In plant organisms, lutein, as an essential component of photosystems, fulfills the functions of light collection and photoprotection, among others. A photosystem consists of an antenna complex or light-collecting complex (light-collecting trap) and a reaction center, and is a collection of proteins and pigment molecules – chlorophylls and carotenoids. It is localized on the inner membrane – thylakoid membrane – of chloroplasts, the sites of photosynthesis. The light-collecting complex of each photosystem is composed of about 250 or 300 protein molecules associated with chlorophyll and carotenoid pigments. Incident light raises the antenna complex to a high-energy, excited state. Lutein and other carotenoids have the task here of absorbing the light quanta and passing its energy from one molecule to the next to the reaction center of the photosystem. Once at the reaction center, the energy is absorbed by chlorophyll-a molecules. These use the energy to produce chemical energy equivalents. The reaction center of the photosystems ultimately provides an irreversible trap for light quanta. In addition, lutein has an antioxidant effect and thus acquires a vital protective function for plant as well as animal cells. It is able to intercept cell-destroying singlet oxygen. Singlet oxygen belongs to the free radicals that can react with lipids, especially polyunsaturated fatty acids and cholesterol, proteins, nucleic acids, carbohydrates as well as DNA and modify or destroy them – oxidative stress. During detoxification of singlet oxygen, lutein acts as an intermediate carrier of energy – it releases the energy in interaction with its environment in the form of heat – process of “quenching”. In this way, reactive singlet oxygen is rendered harmless. Studies on mutant organisms, in which carotenoids, mainly lutein were completely absent, showed that the cells were destroyed in the presence of oxygen. The cell components – lipids, proteins and nucleic acids – were defenceless against the reactive oxygen compounds. The result was cell death.
Lutein and diseases
Lutein and Eye Disease Lutein and zeaxanthin play a significant role in the prophylaxis of cataract (cataract) and age-related macular degeneration (AMD). Both eye diseases are the two leading causes of visual impairment and blindness, ahead of diabetic retinopathy – a disease of the retina of the eye caused by diabetes mellitus. Age-related macular degeneration (AMD)The macula lutea (yellow spot) is located near the center of the retina, a thin, transparent, light-sensitive nerve tissue composed of photoreceptor cells, the rods and cones. The yellow spot is about 5 millimeters in diameter and has the greatest density of rods and cones. From the outer (perifovea) to the inner area (parafovea) of the macula, the proportion of rods decreases, so that in the fovea centralis only cones – visual cells responsible for color perception – are expected. The fovea centralis of the yellow spot is the area of sharpest vision and specialized for highest spatial resolution. Thus, it is obvious that towards the fovea centralis the content of lutein and zeaxanthin increases strongly in order to provide sufficient protection for the sensitive cones. In addition to lutein and zeaxanthin, meso-zeaxanthin was also found in appreciable amounts in the retina. Presumably, meso-zeaxanthin represents a conversion product of lutein. In the fovea centralis, lutein appears to undergo a chemical reaction. By reactive compounds it could oxidize to oxolutein and as a result of reduction be converted to zeaxanthin and meso-zeaxanthin. The enzymes required for this process have not yet been identified. Since the retina of children contains more lutein and less meso-zeaxanthin compared to that of adults, this mechanism does not seem to be so strongly developed in the child’s organism yet. The rods and cones of the retina have a high content of unsaturated fatty acids and are therefore extremely sensitive to lipid peroxidation. They are also exposed to high levels of light radiation – high risk for photooxidative damage. Lutein acts in the retina on the one hand as a light filter and on the other hand as an antioxidant.Xanthophyll has the ability to filter out short-wave blue light rays from the normal spectral range of light. Especially the high-energy blue light is thought to be responsible for the formation of singlet oxygen and other reactive oxygen compounds by converting exo- as well as endogenous photosensitizers into an excited state. Thus, lutein protects the eye from radical attack and photooxidative damage. Furthermore, lutein can inactivate reactive oxygen species – quenching -, interrupt chain reactions of free radicals and thus reduce lipid peroxidation. This prevents the formation of lipofuscin, for example, a photoreactive substance. Lipofuscin belongs to a chemically not clearly defined group of various complex aggregated structures of lipids and proteins. The prooxidant substance increases the risk of age-related macular degeneration. The xanthophyll pigments in the fovea centralis of the yellow spot are preferentially oriented and therefore can absorb polarized light only in certain directions. By preferentially absorbing polarized light from certain angles, lutein may reduce gloss and glare effects. In addition, it is believed that lutein may mitigate the effects of chromatic aberration (aberrations of optical lenses) and thus improve visual acuity, particularly in the short-wavelength range. In patients with congenital retinal degeneration, increased lutein intake through increased consumption of spinach or kale, for example, results in better contrast acuity, less glare, and improved color perception. Studies of deceased AMD patients found that their retinas had significantly reduced lutein and zeaxanthin levels. Finally, high concentrations of lutein and zeaxanthin in the retina are associated with up to 82% lower risk of AMD. Adequate intake of lutein- and zeaxanthin-rich foods therefore plays an essential role. Increased intake of lutein and zeaxanthin can significantly increase the concentrations in the macula lutea of the retina. The levels of xanthophylls in the retina correlate with their serum levels. The accumulation processes require up to several months, so that the increased intake of lutein and zeaxanthin must be long-term. In corresponding studies, the concentrations of both xanthophylls had not significantly increased after only one month. Increased intake of lutein is not associated with side effects such as hypercarotenemia, carotenderma, and changes in hematologic or biochemical processes. Cataract (cataract)Similar to AMD, scientific studies confirm the prophylactic effect of lutein in cataract. In terms of antioxidant property, lutein prevents photochemical generation of reactive oxygen species (ROS) in the various tissues of the eye, which could be the trigger of the disease. Oxygen radicals lead to, among other things, modification of lens proteins, accumulation of glycoproteins, oxidation products of the amino acid tryptophan, and numerous fluorescent molecules from exogenous and endogenous sources. These sensitizers are ultimately held responsible for lens opacification. By significantly reducing the damaging effects of light and oxygen through a long-term, regular, and high intake of lutein-rich foods, the risk of cataract is reduced by up to 50%. Lutein acts synergistically with other antioxidants, such as the enzymes superoxide dismutase, catalase and gluthate peroxidase. High concentrations of lutein as well as zeaxanthin in the retina correlate with transparent lenses. Further epidemiologic studies concluded that individuals with increased intakes of lutein and zeaxanthin, but not other carotenoids or vitamin A, had a significantly reduced risk of cataract surgery. Olmedilla et al 2001 showed that lutein leads to improved vision, a decrease in glare sensitivity, and an increase in visual acuity in cataract patients.
Functions in food
Because lutein is relatively stable in storage during the processing of food, only minor losses occur, lutein as a single substance or component of plant extracts finds application as a food colorant.Lutein provides a yellow-orange color and is found, for example, in soups, sauces, flavored beverages, desserts, spices, confectionery and baked goods. Lutein is also used for indirect coloration via animal feed. In particular, it is added to chicken feed, enhancing the characteristic yellow of the egg yolk. Furthermore, lutein is an important precursor of flavoring substances. Xanthophyll is degraded by cooxidation with the aid of lipoxygenases, by reacting with reactive oxygen compounds and under thermal stress. Carbonyl compounds with low odor threshold are formed from lutein.
