Comparative Radical-scavenging Activity of Curcumin and Tetrahydrocurcumin with Thiols as Measured by the Induction Period Method

Abstract

Background: The in vivo radical-scavenging activity of curcumin (CUR) and THC (tetrahydrocurcumin, a metabolite of CUR) does not occur in isolation, but through an intricate antioxidant network together with co-antioxidants such as glutathiones (GSH). In the present investigation, the radical-scavenging activity of CUR and THC with 2-mercapto-1-methylimidazole (MMI, a thiol) was studied using the induction period method. Materials and Methods: The induction period (IP) and propagation rate (Rp) for mixtures of MMI with THC or CUR were determined by differential scanning calorimetry (DSC) monitoring of the polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of benzoyl peroxide (BPO, a source of peroxy radical, PhCOOØ) or 2,2'-azobisisobutyronitrile (AIBN, a source of alkyl radical, RØ) under nearly anaerobic conditions. Results: The stoichiometric number of PhCOOØ radicals that could be trapped per molecule (n) was 3.4 and 3.3 for CUR and THC, and that of the RØ radical was 3.1 and 2.5, respectively. At a molar ratio of antioxidant:co-antioxidant (MMI) = 1:5, a THC/MMI mixture with PhCOOØ enhanced the total radical-scavenging activity, possibly due to partial regeneration of THC, whereas a CUR/MMI mixture with PhCOOØ reduced it. Similarly, CUR/MMI and THC/MMI mixtures with RØ, particularly the former, reduced the total radical-scavenging activity, possibly due, in part, to destructive interference between the antioxidant and the co-antioxidant. Conclusion: THC oxidized by peroxy radicals may be more antioxidative than the corresponding CUR in the interplay with GSH.