Carotenoid is any of a large class of over 600 organic pigments, including the carotenes and xanthophylls, that are terpenoids (typically tetraterpenoids, derived from 8 five-carbon isoprene units), structured in the form of a polyene chain (properties), widely distributed in nature, and commonly imparting yellow, orange, red, or purple colors. Generally, they are fat-soluble, dissolving in fats and oils but not water, except when complexed with proteins. In plants, they naturally occur in chromoplasts, imparting color to fruits and vegetables, such as carrots, pumpkins, sweet potatoes, and tomatoes. They also are found in some other photosynthetic organisms like algae, some types of fungus, and some bacteria.
In animals such as crustaceans, nudibranches, and echinoderms, carotenoprotein complexes give red, purple, green, blue, and other colors. Animals obtain carotenoids through the diet. For example, the pink color of flamingos and salmon, and the red coloring of lobsters, are due to carotenoids obtained through the diet.
Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage (Armstrong and Hearst 1996). In humans, carotenoids such as beta-carotene are a precursor to vitamin A, a pigment essential for good vision, and carotenoids can also act as antioxidants (Sims and Odle 2005).
During autumn in temperate zones, when deciduous trees stop producing chlorophyll in preparation for winter, the orange, yellow, and red colors of carotenoids are revealed as the green color fades, providing beautiful fall foliage.
Carotenoids are a type of terpenoid (sometimes referred to as isoprenoids), which are hydrocarbons resulting from the combination of several five-carbon isoprene units. Specifically, they are tetraterpenoids, which means they are derived from 8 isoprene units—meaning they typically contain 40 carbon atoms. Structurally they are in the form of a polyene chain that is sometimes terminated by rings. Polyenes are poly-unsaturated organic compounds that contain one or more sequences of alternating double and single carbon-carbon bonds. These double carbon-carbon bonds interact in a process known as conjugation, which results in an overall lower energy state of the molecule. Carotenoids are mainly aliphatic and aliphatic-alicyclic polyenes, with a few aromatic polyenes (McGraw-Hill 2005). (Aromatic compounds contain benzene rings or similar rings of atoms, while aliphatic compounds, such as fat and oil, do not contain aromatic rings; alicyclic are organic compounds that are both aliphatic and cyclic.) Carotenoids absorb blue light.
The color of carotenoids, ranging from pale yellow through bright orange to deep red, is directly linked to their structure. The double carbon-carbon bonds interact with each other in the process of conjugation, which allows electrons in the molecule to move freely across these areas of the molecule. As the number of double bonds increases, electrons associated with conjugated systems have more room to move, and require less energy to change states. This causes the range of energies of light absorbed by the molecule to decrease. As more frequencies of light are absorbed from the short end of the visible spectrum, the compounds acquire an increasingly red appearance.
Carotenoids include two small classes of pigments, xanthophylls and carotenes. Carotenes typically contain only carbon and hydrogen. The unoxygenated (oxygen free) carotenoids such as alpha-carotene, beta-carotene, and lycopene are well-known carotenes. Xanthophylls are carotenoids with molecules containing oxygen. Lutein, zeaxanthin, cryptoxanthin, and astaxanthin are well-known xanthophylls. Xanthophylls are often yellow, hence their class name.
Probably the most well-known and well-studied carotenoid is the one that gives the first group its name, beta-carotene, found in carrots and responsible for their bright orange color. It also is found in pumpkins, peaches, and sweet potatoes (Sims and Odle 2005). Crude palm oil, however, is the richest source of carotenoids in nature (May 2008). Beta-carotene is the primary precursor (provitamin A carotenoid) to vitamin A (Sims and Odle 2005). The body can split one molecule of beta-carotene into two vitamin A molecules (Sims and Odle 2005).
Lycopene also is common and is considered the most common carotenoid in the U.S. diet because it is found in tomato products (Sims and Odle 2005). It does not produce vitamin A. In plants, the xanthophyll lutein also is very common and its role in preventing age-related eye disease is currently under investigation. Lutein and the other carotenoid pigments found in leaves are not obvious because of the presence of other pigments such as chlorophyll. Lutein and zeaxantin are found in kale, spinach, corn, alfalfa, broccoli, and egg yolks (Sims and Odle 2005).
