Activated 7-dehydrocholesterol

As a brief introductory summary, vitamin D substances perform the following fundamental physiological functions (1) promote normal growth (via bone growth) (2) enhance calcium and phosphorus absorption from the intestine (3) serve to prevent rickets (4) increase tubular phosphorus reabsorpiion (5) increase citrate blood levels (6) maintain and activate alkaline phosphatase m bone (7) maintain serum calcium and phosphorus levels. A deficiency of D substances may be manifested in the form of rickets, osteomalacia, and hypoparathyroidism. Vitamin D substances are required by vertebrates, who synthesize these substances in the skin when under ultraviolet radiation, Animals requiring exogenous sources include infant vertebrates and deficient adult vertebrates, Included there are vitamin D (calciferol ergocalciferol) and vitamin D< (activated 7-dehydrocholesterol cholecalciferol). 

In 1918, Mellanby produced experimental rickets in dogs. In 1919, Huldschinsky ameliorated rachitic symptoms in children with ultraviolet radiation. Hess, in 1922, showed that liver oils contain the same antirachitic factor as sunlight In that same year, McCollum increased calcium deposition in rachitic rats with cod liver oil factor. In 1924. Steenbook and Hess demonstrated irradiated foods have antirachitic properties, It was in 1925 that McCollum named antirachitic factor as vitamin D. In 1931, Angus isolated crystalline vitamin D (calciferol). In 1936, Windaus isolated vitamin D3 (activated 7-dehydrocholesterol). 

The international standard for vitamin D is an oil solution of activated 7-dehydrocholesterol (3). The lU is the biological activity of 0.025 jig of pure cholecaldferol. 

Activated 7-dehydrocholesterol Desoxycorticosterone acetate Desoxycorticosterone trimethylacetate Diaminodiphenylsulfone Dibucaine... 

Vitamin D3 (cholecalciferol, Calcitol, activated 7-dehydrocholesterol, (+)-vitamin D3) [67-97-0] 384.6, m 83-85 , [a] Uj +126 (c 2, EtOH). It is converted into its 3,5-dinitrobenzoyI ester and crystallised repeatedly from acetone. The ester is then saportified and the free vitarrrin is isolated. Store it in sealed ampoules urtder argon below 8 . It acts through a receptor which modrrlates differerrtiation and proliferation of normal arrd neoplastic cells. [Laughland Phillips Anal Chem 28 817 1956, DeLuca Schones Ann Rev Biochem 52 411 1983, Beilstein 6 III 2811,6IV 4149 ]... 

Vitamin D consists of a group of sterol derivatives produced by ultraviolet irradiation of the corresponding precursors. The two important members of this group are vitamin D2 or calciferol, which is activated ergosterol, and vitamin D3, which is activated 7-dehydrocholesterol. The latter is the compound formed in the skin on exposure to sunshine or ultraviolet rays. 

Infants maybe sensitive to doses of vitamin A [11103-57-4] in the range of 75,000—200,000 lU (22.5—60 mg), although the toxic dose in adults is probably 2—5 million lU (90.6—1.5 g). Intakes in this range from normal food suppHes without oral supplements are simply beyond imagination (79). Vitamin D [1406-16-2] toxicity is much more difficult to substantiate clinically. Humans can synthesize active forms of the vitamin in the skin upon irradiation of 7-dehydrocholesterol. Toxic symptoms are relatively nonspecific, and dangerous doses seem to He in the range of 1000—3000 lU/kg body wt (25—75 flg/kg body wt) (80). Cases of toxicity of both vitamins E and K have been reported, but under ordinary circumstances these vitamins are considered relatively innocuous (81). 

Vitamin (cholecalcifetol calciol), (5Z,7E)-(33)-9,10-seco-5,7,10(19) cholestatriene-3-ol (4), is the naturally occurring active material found ki all animals. It is produced ki the skin by the kradiation of stored 7-dehydrocholesterol (provitamin E) ), cholesta-5,7-diene-3B-ol (3). 

