Biochemistry of Lycopene

How does it protect against cancer?

Tomatoes are rich in lycopene.
Tomatoes are rich in lycopene. Annabelle Breakey, Getty Images

Lycopene (see chemical structure), a carotenoid in the same family as beta-carotene, is what gives tomatoes, pink grapefruit, apricots, red oranges, watermelon, rosehips, and guava their red color. Lycopene is not merely a pigment. It is a powerful antioxidant that has been shown to neutralize free radicals, especially those derived from oxygen, thereby conferring protection against prostate cancer, breast cancer, atherosclerosis, and associated coronary artery disease.

It reduces LDL (low-density lipoprotein) oxidation and helps reduce cholesterol levels in the blood. In addition, preliminary research suggests lycopene may reduce the risk of macular degenerative disease, serum lipid oxidation, and cancers of the lung, bladder, cervix, and skin. The chemical properties of lycopene responsible for these protective actions are well-documented.

Lycopene is a phytochemical, synthesized by plants and microorganisms but not by animals. It is an acyclic isomer of beta-carotene. This highly unsaturated hydrocarbon contains 11 conjugated and 2 unconjugated double bonds, making it longer than any other carotenoid. As a polyene, it undergoes cis-trans isomerization induced by light, thermal energy, and chemical reactions. Lycopene obtained from plants tends to exist in an all-trans configuration, the most thermodynamically stable form. Humans cannot produce lycopene and must ingest fruits, absorb the lycopene, and process it for use in the body.

In human plasma, lycopene is present as an isomeric mixture, with 50% as cis isomers.

Although best known as an antioxidant, both oxidative and non-oxidative mechanisms are involved in lycopene's bioprotective activity. The nutraceutical activities of carotenoids such as beta-carotene are related to their ability to form vitamin A within the body.

Since lycopene lacks a beta-ionone ring structure, it cannot form vitamin A and its biological effects in humans have been attributed to mechanisms other than vitamin A. Lycopene's configuration enables it to inactivate free radicals. Because free radicals are electrochemically imbalanced molecules, they are highly aggressive, ready to react with cell components and cause permanent damage. Oxygen-derived free radicals are the most reactive species. These toxic chemicals are formed naturally as by-products during oxidative cellular metabolism. As an antioxidant, lycopene has a singlet-oxygen-quenching ability twice as high as that of beta-carotene (vitamin A relative) and ten times higher than that of alpha-tocopherol (vitamin E relative). One non-oxidative activity is regulation of gap-junction communication between cells. Lycopene participates in a host of chemical reactions hypothesized to prevent carcinogenesis and atherogenesis by protecting critical cellular biomolecules, including lipids, proteins, and DNA.

Lycopene is the most predominant carotenoid in human plasma, present naturally in greater amounts than beta-carotene and other dietary carotenoids. This perhaps indicates its greater biological significance in the human defense system.

Its level is affected by several biological and lifestyle factors. Because of its lipophilic nature, lycopene concentrates in low-density and very-low-density lipoprotein fractions of the serum. Lycopene is also found to concentrate in the adrenal, liver, testes, and prostate. However, unlike other carotenoids, lycopene levels in serum or tissues do not correlate well with overall intake of fruits and vegetables.

Research shows that lycopene can be absorbed more efficiently by the body after it has been processed into juice, sauce, paste, or ketchup. In fresh fruit, lycopene is enclosed in the fruit tissue. Therefore, only a portion of the lycopene that is present in fresh fruit is absorbed. Processing fruit makes the lycopene more bioavailable by increasing the surface area available for digestion.

More significantly, the chemical form of lycopene is altered by the temperature changes involved in processing to make it more easily absorbed by the body. Also, because lycopene is fat-soluble (as are vitamins, A, D, E, and beta-carotene), absorption into tissues is improved when oil is added to the diet. Although lycopene is available in supplement form, it is likely there is a synergistic effect when it is obtained from the whole fruit instead, where other components of the fruit enhance lycopene's effectiveness.