Carotenoid-containing unilamellar liposomes loaded with glutathione: a model to study hydrophobic-hydrophilic antioxidant interaction

Unilamellar liposomes are used as a simple two-compartment model to study t he interaction of antioxidants. The vesicle membrane can be loaded with lip ophilic compounds such as carotenoids or tocopherols, and the aqueous core space with hydrophilic substances like glutathione (GSH) or ascorbate, mimi cking the interphase between an aqueous compartment of a cell and its surro unding membrane. Unilamellar liposomes were used to investigate the interaction of GSH with the carotenoids lutein, beta -carotene and lycopene in preventing lipid per oxidation. Lipid peroxidation was initiated with 2,2'-azo-bis-[2,4-dimethyl valeronitrile] (AMVN). Malondialdehyde (MDA) formation was measured as an i ndicator of oxidation; additionally, the loss of GSH was followed. In lipos omes without added antioxidant, MDA levels of 119 +/- 6 nmol/mg phospholipi d were detected after incubation with AMVN for 2 h at 37 degreesC. Consider ably lower levels of 57 +/- 8 nmol MDA/mg phospholipid were found when the liposomal vesicles had been loaded with GSH. Upon incorporation of beta -ca rotene, lycopene or lutein, the resistance of unilamellar liposomes towards lipid peroxidation was further modified. An optimal further protection was observed with 0.02 nmol beta -carotene/mg phospholipid or 0.06 nmol lycope ne/mg phospholipid. At higher levels both these carotenoids exhibited proox idant effects. Lutein inhibited lipid peroxidation in a dose-dependent mann er between 0.02 and 2.6 nmol/mg phospholipid. With increasing levels of lyc opene and lutein the consumption of encapsulated GSH decreased moderately, and high levels of beta -carotene led to a more pronounced loss of GSH. The data demonstrate that interactions between GSH and carotenoids may impr ove resistance of biological membranes towards lipid peroxidation. Differen t carotenoids exhibit specific properties, and the level for optimal protec tion varies between the carotenoids.