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Carotenoids  

2016-09-21 17:15:06|  分类: 博览群书 |  标签: |举报 |字号 订阅

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Carotenoids are familiar to all of us through the orange-red colours of popular foods like oranges, tomatoes and carrots, and the yellow colours of many flowers.

They are also added as colorants to many manufactured foods, drinks and animal feeds, either in the form of natural extracts (e.g annatto) or as pure compounds manufactured by chemical synthesis. Carotenoids are essential to plants for photosynthesis, acting in light-harvesting and, especially, in protection against destructive photooxidation. Without carotenoids photosynthesis in an oxygenic atmosphere would be impossible.

But carotenoids are not simply pigments of terrestrial plants. They occur widely in bacteria, fungi and algae, where they can be useful taxonomic markers. The production of carotenoids in seaweed runs to hundreds of million tons per year.

Some animals use carotenoids for colouration, especially birds (yellow and red feathers), fish (e.g. goldfish and salmon) and a wide variety of invertebrate animals, where complexation with protein may modify their colour to blue, green, purple etc.

Carotenoids are important factors in human health. The essential role of beta-carotene and others as the main dietary source of vitamin A has been known for many years. More recently, protective effects of carotenoids against serious disorders such as cancer, heart disease and degenerative eye disease have been recognized, and have stimulated intensive research into the role of carotenoids as antioxidants and as regulators of the immune response system.

Current carotenoid research encompasses a wide variety of fields and interests including plant physiology, food science, environmental science, taxonomy, industrial chemical synthesis, biotechnology and medical research. All the work must be based on a solid foundation of carotenoid chemistry and reliable methods for handling and analyzing these rather unstable substances.


CAROTENOID

Of the various classes of pigments in nature the carotenoids are among the most widespread and important ones, especially due to their most varied functions. In 1831 Wackenroder isolated carotene from carrots and in 1837 Berzelius named the yellow pigments from autumn leaves xanthophylls. This marks the beginning of carotenoid research and since then continuous developments have taken place. Because of their ubiquitous occurrence, different functions (see below), and interesting properties carotenoids are the subject of interdisciplinary research in chemistry, bio-chemistry, biology, medicine, physics, and many other branches of science.


OCCURRENCE

As already mentioned, the carotenoids are a class of natural pigments that is very widespread and it was demonstrated that they occur in all the three domains of life, i.e. in the eubacteria, the archea and in the eucarya. A rich source for carotenoids are the algae and more than 100 carotenoids have been isolated and characterized from these organisms. For human diet the most important source for carotenoids are plants, where often the brilliant colors of the carotenoids are masked by chlorophylls, e.g. in green vegetables. The carotenoids are responsible for the beautiful colors of many fruits (e.g. pine-apple, citrus fruits, tomatoes, paprika, rose hips) and flowers (e.g. Eschscholtzia, Narcissus), as well as the colours of many birds (e.g. flamingo, cock of rock, ibis, canary), insects (e.g. lady bird), and marine animals (e.g. crustaceans, salmon). Normally carotenoids occur in low concentrations, but this varies enormously from one source to another. The total carotenoid production in nature has been estimated at about 100'000'000 tons a year. It has been demonstrated by the analysis of serum and human breast milk that up to 50 dietary carotenoids from fruits and vegetables may be absorbed and metabolized by humans.

TECHNICAL SYNTHESIS, INDUSTRIAL PRODUCTION

Ever since the elucidation of the structure of β-carotene (III) and other carotenoids much effort has been devoted to the synthesis of carotenoids and other polyenes. The first synthesis of β-carotene (III) was reported independently in 1950 by Karrer & Eugster, Inhoffen & al. and Milas & al.. The Inhoffen synthesis was later developed into an industrial process and since 1954 β-carotene (III) has been produced commercially. Today six synthetic carotenoids (Fig. 6) have become commercially important: 8'-apo-β-caroten-8’-al (XIII)(C30), ethyl 8’-apo-β-caroten-8’-oate (XV)(C32), citranaxanthin (XVI)(C33), β-carotene (III) (C40), canthaxanthin (XI)(C40), and a mixture of the stereoisomers of astaxanthin (XII)(C40).

BIOSYNTHESIS

Carotenoids are synthesized in nature by plants and many microorganisms. Animals can metabolize carotenoids in a characteristic manner, but they are not able to synthesize carotenoids.