Carotenoids can have many classifications. Some are alcohols, ethers, epoxides, ketones, acids, and so forth. They can be classified also into Apo Carotenoids, Nor and Seco Carotenoids, retro Carotenoids, retro Apo Carotenoids, and Higher Carotenoids.
Carotenoids have many physiological functions. Carotenoids appear to be used by plants to protect their leaves and stems from damage from the sun and for photosynthesis. In animals, they also can protect cells from damage from free radicals and for ornamental features and enhancing the vertebrate immune system.
In photosynthetic organisms, carotenoids play a vital role in the photosynthetic reaction center. One the one hand, they may participate in the energy-transfer process. On the other hand, they can protect the reaction center from auto-oxidation. Ultraviolet wavelengths are able to generate free radicals (unstable or highly reactive molecules) that can damage living cells and cartenoids act as antioxidants by donating electrons to neutralize the oxidant radicals (Sims and Odle 2005).
In non-photosynthesizing organisms, carotenoids have been linked to oxidation-preventing mechanisms.
Animals are incapable of synthesizing carotenoids, and must obtain them through their diet, yet they are common and often in ornamental features. It has been proposed that carotenoids are used in ornamental traits because, given their physiological and chemical properties, they can be used as honest indicators of individual health, and hence they can be used by animals when selecting potential mates.
Despite being important in nutrition, some carotenoids are produced by bacteria to protect themselves from immune attack, such as MRSA. The golden pigment of S. aureus allows it to survive competitive attack by Lactobaccillus as well as the human immune system (Liu et al. 2005).
In humans, carotenoids act as antioxidants to protect cells from the danger of free radicals. Such free radicals may be produced during metabolism or by pollution, cigarette smoke, sunlight, radiation, and stress. Every second, tens of thousands are created in the body, and when a free radical captures an electron from a molecule, a new free radical is produced as the second molecule now seeks to capture an electron, leading to a chain reaction that can damage DNA, fats, and proteins. Carotenoids, vitamins A and C, and lipoic acid are some of the antioxidants that help to quench the body of free radical reactions (Sims and Odle 2005).
Consequently, epidemiological studies have shown that people with high beta-carotene intake and high plasma levels of beta-carotene have a significantly reduced risk of lung cancer. However, studies of supplementation with large doses of beta-carotene in smokers have shown an increase in cancer risk (possibly because excessive beta-carotene results in breakdown products that reduce plasma vitamin A and worsen the lung cell proliferation induced by smoke (Alija et al. 2004). Similar results have been found in other animals. Not all carotenoids are helpful, for example, etretinate is a teratogen.
People consuming diets rich in carotenoids from natural foods, such as fruits and vegetables, are considered to be healthier and have lower mortality from a number of chronic illnesses. However, a recent meta-analysis of 68 reliable antioxidant supplementation experiments involving a total of 232,606 individuals concluded that consuming additional beta-carotene from supplements is unlikely to be beneficial and may actually be harmful (Bjelakovic et al. 2007), although this conclusion may be due to the inclusion of studies involving smokers. It is known that taking beta-carotene supplements is harmful for smokers, and the meta-analysis of Bjelakovic et al. (2007) was influenced by inclusion of these studies.
Since most carotenoid-rich fruits and vegetables are low in lipids and since dietary lipids have been hypothesized to be an important factor for carotenoid bioavailability, a 2005 study investigated whether addition of avocado fruit or oil, as lipid sources, would enhance carotenoid absorption in humans. The study found that the addition of both avocado fruit and oil significantly enhanced the subjects' absorption of all carotenoids tested (alpha-carotene, beta-carotene, lycopene, and lutein) (Unlu et al. 2005).
Products of carotenoid degradation such as ionones, damascones, and damascenones are also important fragrance chemicals that are used extensively in the perfumes and fragrance industry. Both beta-damascenone and beta-ionone, although low in concentration in rose distillates, are the key odor-contributing compounds in flowers. In fact, the sweet floral smells present in black tea, aged tobacco, grape, and many fruits are due to the aromatic compounds resulting from carotenoid breakdown.
5',6'-dihydro-5'-apo-18'-nor-b-caroten-6'-one or 6'-methyl-6'-apo-b-caroten-6'-one
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