what is vitamin???

            Vitamin

Vitamin C (chemical names: ascorbic acid and ascorbate) is a six-carbon lactone which is synthesized from glucose by many animals. Vitamin C is synthesized in the liver in some mammals and in the kidney in birds and reptiles.
However, several species—including humans, non-human primates, guinea pigs, Indian fruit bats, and Nepalese red-vented bulbuls—are unable to synthesize vitamin C. When there is insufficient vitamin C in the diet, humans suffer from the potentially lethal deficiency disease scurvy. Humans and primates lack the terminal enzyme in the biosynthetic pathway of ascorbic
acid, l-gulonolactone oxidase, because the gene encoding for the enzyme has
undergone substantial mutation so that no protein is produced.

Role of vitamin C in human metabolic processes

Background biochemistry 

Vitamin C is an electron donor (reducing agent or antioxidant), and probably all of its biochemical and molecular roles can be accounted for by this function. The potentially protective role of vitamin C as an antioxidant is discussed in the antioxidants chapter of this report.

Enzymatic functions 

Vitamin C acts as an electron donor for 11 enzymes. Three of those enzymes are found in fungi but not in humans or other mammals and are involved in reutilization pathways for pyrimidines and the deoxyribose moiety of deoxynucleosides. Of the eight remaining human enzymes, three participate in collagen hydroxylation and two in carnitine biosynthesis; of the three enzymes which participate in collagen hydroxylation, one is necessary for biosynthesis of the catecholamine norepinephrine, one is necessary for amidation of peptide hormones, and one is involved in tyrosine metabolism.
Ascorbate interacts with enzymes having either monooxygenase or dioxygenase activity. The monooxygenases, dopamine b-monooxygenase and peptidyl-glycine a-monooxygenase, incorporate a single oxygen atom into a substrate, either a dopamine or a glycine-terminating peptide. The dioxygenases incorporate two oxygen atoms in two different ways: the enzyme 4-hydroxyphenylpyruvate dioxygenase incorporates two oxygen atoms into one product; the other dioxygenase incorporates one oxygen atom into succinate and one into the enzyme-specific substrate.

Miscellaneous functions

Concentrations of vitamin C appear to be high in gastric juice. Schorah et al. Found that the concentrations of vitamin C in gastric juice were severalfold higher (median, 249mmol/l; range, 43–909mmol/l) than those found in the plasma of the same normal subjects (median, 39mmol/l; range, 14–101mmol/l).
Gastric juice vitamin C may prevent the formation of N-nitroso compounds, which are potentially mutagenic. High intakes of vitamin C correlate with reduced gastric cancer risk, but a cause-and-effect relationship has not been established. Vitamin C protects low-density lipoproteins ex vivo against oxidation and may function similarly in the blood.
A common feature of vitamin C deficiency is anaemia. The antioxidant properties of vitamin C may stabilize folate in food and in plasma; increased excretion of oxidized folate derivatives in humans with scurvy has been
reported. Vitamin C promotes absorption of soluble non-haem iron possibly by chelation or simply by maintaining the iron in the reduced (ferrous,Fe2+) form. The effect can be achieved with the amounts of vitamin C obtained in foods. However, the amount of dietary vitamin C required to increase iron absorption ranges from 25 mg upwards and depends largely on the amount of inhibitors, such as phytates and polyphenols, present in the meal.

 References:

Thurnham DI. b-Carotene, are we misreading the signals in risk groups? Some
analogies with vitamin C. Proceedings of the Nutrition Society, 1994,
53:557–569.

TERIMA KASIH ATAS KUNJUNGAN SAUDARA
Semoga artikel ini bermanfaat bagi saudara. Jika ingin mengutip, baik itu sebagian atau keseluruhan dari isi artikel ini harap menyertakan link dofollow ke http://tugasdancatatanku.blogspot.com/2011/04/what-is-vitamin.html. Terima kasih sudah singgah membaca artikel ini.