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

Abstract

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 Fe(3+) and Cu(2+), using rat liver cytosol and microsomes as biological systems. Our data show that, in the absence of proteins, Cu(2+)/ascorbate elicited more oxygen consumption than Fe(3+)/ascorbate under identical conditions. Presence of cytosolic and microsomal protein, however, differentially altered oxygen consumption patterns. In addition, Cu(2+)/ascorbate increased microsomal lipid peroxidation and decreased cytosolic and microsomal content of thiol groups more efficiently than Fe(3+)/ascorbate. Finally, Fe(3+)/ascorbate and Cu(2+)/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 Cu(2+) 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 Fe(3+)/ascorbate and Cu(2+)/ascorbate but not by Cu(2+) alone. N-Acetylcysteine completely prevented the damage elicited by Cu(2+)/ascorbate, Fe(3+)/ascorbate and Cu(2+) alone. N-Acetylcysteine also completely reversed the damage to thiol groups elicited by Fe(3+)/ascorbate, partially reversed that of Cu(2+)/ascorbate but failed to reverse the damage promoted by Cu(2+) alone. Our data are discussed in terms to the potential damage that the accumulation of iron and copper ions can promote in biological systems.