Carotene
The term carotene is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals. Carotenes are photosynthetic pigments important for photosynthesis. Carotenes contain no oxygen atoms. They absorb ultraviolet, violet, and blue light and scatter orange or red light, and yellow light.
Carotenes are responsible for the orange colour of the carrot, for which this class of chemicals is named, and for the colours of many other fruits, vegetables and fungi. Carotenes are also responsible for the orange colours in dry foliage. They also impart the yellow coloration to milk-fat and butter. Omnivorous animal species which are relatively poor converters of coloured dietary carotenoids to colourless retinoids have yellowed-coloured body fat, as a result of the carotenoid retention from the vegetable portion of their diet. The typical yellow-coloured fat of humans and chickens is a result of fat storage of carotenes from their diets.
Carotenes contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll. They also protect plant tissues by helping to absorb the energy from singlet oxygen, an excited form of the oxygen molecule O2 which is formed during photosynthesis.
β-Carotene is composed of two retinyl groups, and is broken down in the mucosa of the human small intestine by β-carotene 15,15'-monooxygenase to retinal, a form of vitamin A. β-Carotene can be stored in the liver and body fat and converted to retinal as needed, thus making it a form of vitamin A for humans and some other mammals. The carotenes α-carotene and γ-carotene, due to their single retinyl group, also have some vitamin A activity, as does the xanthophyll carotenoid β-cryptoxanthin. All other carotenoids, including lycopene, have no beta-ring and thus no vitamin A activity.
Animal species differ greatly in their ability to convert retinyl containing carotenoids to retinals. Carnivores in general are poor converters of dietary ionone-containing carotenoids. Pure carnivores such as ferrets lack β-carotene 15,15'-monooxygenase and cannot convert any carotenoids to retinals at all ; while cats can convert a trace of β-carotene to retinol, although the amount is totally insufficient for meeting their daily retinol needs.
Molecular structure
Chemically, carotenes are polyunsaturated hydrocarbons containing 40 carbon atoms per molecule, variable numbers of hydrogen atoms, and no other elements. Some carotenes are terminated by hydrocarbon rings, on one or both ends of the molecule. All are coloured to the human eye, due to extensive systems of conjugated double bonds. Structurally carotenes are tetraterpenes, meaning that they are synthesized biochemically from four 10-carbon terpene units, which in turn are formed from eight 5-carbon isoprene units.Carotenes are found in plants in two primary forms designated by characters from the Greek alphabet: alpha-carotene and beta-carotene. Gamma-, delta-, epsilon-, and zeta-carotene also exist. Since they are hydrocarbons, and therefore contain no oxygen, carotenes are fat-soluble and insoluble in water.
Dietary sources
The following foods contain carotenes in appreciable amounts:- carrots
- wolfberries
- cantaloupe
- mangoes
- red bell pepper
- papaya
- spinach
- kale
- sweet potato
- tomato
- dandelion greens
- broccoli
- collard greens
- winter squash
- pumpkin
- cassava
Forms of carotene
The two primary isomers of carotene, α-carotene and β-carotene, differ in the position of a double bond in the cyclic group at one end.β-Carotene is the more common form and can be found in yellow, orange, and green leafy fruits and vegetables. As a rule of thumb, the greater the intensity of the orange colour of the fruit or vegetable, the more β-carotene it contains.
Carotene protects plant cells against the destructive effects of ultraviolet light. β-Carotene is an antioxidant.
β-Carotene and physiology
β-Carotene and cancer
An article on the American Cancer Society says that The Cancer Research Campaign has called for warning labels on β-carotene supplements to caution smokers that such supplements may increase the risk of lung cancer.The New England Journal of Medicine published an article in 1994 about a trial which examined the relationship between daily supplementation of β-carotene and vitamin E and the incidence of lung cancer. The study was done using supplements and researchers were aware of the epidemiological correlation between carotenoid-rich fruits and vegetables and lower lung cancer rates. The research concluded that no reduction in lung cancer was found in the participants using these supplements, and furthermore, these supplements may, in fact, have harmful effects.
