Biochemistry of Vision PDF

# Biochemistry of Vision ## Vitamins - Potent organic compounds required in the diet in small amounts for optimum growth and health of the organism - Not used for energy but for utilization of other nutrients like carbohydrates, proteins, and fats - Most vitamins are not made in the body, they mus...

# Biochemistry of Vision ## Vitamins - Potent organic compounds required in the diet in small amounts for optimum growth and health of the organism - Not used for energy but for utilization of other nutrients like carbohydrates, proteins, and fats - Most vitamins are not made in the body, they must be supplied by the diet - Most vitamins act as co-enzymes ## Classification of Vitamins - **Fat soluble vitamins** - Vitamin A - Vitamin D - Vitamin E - Vitamin K - **Water soluble vitamins** - Vitamin C - Vitamin B complex - Energy releasing: B1, B2, B3, B6, B7, pantothenic acid. - Hematopoietic: folic acid, B12. ## Vitamin A - Vitamin A is a broad term for a number of similar compounds - First recognized fat soluble vitamin ### Two Forms - **Preformed vitamin A:** retinoids (from animals) - **Provitamin A:** carotenoids (predominantly beta-carotene from plants) ### Preformed Vitamin A: Retinoids - Active or usable form - Four categories of retinoids - Retinol - Retinal - Retinoic acid - Retinyl esters - All retinoids are absorbed as retinol. - **Sources:** animal products like liver, fish, fish oils, milk, eggs, etc. - Liver is the richest source. ### Structure of Different Forms of Vitamin A - Unsaturated organic compounds - All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group - Diagram showing the structure of retinol, retinal, retinoic acid, and beta-carotene ### Retinol - Vitamin A alcohol - Pure alcohol form unstable - Present in animal tissues as retinyl ester with long chain fatty acid. ### Retinal - Vitamin A aldehyde - Obtained by oxidation of retinol - Previously known as retinene - Retinal and retinol interconvertible ### Retinoic Acid - Vitamin A acid. - Produced by oxidation of retinal - Cannot give rise to formation of retinal or retinol ### Provitamin A: Carotenoids - Precursor of vitamin A - Predominantly beta carotene - Body has to convert it into active vitamin A after consumption - **Sources:** plant products like carrots, green leafy vegetables, papaya, mango, bringal. ### Types of Carotenes - Alpha carotenes: yields one molecule of Vitamin A. - Beta carotenes: yields two molecules of Vitamin A. - Gamma carotenes: yields one molecule of Vitamin A. - Diagram showing food sources of Vitamin A and beta-carotene. ## Biochemical Functions of Vitamin A - Vision in dim light - Necessary for maintenance of normal epithelium: Synthesis of goblet cells in epithelial tissue, which secrete mucous having antimicrobial component. - Embryonic development & reproduction: During fetal development, retinoic acid allows for development of lungs, hearts, eyes, and ears, and regulates expression of growth hormone gene. - Acts as anti-oxidant: Carotenoids oxidize free radicals and prevent free radical cellular damage that can lead to cancer and other diseases. - Immune function: Ensures working mucosal cells, membranes, and epithelial layers: the body’s first line of defense. Aids in development of lymphocytes and white blood cells. ## Role of Vitamin A in Eye - Formation of rhodopsin - used in night vision - Maintenance of healthy cornea and conjunctival cells - Diagram showing the letters VISION and BIOCHEMISTRY in a green font. ## Can Be Discussed Under 1. Vitamin A absorption and storage 2. Transport from liver to eye 3. Synthesis of visual pigments 4. Light induced changes in visual pigments ## 1. Vitamin A Absorption and Storage - Dietary vitamin A (carotenes in plant food & retinol in animal food) - In intestine vitamin A is reesterified - Reaches blood stream through intestinal lymphatics - Most retinol reaches liver (90% stored as well) - Retinol bound to retinol-binding protein (stable form) ## RPE - RPE in second only to liver in its concentration to vitamin A - Aldehyde and alcohol form of vitamin A is membranolytic, so are stored as esters in RPE. - RPE acquires vitamin A by three ways. - From circulation - Release during bleaching of rhodopsin, and return via the regeneration process - Via phagocytosis of shed photoreceptor outer segment discs - Diagram showing a flow chart of vitamin A absorbed, reesterified and stored in