Transfer of Copper between Bis(thiosemicarbazone) Ligands and Intracellular Copper-Binding Proteins. Insights into Mechanisms of Copper Uptake and Hypoxia Selectivity

Abstract

Bis(thiosemicarbazonato) complexes Cu(II)(Btsc) have attracted interest as promising metallodrugs and, in particular, as copper radiopharmaceuticals. Prototypes Cu(Atsm) and Cu(Gtsm) are membrane-permeable, but their metabolisms in cells are distinctly different: copper that is delivered by Cu(Gtsm) is trapped nonselectively in all cells, whereas copper that is delivered by Cu(Atsm) is retained selectively in hypoxic cells but is "washed out" readily in normal cells. We have studied copper-transfer reactions of these two complexes under various conditions, aiming to model their cellular chemistry. In Me2SO, both complexes exhibited reversible one-electron-reduction processes with Cu(Atsm) being more difficult to reduce than Cu(Gtsm) (E(1/2)'=-0.60 and -0.44 V, respectively, vs AgCl/Ag). Upon introduction of an aqueous buffer into Me2SO, the electrochemical reduction remained chemically reversible for Cu(Atsm) but became irreversible for Cu(Gtsm). However, the estimated difference in their reduction potentials did not change. Chromophoric ligand anions bicinchonate (Bca) and bathocuproine disulfonate (Bcs) were used as Cu(I) indicators to trace the destinations of copper in the reactions and to mimic cellular Cu(I)-binding components ("sinks"). While both BtscH2 ligands have high affinities for Cu(I) (KD in the picomolar range), they cannot compete with Cu(I) sinks such as the copper-binding proteins Atx1 and Ctr1c (or a mimic such as Bcs). In the presence of these proteins, reduction of Cu(II)(Btsc) leads to irreversible transfer of copper to the protein ligands. Endogenous reductants ascorbate and glutathione can reduce Cu(II)(Gtsm) in the presence of such protein ligands but cannot reduce Cu(II)(Atsm). These properties establish a strong correlation between the contrasting cellular retention properties of these complexes and their different reduction potentials. The endogenous reductants in normal cells appear to be able to reduce Cu(II)(Gtsm) but not Cu(II)(Atsm), allowing the latter to be washed out. The more reducing environment of hypoxic cells leads to reduction of Cu(II)(Atsm) and retention of its copper.

Increasing Cu bioavailability inhibits Aβ oligomers and tau phosphorylation

“…Importantly, once Cu II (gtsm) is exposed to the intracellular reducing environment, Cu II is reduced to Cu I , causing it to dissociate from the ligand and to increase Cu bioavailability. The negative control Cu II (atsm) does not reduce and dissociate under normal intracellular conditions, and therefore does not increase Cu bioavailability (18,(23)(24)(25)(26). We found both metal complexes increased cellular Cu levels several hundredfold ( Fig.…”

Copper(II)-Bis(Thiosemicarbazonato) Complexes as Antibacterial Agents: Insights into Their Mode of Action and Potential as Therapeutics

“…8). This occurs upon reduction of the Cu(II) center by cytoplasmic reductants, such as thiols (21). As a consequence of a higher Cu(II)/ Cu(I) reduction midpoint potential for Cu(gtsm) (Fig.…”

“…As a consequence of a higher Cu(II)/ Cu(I) reduction midpoint potential for Cu(gtsm) (Fig. 1), dissociation of Cu(I) ions from Cu(gtsm) is ensured (21). In contrast, Cu(atsm) possesses a lower Cu(II)/Cu(I) midpoint potential, and thus dissociation of Cu(I) ions from Cu(atsm) is not thought to occur, except in hypoxic cells (22,23).…”

Copper Complexation Screen Reveals Compounds with Potent Antibiotic Properties against Methicillin-Resistant Staphylococcus aureus

“…The bioactivities of clinically investigated bis(thiosemicarbazone) ligands have been linked to their redox stability. In particular, the reduction of Cu II to Cu I is thought to destabilize the complex, leading to the release of copper inside cells (42). This process is more likely to occur in complexes with a high redox potential (42,43).…”