Impact of human serum albumin on CuII and ZnII complexation by ATSM (diacetyl-bis(N4-methylthiosemicarbazone)) and a water soluble analogue

Abstract

The chelator diacetyl-bis(N4-methylthiosemicarbazone) (ATSM) and its complexes with Cu^II and Zn^II are becoming increasingly investigated for medical applications such as PET imaging for anti-tumour therapy and the treatment of amyotrophic lateral sclerosis. However, the solubility in water of both the ligand and the complexes presents certain limitations for in vitro studies. Moreover, the stability of the Cu^II and Zn^II complexes and their metal exchange reaction against the potential biological competitor human serum albumin (HSA) has not been studied in depth. In this work it was observed that the ATSM with an added carboxylic group into the structure increases its solubility in aqueous solutions without altering the coordination mode and the conjugated system of the ligand. The poorly water-soluble Cu^II– and Zn^II–ATSM complexes were prevented from precipitating due to the binding to HSA. Both HSA and ATSM show a similar thermodynamic affinity for Zn^II. Finally, the Cu^II-competition experiments with EDTA and the water-soluble ATSM ligands yielded an apparent log K_d at pH 7.4 of about −19. When ATSM was added to Cu^II- and Zn^II-loaded HSA, withdrawing of Zn^II was kinetically favoured, but this metal is slowly substituted by the Cu^II afterwards taken from HSA so that this protein could be considered as a source of Cu^II for ATSM.