the liver. ## 2. Transport from Liver to Eye - Retinol-protein complex enters circulation - Becomes attached to the specific receptors present on the basal surfaces of the retinal pigment epithelial (RPE) cells. - RBP is left outside & retinol only enters RPE ## In Cornea and Conjunctiva - Possible routes of vitamin A transfer to cornea and conjunctiva - Migration of holo-RBP from blood capillaries in limbus region - Direct uptake of vitamin A from tear fluid - Transfer of vitamin A from aqueous humor. ## In Tear - Concentration of retinol 0.1 µmol/litre (5% of plasma) - In aqueous: retinol barely detectable ## 3. Synthesis of Visual Pigments - Retinol remains unchanged in RPE cells - Retinol enters into the outer segments of photoreceptors - Diagram showing the conversion of retinol to retinene, and then the combination of retinene with opsin to form rhodopsin. ## Rhodopsin - Retinene combines with the protein opsin to form rhodopsin - NAD oxidative system (present in RPE) supports the reaction of rhodopsin formation. - Diagram showing a simplified representation of the process of synthesis of rhodopsin, involving the conversion of retinol to retinene, and the combination of retinene with opsin to form rhodopsin. ## 4. Light Induced Changes in Visual Pigments - Light falling on retina absorbed by photoreceptors - Photochemical changes in outer segments of photoreceptors initiate electrical changes. - Light induced changes as studied in rods. - Rhodopsin bleaching - Rhodopsin regeneration - Visual cycle ## I. Rhodopsin Bleaching and Regeneration - Rhodopsin: Opsin (protein) + retinene (vitamin A aldehyde) - Light absorbed by rhodopsin converts its 11-cis retinal into all-trans retinal - Formation of many intermediates ## II. Rhodopsin Regeneration - All-trans retinal enters into the chromophore pool existing in photoreceptor outer segment and RPE cells. - All-trans retinal may be further reduced to retinol by alcohol dehydrogenase, then esterified to re-enter the systemic circulation - All-trans retinal isomerized to 11-cis retinal by retinal isomerase. - 11-cis retinal in outer segments of photoreceptors reunites with opsin to form rhodopsin - Diagram showing the conversion of rhodopsin into all-trans retinal and back to rhodopsin through various intermediate steps. ## III. Visual Cycle - Under constant light stimulation: Photoreceptor Bleaching = Photoreceptor Regeneration - Equilibrium between the photodecomposition and regeneration of visual pigments: visual cycle. - Diagram showing the visual cycle in which rhodopsin is converted to all-trans retinal and back to rhodopsin through various steps. ## Recommended Dietary Allowance (RDA) of Vitamin A - Table showing the groups and their RDA (IU) of Vitamin A. - IU = International Unit - 1 IU = 0.3 µg of retinol ## Vitamin A Deficiency - Diagram showing a flow chart of Vitamin A deficiency in food, intestine, liver, blood, and eye. ## Xerophthalmia - General term applied to all the ocular manifestations of impaired vitamin A metabolism, from night blindness through complete corneal destruction - Xeros – dry - Ophthalmia – eye - Literally means "dry eye" - Conventionally xerophthalmia has become synonymous with vitamin A deficiency. ## Night Blindness - Earliest symptom - Difficulty to see in dusky and dark environment. - Diagram showing a person looking out a window at night. ## Conjunctival Xerosis - Keratinizing metaplasia of conjunctiva. - Loss of goblet cells and mucus. - Lusterless, wrinkled, and pigmentation of bulbar conjunctiva ## Bitot’s Spots - Extension of xerotic process seen in stage X1A - Raised, silvery white, foamy, triangular patch of keratinised epithelium - Situated on bulbar conjunctiva in the inter-palpebral area. - Diagram showing a magnified image of a Bitot’s spot on the conjunctiva. ## Corneal Xerosis - Earliest change in cornea is punctate keratopathy, which begins at the lower nasal quadrant. - Haziness and granular pebbly dryness. - Involved cornea lacks lustre. ## Prevention of Vitamin A Deficiency - Breastfeeding - Vitamin A supplementation - Food fortification - Promotion of vitamin A-rich diets - Diagram showing the letters PREVENTION OF XEROPHTHALMIA in a yellow font. - Diagram showing a flow chart of the effects of vitamin A deficiency on the eye. - Diagram showing a flow chart of how a lack of vitamin A affects the eyes. - Diagram showing a flow chart of how vitamin A affects the eye.