Copper-induced peroxidation of phosphatidylserine-containing liposomes is inhibited by nanomolar concentrations of specific antioxidants

Copper-induced peroxidation of liposomal palmitoyllinoleoyl-phosphatidylcholine (PLPC) is inhibited by -tocopherol at micromolar concentrations. In our previous study we found that when the liposomes contain phosphatidylserine (PS), nanomolar concentrations of Toc were sufficient to inhibit peroxidation. In an attempt to gain understanding of the origin of this extreme antioxidative potency, we tested the antioxidative potency of 36 additional antioxidants and the dependence of their potency on the presence of PS in the liposomes. The results of these studies reveal that only 11 of the tested antioxidants possess similar antioxidative potency to that of Toc. These include trolox, butylated hydroxytoluene (BHT), curcumin, nordihydroguaiaretic acid (NDGA), diethylstilbestrol (DES), 2 of the 13 tested flavonoids (luteolin and 7,3′,4′-trihydroxyflavone; T-414), -naphthol, 1,5-, 1,6- and 1,7-dihydroxynaphthalenes (DHNs). Propyl gallate (PG), methyl syringate, rosmarinic acid, resveratrol, other flavonoids, as well as β-naphthol, 1,2-, 1,3-, 1,4-, 2,3-, 2,6-, and 2,7-DHNs were either moderately antioxidative or pro-oxidative. For liposomes made of PLPC (250 μM) and PS (25 μM) the “lag” preceding copper-induced peroxidation (5 μM copper) was doubled upon addition of 30–130 nM of the “super-active” antioxidants.

We propose that the mechanism responsible for the extreme antioxidative potency against copper-induced peroxidation in PS-containing liposomes involves replenishment of the antioxidant in a ternary PS–copper-antioxidant complex. Based on structure–activity relationship of the 37 tested antioxidants, the “super-antioxidative potency” is attributed to the recycling of relatively stable semiquinone or semiquinone-like radicals.