NO - Besoin d'assistance

Mechanisms and Biomarkers (WG 4) page 43
__________________________________________________________________________________________
Isolated human LDL can be oxidised when they are challenged with copper ions. The
lipoperoxidation of lipids is usually monitored by the TBARS assay or following the
formation of conjugated dienes. Esterbauer (1992) assessed the depletion of carotenoids
during this process. In this in vitro system there is no regeneration of tocopherol by water
soluble antioxidants and as expected the lipid soluble antioxidant was the first to be depleted.
The initial carotenoid to be depleted was lycopene, while β-carotene was relatively stable. If
the carotenoids are located in a position that makes their interaction with tocopherol unlikely.
Then this raises the possibility of a synergystic activity between the carotenoids and the more
polar xanthophylls present, prior to the interaction with tocopherol. Indeed Stahl (1998)
indicated that an antioxidant synergism between lycopene and lutein exists when
multilamellar liposomes were oxidised using AMVN. Again the discussion focused on the
orientation of the xanthophyll, with the polar groups being anchored at polar sites in the
membrane whilst lycopene resided in the inner part of membranes and retained a substantial
degree of mobility.
Many groups have investigated the number of antioxidant components present in a LDL
particle. It has been estimated that there are 3-15 molecules of α-Tocopherol per LDL
particle, whilst β-carotene and lycopene are present at 0.3 and 0.16 molecules respectively.
These figures refer to a healthy subject not consuming vitamin E or carotenoid supplements
(Esterbauer, 1992). Although lycopene may play a role in protecting LDL against
peroxidation, the figures would suggest that carotenoids are not essential. Dietary intervention
studies have also yielded conflicting results as reported in chapter 4.
Prooxidant effects
The potential prooxidant effects of the carotenoids depends upon the formation of carotenoid
peroxyl radical. This specie is easily formed as carbon centred radical and usually very stable
due to extensive delocalisation of electrons within the polyene structure. This radical will
interact with oxygen to form the peroxyl radical.
Car . + O2 → CarOO .
Alternatively the carotenoid may interact with a lipid peroxyl radical to form a carotenoid
radical adduct. This may then go on to react with molecular oxygen to yield reactive peroxyl
radical.