Provitamin precursors

Occurrence. The provitamins, precursors of the vitamin Ds, are distributed widely in nature, whereas the vitamins themselves are less prevalent. The amounts of provitamins D2 and D in various plants and animals are Hsted in Table 2. 

Figure 6.8 Biosynthesis of biologically active vitamin D compounds from their provitamin precursors.

The reaction has been applied successfully to the synthesis of a precursor of provitamin D. 520, which has a homoannular conjugated diene in the B ring[340]. Treatment of the 7a-carbonate 518 with Pd cataJy.st at 40 C afforded the 5.7-diene 520 regioselcctively in good yield. No heteroannular diene 521... 

Biological, spectroscopic, and chromatographic methods have been used to assay vitamin A and the carotenoids. Biological methods have traditionally been based on the growth response of vitamin A—deficient rats. The utiUty and shortcomings of this test have been reviewed (52,53). This test has found apphcabiUty for analogues of retinol (54,55). Carotenoids that function as provitamin A precursors can also be assayed by this test (56). 

Many carotenoids function in humans as vitamin A precursors however, not all carotenoids have provitamin A activity (Table 3). Of the biologically active carotenoids, -carotene has the greatest activity. Despite the fact that theoretically one molecule of -carotene is a biological source of two molecules of vitamin A, this relationship is not observed and 6 p.g -carotene is equivalent to 1 p. vitamin A. Although -carotene and vitamin A have complementary activities, they caimot totally replace each other. Because the conversion of -carotene to vitamin A is highly regulated, toxic quantities of vitamin A cannot accumulate and -carotene can be considered as a safe form of vitamin A (8). 

Provitamin. The chemistry of the D vitamins is intimately involved with that of their precursors, the provitamins. The manufacture of the vitamins and their derivatives usually involves the synthesis of the provitamins, from which the vitamin is then generated by uv irradiation. The chemical and physical properties of the provitamins are discussed below, followed by the properties of the vitamins. 

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... 

In animals, the major function of carotenoids is as a precursor to the formation of vitamin A. Carotenoids with provitamin A activity are essential components of the human diet, and there is considerable evidence that they are absorbed through the diet and often metabohzed into other compounds. Beyond their important role as a source of vitamin A for humans, dietary carotenoids, including those that are not provitamin A carotenoids, have been implicated as protecting against certain forms of cancer and cardiovascular disease. ... 

The underlying mechanisms involved in the activities of carotenoid oxidation products are due either to a possible role as precursors of retinoids that would be the active species for positive effects or to their own specific activities. This latter case is illustrated by the activity of non-provitamin A carotenoid oxidation products such as those derived from lycopene. However, biological effects of carotenoid oxidation products other than retinoids are only hypothesized in vivo in humans, which hypothesis has been used as the basic principle to justify in vitro studies of these compounds. 

Apart from lutein and zeaxanthin, a number of other xanthophylls are present in the retina at low concentrations, including 3 -cpi-lutein, lactucaxanthin, 3 -dehydrolutein, and (3,(3-carotene-3,3 -dione (Khachik et al. 1997a, Bernstein et al. 2001). Interestingly, (3-carotene, the provitamin A precursor of the chromophore of the visual pigments, has only been identified in traces in the retina, if at all. 

Lipids are important components of the diet fatty acids are the higher energetic source as they ensure 9kcal/g. Furthermore, some peculiar fatty acids themselves and several components of the unsaponifiable fraction are biologically active molecules, as they can act as vitamins (tocopherols— vitamin E), provitamins (carotenes—vitamin A, cholecalcipherol—vitamin D), vitamin-like (essential fatty acids), and hormones or hormone precursors (sterols—steroidal hormones). 

PRECURSOR. In biological systems, an intermediate compound or molecular complex present in a living organism which, when activated physiochemically, is converted to a specific functional substance. Sometimes the prefix pro is used to indicate that a compound in question plays the role of a precursor. Examples from the history of vitamin and other essential chemical developments include ergosterol (pro-vitamin D2), which is activated by ultraviolet radiation to form vitamin D carotene (provitamin A) is a precursor of vitamin A prothrombin forms thrombin upon activation in the blood-clotting mechanism. 

