Vitamins

Vitamins

DR Ahmed al taweel Dept. of Biochemistry


Polish biochemist Casimir Funk discovered vitamin B1 in 1912 in rice bran. He proposed the complex be named "Vitamin" (vital amines). By the time it was shown that not all vitamins were amines, the word was already ubiquitous.

Vitamin - definition

An organic compound required as a nutrient in tiny amounts by an organisms. It cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Vitamins have diverse biological function: hormone-like functions as regulators of mineral metabolism (vit. D), regulators of cell and tissue growth and differentiation (some forms of vit. A) antioxidants (vit. E, C) enzyme cofactors (tightly bound to enzyme as a part of prosthetic group, coenzymes)

Vitamin classification

Lipid-soluble vitamins (A, D, E and K) hydrophobic compounds, absorbed efficiently with lipids, transport in the blood in lipoproteins or attached to specific binding proteins, more likely to accumulate in the body, more likely to lead to hypervitaminosis

Vitamin classification

Water-soluble vitamins - 8 B vitamins and vitamin C Function: mainly as enzyme cofactors, hydrophilic compounds dissolve easily in water, not readily stored, excreted from the body, their consistent daily intake is important. Many types of water-soluble vitamins are synthesized by bacteria.

Lipid-soluble vitaminsVitamin A

RetinolBiologically active forms - retinoids: retinol, retinal, retinoid acid.Major vit. A precursors (provitamins) → plants carotenoids.Foodstaf of animals origin contain most of vit. A in the form of esters (retinylpalmi-tates) – retinol and long fatty acid Cyklohexan ring and isoprenoid chain



Retonol esters → hydrolysis by pancreatic enzymes to retinol. b-caroten is cleaved to retinal by b-carotene 15,15ґ dioxygenase (cofactors iron and bile salts). Intestinal cells → esterification of retinol → transported in chylomicrons. Remnants of chylomicrons → liver→ esterification (if the concentration exceeds 100 mg, esters are stored ). Transport of retinol to target organs tightly bound to retinol-binding protein, RBP. Vit. A transport and metabolism

Vitamin A and vision

Vit. A is necessary to form rhodopsin (in rodes, night vision) and iodopsins (photopsins, in cones – color vision) - visual pigment. Retinaldehyd is a prosthetic group of light-sensitive opsin protein. In the retina, all-trans-retinol is isomerized to 11-cis-retinol → oxidized to 11-cis-retinaldehyd, this reacts with opsin (Lys) → to form the holoprotein rhodopsin. Absorption of light → conformation changes of opsin → photorhodopsin.

Vitamin A and vision

The following is a series of izomerisation→ initiation of nerve impulse.The final step is hydrolysis to release all-trans-retinaldehyde and opsin.Deficiency of vit. A leads to night blindness. Vitamin A is an important antioxidant.

Vitamin A and other functions

Transcription and cell differentiationRetinoic acid regulates the transcription of genes - acts through nuclear receptors (steroid-like receptors). By binding to various nuclear receptors, vit. A stimulates (RAR – retinoid acid receptor) or inhibits (RXR- retinoid „X“ receptor) transcription of genes transcription. All-trans-retinoic acid binds to RAR and 9-cis-retinoic acid binds to RXR.Retinoic acid is necessary for the function and maintenance of epithelial tissues. Retinol retinal retinoic acid
Retinol dehydrogease
Retinaldehyde dehydrogenasa

Vitamin A - deficiency

The early sign → a loss of sensitivity to green light,prolonged deficiency → impairment to adapt to dim lightmore prolonged deficiency leads to night blindnessEver escalated deficiency leads to squamous metaplasia - columnar epithelia are transformed into heavily keratinized squamous epithelia.The conjunctiva loses mucus-secreting cells → glykoprotein content of the tears is reduced → xeroftalmia ( „dry eyes“)Often complication - bacterial or chlamidial infection which results in perforation of the cornea and blindness

Vitamin A - deficiency

Transformation of respiratory epithelium – loss of protective airway function (antibacterial properties) → bronchitis.Conversion of the urinary tract epithelium → higher frequency of urinary stone formation ImmunosuppressionImpairment of reproductive function (both in men and women).Worldwide deficiency of vit. A 3 – 10 mil. children become xerophtalmic every year 250 000 to 500 000 go to blindness1 million die from infections


Vitamin A - toxicity
Toxic dose: single dose of more than 200 mg more than 40 mg per dayAcute symptoms - headache, vomiting, impaired consciousness.Chronic intoxication – weight loss, vomiting, pain in joints, muscles, blurred vision, hair loss, excessive bone growth.Both vit. A excess and deficiency in pregnancy are teratogenic – retinoic acid is gene regulator during early fetal developmentCarotenoids are non toxic - accumulation in tissues rich in lipids (the skin of babies overdosed with carrot juice may be orange).

