Unilamellar liposomes are used as a simple two-compartment model to study the interaction of antioxidants. The vesicle membrane can be loaded with lipophilic compounds such as carotenoids or tocopherols, and the aqueous core space with hydrophilic substances like glutathione (GSH) ascorbate, mimicking the interphase between an aqueous compartment of a cell and its surrounding membrane. Unilamellar liposomes were used to investigate the interaction of GSH with the carotenoids lutein, beta-carotene and lycopene in preventing lipid peroxidation. Lipid peroxidation was initiated with 2,2'-azobis-[2,4-dimethylvaleronitrile] (AMVN). Malondialdehyde (MDA) formation was measured as an indicator of oxidation; additionally, the loss of GSH was followed. In liposomes without added antioxidant, MDA levels of 119 +/- 6 nmol/mg phospholipid were detected after incubation with AMVN for 2 h at 37 degrees C. Considerably lower levels of 57 +/- 8 nmol MDA/mg phospholipid were found when the liposomal vesicles had been loaded with GSH. Upon incorporation of beta-carotene, 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 lycopene/mg phospholipid. At higher levels both these carotenoids exhibited prooxidant effects. Lutein inhibited lipid peroxidation in a dose-dependent manner between 0.02 and 2.6 nmol/mg phospholipid. With increasing levels of lycopene 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 improve resistance of biological membranes towards lipid peroxidation. Different carotenoids exhibit specific properties, and the level for optimal protection varies between the carotenoids.