Multifunctional antioxidant activities of alkyl gallates

A series (C₁ to C₁₆) of alkyl gallates was tested for their antioxidant activity for food protection and human health. One molecule of alkyl gallate, regardless of alkyl chain length, scavenges six molecules of 1,1-diphenyl-2-picrylhydrazyl (DPPH). Alkyl gallates inhibited the linoleic acid peroxidation catalyzed by soybean lipoxygenase-1 (EC 1.13.11.12, Type 1) without being oxidized. The progress curves for enzyme reactions were recorded by both spectrophotometric and polarographic methods and the inhibitory activity was a parabolic function of their lipophilicity (log P) and maximized with alkyl chain length between C₁₂ and C₁₆. Tetradecanyl (C₁₄) gallate exhibited the most potent inhibition with an IC₅₀ of 0.06 μM. The inhibition kinetics of dodecyl gallate (C₁₂) revealed competitive and slow-binding inhibition. Alkyl gallates chelate transition metal ions and this chelation ability should be of considerable advantage as antioxidants. Additionally, gallic acid was found to inhibit superoxide anion generated by xanthine oxidase (EC 1.1.3.22) but did not inhibit enzymatically catalyzed uric acid formation.