Antioxidant activity of (all-E)-lycopene and synthetic apo-lycopenoids in a chemical model of oxidative stress in the gastro-intestinal tract

Lycopene, the main pigment of tomato, is known to have protective effects on health. Its metabolites could also be involved in these effects. Potentially bioactive lycopene metabolites namely apo-10′-lycopenol, apo-10′-lycopenal, apo-14′-lycopenol, apo-14′-lycopenal, and apo-11-lycopenoids with alcohol, carboxylic acid, aldehyde and ethyl ester terminal groups were obtained by organic synthesis as (all-E) stereoisomers using HWE condensation reactions. The ability of (all-E)-lycopene and the synthesized apo-lycopenoids to inhibit lipid peroxidation was tested in a chemical model of postprandial oxidative stress in the gastric compartment. Oxidative stress was generated by metmyoglobin, the main form of dietary iron (from red meat), which is able to catalyse the peroxidation of linoleic acid under mildly acidic conditions. In this model, apo-6′-lycopenal and apo-8′-lycopenal were better inhibitors of lipid peroxidation than (all-E)-lycopene itself. For the apo-lycopenoids, a long unsaturated chain and a terminal carboxylic acid group both favour the antioxidant activity. The short-chain apo-lycopenoic acid, apo-14′-lycopenoic acid, was shown to behave like a hydrophilic antioxidant, i.e. by reducing hypervalent iron forms of metmyoglobin. Thus, besides the polyene chain length, the terminal group of the apo-lycopenoids is expected to deeply influence the lipophilic/hydrophilic balance of the molecule (and consequently its distribution between the aqueous and lipid phases) and its affinity for the heme cavity. It can thus be concluded that the polyene chain length and terminal group are two important parameters modulating the mechanism by which apo-lycopenoids express their antioxidant activity.