Antioxidant Activity and Incorporation in Soybean Phosphatidylcholine Membranes of Catechins Protecting α-Tocopherol against Lipid Peroxidation

Abstract

The aim of the present paper is to correlate the relative antioxidant activity of catechins and their incorporation in soybean phosphatidylcholine (sPC) membranes. For the purpose the inhibition effect of the catechins (+)-catechin (C), (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC), and (-)-epigallocatechin gallate (EGCG) on α-Tocopherol (α-Toc) consumption was compared when the lipid oxidation was initiated in the aqueous medium by the water-soluble radical initiator 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) or in the lipid phase by the lipophilic 2,2′-azobis(4-methoxy-2, 4-dimethylvaleronitrile) (Meo-AMVN). The catechins C, EC, EGC, ECG and EGCG at a concentration of 15 μM were added exogenously to the water medium of multilamellar (MLV) liposomal suspensions or were incorporated in the lipid phase of MLV or unilamellar (ULV) liposomes, containing incorporated α-Tocopherol (α-Toc) at the same concentration. The α-Toc and catechin consumption were followed by high-performance liquid chromatography (HPLC) technique. The inhibition the accumulation of lipid hydroperoxides (LOOH) by catechins in ULV oxidized by AAPH also was studied. The antioxidant activity of catechins, expressed by the constant of inhibition k_inh, and the parameters n and kcl_inh were determined. The galloylated catechins ECG and EGCG, inhibiting α-Toc consumption in greater extent in MeO-AMVN - than in AAPH-oxidation, were better incorporated in lipid membranes, and also had higher peroxyl radicals scavenging activity than the non-galloylated C, EC, and EGC. ECG being the most effective catechin protecting α-Toc against lipophilic radicals interacted most efficiently with PC layers as well as ECG was the strongest radical scavenger. In hydrophilic radicals-initiated oxidation EGCG incorporated in MLV layers protected α-Toc better than ECG, and was consumed faster than ECG when incorporated in ULV liposomes. EGCG was situated closer to the lipid/water interface than ECG. The EGCG derived radical was more prone to participate in side propagation reactions. The cis-form EC had higher affinity for lipid membranes than the trans-form C, as EC had greater effect sparing α-Toc consumption in MeO-AMVN oxidation than C. EGC, the least effective among the studied catechins to preserve α-Toc from oxidation, was worst incorporated in lipid layers, and its radical was unstable leading to side propagations. In conclusion, the catechins interacted with PC membranes, and preserved α-Toc from oxidation by cooperating with it to scavenge peroxyl radicals or by regenerating it from its radical.