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Mechanisms and Biomarkers (WG 4) page 42
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Lipid peroxidation of LDL is thought to be initiated by the presence of the hydroxyl radical or
peroxynitrite. The end result is that H is abstracted from a polyunsaturated lipid to form a
radical e.g. arachidonate radical. Following a structural reorganisation of the lipid radical and
the addition of oxygen, a lipid peroxyl is formed.
L . + O2 → LOO .
Peroxyl radicals can react with carotenoids by one of two mechanisms
a) formation of a carotenoid cation radical
LOO . + Car → LOO - + Car +.
b) Addition reaction
LOO . + Car → [Car-OOL] .
The radical formed following the addition reaction can react with either other peroxyl radicals
or with oxygen.
The carotenoid cation radical is then thought to be reduced by α-tocopherols in the LDL
particle.
Car +. + α-TOC → Car + αTOC . + H +
This reaction would be beneficial in two respects in that it terminates an initiation event, and
secondly it would optimise the antioxidant capacity of vitamin E. Vitamin E is quite polar and
resides towards the periphery of the LDL particle. In contrast the structure of the carotenoid
will influence its location within the LDL particle. Lycopene and β-carotene are located
within the hydrophobic core of the LDL, and so would be expected to interact with non-polar
radicals. While zeaxanthin which contains two distal polar groups is located near the
periphery so that it may have greater access to radicals or reactive species that initiate lipid
peroxidation (Rice-Evans, 1997).