Apocarotenoids vitamin

Davies (1976) has reviewed the new system of nomenclature of carotenoids. Figure 1 shows the basic structure and numbering system for (3-, a-, and y-carotenes and for cryptoxanthin, the major carotenoids contributing to the vitamin A activity in animal and human foods. Apocarotenoids are compounds that have been shortened by removal of at least one end of the molecule beyond a designated location. In general, they have less biological activity than P-carotene (see Table I). 

On the basis of structure alone, Simpson and Chichester (1981) estimate that 50-60 carotenoids and apocarotenoid compounds could have provitamin A activity. Only a few of the identified carotenoids, however, have both vitamin A activity and occur in significant amounts in natural foods as commonly eaten by vertebrate animals (Bauemfeind, 1972). Of these, p-carotene has by far the most provitamin A activity. The a- and y-carotenoids and cryptoxanthin (3-hy-droxy-P-carotene), also found in substantial amounts in commonly consumed foods, have about one-half the activity of the P form. Table I contains a list of the provitamin A activity found in the most important mammalian food sources of carotenoids. Table II lists vitamin A-inactive carotenoids commonly found in food. 

The basic and most important biologically active apocarotenoid in animal tissues is aU-trans-retinol also known as axeroftol or vitamin Aj (5-1). Retinol is an isoprenoid with 20 carbon atoms and five conjugated double bonds in the molecule, more precisely a diterpenic ahcychc alcohol with the so- called fl-ionone ring and a side chain of four conjugated double bonds attached to C-6. It is one of 16 possible stereoisomers. 

Apocarotenoids exhibit different biological functions. The most important apocarotenoid is vitamin Aj (all-frans-retinol). 

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