Carotene
The term carotene (also carotin, from the Latin carota, "carrot"
Carotenes are responsible for the orange colour of the
, as a result of the carotenoid retention from the vegetable portion of their diet.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.
Animal species differ greatly in their ability to convert retinyl (beta-ionone) 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 (resulting in carotenes not being a form of vitamin A for this species); while cats can convert a trace of β-carotene to retinol, although the amount is totally insufficient for meeting their daily retinol needs.[4]
Molecular structure
Carotenes are polyunsaturated
Carotenes are found in plants in two primary forms designated by characters from the
History
The discovery of carotene from carrot juice is credited to
Dietary sources
The following foods contain carotenes in notable amounts:[6]
Absorption from these foods is enhanced if eaten with fats, as carotenes are fat soluble, and if the food is cooked for a few minutes until the plant cell wall splits and the color is released into any liquid.[6] 12 μg of dietary β-carotene supplies the equivalent of 1 μg of retinol, and 24 µg of α-carotene or β-cryptoxanthin provides the equivalent of 1 µg of retinol.[6][8]
Forms of carotene
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The two primary isomers of carotene, α-carotene and β-carotene, differ in the position of a double bond (and thus a hydrogen) in the cyclic group at one end (the right end in the diagram at right).
Carotene protects plant cells against the destructive effects of ultraviolet light so β-carotene is an antioxidant.
β-Carotene and physiology
β-Carotene and cancer
An article on the
The New England Journal of Medicine published an article[12] in 1994 about a trial which examined the relationship between daily supplementation of β-carotene and vitamin E (α-tocopherol) 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[13][14] about a trial with a goal to determine if vitamin A (in the form of retinyl palmitate) and β-carotene (at about 30 mg/day, which is 10 times the Reference Daily Intake) 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.[14]
A review of all randomized controlled trials in the scientific literature by the
β-Carotene and photosensitivity
Oral β-carotene is prescribed to people suffering from erythropoietic protoporphyria. It provides them some relief from photosensitivity.[17]
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 (carotenodermia), but not the
Production
Carotenes are produced in a general manner for other terpenoids and terpenes, i.e. by coupling, cyclization, and oxygenation reactions of isoprene derivatives. Lycopene is the key precursor to carotenoids. It is formed by coupling of geranylgeranyl pyrophosphate and geranyllinally pyrophosphate.[18]
Most of the world's synthetic supply of carotene comes from a manufacturing complex located in
Carotenes are also found in palm oil, corn, and in the milk of dairy cows,[20] causing cow's milk to be light yellow, depending on the feed of the cattle, and the amount of fat in the milk (high-fat milks, such as those produced by Guernsey cows, tend to be yellower because their fat content causes them to contain more carotene).
Carotenes are also found in some species of termites, where they apparently have been picked up from the diet of the insects.[21]
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:[22][23]
The second is a
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 (with one β ring and one uncyclized end that is labelled psi) is β,ψ-carotene;
- δ-carotene (with one ε ring and one uncyclized end) is ε,ψ-carotene;
- ε-carotene is ε,ε-carotene
- lycopene is ψ,ψ-carotene
ζ-Carotene is the biosynthetic precursor of neurosporene, which is the precursor of lycopene, which, in turn, is the precursor of the carotenes α through ε.
Food additive
Carotene is used to colour products such as juice, cakes, desserts, butter and margarine.[3] It is approved for use as a food additive in the EU (listed as additive E160a)[25] Australia and New Zealand (listed as 160a)[26] and the US.[27]
See also
References
- ^ Mosby’s Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby-Year Book 1994, p. 273
- ^ "carotene". Online Etymology Dictionary.
- ^ ISBN 978-0471238966.
- PMID 21797934.
- ^ Theodore L. Sourkes, "The Discovery and Early History of Carotene," http://acshist.scs.illinois.edu/bulletin_open_access/v34-1/v34-1%20p32-38.pdf
- ^ a b c d e f g h i j k l m n o "Carotenoids". Micronutrient Information Center, Linus Pauling Institute, Oregon State University. 1 August 2016. Retrieved 19 August 2019.
- PMID 18486400.
- ^ a b c d "Vitamin A: Fact Sheet for Health Professionals". Office of Dietary Supplements, US National Institutes of Health. 9 July 2019. Retrieved 19 August 2019.
- PMID 23931131.
- .
- ^ "British Cancer Organization Calls for Warning Labels on Beta-Carotene". 2000-07-31. Archived from the original on 2006-12-04. Retrieved 2007-03-15.
- PMID 8127329.
- PMID 8901853.
- ^ PMID 8602180.
- PMID 17327526.
- ^
See the letter to JAMA by Philip Taylor and Sanford Dawsey and the replyby the authors of the original paper.
- PMID 900968.
- ISBN 0471238961.
- ISBN 9780128173145.
- PMID 28542353.
- ISBN 9780323144582.
- ^ Wittig G.; Pommer H.: DBP 954247, 1956
- ^ Wittig G.; Pommer H. (1959). Chem. Abstr. 53: 2279
- ^ US patent 2609396, Inhoffen Hans Herloff & Pommer Horst, "Compounds with the carbon skeleton of beta-carotene and process for the manufacture thereof", published 1952-09-02
- ^ UK Food Standards Agency: "Current EU approved additives and their E Numbers". Retrieved 2011-10-27.
- ^ Australia New Zealand Food Standards Code"Standard 1.2.4 – Labelling of ingredients". 8 September 2011. Retrieved 2014-12-22.
- ^ US FDA: "Food Additive Status List". Food and Drug Administration. Retrieved 2014-12-22.
External links
- Carotene at the U.S. National Library of Medicine Medical Subject Headings (MeSH)