Antioxidant and Anti-inflammatory Activity of Eugenol-related Compounds

Objectives: We have previously reported that 2-methoxyphenol dehydrodi-isoeugenol exerts an effective anti-inflammatory effect, as judged in terms of COX-2 inhibition, and a causal link between this and its antioxidant activity has been suggested. Therefore, we investigated the antioxidant activity of eugenol-related compounds using the induction period method and the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, and addressed the relationship between this and their anti-inflammatory activity. Methods: The activity of compounds was evaluated by the induction period method from the kinetics of polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of BPO (peroxy radical, PhCOO^.), using differential scanning calorimetry. The DPPH assay was carried out in ethanol solution. Lipopolysaccharide (LPS)-stimulated activator protein-1 (AP-1) binding to the 12-tetradecanoylphorbol-13-acetate-responsive element (TRE) sequence was evaluated by gel mobility shift assay using a ³²P-labeled double-strand oligonucleotide containing the TGACTCA sequence. Results: PhCOO^.-scavenging activity declined in the order alpha-di-isoeugenol > bis-eugenol > isoeugenol > o-methoxyphenol > eugenol > dehydrodi-isoeugenol > vaniline. DPPH^.-scavenging activity declined in the order bis-eugenol > isoeugenol > alpha-di-isoeugenol > eugenol > o-methoxyphenol > dehydrodi-isoeugenol > vaniline. Dehydrodi-isoeugenol showing an effective anti-inflammatory action with less antioxidant activity. To clarify the reasons for these findings, the phenolic O-H bond-dissociation enthalpy (BDE) of eugenol-related compounds was calculated by the DFT method. The phenolic O-H BDE of the most potent compound, dehydrodi-isoeugenol, was 85.99 kcal/mol. The values for eugenol and isoeugenol were < 85 kcal/mol. The non-steroidal anti-inflammatory drug (NSAID)-like activity of dehydrodi-isoeugenol appeared to be related to its phenol function. Binding of AP-1 to the TRE sequence in LPS-stimulated cells was inhibited by dehydrodi-isoeugenol at 0.1 mM, but not by isoeugenol and eugenol. Conclusion: No relationship between the anti-inflammatory and antioxidant activities was found. Compounds with effective anti-inflammatory activity acted as a phenol function.