release from the liver: When needed, retinol is released from the liver and is transported to extrahepatic tissues by the plasma retinol-binding protein (RBP). The RBP complex attaches to specific receptors on the surface of the cells of peripheral tissues, permitting retinol to enter.  Functions of vitamin A Visual cycle: Vitamin A is a component of the visual pigments of rod and cone cells. Rhodopsin, the visual pigment of the rod cells in the retina, consists of 11- cis retinal specifically bound to the protein opsin. When light falls: All cis retinal converted to trans retinal. This process triggers a nerve impulse that is transmitted by the optic nerve to the brain. In the absence of light Regeneration of rhodopsin requires isomerization of all trans retinal back to 11- cis retinal. Trans retinal is isomarized to 11-cis retinal, which spontaneously combines with opsin to form rhodopsin, thus completing the cycle.  Biochemical functions of Vitamin A 1. Vision : retinal is the active form required for normal vision 2. Growth: Retinoic acid is implicated in growth and differentiation of tissues 3. Reproduction: Retinol are essential for normal reproduction, supporting spermatogenesis in the male and preventing fetal resorption in the female. (deficiency of retinol or retinal causes impotence) 4. Antioxidant property Fresh vegetable containing carotenoides were shown to reduce the incidence of cancer Dietary deficiency of vitamin A: Night blindness Is one of the earliest signs of vitamin A deficiency. The visual threshold is increased, making it difficult to see in dim light. Prolonged deficiency leads to an irreversible loss in the number of visual cells.  Xerophthalmia: Severe vitamin A deficiency leads to Xerophthalmia, a pathologic dryness of the conjunctiva and cornea. If untreated, xerophthalmia results in corneal ulceration and, ultimately, in blindness.  3. Acne and psoriasis: Dermatologic problems such as acne and psoriasis are effectively treated with retinoic acid or its derivatives.  Toxicity of retinoids Excessive intake of vitamin A produces a toxic syndrome called hypervitaminosis A. Amounts exceeding 7.5 mg/day of retinol should be avoided. Early signs of chronic hypervitaminosis A are reflected in: skin: which becomes dry and pruritic (due to a decrease in keratin synthesis). liver: which becomes enlarged and can become cirrhotic. nervous system: where a rise in intracranial pressure may mimic the symptoms of a brain tumor. Pregnant women: particularly should not ingest excessive quantities of vitamin A because of its potential for causing congenital malformations in the developing fetus. Reference Book: Vasudevan, D. M., Sreekumari, S., and Kannan, V.., 2011. Textbook of biochemistry for medical students, 6th Edition. 20