Provitamin A activity

An important function of certain carotenoids is their provitamin A activity. Vitamin A may be considered as having the stmcture of half of the P-carotene molecule with a molecule of water added at the end position. In general, all carotenoids containing a single unsubstituted P carotene half have provitamin A activity, but only about half the activity of P carotene. Provitamin A compounds are converted to Vitamin A by an oxidative enzyme system present in the intestinal mucosa of animals and humans. This conversion apparendy does not occur in plants (see Vitamins, VITAMIN a). 

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). 

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. ... 

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. 

Heinonen, M.I., Carotenoids and provitamin A activity of carrot (Daucus carota L.) cultivars, J. Agric. Food Chem., 38, 609,1990. 

Ahneida, L.B. and Penteado, M.V.C., Carotenoids with provitamin A activity of carrots Daucus carota L.) consumed in Sao Paulo, Brazil, Rev. Farm. Bioquim. Univ. S. Paulo, 23, 133, 1987. 

Structurally, vitamin A (retinol) is essentially one half of the molecule of (3-carotene. Thus, (3-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted (3 ring with a Cn polyene chain is the minimum requirement for vitamin A activity, y -Carotene, a-carotene, (3-cryptoxanthin, a-cryptoxanthin, and (3-carotene 5,6-epoxide, all having one unsubstituted ring, have about half the bioactivity of (3-carotene (Table7.4) On the other hand, the acyclic carotenoids, devoid of (3-rings, and the xanthophylls, in which the (3-rings have hydroxy, epoxy, and carbonyl substituents, are not provitamin A-active for humans. 

Retinol (vitamin A) is found in foods of mammalian origin in the form of retinyl ester, or in fruits and vegetables as carotenoids with provitamin A activity, especially P-carotene (provitamin A). In enterocytes, retinol binds to cellular retinol-binding protein type II (CRBPII), which directs the esterification by the enzyme lecithin retinol acyltransferase (LRAT). 

Table 1 Chemical Nomenclature and Provitamin A Activity of Some Common Carotenoids of Plant Foods...

Awareness of lycopene as a potentially beneficial carotenoid in the nutritional and medical sciences is less than 10 years old. Therefore, comparatively few scientific data are available from animal and human studies. But in this short time, a number of features of lycopene have emerged that are unique among the common carotenoids, quite apart from its exceptionally high singlet oxygen quenching capacity in vitro. The most obvious distinction is that unlike the other hydrocarbon carotenoids, a- and P-carotene, lycopene has no provitamin A activity. 

Since lycopene lacks the (3-ionic ring structure, unlike (3-carotene, it lacks provitamin A activity. The biological activity of lycopene is thought to be primarily due to its antioxidant properties. However, other mechanisms, such as facilitating gap junction communication (GJC) (Aust et al, 2003 Heber, 2002 Wertz et al, 2004 Zhang et al, 1991, 1992), stimulation of the immune system (Chew and Park, 2004 Heber, 2002 Heber and Lu, 2002 Kim et al, 2004 Wertz et al, 2004), endocrine-mediated pathways... 

The biological activities of carotenoids, such as (3-carotene, are related to their provitamin A activity within the body (Clinton, 1998). Since lycopene lacks the (3-ionic ring structure, it does not have any provitamin A activity (Stahl and Sies, 1996). The biological effects of lycopene in humans have therefore been attributed to mechanisms other than vitamin A. Two major hypotheses have been proposed to explain the anticarcinogenic and antiatherogenic activities of lycopene oxidative and nonoxidative mechanisms. The proposed mechanisms for the role of lycopene in the prevention of chronic diseases are summarized in Figure 6. 

In chilli, 95% of the total provitamin A is in green pod and (3-carotene accounts for 93% in mature red pods. When mature red pods were measured, the cultivars with the highest and the lowest provitamin A activity were both yellow max pod types. In the matured red pods, the a-, (3-carotene and provitamin A activity increased by 344, 255 and 229%, respectively, as the pods matured. 

Although there is some evidence that 8-carotene and other carotenoids may have actions in their own right, apart from their provitamin A activity (Section 2.6.3), there is no evidence on which to base any recommendations... 

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