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Mechanisms and Biomarkers (WG 4) page 41
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carotenoid resulting in ground state oxygen and excited triple state carotenoid. The energy is
dissipated through rotational and vibrational interactions between the excited carotenoid and
the surrounding solvent to yield ground state carotenoid and thermal energy. Isomerisation of
the carotenoid may occur during this process. The quenching activity will largely depend
upon the number of conjugated double bonds and is influenced to a lesser extent by acyclic or
cyclic end-groups. Lycopene has 11 conjugated and 2 non-conjugated double bonds, and is
one of the most efficient singlet oxygen quenchers.
Chemical quenching is a minor process but can lead to the decomposition of lycopene to yield
2-methyl-hepten-6-one and apo-6’-lycopenal (Ukai, 1994). The decomposition products may
offer some biological activity, but has this any physiological relevance? Bearing in mind the
minor importance of singlet oxygen in humans, and the low incidence of the chemical
quenching process.
As previously reported lycopene appears to be an excellent quencher of singlet oxygen (Di
Mascio, 1989; Conn, 1991) in vitro (17x 10 9 M -1 s -1 ) compared to β-carotene (13 x10 9 M -1 s -1 ).
Exposure to sunlight can deplete carotenoid levels in the plasma and skin, and this scavenging
mechanism may be important in the eye.
Carotene-oxygen radical interactions have been studied with lycopene and β-carotene. The
second order rate constants for the electron transfer from a carotenoid radical anion to oxygen
have been determined (Conn, 1992).
C .- + O2 → C + O .- 2
For lycopene the rate constant was 2 x 10 8 M -1 s -1 and for β-carotene it was 25 x 10 8 M -1 s -1 .
This means that for β-carotene the equilibrium lies to the right and more in favour of the
formation of superoxide. For lycopene the reaction is less efficient and electron transfer is
observed in both directions. The electron transfer rate constant for lycopene is 1/10 th that of
β-carotene, and will be important in evaluating their antioxidant properties.
There is some tentative evidence to suggest that lycopene and other carotenoids can also
interact with peroxynitrite. Panasenko et al. (2000) demonstrated that lyopene was an
effective scavenger of peroxynitrite in a model system employing LDLs.
Lycopene is largely transported around the body bound to LDL particles. The oxidative
modification of LDL is thought to be central to several theories accounting for the initiation
of atherosclerotic events. Lipid peroxidation is an essential mechanism in LDL oxidation, and
it is thought that lycopene and β-carotene play a role in the prevention of lipid peroxidation.