Carotenoids

The carotenoids are red, orange, and yellow pigments synthesized by all green plants and some microbes. They have an essential function in photosynthesis and in attracting the attention of animals. Several of these pigments also have an important nutritional function for animals and some of the familiar colors of animals are derived from plant or microbial carotenoids they consume.

Structure and Occurrence

Carotenoids are synthesized in the plastids of a plant cell and typically contain forty carbon atoms derived from eight subunits of the five-carbon compound, isoprene. Larger and smaller carotenoids do occur. Two categories of carotenoids occur in nature. These are the carotenes that contain only carbon and hydrogen, and the xanthophylls (also termed oxycarotenoids) that contain carbon, hydrogen, and oxygen atoms. Each carotenoid has its own distinctive color. Their chemical structure makes carotenoids very insoluble in water, but they are fat-soluble. Therefore they are usually associated with cell membranes and lipids, the primary water-insoluble component of cells. Some plant carotenoids occur as crystals in a protein matrix, and in some animals carotenoids occur with proteins. These animal carotenoproteins can be a very different color than their component carotenoids. For example, the carotenoprotein responsible for the distinctive blue color of some live lobsters breaks into a bright red carotenoid and colorless protein upon heating. Over six hundred carotenoids occur in plants, animals, and microbes. Since only plants and some microbes can synthesize carotenoids, those carotenoids in animals all come from their dietary sources. Typically several different carotenoids occur in plant tissue containing this class of pigments. For example, orange carrots contain at least six different carotenes that account for their color. All green leaves contain beta-carotene and three xanthophylls, lutein, neoxanthin, and violaxanthin. Most leaves also contain alpha-carotene and several other xanthophylls as well.

Both chlorophylls and carotenoids occur in all green leaves, but chlorophylls mask the carotenoids to the human eye. When the chlorophylls break down as leaves senesce (mature), the yellow and orange carotenoids persist and the leaves turn yellow.

Role in Plants

In the process of photosynthesis, potentially harmful oxidizing compounds are generated. The carotenoids occur in photosynthetic tissues along with chlorophyll to protect them from photooxidative damage. In fact, this protection is essential for photosynthesis. The photoprotective role of carotenoids is demonstrated in plant mutants that cannot synthesize essential leaf carotenoids. These mutants are lethal in nature since without carotenoids, chlorophylls degrade, their leaves are white in color, and photosynthesis cannot occur. Carotenoids also assist chlorophylls in harvesting light. Carotenoids absorb wavelengths of blue light that chlorophylls do not. The energy that carotenoids harvest in the blue range of the spectrum and transfer to chlorophyll contributes significantly to photosynthesis. The growth and development of plants is often stimulated by light, and carotenoids have sometimes been implicated as the photoreceptors of light to trigger these responses.

Outside of photosynthesis, plant carotenoids also serve as one of the pigments, along with anthocyanins and betalains, that provide color to flowers, ripening fruit, and other plant parts. Familiar examples of carotenoids having this role are found in sunflowers, marigolds, bananas, peaches, oranges, tomatoes, peppers, melons, and yellow corn. Two root crops, carrots and sweet potatoes, also acquire their color from carotenoids. These colors attract insects, birds, and bats for pollinating flowers, and they attract a wide range of animals to aid in the dispersal of seeds and fruits.

Role in Animals

Beyond their role in attracting animals, carotenoids are important nutrients and colorants for animals that consume them. Vitamin A is an essential nutrient for humans and animals, and all vitamin A ultimately comes from dietary carotenoids. A small subset of all carotenoids, including betacarotene, can be converted to vitamin A by animal metabolic systems. Animals cannot synthesize these provitamin A carotenoids or vitamin A itself from scratch, and humans around the world obtain about two-thirds of their vitamin A in this provitamin form from plants they eat. The rest is consumed as vitamin A from meat, eggs, and dairy products that ultimately came from carotenoids in the diets of these animal sources.

Animal products and tissues containing vitamin A, which has no color, often contain carotenoids as well. This is the case in egg yolk, butterfat, and the pink or red flesh color of certain fish, such as salmon and trout. Dietary carotenoids are readily visible in the striking and familiar colors of flamingoes, canaries, lobsters, prawn, goldfish, and ladybugs. Some of the coloration in human skin is also due to the carotenoids consumed. 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. Two carotenoids that appear to impart health benefits are lycopene and lutein. Lycopene is the red carotene that accounts for the typical color of tomatoes and watermelon. Lutein is a yellow xanthophyll common in all green leaves and responsible for much of the color of milk, butter, and egg yolks. The antioxidant properties that make carotenoids essential for photosynthesis may provide a similar type of protection in preventing human disease.

Commercial Importance

The color of food is an important variable contributing to its selection for consumption. Carotenoids in plant extracts such as red palm oil, saffron, annatto, and paprika have been used as food colors through much of history. More recently, industrially produced (synthetic) carotenoids have also been used as food color. Poultry, fish, and mammalian food animal diets also are frequently supplemented with natural or synthetic carotenoids to not only provide a dietary vitamin A source, but primarily to color meat and animal products and make them more appealing for consumers. Medicines and cosmetic products are often colored with carotenoids to enhance their appeal.

see also Pigments; Plastids.

Philipp W. Simon

Bibliography

Bauernfeind, J. Christopher, ed. Carotenoids as Colorants and Vitamin A Precursors. New York: Academic Press, 1981.

Britton, George, Synnove Liaaen-Jensen, and Hanspeter Pfander, eds. Carotenoids, Vol. 1. Basel: Birkhauser Verlag, 1995.

Packer, Lester, ed. Carotenoids, Methods in Enzymology, Vols. 213 and 214. New York:Academic Press, 1993.

Genetic engineering of rice may allow high expression of carotenoids. However, the normally white rice becomes yellow-orange in the process, which may not be appealing to consumers.