Synergistic effect of glycolic acid on the antioxidant activity of alpha-tocopherol and melatonin in lipid bilayers and in human skin homogenates

Considerable interest has been raised concerning the use of natural compounds in preventing skin aging and photoaging. In the idea that the combined action of agents increasing epidermal turnover with antioxidants could be advantageous in cosmetic and therapeutic treatments, we first investigated if alpha-glycolic acid affected or prevented the antioxidant activity of vitamin E and of melatonin, two compounds found beneficial as topical photoprotectant. Assays were carried out in vitro either in a biomimetic liposomal system, or in human skin homogenates. Lipid peroxidation was monitored spectrophotometrically by the time course of lipid hydroperoxide production in liposomes and by formation of TBA reactive substances (TBARS) in skin homogenates. Glycolic acid, at 25 microM to 1 mM, showed a mild, concentration-dependent antioxidant effect in liposomes, as evaluated by a slight decrease of the peroxidation rate, while, at 1 mM, reduced TBARS production in skin homogenates by 14%. Combinations of either vitamin E or melatonin with glycolic acid, in a 1:5 to 1:200 molar ratio, resulted in a clear synergistic protection of liposomes, more evident for the combination of glycolic acid with vitamin E. An amount of synergism up to 250% and up to 80% was evaluated with vitamin E and melatonin, respectively. Consumption rate of vitamin E during peroxidation of liposomes, in the absence or in the presence of glycolic acid, suggests that regeneration of vitamin E may in part explain the observed synergism. Synergistic antioxidant activity between vitamin E and glycolic acid was also observed in skin homogenates, whereas the effect of glycolic acid on the antioxidant activity of melatonin appeared additive. However, the combination of these three compounds inhibited TBARS production almost completely. Our data provide evidence that glycolic acid can strongly potentiate the antioxidant action of melatonin and vitamin E. This may suggest the advantage of combining alpha-glycolic acid with these antioxidants in skin designed preparations, both to improve penetration and availability of antioxidants to epidermal layers and to enhance their protective potential.