The Glutathione/Metallothionein System Challenges the Design of Efficient O2 -Activating Copper Complexes

Abstract

Copper complexes are of medicinal and biological interest, including as anticancer drugs designed to cleave intracellular biomolecules by O₂ activation. To exhibit such activity, the copper complex must be redox active and resistant to dissociation. Metallothioneins (MTs) and glutathione (GSH) are abundant in the cytosol and nucleus. Because they are thiol-rich reducing molecules with high Cu^I affinity, they are potential competitors for a copper ion bound in a copper drug. Herein, we report the investigation of a panel of Cu^I /Cu^II complexes often used as drugs, with diverse coordination chemistries and redox potentials. We evaluated their catalytic activity in ascorbate oxidation based on redox cycling between Cu^I and Cu^II , as well as their resistance to dissociation or inactivation under cytosolically relevant concentrations of GSH and MT. O₂ -activating Cu^I /Cu^II complexes for cytosolic/nuclear targets are generally not stable against the GSH/MT system, which creates a challenge for their future design.

Keywords: bioinorganic chemistry; copper-based drugs; glutathione; metallothionein; redox activity.

Figure 1.

Schematic representation of the mechanism of copper-catalyzed ROS production with O₂ and AscH⁻. Copper undergoes redox cycling between Cu^II/Cu^I redox-states, through AscH⁻ oxidation to AscH^•−. O₂ is reduced by Cu^I to O₂^•−, H₂O₂ and HO^• via one electron events.

Figure 2.

Time course of AscH⁻ oxidation monitored by absorbance spectroscopy at λ_max= 265 nm (representative data sets). The reaction of AscH⁻ oxidation was triggered with the addition of preformed Cu^II/Cu^I-complexes after 10 min (black arrow). Experimental conditions: preformed copper-complexes CuL_1/2 5 μM concentration, AscH⁻ 100 μM, in HEPES 50 mM, pH 7.4.