Toxicity, carotenoids vitamin

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). 

Carotenoids have two general characteristics of importance to the food iadustry they are not pH sensitive ia the normal 2—7 range found ia foods, and they are not affected by vitamin C, making them especially important for beverages. They are more expensive than synthetic food dyes and have a limited color range. In their natural environment they are quite stable, but they become more labile when heated or when they are ia solution. Under those conditions, there is a tendency for the trans-double bonds to isomerize to the cis-stmcture with a subsequent loss of color iatensity. The results of controlled tolerance and toxicity tests, usiag pure carotenoids, iadicate that they are perfecdy safe as food colors (132). 

Crocetin (C20) is a yellow (IP)2—(PI)2-derived dicarboxylic, acid (generalized structure -OOG-Gig-GOO-) from the styles of Crocus sativus (Iridaceae) (the saffron of Indian cooking and Buddhist robes). Grocin, the digentiobiose ester of crocetin, is water soluble, unlike other carotenoids which are lipophilic (fat soluble). Crocetin is a protein kinase inhibitor. Excess vitamin A (or excess pro-vitamin A) ingestion is toxic (dog liver consumption having caused the death of Sir Douglas Mawson s explorer companions in the Antarctic by this mechanism). 

Preformed vitamin A is found only in animals and a small number of bacteria. A number of the carotenoid pigments in plants can be cleaved oxidatively to yield retinol /S-carotene is quantitatively the most important of these provitamin A carotenoids. Although preformed retinol is both acutely and chronically toxic in excess, carotene is not, because there is only a limited capacity to cleave it to retinol. 

Many medicinal and food plants contain large amounts of antioxidants other than vitamin C, vitamin E, and carotenoids. The antioxidative effects are mainly due to phenolic compounds phenolic acids, flavonoids, and phenolic diterpenes. These namral antioxidants can exert considerable protection, in humans, against aging and cancer caused by free radicals, and can replace synthetic antioxidants such as butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), which are suspected to have toxic and carcinogenic effects on humans. 

P, P-carotenoid astaxanthin (Table 8). p,p-Carotene and some other carotenoids are vitamin A active as they are degraded to vitamin A aldehyde in the human and mammal organism trans-cis isomerization of vitamin A aldehyde bound to the protein opsin in rhodopsin in the retina of the eyes is the key step of the visual process (section 4.2). As non-toxic natural compounds giving no cause for concern, P,P-carotene and some other carotenoids are used as coloring agents for foods and cosmetics, and/or as vitamin A precursors and antioxidants in medicines. 

There is no evidence that the carotenoids, as such, exhibit toxic effects. Loading the patient with carotenoids does not elevate sertun vitamin A it may even lower vitamin A to some extent. Excess carotenoid ingestion does cause yellow pigmentation (xanthosis) in children. 

The parent compound in the vitamin A group is called sdl-trans retinol (Fig. lA) [4]. Its aldehyde and acid forms are retinal (Fig. IB) and retinoic acid (Fig. 1C). The active form of vitamin A in vision is ll-cis retinal (Fig. ID), and a therapeutically useful form (accutane, isotretinoin) is l3-cis retinoic acid (Fig. IE). Retinyl palmitate (Fig. IF) is a major storage form, and retinoyl p-glucuronide is a biologically active, relatively non-toxic water-soluble metabolite (Fig. IG). A synthetic aromatic analog (etretin, acitretin), shows therapeutic usefulness (Fig. IH). Finally, p-carotene, a major provitamin A carotenoid, is shown in Figure II. 

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