Epigallocatechin gallate protects curcumin against oxidative degradation and mechanistic studies

Abstract

Curcumin (CM) and Epigallocatechin gallate (EGCG), both are natural antioxidants and known to produce beneficial pharmacological activity [1]. CM has been reported to have a low stability toward oxidants; its oxidative status could be repaired by EGCG [2]. EGCG has shown a stronger DPPH-scavenging potency (IC0.200) than CM (2.9 vs. 13.6µM) [3]. To study the mechanism for the stabilizing effect of EGCG on CM, CM (10µM) was incubated in an egg phosphatidylcholine (EPC) solution (0.9mg/mL, pH 6.5), with or without EGCG (20µM), at 37°C for 72 hours and 2,2-azobis (2-methylpropionamidine) dihydrochloride (AAPH) was added to provide free radical species for degradation of CM. With the addition of EGCG, the degradation of CM was reduced by more than 2 folds (from 88.4% to 40.4%). In the presence of AAPH (10mM), CM was observed to decline to baseline within 2 hours; while EGCG was found to slow down the degradation of CM by 3 times. While EGCG had shown to prolong the decline of CM to 6 hours, itself was declined to the baseline in 2 hours. On the other hand, CM was found to produce no protection on EGCG against the AAPH. With or without the coexistence of CM, EGCG was found to degrade at a rate of 78.7%/hr, while EGCG was degraded at a much slow rate of 8.3%/hr in the absence of AAPH. Furthermore, EGCG, CM and their combination were found effective in delaying the AAPH-induced lipid peroxidation of EPC, as assayed by thiobarbituric acid-reactive substances, for 2, 2, and 4 hours, respectively, which roughly corresponded to the length of time required to deplete CM and EGCG. Taken together, EGCG could provide a protective action to CM against the oxidative degradation of free radical, and this protection appears to be attributed to the radical-scavenging power of EGCG.