Carotenoids

A number of carotenoids that have an unsubstituted f-ionone ring can be cleaved oxidatively to yield retinaldehyde, which is then reduced to retinol or oxidized to retinoic acid. Such compounds are known collectively as provitamin A carotenoids. Because relatively few foods are especially rich sources of retinol - and these are all animal foods - they are nutritionally important, accounting for a significant proportion of total vitamin A intake in most countries. Even in developed countries, with a relatively high intake of animal foods and fortified products, 25% to 30% of dietary vitamin A is derived from carotenoids; in developing countries, 80% or more of the potentially available vitamin A is from carotenoids. Only a small number of the several hundred carotenoid pigments found in plants have an unsubstituted f -ionone ring and are therefore capable of cleavage to yield retinaldehyde. The major dietary carotenoids are shown in Figure 2.2.

Carotenoids are classified in two ways:

1. Nutritionally, on the basis of whether or not they have an unsubstituted f -ionone ring and can therefore act as precursors of retinol (the provitamin A carotenoids)

2. On the basis of their chemistry, as:

(a) hydrophobic hydrocarbon carotenoids, of which a- and f -carotene and lycopene are the most important in human tissues; lycopene is not a precursor of vitamin A in mammals, although it is the acyclic precursor of f -carotene in plants

(b) monohydroxycarotenoids, of which f -cryptoxanthin is the most important in human tissues

(c) dihydroxycarotenoids, of which lutein (dihydroxy a-carotene) and zeaxanthin (dihydroxy f -carotene) are the most important in human tissues.

Figure 2.2. Major dietary carotenoids. a-, p-, and y-carotenes and p-cryptoxanthine have vitamin A activity; zeaxanthin, lutein, canthaxanthin, and lycopene do not. Relative molecular masses (Mr): a-, p-, and y-carotenes, 536.9; p-cryptoxanthine, 552.9; zeaxanthin, 568.9; lutein (xanthophyll), 568.85; canthaxanthin, 564.8; and lycopene, 536.9.

Figure 2.2. Major dietary carotenoids. a-, p-, and y-carotenes and p-cryptoxanthine have vitamin A activity; zeaxanthin, lutein, canthaxanthin, and lycopene do not. Relative molecular masses (Mr): a-, p-, and y-carotenes, 536.9; p-cryptoxanthine, 552.9; zeaxanthin, 568.9; lutein (xanthophyll), 568.85; canthaxanthin, 564.8; and lycopene, 536.9.

In plasma, the more hydrophobic carotenoids are deep within chylomicrons or very low-density lipoproteins, whereas the more polar hydroxycarotenoids are at the surface, and therefore potentially available for transfer between plasma lipoproteins and uptake into tissues.

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