Proton and Iron Binding by the Cyanobacterial Toxin Microcystin-LR

摘要

Microcystins (MCs) are a group of hepatotoxins produced by rncyanobacteria that have not had their functional role or the environmental factorsrnthat trigger production clearly determined. One suggestion is that microcystins arernsiderophores (i.e., ligands with an extremely high affinity with iron, typically withrnstability constants substantially greater than 10~(25)). In this work, we explore protonrnand iron binding with microcystin-LR (MC-LR). Using UV-visible spectroswpyrnand a HPLC peak retention time-based method, the two acid dissociation constantsrnassociated with the carboxylic groups of MC-LR were determined to be: pK_(a1) = 2.17rnand pK_(a2) = 3.96. Cyclic voltammetry provides evidence for the formation of at leastrntwo Fe~(Ⅲ)-MC-LR wmplexes, with the Fe~(Ⅲ) reduction peak significantly shifted to more reducing potentials in the presence ofrnMC-LR. These complexes have been interpreted as a rapidly formed initial complex (wmplex 1) and a more stable and slowerrnforming, Complex 2. The stability constant for Fe~(Ⅲ)-MC-LR (Complex 2) was estimated to be approximately 10~(13) in 60% v/vrnMeOH/water at 0.1 M ionic strength. The electrochemical experiments provide no evidence for the formation of a complexrnbetween Fe~(2+) and MC-LR. Given that most MC-LR is released only upon cell lysis, and coupled with the moderate strength ofrnthe stability cownstant with Fe~(Ⅲ) determined in this study, it appears unlikely that that MC-LR is an extracellular siderophore. IfrnMC-LR is involved in iron regulation in cyanobacteria, it is more likely as a shuttle for iron across the cell membrane or inrnintracellular processes.