Title: Lycopene compared with other dietary carotenoids

Key words: reactive oxygen species, substrate, free radicals, LDL-cholesterol, beta-carotene, alpha-carotene, tocopherols,

Date: May 2001

Category: Micronutrients

Type: Article

Author: Pam Brown

Lycopene compared with other dietary carotenoids

Why is lycopene an antioxidant?

Halliwell and Gutteridge define an antioxidant as ‘any substance that, when present at low concentrations compared with those of an oxidizable substrate, significantly delays or prevents oxidation of that substrate1’. The antioxidant capacity of a substance depends on the type of reactive oxygen species (ROS) or reactive nitrogen species (RNS) mounting the attack, and the substrate under attack eg protein, lipid or DNA.

Carotenoids including lycopene are antioxidants because they decrease production of, and help quench reactive oxygen species (ROS) and scavenge free radicals1. Lycopene has no provitamin A activity, and its main antioxidant activities are quenching of singlet oxygen species and peroxyl radical scavenging2, although other researchers have found that it also reacts with nitrogen dioxide and other radicals3. Lycopene’s bioavailability is improved by heating and processing and as it is hydrophobic, absorption requires fat. Antioxidant activity of lycopene has been demonstrated in vitro and in vivo4,5.

In vitro studies show that when copper-mediated oxidation is induced in LDL samples, carotenoids were used up before lipid peroxidation occurred, with lycopene used up first, suggesting that lycopene acts as an antioxidant6.

Feeding tomato puree results in increased plasma and lymphocyte carotenoids, and improved lymphocyte resistance to oxidative stress and a 33-50% reduction in DNA damage7,8. In another study, a single serving of tomato lowered oxidative damage of white cell DNA within 24 hours, and levels of 8-hydroxyguanine decreased in those with high initial levels. However, other studies have failed to find an increase in total antioxidant capacity of plasma after increases in dietary lycopene9,10. If lycopene acts as an antioxidant in vivo, then oxidative metabolites should be identifiable; Khachik et al identified oxidative metabolites in serum and milk11, as did Paetau12. Plasma and tissue levels in humans are compatible with antioxidant effects. Some of lycopene’s beneficial in vivo effects may result from other mechanisms eg increased gap junctional communication, inhibition of tumour growth factors or decreased cholesterol synthesis rather than antioxidant activity1. Or they may be due to some other phytochemical in tomatoes13.

Comparison with other carotenoids

All carotenoids are hydrophobic. There is no best antioxidant since this depends on the substrate being protected. Ability of carotenoids to scavenge radicals depends on the number of double bonds in their structure, and lycopene, with 13 has more than other carotenoids, giving it the potential to be more potent. If the trolox equivalent antioxidant capacities (TEAC) are compared, lycopene has a TEAC of 2.9mM, compared with 1.5 for beta-carotene, 1.3 for alpha-carotene and 1.5 for lutein14. Murakoshi found alpha-carotene 38% stronger as an antioxidant than beta-carotene15, with lycopene even more potent. Lycopene is a more potent quencher of singlet oxygen species than other carotenoids in vitro, with double the activity of beta-carotene16. However, when the plasma levels are taken into account the plasma singlet oxygen quenching capacities of lycopene (0.7microM) beta-carotene (0.5microM) and alpha-tocopherol (22 microM) are comparable16. Mortensen established an antioxidant hierarchy with lycopene similar to beta and gamma-tocopherols, and greater than beta-carotene, >zeaxanthin, > lutein>>canthaxanthin17. Lycopene supplementation also provides greater protection against oxidative damage to lymphocyte DNA by atmospheric oxygen than beta-carotene18.

Lycopene’s antioxidant effects are improved in the presence of other antioxidants such as the tocopherols and other carotenoids. Cis and trans isomers of lycopene may have different biological activities19.

In the EURAMIC study evaluating the relationship between antioxidant status and MI, lycopene was the carotenoid with the major effect20. However other workers found that this relationship was strongest in non-smokers, contrary to expectations if the effects were purely due to lycopene’s antioxidant abilities, since oxidative stress should be higher in smokers.

 

References

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