P-Hydroxy steroids which contain the 5,7-diene system and can be activated with uv light to produce vitamin D compounds are called provitamins. The two most important provitamins are ergosterol (1) and 7-dehydrocholesterol (3). They are produced in plants and animals, respectively, and 7-dehydrocholesterol is produced synthetically on a commercial scale. Small amounts of hydroxylated detivatives of the provitamins have been synthesized in efforts to prepare the metaboHtes of vitamin D, but these products do not occur naturally. The provitamins do not possess physiological activities, with the exception that provitamin D is found in the skin of animals and acts as a precursor to vitamin D, and synthetic dihydroxalated... 

FIGURE 18.37 (a) Vitamin D3 (cholecalciferol) is produced in the skin by the action of sunlight on 7-dehydrocholesterol. The successive action of mixed-function oxidases in the liver and kidney produces 1,25-dihydroxyvitamin D3, the active form of vitamin D. 

Actually, neither vitamin D2 nor D3 is present in foods. Rather, foods contain the precursor molecules 7-dehydrocholesterol and ergosterol. In the presence of sunlight, however, both precursors are converted under the skin to the active vitamins, hence the nickname for vitamin D, the "sunshine vitamin. 7... 

The active vitamins are produced by conversion of provitamins by ultraviolet light. Ergosterol, a yeast sterol, is converted to its active form, ergocalciferol (vitamin D2), and 7-dehydrocholesterol, which is found in many natural foods and is also synthesized in man, is converted to cholecalciferol (vitamin D3). Fish liver oils are virtually the only source of vitamin D3 in nature. The most active form of vitamin D3 is 1,25-dihydroxycholecalciferol and this is produced by the hydroxylation of cholecalciferol at position 25 in the liver and then at position 1 in the kidney. 

The precursor, 7-dehydrocholesterol is converted by a non-enzymatic reaction to cholecalciferol (calciol). This reaction occurs in skin exposed to sunlight due to irradiation by UV-B light at a wavelength of about 300 nm. Cholecalciferol is transported via carrier proteins to the liver where hydroxylation at carbon-25 occurs in a reaction catalysed by a microsomal cytochrome P450 hydroxylase to form calcidiol. This compound travels to the kidney attached to specific binding proteins, where another cytochrome P450 enzyme, mitochondrial 1-a-hydroxylase, introduces a second hydroxyl group in to the molecule to form the active calcitriol. 

Activation of 7-dehydrocholesterol by UV light in the skin produces cholecalciferoi (vitamin Dj) This step is insufficient in many people in cold, doudy dimates, and vitamin Dj... 

Vitamin D hormone is derived from vitamin D (cholecalciferol). Vitamin D can also be produced in the body it is formed in the skin from dehydrocholesterol during irradiation with UV light. When there is lack of solar radiation, dietary intake becomes essential, cod liver oil being a rich source. Metaboli-cally active vitamin D hormone results from two successive hydroxylations in the liver at position 25 ( calcifediol) and in the kidney at position 1 ( calci-triol = vit. D hormone). 1-Hydroxylation depends on the level of calcium homeostasis and is stimulated by parathormone and a fall in plasma levels of Ca or phosphate. Vit D hormone promotes enteral absorption and renal reabsorption of Ca and phosphate. As a result of the increased Ca + and phosphate concentration in blood, there is an increased tendency for these ions to be deposited in bone in the form of hydroxyapatite crystals. In vit D deficiency, bone mineralization is inadequate (rickets, osteomalacia). Therapeutic Liillmann, Color Atlas of Pharmacology... 

Vitamin D is synthesized in the skin in the presence of ultraviolet light, and it is unusual to become dependent on dietary intake except when exposed to inadequate UV light. The active form of vitamin D is 1,25-dihydroxycholecalciferol (1,25-OHCC), also termed calcitriol. For vitamin D synthesis, cholecal-ciferol (also termed vitamin D3) is synthesized in the skin from cholesterol via 7-dehydrocholesterol, and is 25-hydroxylated in the liver and 1-hydroxylated in the kidney. Dietary vitamin D is actually a mixture of sterols which includes 7-dehydrocholesterol, and is mainly found in fish and eggs. 

The primary supply of vitamin in humans is not obtained from the diet but rather is derived from the ultraviolet photoconversion of 7-dehydrocholesterol to vitamin Ds in skin. Thus, vitamin Dj synthesis varies with the seasons. D3 is a prohormone and requires further metabolic conversion to exert biological activity in its target organs (Fig. 66.2). The liver and the kidney are the major sites of metabolic activation of this endogenous sterol hormone. The initial transformation of D3 occurs in the liver and is catalyzed by the enzyme 25-OH-D3-hydroxylase... 

