Vitamer

A vitamer of a particular vitamin is any of a number of chemical compounds, generally having a similar molecular structure, each of which shows vitamin-activity in a vitamin-deficient biological system.

For instance, vitamin A has at least six vitamer chemicals that all qualify as "vitamin A", each with slightly different properties. In such a system, "vitamin A" is termed the "generic descriptor" of the vitamin, which is defined by its biological properties in a vitamin deficient organism, not by its chemical structure. In the "vitamin A" system, four of those found naturally in plant origin foods are chemically carotenoids (three carotenes and one is a xanthophyll). However, the retinol and retinal forms, which occur in animal-based foods, are several times as effective in humans, per microgram, than are the plant-based forms. In some cases these differences are extreme: for example the carotenoid forms of vitamin A cannot be absorbed by cats or ferrets at all, and therefore do not have vitamin A activity in these species. The retinoids are pharmaceutical forms of vitamin A.

Typically, the vitamin activity of multiple vitamers is due to the body's limited ability to convert one vitamer to another, or many vitamers to the same enzymatic cofactor(s), which is active in the body as a form of the vitamin.

As part of the definition of vitamin, the body cannot completely synthesize an optimal amount of vitamins from simple foodstuffs, without some minimal amount of a vitamer molecule as a foundation.

Typically not all vitamers possess exactly the same vitamin potency, per mass. This is due to differences in absorption and interconversion of the various vitamers of a vitamin. Often for the same reason, the toxicity of vitamers varies by molecule, as is the case with vitamin E.

A set of chemicals may be (but is not always) grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A". Other examples of vitamers include cyanocobalamin, hydroxocobalamin, methylcobalamin, and 5-deoxyadenosylcobalamin (adenosylcobalamin—AdoB-12), which are all vitamers of B₁₂, and thus all possess "B₁₂ activity". Another example is that both niacinamide and nicotinic acid (niacin) have vitamin B₃ activity.

List of vitamins by generic descriptor, with some of their vitamers including active forms

Vitamin generic descriptor name	Vitamer chemical name(s) or chemical class of compounds (list not complete)
Vitamin A	Retinol, retinal, and four carotenoids: the carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin
Vitamin B1	Thiamine, Thiamine pyrophosphate
Vitamin B2	Riboflavin, Flavin mononucleotide (FMN), Flavin adenine dinucleotide (FAD)
Vitamin B3	Niacin, niacinamide
Vitamin B5	Pantothenic acid, panthenol, pantethine
Vitamin B6	Pyridoxine, pyridoxamine, pyridoxal, pyridoxal 5-phosphate
Vitamin B7	Biotin
Vitamin B9	Folic acid, folinic acid, 5-Methyltetrahydrofolate
Vitamin B12	Cyanocobalamin, hydroxocobalamin, methylcobalamin, adenosylcobalamin
Vitamin C	Ascorbic acid, dehydroascorbic acid, calcium ascorbate, sodium ascorbate, other salts of ascorbic acid
Vitamin D	Calcitriol, ergocalciferol (D2), cholecalciferol (D3)
Vitamin E	Tocopherols (d-alpha, d-beta, d-gamma, and d-delta-tocopherol), tocotrienols (alpha, beta, gamma, delta tocotrienols)
Vitamin K	phylloquinone(K1), menaquinones (K2), menadiones (K3)

See also

• Isomer
• Provitamin

External links

• Dictionary definition of vitamer. Referenced Jan. 4, 2008