Radical Production and Cytotoxic Activity of tert -Butyl-Substituted Phenols

2,4,6-Tri-tert-butylphenol (TBP)-related compounds are used for stabilizing plastics by making them resistant to oxidation. However, the cytotoxic activity of these compounds has not yet been established. TBP produced phenoxyl radicals at pH ≥ 9.0 and 2,4-di-t-butylphenol (DBP) at pH 12.5, but 3,3′,5,5′-tetra-t-butyl-1,1′-biphenyl-2,2′-diol (bisDBP) did not, using ESR spectroscopy. Both superoxide anion radical (O₂^-) scavenging activity and reactive oxygen species (ROS) production activity declined in the order of TBP > DBP > bisDBP. The cytotoxic activity against human oral tumor cell lines (HSC-2, HSG) and human gingival fibroblast cells (HGF) declined in the order of DBP >> bisDBP = TBP = TBP-OOH (2,4,6-tri-t-butyl-4-hydroperoxy-2,5-cyclohexadiene-1-one). The cytotoxic activity of TBP, but not of DBP or bisDBP was significantly enhanced after visible light (VL)-irradiation for 10 min. The cytotoxicity of irradiated TBP was significantly higher than that of either original TBP or TBP-OOH, the oxidative metabolite of TBP, possibly due to the formation of TBP stable radical and ROS via oxidation. In contrast, the cytotoxic activity of DBP and bisDBP was independent of radical production, and therefore, may be intrinsic. A non-enzymatic oxidation decomposition of DBP or TBP was estimated from the formation of reaction enthalpy (ΔH) using a modified neglect of diatomic overlap, parametric method 3 (MNDO-PM3) semi-empirical method, suggesting that O₂ is capable of activating DBP to a reactive quinone or dimer and that TBP phenoxyl radicals via oxidation directly affect extra- or intracellular bioactive materials, resulting in the induction of cytotoxicity.