Organic ligands reduce copper toxicity in Pseudomonas syringae

Abstract

Pseudomonas syringae cells were exposed to 100 μM copper alone, or to previously equilibrated copper sulfate‐ligand solutions. Ligand concentrations were determined experimentally as those that reduced the free copper concentration to 5 μM (determined with a Cu²⁺‐selective electrode). These values were in agreement with those calculated by computational equilibrium simulation based on published stability constants. Exposure of P. syringae cells to copper sulfate, chloride, or nitrate resulted in similar high mortality, suggesting that copper was responsible for cell death. Acetate, succinate, proline, lysine, cysteine, and EDTA significantly reduced both the amount of copper bound to the cells and cell death, indicating that not only strong chelating agents but also weak and moderate copper ligands can effectively antagonize copper toxicity. However, cysteine and EDTA were considerably more effective than acetate, succinate, proline, and lysine, indicating that copper toxicity is not simply a function of free copper concentration but depends on the nature of the ligand. The results suggested that a significant fraction of copper bound to acetate, succinate, proline, or lysine was displaced to the bacteria or, alternatively, mixed copper‐ligand‐cell complexes could be formed. On the contrary, none of these phenomena occurred for the copper complexes with cysteine or EDTA.