Metabolic functions of vitamin A

Vision Gene transcription Immune function Embryonic development and reproduction Bone metabolism Haematopoieis Skin health Antioxidant activity

Sources of vitamin A

cod liver oil meat egg milk dairy products
carrot broccoli spinach papaya apricots
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Vitamin D

Calciol, vitamin D2 (cholecalciferol) → precursor of calcitriol, D3 (1,25-dihydroxycalciferol).Regulates with PTH calcium and phosphate level (absorption, reabsorption, excretion).Synthesis in the skin (7-dehydrocholesterol) UV → further transformation in the liver and kidneys .

Synthesis

Non-enzymatic reaction in the skin
Transport to the liver
UV irradiation 270 – 300 nm Photolysis

Liver

Kidneys
Inactive form

Effects of vitamin D

Transported in the blood on a carrier (vitamin-D binding protein, VDBP). 1,25(OH)2D binds to intracellular receptors (intestine, bone, kidney). The main function is to maintain plasma levels of calcium (essential for neuromuscular activity) and phosphate levels: increase Ca absorption in the intestine, reduce the excretion of calcium (stimulates parathyroid hormone-dependent Ca reabsorption in the distal tubule), mobilizing bone mineral, together with parathyroid hormone

Vitamin D - deficincy

Failure of absorption in the intestine. The lack of the liver and the renal hydroxylation of vit. D (congenital deficiency of 1-hydroxylase). The lack of UV irradiation. The main manifestation - impaired ossification of the newly created osteiod, abundance of non mineralized matrix. Vit. D is necessary for the prevention of skeletal changes (rickets in growing individuals, osteomalacia in adults).

Vitamin D and imunity

It increases the activity of natural killer cells (cytotoxic lymphocytes).Increases the phagocytic ability of macrophages .Reduces the risk of virus diseases (colds, flu). Reduces the risk of many cancers (colon, breast and ovarian cancer).Reduces the risk of cardiovascular disease → have a positive impact on the composition of plasma lipids.

Sources of vitamin D

In addition to sunbathing: various fish species (salmon, sardines and mackerel, tuna, catfish, eel), fish oil, cod liver eggs, beef liver, mushrooms

Vitamin E

Vitamin E is a famil of a-, b-, g-, d- tocopherols and corresponding tocotrienols izomers. They are formed from chroman ring and hydrofobic fytyl side chain. The highest biological activity has a-tokoferol.

Vitamin E

Adsorbtion from the small intestine.Its absorption is dependent on the presence of lipids in the diet.Associated with plasma lipoproteins → liver uptake through receptors for apolipoprotein E. a-tocopherol is bind to a-tocopherol transport protein (a-TTP) → transported to the target organs (the excess is stored in adipocytes, in muscle, liver).b-, g- a d-tocopherols are transferred into the bile and degraded.

Vitamin E as antioxidant

Stops free radical reactions (peroxyl radicals ROO , oxygen radicals HO, lipoperoxid radicals LOO). Chroman ring with OH group → uptake radicals.


PUFA-H = polyunsaturated fatty acid PUFA-OO = peroxyl radical of polyunsaturated fatty acid PUFA-OOH = hydroxyperoxy polyunsaturated fatty acid PUFA-OH = hydroxy polyunsaturated fatty acid Free radical chain reaction
Interaction and synergism between antioxidant systems operating in the lipid phase (membranes) and the aqueous phase (cytosol)

Vitamin E as enzyme cofactor

a-tocopherol quinon generated by oxidation of a-tocopherol can acts as a cofactor of mitochondrial unsaturated fatty acids .a-tocopherol quinon + cytochrom B5 + NADH+H+ initiate formation of double bonds in FA – temporarily changes to a-tocopherol-hydroquinon (in the presence of O2 changes back to a-tocopherol quinon).

