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Vitamins are essential organic (carbon containing) compounds, needed in small amounts for normal functioning of the human body's metabolism, growth and reproduction.

A vitamin is called "essential" because either we can't make it in the body (called synthesis) or that we make too little of it for good health. For example, humans can't synthesize vitamin C (ascorbic acid) and therefore, we must obtain it through our diet. On the other hand, we can synthesize enough vitamin D in our skin if we can get sufficient exposure to sunlight. However, if we can't get enough sun, we may find ourselves deficient in vitamin D and therefore should obtain vitamin D as part of our diet.

The 13 or so vitamins that we use all play important roles in sustaining life. Fat-soluble vitamins that we need include vitamins A, D, E and K. Fat-soluble vitamins dissolve in fat and therefore can be stored in the fatty tissues of the body. As a consequence, we do not need to ingest fat-soluble vitamins daily. The body can use these stores when the intake of fat-soluble vitamins is low.

An assortment of B vitamins and vitamin C are the water-soluble vitamins that we require. Water-soluble vitamins dissolve in water and not in fat and therefore cannot be stored in the fatty tissues of the body. Because water-soluble vitamins mix easily with intracellular fluids, which are mostly water, they are fairly rapidly excreted via the skin (sweat) and the kidneys (urine). In order to avoid a deficiency, water-soluble vitamins are needed on nearly a daily basis.

Did you know that the word vitamin comes from the word "vitamine?" It's true! Vitamine, which means "vital amine," was coined in 1911 by the Polish-American biochemist Casimir Funk because it was thought at the time that these substances contained amines, compounds that have nitrogen (N) as part of their molecular structure. The letter "e" in vitamine was later dropped when it was determined that not all vitamins contained nitrogen. It was Casimir Funk who discovered vitamin B1 (thiamine).



Vitamin A

____Vitamin A has many Important Functions:



Vitamin A (also called retinol) is fat-soluble, and as such, can be stored in the fatty tissues of the body, although much the body's vitamin A is stored in the liver as retinyl palmitate. We obtain vitamin A either directly from foods that are substantial in vitamin A (beef liver, fish liver oils, egg yolks and butter, for example) or by converting a substance called beta-carotene into vitamin A.


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Beta-carotene is the provitamin for vitamin A, that is, it can be converted to vitamin A. Provitamins are substances that are transformed into vitamins in the body. The amino acid tryptophan is another example of a provitamin. Tryptophan can be converted to vitamin B3 (niacin).

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Beta-carotene comes from a group of compounds called the "carotenoids." These are substances, which exhibit characteristic yellow and orange colors. Carrots, for example, are high in beta-carotene and other carotenoids. In addition to the role it can play as a provitamin for vitamin A, beta-carotene also functions as an important antioxidant compound, protecting cells from the harmful effects of free radicals.

Dark-green leafy vegetables (spinach and mustard greens) and yellow-orange fruits (papaya, apricots and mango) and vegetables (carrots, yams, yellow squash and sweet potatoes) are all good sources of beta-carotene.

You need to eat about six times as much beta-carotene to get the same amount of vitamin A as in retinol.




Vitamin D ______ "The Sun Vitamin"

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Vitamin D3 is synthesized in the body when ultraviolet light (mostly UV-B radiation) strikes our skin. Fair-skinned people need 20 to 30 minutes a day in bright sunlight to meet their vitamin D needs. The darker your skin the longer you will need to be in the sun in order to make enough vitamin D. Overcast skies, smog, clothing and sunscreens all decrease your exposure to UV-B radiation and hence decrease the amount vitamin D you can produce from sunlight. Although overexposure to sunlight can give you sunburn, it will not cause vitamin D toxicity. We can also meet our vitamin D needs with vitamin D fortified foods (milk, for example is one) and vitamin D supplements.


Some of the many useful functions of Vitamin D:

Vitamin D is actually a family of fat-soluble sterol compounds:

For humans, the two most important forms of vitamin D are vitamin D2 and vitamin D3. Vitamin D2 (ergocalciferol) is derived from plants and irradiated yeast and fungi. Vitamin D3 is synthesized in the body when our skin is exposed to sunlight and we can obtain vitamin D3 from foods like milk, fortified cereals, tuna, salmon and fish oils.

Vitamin D2 and vitamin D3 have equal biological activity. They both can be converted first to calcifediol in the liver and then to calcitriol (1,25-dihydroxycholecalciferol) in the kidneys. Calcitriol, which is the most active form of vitamin D3, is then transported via a carrier protein to the various sites in the body where it is needed.


Steps in the synthesis of Calcitriol (the active form of vitamin D3):


______Calcitriol (active Vitamin D3)




Vitamin E

____The Antioxidant Vitamin


When we talk about fat-soluble Vitamin E, we're really talking about a family of eight different vitamin E molecules.