Cholecalciferol is pure vitamin D3 derived from the ultraviolet conversion of 7-dehydrocholesterol to cholecalciferol. Ergocalciferol vitamin D2) is a sterol derived from yeast and fungal ergosterol. Calcitriol [Rocaltrol, 1,25-(0H)2D3] is the metabolically active vitamin D3 compound. Dihydrotachysterol is a synthetic compound that may act somewhat more quickly than either vitamin D2 or D3. 

T FIGURE 10-20 Vitamin D3 production and metabolism, (a) Cholecalciferol (vitamin D3) is produced in the skin by UV irradiation of 7-dehydrocholesterol, which breaks the bond shaded pink. In the liver, a hydroxyl group is added at C-25 (pink) in the kidney, a second hydroxylation at C-1 (pink) produces the active hormone, 1,25-dihydroxycholecalciferol. This hormone regulates the metabolism of Ca2+ in kidney, intestine, and bone, (b) Dietary vitamin D prevents rickets, a disease once common in cold climates where heavy clothing blocks the UV component of sunlight necessary for the production of vitamin D3 in skin. On the left is a 21/2-year-old boy with severe rickets on the right, the same boy at age 5, after 14 months of vitamin D therapy. 

Vitamin D3, also called cholecalciferol, is normally formed in the skin from 7-dehydrocholesterol in a photochemical reaction driven by the UV component of sunlight (Fig. 10-20). Vitamin D3 is not itself biologically active, but it is converted by enzymes in the liver and kidney to 1,25-dihydroxycholecalciferol, a hormone that regulates calcium uptake in the intestine and calcium levels in kidney and bone. Deficiency of vitamin D... 

Vitamin D is obtained in the diet or by photolysis of 7-dehydrocholesterol in skin exposed to sunlight. Calcitriol works in concert with parathyroid hormone in Ca2+ homeostasis, regulating [Ca2+] in the blood and the balance between Ca2+ deposition and Ca2+ mobilization from bone. Acting through nuclear receptors, calcitriol activates the synthesis of an intestinal Ca2+-binding protein essential for uptake of dietary Ca2+. Inadequate dietary vitamin D or defects in the biosynthesis of calcitriol result in serious diseases such as rickets, in which bones are weak and malformed (see Fig. 10-20b). 

However, it was not until 1924, when Steenbock and Hess showed that irradiation of certain foods generated protective activity against the disease, that vitamin D (calciferol) was recognized as a second lipid-soluble vitamin. Vitamin D is a family of compounds formed by the irradiation of A5/7-unsaturated sterols such as ergosterol and 7-dehydrocholesterol. The former yields ergocalciferol (vitamin D2) and the latter cholecalciferol (vitamin D3). 

Vitamin D3 is a precursor of the hormone 1,25-dihy-droxyvitamin D3. Vitamin D3 is essential for normal calcium and phosphorus metabolism. It is formed from 7-dehydrocholesterol by ultraviolet photolysis in the skin. Insufficient exposure to sunlight and absence of vitamin D3 in the diet leads to rickets, a condition characterized by weak, malformed bones. Vitamin D3 is inactive, but it is converted into an active compound by two hydroxylation reactions that occur in different organs. The first hydroxylation occurs in the liver, which produces 25-hydroxyvita-min D3, abbreviated 25(OH)D3 the second hydroxylation occurs in the kidney and gives rise to the active product 1,25-dihydroxy vitamin D3 24,25 (OH)2D3 (fig. 24.13). The hydroxylation at position 1 that occurs in the kidney is stimulated by parathyroid hormone (PTH), which is secreted from the parathyroid gland in response to low circulating levels of calcium. In the presence of adequate calcium, 25(OH)D3 is converted into an inactive metabolite, 24,25 (OH)2D3. The active derivative of vitamin D3 is considered a hormone because it is transported from the kidneys to target cells, where it binds to nuclear receptors that are analogous to those of typical steroid hormones. l,25(OH)2D3 stimulates calcium transport by intestinal cells and increases calcium uptake by osteoblasts (precursors of bone cells). 

