Antioxidant activity of tea catechins in different lipid systems

Tea catechins showed different trends in relative antioxidant activity in different lipid systems. In corn oil triglycerides oxidized at 50 degrees C, epigallocatechin (EGC), epigallocatechin gallate (EGCG), and epicatechin gallate (ECG) were better antioxidants than epicatechin (EC) and catechin at 140 mu M. Used as reference compounds, gallic acid (GA) was more active than propyl gallate (PG), and both were more effective than EC and catechin. However, in the corresponding corn oil-in-water emulsions, all tea catechins, GA, and PG were prooxidants at 5 and 20 mu M by accelerating hydroperoxide and hexanal formation. In contrast, in soy lecithin Liposomes oxidized at 50 degrees C, EGCG and PG were the best antioxidants, followed by EC, EGC, ECG, catechin, and GA at 20 mu M. In liposomes oxidized at 37 degrees C with 10 mu M cupric acetate, catechin and EC were better antioxidants than ECG, but EGCG, EGC, PG, and GA promoted lipid oxidation. The improved antioxidant activity observed for tea catechins in liposomes compared to emulsions can be explained by the greater affinity of the polar catechins toward the polar surface of the lecithin bilayers, thus affording better protection. The marked variation in activity among tea catechins may be partly explained by their different reducing potentials, stabilities, and relative partitions between phases in different lipid systems.