PROVITAMIN. The precursor of a vitamin. Examples are carotene and ergosterol, which upon activation become Vitamin A and Vitamin D, respectively. 

In the biosynthesis of vitamin D substances, precursors include cholesterol (skin + ultraviolet radiation) in animals ergosterol (algae, yeast + ultraviolet radiation), Intermediates in the biosynthesis include preergocaldferol, tachysterol, and 7-dehydrocholesterol. Provitamins in very small quantities are generated in the leaves, seeds, and shoots of plants. In animals, the production site is the skin. Target tissues in animals are bone, intestine, kidney, and liver. Storage sites in animals are liver and skin. 

Among the several vitamins in this classification, only vitamin A is present in appreciable quantity as carotenoid provitamin A in citrus (38). No vitamin D has ever been reported in citrus nor any plant vitamin D precursors, such as ergosterol. Several of the sterols present in citrus fruits are reported (39, 40, 41), but they are not related to vitamin D. 

It is well-known that plants do not synthesize vitamin A. Also, animals can only synthesize vitamin A from p-carotene or carotenoids in which one-half of the molecule is like p-carotene. In nature, the vitamin A precursor comes either from plants or microorganisms. The most common sources of vitamin A in citrus are a- and p-carotenes and p-cryptoxanthin. In addition to the above name carotenoids, p-apo-8 -carotenal in citrus peel could be a source of provitamin A. However, the peel is not usually consumed. Provitamin A compounds are cleaved to form vitamin A aldehyde in the intestine by p-carotene 15,15 -oxygenase (Figure 4) (54). Aldehyde reductase reduces the aldehyde to the all trans-vitamin A. p-Carotene is cleaved between the 15,15 carbon... 

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. 

Occurrence in feeds Provitamins or precursors may be present No precursors known (except tryptophan can be converted to niacin)... 

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. 

Until recently it was thought that the sole function of carotenoids in animals was to act as precursors of vitamin A, the so-called provitamins A. It has been known... 

Vitamin D3 is not an essential exogenous micronutrient as such because it is made endogenously from a precursor in skin, 7-dehydrocholesterol (provitamin D3), by exposure to the high-energy ultraviolet B (UVB) photons (290-315 nm) of the solar spectrum [33]. The photons penetrate the epider-... 

In addition to its provitamin A role, p - carotene is a radical trapping antioxidant and may be nutritionally important in its own right both as an antioxidant and possibly also through direct actions that are independent of retinoids. Other carotenoids that occur in foods, and circulate in the bloodstream, also have free radical trapping activity, and, hence, potential metabolic significance, whether or not they are metabolic precursors of vitamin A. 

Last are Car precursors for important metabolites. Only three examples shall be given. The first example is retinal (Fig. 3), which is the chromophore of the visual pigment rhodopsin (23) and is derived from P,P -carotene. Because the latter cannot be synthesized by mammals, they need it to be supplied as provitamin A. Retinal derivatives are also required for other regulatory functions. The second example is abscisic acid (Fig. 3), which is the plant hormone involved in the shedding of leaves in fall and in fruit ripening it is derived from violaxanthin. Finally, certain fragrances of roses are not synthesized directly, but they are breakdown products of the flowers Cars. 

Cholesterol is also the precursor of vitamin D, which plays an essential role in the control of calcium and phosphorus metabolism. 7-Dehydrocholesterol (provitamin D is photolyzed by the ultraviolet light of sunlight to previtamin D, ... 

Vitamins are low molecular weight organic compounds required in small amounts in the diet. Most of the vitamins are not synthesized in the human body but are synthesized by the plants. Hence these essential nutrients are mainly obtained through the food. Though most of them are present in the diet as such, some are present as precursors known as provitamins. 

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