Vitamin D

Vitamin D3 (colecalciferol, cholecalciferol)*

is a sterol metabolite formed photochemically in animals from 7-dehydrocholesterol by the sun's irradiation of the skin (Figure 5.106). 7-Dehydrocholesterol is the immediate A5'7 diene precursor of cholesterol (see page 234), and a photochemical reaction allows ring opening to precholecalciferol. A thermal 1,7-hydrogen shift follows to give colecalciferol (vitamin D3). Vitamin D3 is also manufactured photosynthetically by the same route. Vitamin D2 (ergocalciferol)* may be obtained from ergosterol in exactly the same way, and, although found in plants and yeasts, large amounts are obtained semi-synthetically by the sequence shown in Figure 5.106, using ergosterol from yeast

Fusidic Acid

Fusidic acid (Figure 5.105) is a steroidal antibiotic produced by cultures of the fungus Acremonium fusidioides (formerly Fusidium coccineum). It has also been isolated from several Cephalosporium species. Fusidic acid and its salts are narrow-spectrum antibiotics active against Gram-positive bacteria. It is primarily used, as its sodium salt, in infections caused by penicillin-resistant Staphylococcus species, especially osteomyelitis since fusidic acid concentrates in bone. It is usually administered in combination with another antibiotic to minimize development of resistance. Fusidic acid reversibly inhibits protein biosynthesis at the translocation step by binding to the larger subunit of the ribosome (see page 407).

absorption of light energy by n-electron system allows electrocyclic ring opening

1,7-hydrogen shift; this should be considered as an extended version of an allylic isomerization:

however, there is intramolecular transfer of the proton rather than employing an external source absorption of light energy by n-electron system allows electrocyclic ring opening

however, there is intramolecular transfer of the proton rather than employing an external source

Photosynthesis Vitamin

calcitriol (1a,25-dihydroxyvitamin D3)

calcidiol (25-hydroxyvitamin D3)

colecalciferol (cholecalciferol) (vitamin D3)

calcitriol (1a,25-dihydroxyvitamin D3)

calcidiol (25-hydroxyvitamin D3)

colecalciferol (cholecalciferol) (vitamin D3)

ergosterol
Pre Ergocalciferol

pre-ergocalciferol

Figure 5.106

ergocalciferol

(vitamin D2)

pre-ergocalciferol

Figure 5.106

ergocalciferol

(vitamin D2)

(Saccharomyces cerevisiae ). Vitamin D3 is not itself the active form of the vitamin, and in the body it is hydroxylated first to calcidiol and then to calcitriol (Figure 5.106). Colecalciferol and calcitriol have also been found in several plant species.

Systematic nomenclature of vitamin D derivatives utilizes the obvious relationship to steroids, and the term seco (ring opened) is incorporated into the root name (compare secologanin as a ring-opened analogue of loganin, page 189). The numbering system for steroids is also retained, and vitamin D3 becomes a derivative of 9,10-seco-cholestane, namely (5Z,7E)-9,10-secocholesta-5, 7,10(19)-trien-3P-ol, '9,10' indicating the site of ring cleavage. Note that it is necessary to indicate the configuration of two of the double bonds, and the somewhat confusing ^-configuration for the 3-hydroxy shows it is actually the same as in cholesterol.

Vitamin D

Vitamin D3 (colecalciferol, cholecalciferol) (Figure 5.106) is the main form of the fat-soluble vitamin D found in animals, though vitamin D2 (ergocalciferol) (Figure 5.106) is a constituent of plants and yeasts. Vitamin D3 is obtained in the diet from liver and dairy products such as butter, cream, and milk, whilst large amounts can be found in fish liver oils, e.g. cod liver oil and halibut liver oil (Table 3.2). Further requirements are produced naturally when the sterol 7-dehydrocholesterol is converted into colecalciferol by the effects of UV light on the skin. With a proper diet, and sufficient exposure to sunshine, vitamin D deficiency should not occur. Vitamin D deficiency leads to rickets, an inability to calcify the collagen matrix of growing bone, and is characterized by a lack of rigidity in the bones, particularly in children. In adults, osteoporosis may occur. In most countries, foods such as milk and cereals are usually fortified with vitamin D3, obtained commercially by UV irradiation of 7-dehydrocholesterol which is produced in quantity by semi-synthesis from cholesterol. Vitamin D2 has a similar activity in humans and is manufactured by UV irradiation of yeast, thereby transforming the ergosterol content. Other compounds with vitamin D activity have also been produced: vitamin D4 from 22,23-dihydroergosterol, vitamin D5 from 7-dehydrositosterol, vitamin D6 from 7-dehydrostigmasterol, and vitamin D7 from 7-dehydrocampesterol. Vitamin D1 was an early preparation, later shown to be a mixture of vitamin D2 and a photochemical by-product lumisterol (9p,10a-ergosterol). Vitamin D is unstable to heat, light, and air.

Vitamin D3 is not itself the active form of the vitamin, and in the body it is hydroxylated firstly to 25-hydroxyvitamin D3 (calcidiol) (Figure 5.106) by an enzyme in the liver, and then to 1a,25-dihydroxyvitamin D3 (calcitriol) by a kidney enzyme. Calcitriol is then transported to the bones, intestine, and other organs. It stimulates the absorption of calcium and phosphate in the intestine and the mobilization of calcium from bone. Calcitriol and other analogues, e.g. alfacalcidol and dihydrotachysterol (Figure 5.107) are available for use where chronic vitamin D deficiency is due to liver or kidney malfunction. The long term use of calcitriol and alfacalcidol (1a-hydroxyvitamin D3) in the treatment of osteoporosis may lead to toxic effects arising from elevated serum calcium levels. Toxicity is much reduced in the related 1a-hydroxyvitamin D2> and this agent is being investigated further.

Vitamin D is also known to have other physiological functions, including a role in immune suppression, hormone secretion, and the differentiation of both normal and malignant cells.

Precholecalciferol

alfacalcidol dihydrotachysterol calcipotriol tacalcitol alfacalcidol dihydrotachysterol calcipotriol tacalcitol

" (Continued)

Two vitamin D derivatives, calcipotriol and tacalcitol (Figure 5.107) are widely used in the topical treatment of psoriasis, to inhibit the cell proliferation characteristic of this condition.

Vitamin D2 is also employed as a rodenticide. High doses are toxic to rats and mice, the vitamin causing fatal hypercalcaemia.

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Responses

  • SAGRAMOR
    What is preergocalciferol?
    8 years ago
  • caroline
    What vitamins are good for bonds?
    7 years ago
  • nicla
    Is vitamin d a single or double bond?
    9 months ago

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