Lycopene
Lycopene is a bright red carotenoid hydrocarbon found in tomatoes and other red fruits and vegetables, such as red carrots, watermelons, grapefruits, and papayas, but it is not present in strawberries or cherries. Although lycopene is chemically a carotene, it has no vitamin A activity. Foods that are not red may also contain lycopene, such as asparagus, guava and parsley.
In plants, algae, and other photosynthetic organisms, lycopene is an intermediate in the biosynthesis of many carotenoids, including beta-carotene, which is responsible for yellow, orange, or red pigmentation, photosynthesis, and photoprotection. Like all carotenoids, lycopene is a tetraterpene. It is insoluble in water. Eleven conjugated double bonds give lycopene its deep red color. Owing to the strong color, lycopene is useful as a food coloring and is approved for use in the USA, Australia and New Zealand and the European Union.
Structure and physical properties
Lycopene is a symmetrical tetraterpene assembled from eight isoprene units. It is a member of the carotenoid family of compounds, and because it consists entirely of carbon and hydrogen, is also a carotene. Isolation procedures for lycopene were first reported in 1910, and the structure of the molecule was determined by 1931. In its natural, all-trans form, the molecule is long and straight, constrained by its system of 11 conjugated double bonds. Each extension in this conjugated system reduces the energy required for electrons to transition to higher energy states, allowing the molecule to absorb visible light of progressively longer wavelengths. Lycopene absorbs all but the longest wavelengths of visible light, so it appears red.
Plants and photosynthetic bacteria naturally produce all-trans lycopene. When exposed to light or heat, lycopene can undergo isomerization to any of a number of cis-isomers, which have a bent rather than linear shape. Different isomers were shown to have different stabilities due to their molecular energy. In human blood, various cis-isomers constitute more than 60% of the total lycopene concentration, but the biological effects of individual isomers have not been investigated.
Carotenoids like lycopene are found in photosynthetic pigment-protein complexes in plants, photosynthetic bacteria, fungi, and algae. They are responsible for the bright orange–red colors of fruits and vegetables, perform various functions in photosynthesis, and protect photosynthetic organisms from excessive light damage. Lycopene is a key intermediate in the biosynthesis of carotenoids, such as beta-carotene, and xanthophylls.
Dispersed lycopene molecules can be encapsulated into carbon nanotubes enhancing their optical properties. Efficient energy transfer occurs between the encapsulated dye and nanotube — light is absorbed by the dye and without significant loss is transferred to the nanotube. Encapsulation increases chemical and thermal stability of lycopene molecules; it also allows their isolation and individual characterization.
Biosynthesis
The unconditioned biosynthesis of lycopene in eukaryotic plants and in prokaryotic cyanobacteria is similar, as are the enzymes involved. Synthesis begins with mevalonic acid, which is converted into dimethylallyl pyrophosphate. This is then condensed with three molecules of isopentenyl pyrophosphate, to give the 20-carbon geranylgeranyl pyrophosphate. Two molecules of this product are then condensed in a tail-to-tail configuration to give the 40-carbon phytoene, the first committed step in carotenoid biosynthesis. Through several desaturation steps, phytoene is converted into lycopene. The two terminal isoprene groups of lycopene can be cyclized to produce beta-carotene, which can then be transformed into a wide variety of xanthophylls.Staining and removal
Lycopene is the pigment in tomato sauces that turns plastic cookware orange. It is insoluble in plain water, but it can be dissolved in organic solvents and oils. Because of its non-polarity, lycopene in food preparations will stain any sufficiently porous material, including most plastics. To remove this staining, the plastics may be soaked in a solution containing a small amount of chlorine bleach.The bleach oxidizes the lycopene, thus allowing the now-polarized metabolite to dissolve.
Diet
Consumption by humans
Absorption of lycopene requires that it be combined with bile salts and fat to form micelles. Intestinal absorption of lycopene is enhanced by the presence of fat and by cooking. Lycopene dietary supplements may be more efficiently absorbed than lycopene from food.Lycopene is not an essential nutrient for humans, but is commonly found in the diet mainly from dishes prepared from tomatoes. The median and 99th percentile of dietary lycopene intake have been estimated to be 5.2 and 123 mg/d, respectively.
Safety
In humans, the Observed Safe Level for lycopene is 75 mg/day, according to one preliminary study.Adverse effects
Lycopene is non-toxic and commonly found in the diet, mainly from tomato products. There are cases of intolerance or allergic reaction to dietary lycopene, which may cause diarrhea, nausea, stomach pain or cramps, gas, and loss of appetite. Lycopene may increase the risk of bleeding when taken with anticoagulant drugs. Because lycopene may cause low blood pressure, interactions with drugs that affect blood pressure may occur. Lycopene may affect the immune system, the nervous system, sensitivity to sunlight, or drugs used for stomach ailments.Lycopenemia is an orange discoloration of the skin that is observed with high intakes of lycopene. The discoloration is expected to fade after discontinuing excessive lycopene intake.
Research and potential health effects
A 2017 review concluded that tomato products and lycopene supplementation had small positive effects on cardiovascular risk factors, such as elevated blood lipids and blood pressure. A 2010 review concluded that research has been insufficient to establish whether lycopene consumption affects human health. Lycopene has been studied in basic and clinical research for its potential effects on cardiovascular diseases and prostate cancer, although there is no good evidence of benefit in prostate cancer.Regulatory status in Europe and the United States
In a review of literature on lycopene and its potential role as a dietary antioxidant, the European Food Safety Authority concluded that evidence was insufficient for lycopene having antioxidant effects in humans, particularly in skin, heart function, or vision protection from ultraviolet light.Although lycopene from tomatoes has been tested in humans for cardiovascular diseases and prostate cancer, no effect on any disease was found. The US Food and Drug Administration, in rejecting manufacturers' requests in 2005 to allow "qualified labeling" for lycopene and the reduction of various cancer risks, provided a conclusion that remains in effect :
"...no studies provided information about whether lycopene intake may reduce the risk of any of the specific forms of cancer. Based on the above, FDA concludes that there is no credible evidence supporting a relationship between lycopene consumption, either as a food ingredient, a component of food, or as a dietary supplement, and any of these cancers."