Four of the eight vitamin E molecules are called the tocopherols (alpha, beta, gamma and delta). Foods containing significant amounts of tocopherols include a number of oils (corn, safflower, soybean, cottonseed and canola), nuts (almonds, hazelnuts, and walnuts), wheat germ and vegetables like spinach, kale, sweet potatoes and yams.


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The remaining four Vitamin E molecules are called tocotrienols (alpha, beta, gamma and delta). Commercial sources of tocotrienols include rice bran oil, palm oil and extracted oils from annatto beans.



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Table for naming Vitamin E molecules:


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Vitamin E products labeled "Natural Tocopherol," for example, are a mixture of alpha-, beta-, gamma- and delta-tocopherol. In humans, d-alpha-tocopherol (also known as RRR-alpha-tocopherol) appears to be the most biologically active of the tocopherols because it is found in the highest quantities in the blood and tissues of the body. As a consequence, d-alpha-tocopherol is also the only form of vitamin E that meets the latest Recommended Dietary Allowance (RDA) for vitamin E.



Ranking the Biological Activities of the Tocopherols and Tocotrienols


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Recommended Dietary Allowance (RDA) for d-alpha-tocopherol:

Children 1-3 years_________6 mg _(9 IU)
Children 4-8 years_________7 mg _(10.5 IU)
Children 9-13 years________11 mg (16.5 IU)
Adolescents 14-18 years____15 mg (22.5 IU)
Adults___________________15 mg (22.5 IU)


IU stands for "International Units" and its measure for a particular vitamin depends on the biological activity or "potency" of the substance from which the vitamin is obtained as agreed upon by international standards. International units for various vitamin E containing subtances, both "natural" and "synthetic," have been determined:


Natural Vitamin E

1 IU = 0.735 mg d-alpha tocopheryl acetate
1 IU = 0.671 mg d-alpha tocopherol
1 IU = 0.861 mg d-alpha tocopheryl succinate

Synthetic Vitamin E

1 IU = 1 mg dl-alpha tocopheryl acetate
1 IU = 0.91 mg dl-alpha tocopherol


** Synthetic vitamin E (sometimes labeled dl-alpha-tocopherol or as all-rac-alpha-tocopherol) contains both active and inactive forms of alpha-tocopherol.



In foods, alpha-tocopherol is in the form of d-alpha-tocopherol, which is the vitamin E isomer preferred by the body. Fat-soluble vitamin E is present in animal fats, cereal grains, and nuts. d-alpha-tocopherol is found in a number of oils, including safflower and sunflower. It is also found in wheat germ. Soybean and corn oils contain mainly gamma-tocopherol.


d-Alpha-Tocopherol




Alpha-tocopherol is a yellow-colored oil that turns dark (due to oxidation) when exposed to air. To protect vitamin E supplements from oxidation by the air, vitamin E is esterified and is generally available to vitamin consumers either as alpha-tocopheryl succinate or as alpha-tocopheryl acetate. After ingestion, the succinate or acetate is removed from the vitamin E supplement via the digestive system and the alpha-tocopherol is made available to be absorbed through the small intestine.


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Some of the Important Functions of Vitamin E



How Antioxidants Work



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Vitamin K

____The Anti-Hemorrhagic Vitamin


Vitamin K, yet another of the fat-soluble vitamins (A, D, E and K) was first isolated in 1939 by the Danish biochemist Henrik Carl Peter Dam. Dr. Dam, won the Nobel Prize in Medicine along with the American biochemist Edward Adelbert Doisy for their work, involving vitamin K. Ten years earlier, it was shown that this fat-soluble substance present in green leafy vegetables was required for normal coagulation of the blood.


The "K" in vitamin K comes from the German word "koagulation," which refers to blood clotting (coagulation). Vitamin K is essential for the functioning of several proteins involved in normal blood clotting. More specifically, vitamin K is needed for the body to make four of the blood's coagulation factors, including prothrombin (also known as factor II), proconvertin (factor VII), Christmas factor (factor IX) and the Stuart-Power factor (factor X).


There are several forms of vitamin K, namely:



Vitamin K1 or "phylloquinone," which is the major dietary source of vitamin K, is found in green leafy vegetables like lettuce, kale, parsley, spinach and various greens (turnip, beet and mustard). Broccoli is also a good source of vitamin K1 as are certain vegetable oils (soybean, cottonseed, canola, and olive).


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Vitamin K2 is actually a group of compounds called the "menaquinones." Individual menaquinone compounds are generally designated either by the number of isoprene residues in the side chain of the vitamin K2 molecule or by the number of carbons in the side chain. For example, menaquinone-4 (MK-4) has four isoprene units in the side chain of vitamin K2, whereas menaquinone-7 (MK-7) has seven isoprene units in the side chain.


