Mechanisms underlying iron and copper ions toxicity in biological systems: Pro-oxidant activity and protein-binding effects

Iron and copper ions, in their unbound form, may lead to the generation of reactive oxygen species via Haber–Weiss and/or Fenton reactions. In addition, it has been shown that copper ions can irreversibly and non-specifically bind to thiol groups in proteins. This non-specific binding property has not been fully addressed for iron ions. Thus, the present study compares both the pro-oxidant and the non-specific binding properties of Fe3+ and Cu2+, using rat liver cytosol and microsomes as biological systems. Our data show that, in the absence of proteins, Cu2+/ascorbate elicited more oxygen consumption than Fe3+/ascorbate under identical conditions. Presence of cytosolic and microsomal protein, however, differentially altered oxygen consumption patterns. In addition, Cu2+/ascorbate increased microsomal lipid peroxidation and decreased cytosolic and microsomal content of thiol groups more efficiently than Fe3+/ascorbate. Finally, Fe3+/ascorbate and Cu2+/ascorbate inhibited in different ways cytosolic and microsomal glutathione S-transferase (GST) activities, which are differentially sensitive to oxidants. Moreover, in the absence of ascorbate, only Cu2+ decreased the content of cytosolic and microsomal thiol groups and inhibited cytosolic and microsomal GST activities. Catechin partially prevented the damage to thiol groups elicited by Fe3+/ascorbate and Cu2+/ascorbate but not by Cu2+ alone. N-Acetylcysteine completely prevented the damage elicited by Cu2+/ascorbate, Fe3+/ascorbate and Cu2+ alone. N-Acetylcysteine also completely reversed the damage to thiol groups elicited by Fe3+/ascorbate, partially reversed that of Cu2+/ascorbate but failed to reverse the damage promoted by Cu2+ alone. Our data are discussed in terms to the potential damage that the accumulation of iron and copper ions can promote in biological systems.