Molecular mechanism of the unusual biphasic effects of wood preservative copper-ortho-phenanthroline and tetrachlorocatechol on cellular DNA damage.

Tetrachlorocatechol (TCC) is the primary metabolite of wood preservative pentachlorophenol (PCP) in the human body, while copper-ortho-phenanthroline (Cu(OP)₂) is another widely used environmentally friendly wood preservative. TCC and Cu(OP)₂ can coexist in the human body, potentially exerting combined toxicity. This study explores the molecular mechanisms underlying their biphasic effects on intracellular DNA, providing insights into their environmental health risks. Results show that at lower TCC-Cu(OP)₂ ratios (TCC:Cu(OP)₂ ≤ 3), significant intracellular DNA double-strand breaks occur, whereas DNA damage is suppressed when the ratio exceeds 3:1. This biphasic effect aligns with changes in intracellular/nuclear copper concentration and reactive oxygen species (ROS) generation. TCC was found to interact with Cu(OP)2 to form lipophilic TCC-Cu(OP)₂ complexes with different stoichiometry and redox-activity by complementary applications of various analytical methods. Taken together, we propose that the enhancement of TCC-Cu(OP)₂-induced cellular DNA damage at low ratios (<3:1) was due to formation of lipophilic and redox-active copper complexes which facilitated cellular copper uptake and •OH production, while the inhibition at ratios higher than 3 was due to formation of redox-inactive copper complexes. Thus, the biphasic effect is attributed to two physicochemical factors: (1) copper bioavailability and (2) copper redox activity. At low TCC-to-Cu(OP)₂ ratios, the formation of lipophilic copper complexes promotes Cu(OP)₂ entry into cells, inducing ROS generation and DNA damage. In contrast, at high TCC-Cu(OP)₂ ratios, Cu(OP)₂ entry into cells is inhibited, and its reactivity is significantly suppressed, thereby failing to induce DNA damage via ROS generation. This study holds significant implications in the field of environmental health, offering new perspectives for assessing the environmental risks of related compounds.