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Vitamin K2, which is the most biologically active form of vitamin K, is found in egg yolks, butter, liver, cheddar cheese and yogurt. If you can stand the strong smell and taste, a product called natto (fermented soybeans) is an especially good source of vitamin K2. Vitamin K2 is also produced by certain "friendly" intestinal bacteria in humans and it has been suggested that products like yogurt, kefir and acidophilus milk (fermented milk) may help to increase the functioning of these useful bacteria.



Vitamin K3 or "menadione" is a fat-soluble synthetic (man-made) vitamin K compound, used mainly in animal feed and pet foods. Although vitamin K3 is converted to vitamin K2 in the body, it is generally not recommended for use in humans.

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B-vitamins (B1, B2, B3, B5, B6, B7, B9 and B12)


The B vitamins are a group of eight water-soluble vitamins: Thiamin (B1), Riboflavin (B2), Niacin (B3), Pantothenic Acid (B5), Pyridoxine (B6), Biotin (B7), Folic Acid (B9) and Cobalamin (B12). B-Complex vitamin formulas, for example, contain all of the B-vitamins and several vitamin-like compounds: Choline, Inositol, and Para-aminobenzoic Acid (PABA).

B-vitamins are essential for a wide range of biological processes. B-vitamins act as coenzymes and unite with certain protein molecules to form active enzymes. These enzymes function as catalysts in important biochemical reactions, such as turning carbohydrates into energy and metabolizing proteins and fats. A catalyst speeds up or slows down a chemical reaction without the catalyst itself being consumed in the reaction.

Because the B-vitamins are water-soluble, they tend not to be stored in the body for long periods to time like the fat-soluble vitamins, which can be stored in the fatty tissues of the body. Consequently, it is important that we continually replenish B-vitamins by eating a well-balanced diet that is high in essential nutrients.



Let's Examine the B-vitamins !



Vitamin B1 (Thiamine)


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Vitamin B1, known as thiamine is also sometimes called aneurine. Thiamine, which was first isolated in the 1930's, was one of the first organic compounds to be recognized as a vitamin. In the human body, thiamine exists both as free thiamine and in several phosphorylated forms, including thiamine monophosphate, thiamine diphosphate (more commonly known as thiamine pyrophosphate) and thiamine triphosphate.


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Phosphorylation in this case, simply means adding phosphate (-PO43-) groups to thiamine. The phosphate groups come from a molecule called ATP or "adenosine triphosphate." Actually, to make thiamine pyrophosphate from thiamine (vitamin B1) requires not only ATP, it requires magnesium and an enzyme called thiamine pyrophosphokinase, as well.


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Did you know that ATP is regarded by many scientists to be the most important substance in biochemistry?


Thiamine pyrophosphate is a cofactor (also called a coenzyme) for several very important enzymes that help convert foods into energy. Cofactors may be organic or inorganic and may be loosely or tightly bound to the enzyme. A tightly bound cofactor (for example, heme in hemoglobin) is called a prosthetic group. Thiamine is essential, for example, for the metabolism of complex carbohydrates into the simple sugars (glucose).

Coenzymes are the functional part of enzymes, turning "inactive" enzymes (called apoenzymes) into "active" enzymes (called holoenzymes). When we say active, we mean catalytically active as enzymes are catalysts in biochemical reactions. Catalysts speed up or slow down chemical reactions, but are not consumed in the overall process. The protein part of an enzyme is called the apoenzyme. When a coenzyme chemically bonds to the inactive apoenzyme an active holoenzyme is formed. Many coenzymes are derived from vitamins, which is why vitamins are vitally important to our survival.

For enzyme-catalyzed reactions, a cofactor is something other than the enzyme itself that is required. Cofactor is a general term. A cofactor may be organic or inorganic (e.g., metal ions), and may be loosely or tightly (even covalently) bound to the enzyme. An organic cofactor is called a coenzyme; NADH and heme are common examples. NADH is loosely bound to the enzyme. Heme is covalently bound; a tightly bound cofactor such as this is called a prosthetic group. These terms have developed over time, and are not always used precisely. Note that heme is a coenzyme that is also a prosthetic group.

Thiamine is stored in small amounts (25 to 30 mg) in organs with high metabolic needs, such as the skeletal muscles, heart, brain, liver and kidneys. Because these stores are depleted quickly (2 to 3 weeks), thiamine should be a regular part of our diet.

Thiamine is also important for the proper functioning of nerve and muscle cells. As a coenzyme thiamine plays an important role in chemical reactions that stimulate the release of acetylcholine, an important neurotransmitter.


________Acetylcholine

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Neurotransmitters like acetylcholine are molecules that carry signals from one nerve cell to another, traveling across the space or gap between nerve cells. The gap between nerve cells (also known as neurons) is called a synapse.