Carotenoids, being terpenoids, are synthesized from the basic C5-terpenoid precursor, isopentenyl diphosphate (IPP) (XVII) (Fig.1). This compound is converted to geranylgeranyl diphosphate (C20) (XVIII). The dimerization of XVIII leads to phytoene (XIX) and the stepwise dehydrogenation via phytofluene (XX), ζ-carotene (XXI), and neurosporene (XXII) gives lycopene (I). Subsequent cyclizations, dehydrogenations, oxidations etc., lead to the individual naturally occurring carotenoids. Little is known about the biochemistry of the many interesting final structural modifications that give rise to the hundreds of diverse natural carotenoids.

FUNCTIONS

The many various biological properties and functions of the carotenoids are besides their ubiquitous occurrence the main reason for the importance of this class of compounds and the main aspects have recently been reviewed (Krinsky, 1994).

In photosynthesis the energy transfer involves the direct excitation of carotenoids by light to form the first excited singlet state, and the subsequent transfer of this excitation energy to chlorophyll to initiate the process of photosynthesis. This type of process can effectively extend the wavelength of light available to an organism for photosynthesis.

Carotenoids also play a major role in the photoprotection of cells and tissues. This ability is the result of energy transfer reactions in which the energy of triplet state sensitizers or singlet oxygen is transferred to carotenoid molecules in the ground state, forming triplet state carotenoid molecules. The energy acquired by the carotenoids is then lost as heat and the ground state carotenoid is regenerated to undergo another cycle of photoprotection.

In humans and in those animals that require vitamin A for normal growth and development the most important source is the ingestion and metabolism of carotenoids that can be converted to vitamin A, i.e. compounds with an unsubstituted beta-ring, especially beta-carotene (III). In Western countries the supply of vitamin A is not critical but in countries of the third world it is still a severe problem. According to an estimate of the World Health Organization (WHO), 250’000-500’000 children go blind every year due to a deficiency of vitamin A. It was demonstrated that the formation of vitamin A from beta-carotene (III) can occur either by central or by excentric cleavage of beta-carotene (III).

The ability of carotenoids to act as antioxidants has been known for a long time and at the moment it is of great interest whether carotenoids behave as antioxidants in low-density lipoproteins (LDL), inasmuch as the oxidation of LDL is now considered to be an important causative agent in coronary heart disease, but the results of studying carotenoid involvement in preventing LDL oxidation remain controversial.

There have been many reports of a positive effect of dietary carotenoids on improving fertility or reproduction capacity in a number of animals, but additional evidence is still required for this proposed function of the carotenoids

There are a few example where it was demonstrated that carotenoids can alter the activity of a specific enzyme (e.g. aryl hydrocarbon hydroxylase) and this could be of importance in view of the detoxification of potential carcinogens.

Some years ago it was demonstrated that various carotenoids, such as lycopene (I), beta-carotene (III), alpha-carotene (IV), lutein (VII) and canthaxanthin (XI) can decrease the extent of malignant transformation of cells. It was shown that the molecular actions occur via up-regulation of the connexin43 gene, the gene responsible for the production of one of the important components of the gap junction.

Based on epidemiological data it can be assumed that diets rich in carotenoid-containing fruits and vegetables are associated with significantly decreased risks for a variety of degenerative diseases. However in dietary epidemiology it is always difficult to pinpoint the components which may be related to the lowered risk.

Several epidemiological studies have supported the observation that a high content of blood carotenoids decrease the risk of cataract formation. This is important in view of dietary aspects, particularly of the growing elderly population.

Age-related macular degeneration (ARMD), associated with aging can lead to total blindness in otherwise healthy people. A significant reverse relationship between the incidence of ARMD and the ingestion of fruits and vegetables rich in provitamin A carotenoids was demonstrated and it was shown that there are very significant reductions in the risk of developing neovascular ARMD as a function of plasma levels of alpha-carotene (IV), beta-carotene (III), cryptoxanthin (V) and lutein (VII)/zeaxanthin (VI).

Coronary heart disease (CHD) remains the major cause of death in Western societies. There is epidemiological evidence for an inverse association between serum levels and ischemic heart disease, but it is still unclear whether a single antioxidant plays the essential role or whether the sum of the antioxidants are responsible for preventing this disease in humans.

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