Vitamins: Classification and Properties

Vitamins

• Vitamins are organic compounds that cannot be synthesized in adequate quantities by humans and must be supplied by the diet.
• They are classified as water-soluble and fat-soluble.

Water Soluble Vitamins

• Include folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid.
• Readily excreted in the urine, and toxicity is rare.
• Deficiencies can occur quickly.

Fat Soluble Vitamins

• Include vitamins A, D, K, and E.
• Released, absorbed, and transported with dietary fat.
• Not readily excreted, and significant quantities are stored in the liver and adipose tissue.
• Consumption of vitamins A and D in excess of the dietary reference intakes can lead to accumulation of toxic quantities.

Ascorbic Acid (Vitamin C)

• The active form of vitamin C is ascorbic acid.
• A crucial reducing agent in the body.
• Vitamin C is a coenzyme in hydroxylation reactions, required for the maintenance of normal connective tissue and wound healing.
• Facilitates the absorption of dietary nonheme iron from the intestine by reduction of the ferric form (Fe+3) to Fe+2.

Vitamin C Deficiency

• Ascorbic acid deficiency results in scurvy, characterized by sore and spongy gums, loose teeth, fragile blood vessels, hemorrhage, swollen joints, bone changes, and fatigue.
• Many of the deficiency symptoms can be explained by the decreased hydroxylation of collagen, resulting in defective connective tissue.
• A microcytic anemia caused by decreased absorption of iron may also be seen.

Folic Acid (Vitamin B9 or Folat)

• Plays a key role in one-carbon metabolism.
• Tetrahydrofolate (THF), the reduced, coenzyme form of folate, receives one-carbon fragments from donors and transfers them to intermediates in the synthesis of amino acids, purine nucleotides, and thymidine monophosphate (TMP).

Folic Acid Deficiency

• Inadequate serum levels of folate can be caused by increased demand, poor absorption, alcoholism, treatment with certain drugs, and a folate-free diet.
• A primary result of folic acid deficiency is megaloblastic anemia, caused by diminished synthesis of purine nucleotides and TMP.
• Leads to an inability of cells to make DNA and divide.

Cobalamin (Vitamin B12)

• Synthesized only by microorganisms, and not present in plants.
• Required in humans for two essential enzymatic reactions: the re-methylation of homocysteine and the isomerization of methyl-malonyl coenzyme A.
• Deficiency can lead to the accumulation of unusual fatty acids, which may account for some of the neurologic manifestations of vitamin B12 deficiency.

Folate Trap in Vitamin B12 Deficiency

• The rapidly dividing cells or tissues need both the N5,N10-methylene and N10-formyl forms of THF for the synthesis of nucleotides required for DNA replication.
• In vitamin B12 deficiency, the utilization of the N5-methyl form of THF is impaired, leading to a deficiency of the THF forms needed in purine and TMP synthesis.
• Results in the symptoms of megaloblastic anemia.

Vitamin B12 Deficiency

• Significant amounts of vitamin B12 are stored in the body, so it may take several years for clinical symptoms to develop.
• Deficiency can result in pellagra-like symptoms.

Riboflavin (Vitamin B2)

• The two biologically active forms are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).
• FMN and FAD are each capable of reversibly accepting two hydrogen atoms, forming FMNH2 or FADH2.
• Bound tightly, sometimes covalently, to flavoenzymes that catalyze the oxidation or reduction of a substrate.

Biotin (Vitamin B7)

• A coenzyme in carboxylation reactions, serving as a carrier of activated carbon dioxide (CO2).
• Deficiency does not occur naturally, but can be induced by consuming raw egg whites, which contain avidin that binds to biotin.

Pantothenic Acid (Vitamin B5)

• A component of coenzyme A (CoA), which functions in the transfer of acyl groups.
• Also a component of the acyl carrier protein domain of fatty acid synthase.
• Deficiency is not well characterized in humans.

Vitamin A

• A fat-soluble vitamin that comes primarily from animal sources as retinol, a retinoid.
• Essential for vision, reproduction, growth, and maintenance of epithelial tissues.

Vitamin K

• The principal role is in the posttranslational modification of proteins involved in blood clotting.
• Exists in several active forms, including phylloquinone, menaquinone, and menadione.
• Required in the hepatic synthesis of the blood clotting proteins: prothrombin, factor VII, factor IX, and factor X.

Vitamin E

• Consists of eight naturally occurring tocopherols, of which α-tocopherol is the most active.
• Functions as an antioxidant in preventing nonenzymic oxidations.
• Regenerated by vitamin C.