Of provitamins

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... 

The observation that the uv spectmm of provitamin D changed with uv inradiation and also produced antirachitic activity led to the conclusion that vitamin D was derived from the provitamin. Windaus found the vitamin D2 formula to be isomeric with the provitamins. 

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. 

Rosenheim reaction CHCI3 + lead tetraacetate in green fluorescence not given by esters of provitamin D can... 

POTRYKUS I (1997) Transgenic rice Oryza sativa) endosperm expressing daffodil Narcissuspseudonarcissus) phytoene synthase accumulates phytoene, a key intermediate of provitamin A biosynthesis , Plant J, 11, 1071-78. 

Rodriguez-Amaya, D.B., Carotenoids and Food Preparation The Retention of Provitamin A Carotenoids in Prepared, Processed, and Stored Foods, USAID/OMNI, Washington, D.C., 1997. 

In order to exhibit provitamin A activity, the carotenoid molecule must have at least one unsubstituted p-ionone ring and the correct number and position of methyl groups in the polyene chain. Compared to aU-trans P-carotene (100% provitamin A activity), a-carotene, P-cryptoxanthin, and y-carotene show 30 to 50% activity and cis isomers of P-carotene less than 10%. Vitamin A equivalence values of carotenoids from foods have been recently revised to higher ratio numbers (see Table 3.2.2) due to poorer bioavailability of provitamin A carotenoids from foods than previously thought when assessed with more recent and appropriate methods. 

Kiefer, C. et al.. Identification and characterization of a mammalian enzyme catalyzing the asymmetric oxidative cleavage of provitamin A, J. Biol. Chem., 276, 14110, 2001. 

Among thermal processes, canning caused the largest trans-to-cis isomerization of provitamin A carotenoids, increasing the total cis isomers by 39% for sweet potatoes, 33% for carrots, 19% for collards, 18% for tomatoes, and 10% for peaches 13-di-P-carotene was the isomer formed in highest amonnts. ... 

Losses of 45 to 48% in the P-carotene contents and formation of cis isomers were also verified by pasteurization of carrot juice at 110 and 120°C for 30 sec. No significant effects on trans-to-cis isomerization of a- and P-carotene isomers were observed after acidification and heating of carrot juice at 105°C for 25 sec. In addition, an increase of only 3% in the cis isomers of provitamin A carotenoids was observed after orange juice pasteurization. " ... 

Lessin, W.J., Catigani, G.L., and Schwartz, S.J., Quantification of ds-trans isomers of provitamin A carotenoids in fresh and processed fruits and vegetables, J. Agric. Food Chem., 45, 3728, 1997. 

Godoy, H.T. and Rodriguez-Amaya, D.B., Occurrence of cA-isomers of provitamin A in Brazilian fruits, J. Agric. Food Chem., 42, 1306, 1994. 

During, A, and Harrison, EH, 2007. Mechanisms of provitamin A (carotenoid) and vitamin A (retinol) transport into and out of intestinal Caco-2 cells. J Lipid Res 48, 2283-2294. 

Hulshof PJM, Xu C, van de Bovenkamp P, Muhilal and West CE. 1997. Application of a validated method for the determination of provitamin A carotenoids in Indonesian foods of different maturity and origin. J Agric Food Chem 45 1174-1179. 

Carotene content is a critical factor for several fruits and vegetables such as carrots, because consumers consider this vegetable as a major single source of provitamin A, providing 17% of the total vitamin A consumption (Barry-Ryan and O Beirne 2000). Recently, the demand for carotenoids, especially (3-carotene, has increased because of its health benefits (Ruiz-Cruz and others 2007). 

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. 

Vegetables are the major sources of provitamin A carotenoids, followed by fruits. Milk products, egg yolk, shellfish, and Crustacea also contain active carotenoids, which are derived from the animal s diet. The carotenoid distribution in green leafy vegetables is generally constant, whereas fruits contain a greater variety of carotenoids in varying concentrations. In ripening fruits, the decrease in chlorophylls is frequently accompanied by an increase in the concentration of carotenoids and an increase in the ratio of carotenes to xanthophylls. 

Vitamin D is represented by cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2), which are structurally similar secosteroids derived from the UV irradiation of provitamin D sterols. In vertebrates, vitamin D3 is produced in vivo by the action of sunlight on 7-dehydrocholesterol in the skin. Vitamin D2 is produced in plants, fungi, and yeasts by the irradiation of ergosterol. On irradiation, the provitamins are converted to previtamin D, which undergoes thermal transformation to vitamin D. 

To assess the effects of processing on the nutritional value of a plant food with respect to vitamin A activity, the various isomeric forms of provitamin A carotenoids present in both the fresh and processed states must be accurately measured. In such investigations it must be demonstrated that the analytical procedure does not itself cause trans-cis isomerization of carotenoids. 

Table 9 HPLC Methods Used for the Determination of Provitamin A Carotenoids in Food... 

DB Rodriguez-Amaya. Critical review of provitamin A determination in plant foods. J Micronutr Anal 5 191-225, 1989. 

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