Vitamin E – deficiency and toxicity The lack of a-tocopherol in plasma is often associated with impaired fat absorption or distribution (in patients with cystic fibrosis, in patients with intestine resection) deficit of vit. D exhibit - neurological problems, impaired vision, eye muscle paralysis, platelet aggregation, impairment of fertility in men, impaired immunity. Toxicity is relatively small.

Sources of vitamin E

fortified cereals seeds and seed oils, like sunflower nuts and nut oils, like almonds and hazelnuts green leafy vegetables, broccoli cabbage celery
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Vitamin K

Vitamin K is a group of lipophilic, hydrophobic vitamins. They are needed for the postranslation modification of proteins required for blood coagulation, They are involved in metabolism pathways, in bone mineralisation, cell growth, metabolism of blood vessel wall.

Vitamin K1

Vitamin K2
Vitamin K
Vitamin K1 (phylloquinon) – plant originVitamin K2 (menaquinon) – normally produced by bacteria in the large intestineK1 a K2 are used differently in the body K1 – used mainly for blood clothing K2 – important in non-coagulation actions - as in metabolism and bone mineralization, in cell growth, metabolism of blood vessel walls cells. Synthetic derivatives of Vit.K

Vitamin K - function

Cofactor of liver microsomal carboxylase which carboxylates glutamate residues to g-carboxyglutamate during synthesis of prothrombin and coagulation factors VII, IX a X (posttranslation reaction).Carboxylated glutamate chelates Ca2+ ions, permitting the binding of blood clotting proteins to membranes.Forms the binding site for Ca2+ also in other proteins – osteocalcin.

Vitamin K - deficiency

Deficiency is caused by fat malabsorption or by the liver failure. Blood clotting disorders – dangerous in newborns, life-threatening bleeding (hemorrhagic disease of the newborn).Osteoporosis due to failed carboxylation of osteokalcin and decreased activity of osteoblasts.Under normal circumstances there is not a shortage, vit. K is abundant in the diet.

Sources of vitamin K

Green leafy vegetables vegetable oil broccoli cereals
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Vitamin B1 (thiamine) Vitamin B2 (riboflavin) Vitamin B3 or Vitamin P or Vitamin PP (niacin) Vitamin B5 (panthotenic acid) Vitamin B6 (pyridoxine and pyridoxamine) Vitamin B7 or Vitamin H (biotin) Vitamin B9 or Vitamin M and Vitamin B-c (folic acid) Vitamin B12 (cobalamin)
Water soluble vitamins

Vitamin B1 (thiamine)

Thiamin has a central role in energy-yielding metabolism.Composed of a substituted pyridine and thiazole ring. Active form is thiamine diphosphate (thiamin pyrophosphate, TPP), a coenzyme for three multi-enzyme complex →This complex catalyses oxidative decarboxylation of a-ketoacids → pyruvate dehydrogenase in carbohydrate metabolism, a-ketoglutarate dehydrogenase → cytric acid cycle,Branched-chain keto-acid dehydrogenase .TPP is coenzyme for transketolase – pentose phosphate pathway.

Vitamin B1 - deficiency

Mild deficiency – leads to gastrointestinal complients, weaknessModerate deficiency - peripheral neuropathy, mental abnormalities, ataxiaFull-blown deficiency - beri-beri – characterized with severe muscle weakness, muscle wasting and delirium, paresis of the eye muscles, memory loss.Degeneration of the cardiovascular system. .Beri-beri causes long-term consumption of foods rich in carbohydrates but poor in thiamine - husked rice, white flour and refined sugar.


Source of vitamin B1
paddy grains, cerealsmeatyeasthoneynuts

Vitamin B2 (riboflavin)

Yellow to orange-yellow natural dye slightly soluble in water. Has a central role in energy-yielding metabolism. Provides the reactive moieties of the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotid (FAD). Flavin coenzymes are electron carries in oxidoreduction reaction.

Vitamin B2

FMN → ATP-dependent phosphorylation of riboflavinFAD → further reaction with ATP in which its AMP moiety is transferred to FMN.