FIGURE 66.1 Vitamins D3 and D2 are produced by ultraviolet irradiation of animal skin and plants, respectively. The precursor of vitamin D3 in skin is 7-dehydrocholesterol, or provitamin D. In humans, the storage, transport, metabolism, and potency of vitamins D2 and D3 are identical, and the net biologic activity of vitamin D in vivo results from the combined effects of the hydroxylated derivatives of vitamins D2 and D3. 

The chemical structure of vitamin D3 is closely related to its precursor, 7-dehydrocholesterol, from which it is produced by a photochemical reaction. Therefore, vitamin Do is closely related structurally to the four-ring nucleus of steroids derived from the cyclopentanoperhydro-phenanthrene ring system. No vitamin D activity is noticed until the B ring of 7-dehydrocholesterol is opened between C-9 and C-10. Thus, vitamin D3 is a 9,10-seco steroid and its carbon skeleton is numbered accordingly (Scheme I). The important aspects of its chemistry center about the 5,6,7-cis-triene structure. The formula for vitamin D3 is C27H44O and its formula weight is 384.64. 

Fig. 1. Vitamin D endocrine system. Vitamin D, obtained either through the diet or by conversion of 7-dehydrocholesterol in the skin, is sequentially hydroxylated in the liver and kidney to produce the active metabolites la,25(OH)2D, and 24R,25(OH)2D,.

Vitamin D-active substances are required in the diets of growing children and pregnant women, but normal adults receiving sufficient doses of sunshine can manufacture sufficient amounts of these compounds to meet their needs. Active vitamin D compounds can by synthesized in such individuals from 7-dehydro-cholesterol (see Table 6.2), an intermediate in cholesterol biosynthesis. Dietary sources also include cholecalciferol, which is produced from 7-dehydrocholesterol and ergosterol (Table 6.2). 7-Dehydrocholesterol and ergosterol are often referred to as provitamins. 

The precursors of vitamins D2 and D3 are ergosterol and 7-dehydrocholesterol, respectively. These precursors or provitamins can be converted into the respective D vitamins by irradiation with ultraviolet light. In addition to the two major provitamins, there are several other sterols that can acquire vitamin D activity when irradiated. The provitamins can be converted to vitamin D in the human skin by exposure to sunlight. Because very few foods are good sources of vitamin D, humans have a greater likelihood of vitamin D deficiency than of any other vitamin deficiency. Enrichment of some foods with vitamin D has significantly helped to eradicate rickets, which is a vitamin D deficiency disease. Margarine and milk are the foods commonly used as carrier for added vitamin D. 

In osteoblasts, keratinocytes, and colonocytes, andpossibly other cells, calcitriol acts via cell surface receptors linked to phospholipase C, resulting in release of diacylglycerol and inositol trisphosphate (Section 14.4.1), followed by opening of intracellular calcium channels and activation of protein kinase C and mitogen-activated protein (MAP) kinases. The effect of this is inhibition of cell proliferation and induction of differentiation. A variety of analogs of calcitriol that do not bind to the nuclear receptor do bind to, and activate, the cell surface receptor, including l,25-dihydroxy-7-dehydrocholesterol and 1,25-dihydroxylumisterol. The rapid nongenomic responses to vitamin D can be demonstrated in knockout mice that lack the vitamin D nuclear receptor (Farach-Carson and Ridall, 1998 Nemere and Farach-Carson, 1998). 

Figure 26.29. Vitamin D Synthesis. The pathway for the conversion of 7-dehydrocholesterol into vitamin D3 and then into calcitriol, the active hormone.

Some 20% of the daily vitamin D requirement are supplied by food intake, while about 80% are produced via endogenous synthesis through transformation of 7-dehydrocholesterol under the influence of ultraviolet light. Witliin the liver, D3 (= cholecalciferol) is activated by 25-hydroxylation. It is excreted into the bile to become pmrt of an enterohepmtic circulation. In liver cirrhosis, and particularly in cholestasis, this circulation is impaired, so that vitamin D metabolites are activated in larger amounts and excreted in the faeces. The resulting clinical picture is usually a mixture of osteoporosis and osteomalacia. 

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