New insights into iron release from ferritin: direct observation of the neurotoxin 6-hydroxydopamine entering ferritin and reaching redox equilibrium with the iron core

Abstract

Iron release from the iron storage protein ferritin has been studied extensively because of its important role in oxidative stress and its possible role in the progression of Parkinson's disease. For many years external indicators, notably strong iron(II) chelators, have been used to investigate this reaction. Such chelators can, however, drastically affect the electrochemical and thermodynamic properties of iron. The present study is unique in that it has been possible to follow a reaction taking place within the ferritin shell. This was made possible by our serendipitous discovery that, at physiological pHs, the oxidation product of 6-hydroxydopamine (a deprotonated quinone) acts as its own indicator (G. N. L. Jameson and W. Linert, J. Chem. Soc., Perkin Trans. 2, 2001, 563–568). The redox equilibrium data and the kinetics of the formation of this red-coloured species can only be explained on the basis that reduction of the iron(III) takes place within the ferritin shell. This is, in fact, the first time that a reaction actually taking place inside the ferritin shell has been followed. It has also been established that, at least in vitro, all eight hydrophilic channels are capable of being simultaneously involved in the reaction. It has also been possible to calculate the rate of oxidation of the 6-hydroxydopamine within the ferritin and it is demonstrated that a redox equilibrium is established within the protein. Finally, evidence is provided confirming that chelators are in fact intrinsically linked to iron removal from ferritin.