FMN a FAD function

FMN and FAD act as prosthetic groups of many oxidoreduction enzymes, flavoprotein:oxydase of a-amino acids – degradation of amino acidsxantinoxidase – degradation of purinesaldehyde dehydrogenasmitochondrial glycerol-3-phosphate dehydrogenase – transport of reducing unit (H+) from mitochondra to cytosolsuccinate dehydrogenas – citric acid cyclesuccinyl CoA-dehydrogenase – b-oxidation of FANADH-dehydrogenase – part of respiratory chain in mitochondriacoenzymes in hydrogen transfer – formation of reducing forms - FMNH2 a FADH2

Vitamin B2 absorption

Riboflavin is absorbed in the proximal intestine. Riboflavin is stored mainly in the liver, kidney and heart in the form of FAD (70- 90%) or FMN.

Causes of vitamin B2 deficiency

Lack of dietary vitamin B. A result of conditions that affect absorption in the intestine. The body not being able to use the vitamin. An increase in the excretion of the vitamin from the body.

Vitamin B2 – symptoms of deficiency Cracked and red lips. Inflammation of the lining of mouth and tongue. Dry and scaling skin- keratitis, dermatitis and iron-deficiency anemia

Sources of vitamin B2

foods of animal origin (liver, pork and beef, milk, dairy products, fish eggs) cocoa, nuts, yeast, of smaller quantities in cereals.
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Vitamin B3 - niacin

Active form – nikotinic acid and nikotinamid.NAD a NADP → key components of the metabolic pathways of carbohydrates, lipids, amino acids.Nicotinic acid prevents the release of fatty acids from adipose tissue, decreases lipoproteins VLDL, IDL a LDL.High dose of niacin dilates blood vessels .

Vitamin B3 - niacin

Absorption: At low concentration by active transport. At high concentration by passive diffusion. Transportation: Both nicotinic acid (NA) and nicotinamide (NAm) bind to plasma proteins for transportation. Biosynthesis: The liver can synthesize Niacin from the essential amino acid tryptophan, but the synthesis is extremely slow and requires vitamin B6 (60 mg of Tryptophan= 1mg of niacin). Bacteria in the gut may also perform the conversion but are inefficient.

Vitamin B3 - deficiency

Pellagra: A serious deficiency of niacin. The main results of pellagra can easily be remembered as "the four D's": diarrhea, dermatitis, dementia, and death. Pelagra is very rare now, except in alcoholics, strict vegetarians, and people in areas of the world with very poor nutrition. Milder deficiencies of niacin can cause dermatitis around the mouth and rashes, fatigue, irritability, poor appetite, indigestion, diarrhea, headache.

Sources of vitamin B3

foods of animal origin yeast sunflower seeds, beans, peas green leafy vegetable broccoli, carrots
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Vitamin B5 – panthotenic acid Part of acetyl-CoA – consists of pantoic acid and b-alaninem.

Vitamin B5 – panthotenic acid Co-enzyme A assists the following reactions: formation of sterols (cholesterol and 7-dehydrocholesterol). formation of fatty acids. formation of keto acids such as pyruvic acid. Other reactions are acylation, acetylation, signal transduction deamination

Vitamin B5 - deficiency

Rare to occur.When occur it leads to paresthesias.Disorders of the synthesis of acetylcholine – neurological symptoms (parestesie).

Sources of vitamin B5

meat, foods of animal origin, yeast, wholemeal bread, broccoli, avocado royal gelly

Vitamin B6

Prekursor of active coenzyme pyridoxalphosphate – PPL.

Vitamin B6

Vitamin B6 is needed for more than 100 enzymes involved in protein metabolism. It is also essential for red blood cell metabolism and hemoglobin formation. The nervous and immune systems need vitamin B6 to function efficiently. It is also needed for the conversion of tryptophan to niacin (vitamin B3). Vitamin B6 also helps maintain blood glucose within a normal range. When caloric intake is low, vitamin B6 helps to convert stored carbohydrate or other nutrients to glucose to maintain normal blood sugar levels.

Transamination reaction

Vitamin B6 deficiency
Signs of vitamin B6 deficiency include: Skin: dermatitis (skin inflammation), stomatitis (inflammation of the mucous lining of any of the structures in the mouth), glossitis (inflammation or infection of the tongue ). Neurological abnormalities: Depression, confusion, and convulsions. Vitamin B6 deficiency also can cause anemia.