___Nerve transmission at the synapse




Excellent Sources of Vitamin B1:

Great sources of vitamin B1 (thiamine) include meat, pork, beef liver, egg yolks and salmon, whole grain cereals and flours, wheat germ, brown rice, soy, navy, kidney and garbanzo (chickpeas) beans, sunflower seeds and peanuts and brewer's yeast (if you can stand the taste).



Vitamin B2 (Riboflavin)

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Vitamin B2 (Riboflavin) is another important B-vitamin involved with the metabolism of carbohydrates, proteins and fats. Riboflavin was once known as vitamin G.


_______Riboflavin (Vitamin B2)


Riboflavin is composed of an isoalloxazine ring system linked to ribitol. Ribitol is a 5-carbon alcohol (C5H12O5) formed by the reduction of ribose, an important 5-carbon sugar (pentose).

isoallox1.jpg__________

____________________________________________________Ribitol


The isoalloxazine ring in vitamin B2 is both an electron donor and an electron acceptor and this is why vitamin B2 (riboflavin) is a component of the flavin coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These two important flavin molecules are required for enzyme catalyzed oxidation-reduction reactions connected with energy metabolism. [Flavin mononucleotide (FMN) is also called riboflavin phosphate.]


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__Flavin Mononucleotide (FMN)________Flavin Adenine Dinucleotide (FAD)


For a biochemist, oxidation-reduction reactions are electrochemical reactions that are generally concerned with the addition of either oxygen or hydrogen to a biomolecule. In chemistry, oxidation is a term that is used to signify a loss of electrons from an atom or molecule, while reduction is a term used to describe a gain of electrons.

For example, if iron (II) is oxidized to iron (III), then the following oxidation reaction has taken place:

Fe+2_ _Fe+3 + e-

Oxidation has increased the oxidation state of iron (Fe) from +2 to +3.


The reverse reaction is a reduction reaction, whereby iron (III) gains an electron to form iron (II).

Fe+3 + e-_ _Fe+2

Reduction has decreased the oxidation state of iron (Fe) from +3 to +2.


There's a fun way for you to remember the difference between oxidation and reduction and it's called, "LEO the lion says GER."

LEO stands for Loss - of - Electrons - is - Oxidation

GER stands for Gain - of - Electrons - is - Reduction


Oxidation-reduction reactions come in pairs and are known collectively as "redox" reactions, for short. In other words, redox reactions match an oxidation reaction with a corresponding reduction reaction. If one species in the redox pair is being oxidized then the other species in the pair must be reduced and vice versa.


Flavin adenine dinucleotide (FAD) is an electron acceptor. FAD is the oxidized form of the coenzyme required for a number of enzyme catalyzed redox reactions connected with carbohydrate and fat metabolism. When FAD accepts two electrons (2 e-) as shown by the following reaction, it becomes FADH2, which is the reduced form of the molecule. FADH2 functions as an electron carrier in these reactions.


__+ _2H++_2e-
_____FAD___________________________________FADH2




Excellent Sources of Vitamin B2:

Milk is perhaps the best single source of vitamin B2 (riboflavin), containing nearly two milligrams of riboflavin per quart, which is enough riboflavin per day for both children and adults.

Other good sources of riboflavin include various cheeses, yogurt, eggs, liver, beef, chicken, pork, tuna, dark green vegetables such as broccoli and spinach, cereals, breads, wheat germ, wild rice, mushrooms, soybeans and brewer's yeast (again, if you can stand the taste). Eat up !!!



Vitamin B3 (Niacin)


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Vitamin B3 (also known as Niacin) is yet another of the B-vitamins needed for the conversion of foods into energy. Vitamin B3 was originally known as nicotinic acid before the name was changed to niacin, which is actually a term used to describe both nicotinic acid and nicotinamide.


Vitamin B3 (Niacin)


___nicotinicacid1.gif_________nicotinamide1.jpg_

__Nicotinic Acid__________Nicotinamide
Of the two forms of niacin it is nicotinamide that serves as a component in two important coenzymes required for energy metabolism: NAD (nicotinamide adenine dinucleotide) and NADP (nicotinamide adenine dinucleotide phosphate). Because of the positive charge on the nitrogen (N) atom in the nicotinamide ring (as shown in the figures below) the oxidized forms of NAD and NADP are often depicted as NAD+ and NADP+, respectively.

NADP+ is synthesized from NAD+ by phosphorylating the ribose ring of NAD+ through the use of ATP. You can see from the two structures given below, how NADP+ is simply NAD+ with a third phosphate group (PO43-) attached to the ribose ring.


NAD+ (Nicotinamide Adenine Dinucleotide)


______NADox1.jpg____NAD+





NADP+ (Nicotinamide Adenine Dinucleotide Phosphate)


_______NADP+


Coenzymes NAD+ and NADP+