Proton and Iron Binding by the Cyanobacterial Toxin Microcystin-LR

Microcystins (MCs) are a group of hepatotoxins produced by cyanobacteria that have not had their functional role or the environmental factors that trigger production clearly determined. One suggestion is that microcystins are siderophores (i.e., ligands with an extremely high affinity with iron, typically with stability constants substantially greater than 1025). In this work, we explore proton and iron binding with microcystin-LR (MC-LR). Using UV–visible spectroscopy and a HPLC peak retention time-based method, the two acid dissociation constants associated with the carboxylic groups of MC-LR were determined to be: pKa1 = 2.17 and pKa2 = 3.96. Cyclic voltammetry provides evidence for the formation of at least two FeIII-MC-LR complexes, with the FeIII reduction peak significantly shifted to more reducing potentials in the presence of MC-LR. These complexes have been interpreted as a rapidly formed initial complex (Complex 1) and a more stable, and slower forming, Complex 2. The stability constant for FeIII-MC-LR (Complex 2) was estimated to be approximately 1013 in 60% v/v MeOH/water at 0.1 M ionic strength. The electrochemical experiments provide no evidence for the formation of a complex between Fe2+ and MC-LR. Given that most MC-LR is released only upon cell lysis, and coupled with the moderate strength of the stability constant with FeIII determined in this study, it appears unlikely that that MC-LR is an extracellular siderophore. If MC-LR is involved in iron regulation in cyanobacteria, it is more likely as a shuttle for iron across the cell membrane or in intracellular processes.