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Eric C. Niederhoffer, Ph.D. Associate Professor of Biochemistry and Molecular Biology Southern Illinois University School of Medicine 600 Agriculture Drive, Carbondale, IL 62901-6503 Rm 112 Lindegren, 618-453-6467 eniederhoffer@siumed.edu Copyright 2000- , E.C. Niederhoffer. All Rights Reserved. All trademarks and copyrights are the property of their respective owners.

Vitamins

Vitamins are low molecular weight organic molecules that are needed to promote a variety of biological processes important to life. They are generally not made in sufficient quantities in our cells and therefore must be obtained through the diet. Information concerning dietary intake requirements and food sources may be obtained from the Food and Nutrition Information Center (United States Department of Agriculture).

Classification and Nomenclature

The first representatives were initially discovered through separation of water- and fat-soluble components of butter fat This led to classifiying vitamins as water-soluble and fat-soluble.

The original nomenclature was alphabetical in order of discovery (based on correcting some disease or problem) with subscripts added for related molecules. As the structure of each vitamins was determined, chemical names were provided.

Fat Soluble Vitamins

Vitamin A

Vitamin A is most commonly associated with rhodopsin (cis-retinal plus the photoreceptor protein opsin) of the visual system. The provitamin is obtained through the diet from plants rich in carotenoids (-, -, and -carotenes), which is then processed in the body, and from animals that have processed the precursor into the active form. Retinoic acid acts as a ligand for specific nuclear receptors (retinoic acid receptors and retinoid X receptors) that regulate gene transcription. Excessive amounts of the vitamin are toxic (150 mg single dose or 5000 IU/day for several years).

Vitamin D is also know as the "sunshine" vitamin because sun light converts the precursor 7-dehydrocholesterol, which is found in the skin of animals and humans, into vitamin D3. Ricketts was found to be cured by this fat-soluble factor. Ergocalciferol (D2) is obtained from the irradiation of the plant steroid ergosterol. Both vitamin D3 and D2 are converted in the body through a series of hydroxylations, first to 25-hydroxycholecalciferol (25-ergocalciferol) in the liver, and then 1,25-dihydroxycholecalciferol (1,25-dihydroxyergocalciferol) in the kidneys, the active form (also denoted as calcitriol). The active calcitriol functions are a hormone, binding to vitamin D receptors (nuclear receptors).

Vitamin E

Vitamin E was found during studies on the impact of plant oils on reproduction and provided the name tocopherol (tokos = child birth; pherein = to bear; with "ol" at the end to signify an alcohol). The most active forms of the vitamin have been isolated from plant oils and are indicated in the table below. As a fat-soluble molecule that is easily oxidized, it is found associated with cell membranes where it serves as a lipid antioxidant.

Vitamin K

Vitamin K (K for koagulation) was found to promote blood clotting (coagulation). Three forms exist. K1 was isolated from alfalfa (green plants) and is called phylloquinone, phytylmenaquinone, or phytomenadione. K2 was isolated from putrefied fish meal and group of related molecules are called menaquinone-4 to menaquinone-13, indicating the number of isoprenyl units associated with the molecule. K3 is a synthetic form containing no isoprenyl groups (menadione). The vitamin is involved in carboxylase mediated reactions that convert glutamic acid residues to -carboxyglutamic acid (posttranslational modifications), a residue with side groups that bind Ca²⁺. The well known coagulation proteins prothrombin, factors VII, IX, and X are required for blood clotting.

Water Soluble Vitamins

Vitamin B1

Beriberi is a paralyzing disease (Singhalese, beri = weakness) that was cured by providing less polished rice (vitamin is associated with the germ). Thiamin (or thiamine) is converted into the active form (thiamin pyrophosphate, TPP) by the action of thiamin pyrophosphate kinase:

Thiamin is required for pyruvate dehydrogenase (link between glycolysis and citric acid cycle), -ketoglutarate dehydrogenase (citric acid cycle), -ketoacid dehydrogenases (branched-chain amino acid oxidation), and transketolase (pentose phosphate pathway). Thiamin is rapidly excreted in the urine along with its metabolic products (major products are pyridine acetic acid and thiazole acetic acid). Thiamin levels can be obtained by colorimetric assay of the urine or, in a more accurate assessment, functional measurement of metabolic reations. The most useful for the latter approach is determining transketolase activity in red blood cells.

Vitamin B2

Vitamin B2 (riboflavin) was initially discovered as the heat-stable yellow-green fluorescent component of water-soluble vitamin B. Riboflavin is processed in the body to form the two prosthetic groups flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Note that the sugar derivative connected to the flavin is a reduced sugar (polyhydric alcohol) rather than the ribose of a typical nucleotide. Interconversions between riboflavin, FMN, and FAD are illustrated below.

By definition, prosthetic groups FMN and FAD are tightly or covalently bound to their respective proteins. These flavoproteins include acyl CoA dehydrognease (-oxidation), amino acid oxidases, cytochrome c reductase, glutathione reductase (oxidative stress defense), -ketoacid dehydrogenases,-ketoglutarate dehydrogenase, pyruvate dehydrogenase, succinate dehydrogenase (complex I of the respiratory chain), xanthine oxidase (purine catabolism).

Vitamin B3

Vitamin B3 (niacin, nicotinic acid) is not strictly a vitamin because it can be produced from the metabolism of tryptophan. It is associated with the disease pellegra (pelle=skin, agra=rough), which in chronic cases is characterized by dermatitis, diarrhea, and dementia. High doses of niacin are useful as an antihyperlipidemic. The redox pair NAD⁺/NADH associations include glyceraldehyde-3-phosphate dehydrogenase (glycolysis), lactate dehydrogenase (anaerobic glycolysis), pyruvate dehydrogenase, isocitrate dehydrogenae, -ketoglutarate dehydrogenase, glucose-6-phosphate dehydrogenase (NADPH, pentose phosphate pathway), 6-phosphogluconate dehydrogenase (NADPH, pentose phosphate pathway), complex I of the respiratory chain, and enzymes of both amino acid and fatty acid oxidation.

Vitamin B5

Vitamin B5 (pantothenic acid, pantos = everywhere) consists of pantoic acid and -alanine. It is processed in the body to become a component of coenzyme A (amino acid, carbohydrate, and lipid metabolism) and acyl carrier protein (fatty acid synthesis).

Vitamin B6

Vitamin B6 (pyridoxine, pyridoxal, pyridoxamine) along with their 5'-phosphates) are involved in amino acid metabolism (alanine aminotransferase, aspartate aminotransferase, branched-chain amino acid aminotransferases, cystathionine synthase, kynurenininase, and aromatic amino acid decarboxylase), glycogen degradation (glycogen phosphorylase), porphyrin synthesis (-aminolevulinic acid synthetase), and in removing steroid hormone-receptor complex from DNA. There are kinases and phosphatases involved in adding or removing the phosphate group (OPO₃^-) and pyridoxine phosphate oxidase and aminotransferase for interconverting pyridoxine-, pyridoxal-, and pyridoxamine-5'-phosphate.

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