NO - Besoin d'assistance

Mechanisms and Biomarkers (WG 4) page 15
__________________________________________________________________________________________
radicals including N-acetylcysteine acting through elevation of intracellular cysteine (hence
glutathione) and homocysteine which can be involved in the control of the plasma redox thiol
status (Brooks et al., 1991).
Ubiquinone (Coenzyme Q10): this powerful antioxidant is a fat soluble quinone mainly found
in the inner membrane of mitochondria. Its level in mitochondrial membrane is 10 times
higher than in plasma membrane when compared with another lipid soluble antioxidant such
as vitamin E (Takada et al., 1982). It is as efficient as vitamin E in chain-breaking antioxidant
reactions. However, it is believed to regenerate vitamin E radical (tocopheroxyl) formed while
preventing lipid peroxidation (Frei et al., 1990). The oxidised product of ubiquinone
(ubisemiquinone) is reduced back by the electron transport chain of mitochondrial respiratory
chain.
Vitamin E provided by the food is a fat soluble antioxidant found in all lipid structures
(membranes and HDL/LDL). The plasma level is about 15 µg/mL and up to 7 molecules of
vitamin E was found per molecule of LDL (Esterbauer et al., 1989). Vitamin E comprises 8
naturally occurring homologues defined as tocopherols. The most abundant α-tocopherol is a
chain breaking agent reacting with lipid oxides and preventing lipid peroxidation (Diplock et
al., 1989). Such a reaction leads to the formation of tocopheroxyl radical which, by spreading
the energy of the unpaired electron into the entire structure, increases the half life of the
radical and hence blocks the reactivity of the captured electron. Tocopheroxyl can react either
with another tocopheroxyl or lipid peroxide radical to form stable vitamin E dimer and
vitamin E-lipid adduct respectively. The tocopheroxyl radical can also be regenerated to
tocopherol either by ubiquinone from inner mitochondrial membrane or by vitamin C from
the aqueous compartment of cell cytoplasm (Kagan et al., 1990). This latter reaction is an
important way to maintain the level of α-tocopherol in the membrane at the expense of
vitamin C.
Vitamin C (or ascorbic acid) is a water soluble compound found at relatively low
concentration in the plasma (40-140 µmol/L) and can attain millimolar level in tissues. It is
essential for humans as it cannot be synthesised and is obtained from the diet (and/or
supplement). Besides of several important metabolic roles, ascorbate also has the capacity to
scavenge in vivo almost all reactive oxygen and nitrogen radical species (Rose and Bode,
1993). Its antioxidant property results from the formation of semidehydroascorbate which is
less reactive than the free radicals scavenged (Bendish et al., 1986). As discussed above, one
of the roles of vitamin C is to act synergistically with vitamin E through the reduction of α-