Vitamin B6 – narural sources cereals, beans, meat, liver, fish, yeast, nuts and some fruits as banana potatoes. It is also produced by bacterial flora in the colon.

Vitamin B7 - biotin

Prosthetic group of pyruvate carboxylase, acetyl-CoA carboxylase and other ATP-dependent carboxylases.


Biotin – natural source liver meat kidney yeast egg yolk mushrooms milk and diary products.


Vitamin B9 – folic acid Consist of pteroic acid - pteridine + paraaminobenzoic acid (PABA) + glutamic acid


Vitamin B9 – folic acid Active metabolit of folic acid is tetrahydrofolate (THF) . THF is coenzym of transferases carrying one carbon units. This reaction participate in nucleotide and nucleic acid synthesis N5,N10-THF carries one carbon units (methylen or methenyl).

Folic acid deficiency

Deficiency results in elevated levels of homocystein. Deficiency in pregnant women can lead to birth defects.

Sources of vitamin B9

sources of animal origin milk and milk products yeast greens


Vitamin B12 - cobalamin
Chemically most complex vitamin Complex of organic compounds atom within the molecule is Co, similar to the heme. In man there are two metabolically active forms: methylkobalamin a adenosylkobalamin.

Vitamin B12 - cobalamin

Cobalamin catalyses two reactionsCytoplasmic methylation of homocystein to methionin.Mitochondrial methylmalonyl-CoA mutase (methylmalonyl-CoA → sukcynyl-CoA) needs deoxy adenosylkobalamin.

Vitamin B12 – cobalamin Essential for the maturation of erythrocytes. Protects against pernicious anemia. Essential for cell growth and reproduction. Essential for the formation of myelin and nucleoproteins.

Vitamin B12 – cobalamin Vitamin B12 in food is bound to the protein. Hydrochloric acid in the stomach releases free vitamin B12. Once released vitamin B12 combines with a substance called intrinsic factor (IF). This complex can then be absorbed by the intestinal tract.

Sources of vitamin B12

fish and shellfish, meat (especially liver), poultry, eggs, milk, and milk products

while lacto-ovo vegetarians usually get enough B12 through consuming diary products, vegan will lack B12

Vitamin C

Vitamin C is a water-soluble vitamin. Almost all animals and plants synthesize their own vitamin C, not man. Vitamin C was first isolated in 1928 and in 1932 it was proved to be the agent which prevents scurvy.

Vitamin C

Vitamin C is a weak acid, called ascorbic acid or its salts “ascorbates”. It is the L-enantiomer of ascorbic acid. The D-enantiomer shows no biological activity.


The role of vitaminC
Cofactor in the synthesis of norepinephrine from dopamine.Involved in a variety of metabolic processes (oxidation-reduction reactions and cellular respiration, carbohydrate metabolism, synthesis of lipids and proteins).antioxidant and free radical scavenger → maintain proper immune system.

The role of vitaminC

T-lymphocyte activity, phagocyte function, leukocyte mobility, and possibly antibody and interferon production seem to be increased by vitamin C. Involved in the synthesis of collagen, the major component of ligaments, tendons, cartilages and skin. Involved in tyrosine metabolism.

Deficiency of vitaminC

Fatigue, personality changes, decline in psychomotor performance and motivation. Vitamin C deficiency over 3-5 months results in symptomatic scurvy. Scurvy leads to the formation of liver spots on the skin, spongy gums, and bleeding from all mucous membranes. In advanced scurvy there are open, suppurating wounds and loss of teeth. Severe scurvy may progress to neuritis, jaundice, fever, dyspnea, and death.

Vitamin C as antioxidant

Vitamin C as antioxidant

Vitamin C as pro-oxidant

Ascorbic acid reduces transition metals - Cu2+, to Cu+, and Fe3+ to Fe2+ during conversion from ascorbate to dehydroascorbate. This reaction can generate superoxide and other ROS:Fentonґs reaction:(1) Fe2+ + H2O2 → Fe3+ + OH· + OH−(2) Fe3+ + H2O2 → Fe2+ + OOH· + H+2 Fe2+ + 2 H2O2 → 2 Fe3+ + 2 OH· + 2 OH−2 Fe3+ + ascorbate → 2 Fe2+ + dehydroascorbate