The Journal of the National Cancer Institute and The New England Journal of Medicine published articles in 1996 about a trial with a goal to determine if vitamin A and β-carotene supplements had any beneficial effects to prevent cancer. The results indicated an increased risk of lung and prostate cancers for the participants who consumed the β-carotene supplement and who had lung irritation from smoking or asbestos exposure, causing the trial to be stopped early.
A review of all randomized controlled trials in the scientific literature by the Cochrane Collaboration published in JAMA in 2007 found that synthetic β-carotene increased mortality by 1-8 %. However, this meta-analysis included two large studies of smokers, so it is not clear that the results apply to the general population. The review only studied the influence of synthetic antioxidants and the results should not be translated to potential effects of fruits and vegetables.
β-Carotene and cognition
A recent report demonstrated that 50 mg of β-carotene every other day prevented cognitive decline in a study of over 4000 physicians at a mean treatment duration of 18 years.β-Carotene and photosensitivity
Oral β-carotene is prescribed to people suffering from erythropoietic protoporphyria. It provides them some relief from photosensitivity.Carotenemia
Carotenemia or hypercarotenemia is excess carotene, but unlike excess vitamin A, carotene is non-toxic. Although hypercarotenemia is not particularly dangerous, it can lead to an oranging of the skin, but not the conjunctiva of eyes. It is most commonly associated with consumption of an abundance of carrots, but it also can be a medical sign of more dangerous conditions.β-Carotene and nanotechnology
β-Carotene and lycopene molecules can be encapsulated into carbon nanotubes enhancing the optical properties of carbon nanotubes. Efficient energy transfer occurs between the encapsulated dye and nanotube — light is absorbed by the dye and without significant loss is transferred to the single wall carbon nanotube. Encapsulation increases chemical and thermal stability of carotene molecules; it also allows their isolation and individual characterization.Production
Most of the world's synthetic supply of carotene comes from a manufacturing complex located in Freeport, Texas and owned by DSM. The other major supplier BASF also uses a chemical process to produce β-carotene. Together these suppliers account for about 85% of the β-carotene on the market. In Spain Vitatene produces natural β-carotene from fungus Blakeslea trispora, as does DSM but at much lower amount when compared to its synthetic β-carotene operation. In Australia, organic β-carotene is produced by Aquacarotene Limited from dried marine algae Dunaliella salina grown in harvesting ponds situated in Karratha, Western Australia. BASF Australia is also producing β-carotene from microalgae grown in two sites in Australia that are the world's largest algae farms. In Portugal, the industrial biotechnology company Biotrend is producing natural all-trans-β-carotene from a non-genetically-modified bacteria of the genus Sphingomonas isolated from soil.Carotenes are also found in palm oil, corn, and in the milk of dairy cows, causing cow's milk to be light yellow, depending on the feed of the cattle, and the amount of fat in the milk.
Carotenes are also found in some species of termites, where they apparently have been picked up from the diet of the insects.
Total synthesis
There are currently two commonly used methods of total synthesis of β-carotene. The first was developed by BASF and is based on the Wittig reaction with Wittig himself as patent holder:The second is a Grignard reaction, elaborated by Hoffman-La Roche from the original synthesis of Inhoffen et al. They are both symmetrical; the BASF synthesis is C20 + C20, and the Hoffman-La Roche synthesis is C19 + C2 + C19.
Nomenclature
Carotenes are carotenoids containing no oxygen. Carotenoids containing some oxygen are known as xanthophylls.The two ends of the β-carotene molecule are structurally identical, and are called β-rings. Specifically, the group of nine carbon atoms at each end form a β-ring.
The α-carotene molecule has a β-ring at one end; the other end is called an ε-ring. There is no such thing as an "α-ring".
These and similar names for the ends of the carotenoid molecules form the basis of a systematic naming scheme, according to which:
- α-carotene is β,ε-carotene;
- β-carotene is β,β-carotene;
- γ-carotene is β,ψ-carotene;
- δ-carotene is ε,ψ-carotene;
- ε-carotene is ε,ε-carotene
- lycopene is ψ,ψ-carotene