Provitamins from cholesterol

Irradiated ergosterol was found not to be as antirachitic in the chick as in the rat, whereas the chick could be protected by direct kradiation. The provitamin in cholesterol was shown not to be ergosterol. Rygh (14) in 1935 found that 1 rat unit of cod Hver oil was 100 times more potent in chicks than 1 rat unit of vitamin D2. Brockmann (15) in 1936, prepared the pure crystalline 3,5-dinitrobenzoate derivative of vitamin D obtained from tuna Hver oil... 

Provitamin D. Provitamin is made from cholesterol, and its commercial production begias with the isolation of cholesterol from one of its natural sources. Cholesterol occurs ia many animals, and is generally extracted from wool grease obtained by washing wool after it is sheared from sheep. This grease is a mixture of fatty-acid esters, which contain ca 15 wt % cholesterol. The alcohol fraction is obtained after saponification, and the cholesterol is separated, usually by complexation with 2iac chloride, followed by decomplexation and crystallisation. Cholesterol can also be extracted from the spiaal cords and brains of animals, especially catde, and from fish oils. 

Whether the synthesis or storage of provitamin D3 can occur in the human is uncertain, but it can certainly occur in animals. Glover et al. (G3) have shown that this provitamin is present in the tissues even when the animals are fed on a practically sterol-free diet. The conclusion is reached that the animal is not dependent on dietary provitamin D but has the power to synthesize it from cholesterol. The small intestine is especially rich in provitamin. The same authors noted that, in the guinea pig, infection of the liver with Pasteurella pseudotuberculosis is associated with an increased concentration of provitamin in the intestinal mucosa, an interesting observation in view of the possible role of infection in idiopathic hypercalcemia in infancy. 

In a classical sense, vitamin D3. the form produced in animals, is not a true vitamin" because it is produced in the skin from 7-dehydrochole.sterol by UV radiation in the range of 290 to 3(X) nm. 7-Dehydrocholesterol is produced from cholesterol metabolism. Only when exposure to sunlight is inadequate does vitamin Di become a vitamin in the historical. sense. Further, vitamin D3 is now termed a provitamin because it requires hydroxylation by the liver and the kidney to be fully active. 

Cholecalciferol (vitamin D3, I) is formed from cholesterol in the skin through photolysis of 7-dehydrocholesterol (provitamin D3) by ultraviolet light ( sunshine vitamin cf. 3.8.2.2.2). Similarly, vitamin D2 (ergocalciferol, II cf. Formula 6.2) is formed from ergosterol. 

Ergosterol (provitamin Dj) differs from cholesterol by an additional methyl group and two double bonds, one of which is in the side-chain. Stigmasterol differs from cholesterol only in the side-chain, which has a double bond and an additional ethyl group. 

Chloroform-methanol extracts of Borrelia burgdorferi were used for the identification of lipids and other related components that could help in the diagnosis of Lyme disease [58]. The provitamin D fraction of skin lipids of rats was purified by PTLC and further analyzed by UV, HPLC, GLC, and GC-MS. MS results indicated that this fraction contained a small amount of cholesterol, lathosterol, and two other unknown sterols in addition to 7-dehydrocholesterol [12]. Two fluorescent lipids extracted from bovine brain white matter were isolated by two-step PTLC using silica gel G plates [59]. PTLC has been used for the separation of sterols, free fatty acids, triacylglycerols, and sterol esters in lipids extracted from the pathogenic fungus Fusarium culmorum [60]. 

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. 

A-10 Provitamin D is a simple derivative of cholesterol, which occurs when a hydrogen is removed from the number 7 carbon, which then forms a double bond with the number 8 carbon, in the second, or B ring of the cholesterol molecule. 

Cholesterol can be converted to vitamin D photochemically from 7-dehydrocholes-terol (provitamin D) (Special Topic 6.4) and this procedure is still used in industry.616 Vitamin D can also be made by irradiating yeasts rich in ergosterol. In addition, vitamin A (retinol) (see Special Topic 6.1) is synthesized by E Z photoisomerization (Section 6.1.1), sensitized by chlorophyll or other chromophores (Section 6.8) of its 11 -cis isomer, which is produced industrially by conventional synthetic steps.1310... 

Further products have been identified from the irradiation of 7-dehydro-cholesterol [(130), provitamin D3]. In ether or alcohol the main component of the photolysate was assigned structure (131) which results from the photochemical cyclization of the trans-Z-cis-conformation of the triene (132) formed by ringopening of the cholesterol.92 From reaction in ethanol two alcohol-addition products (133) and (134) have been identified. Two other products of the toxisterol type have been assigned structures (135), the difference between them... 

Early investigators showed that superficially purified cholesterol and activatable foods contained a provitamin, or precursor, which was converted into the vitamin on irradiation. Ultimately, two vitamin D precursors were identified. Provitamin D2 was found to be ergosterol (la). WiNDHAUS 194) and Askew (6) independently isolated pure crystalline vitamin D2 (2 a) via its 3,5-dinitrobenzoate from the complex and unstable irradiation products of ergosterol. Provitamin D3 was more difficult to identify, but it was found by Windhaus 195) to be 7-dehydro-cholesterol (lb). On irradiation crystalline vitamin D3 (2b) was obtained, which was identical with that isolated from tuna-liver oil by Brock-MANN 29). Vitamins D4 and D5 have been found in nature, each one differing from the other and the ones mentioned above in the constitution of the side chain. 

CHEMISTRY, METABOLISM, PROPERTIES. Akhough about 10 sterol compounds with vitamin D activity have been identified, only two of these, known as provitamins D or precursors, are of practical importance today from the standpoint of their occurrence in foods—ergocalciferol (vitamin D, calciferol, or viosterol) and cholecalciferol (vitamin D,) the name cholecalciferol of the latter is a reflection of its cholesterol precursor. Because these substances are closely related chemically, the term vitamin D is used collectively to indicate the group of substances that showthis vitamin activity. 

Calciferol is the anti-rachitic steroid obtained artificially. It was first regarded as being identical with vitamin D, and ergosterol was accepted as being the natural provitamin. However, in 1934, Waddell found that both irradiated cholesterol and cod-liver oil concentrates were more effective than calciferol, when given in equivalent dosage to rachitic chickens, and now it is believed that several different vitamins of the D type exist, including D, the anti-rachitic factor in natural liver oils, Dg, or calciferol, and Dg, the factor obtained from 7-dehydrocholesterol, which is its provitamin in crude cholesterol. 

Sterols.—Cholesterol is readily and preferentially absorbed, and circulates as an ester phytosterols are not absorbed, probably because the necessary esterifying system is absent from the intestinal mucosa. The D vitamins and the provitamin ergosterol are rapidly absorbed, but the potency of these compounds is so great that it is not easy to show how far their absorption is specific and quantitative. 

The provitamin D in animal skin had a side-chain that was identical to cholesterol, i.e., it did not contain either a double bond or methyl group on carbons 22-23 and 24, respectively, and was identified as 7-dehydrocholesterol (provitamin D3) (Figure 2). The vitamin Ds generated from ergosterol and 7-dehydrocholesterol were called ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3), respectively. 

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