Sulfide is an efficient iron releasing agent for mammalian ferritins

The most prominent role of mammalian ferritins is to provide an extensive iron-buffering capacity to cells. The large ferritin iron stores can be mobilized in vitro, but the functional relevance of the most efficient iron releasing agents remains elusive. Sulfide is a strongly reducing chemical generated by a series of enzymes. In the presence of limited amounts of sulfide a continuous rate of iron release from ferritin was observed and a majority of the protein iron core was recovered in solution. The rate constants for iron efflux triggered by several reducing or chelating compounds have been measured and compared. Although not as efficient as reduced flavins, sulfide displayed kinetic parameters which suggest a potential physiological role for the chalcogenide in converting the iron storage protein into apoferritin. To further probe the relevance of sulfide in the mobilization of iron, several enzymes, such as NifS, rhodanese, or sulfite reductase generating reduced forms of sulfur by different mechanisms, have been assayed for their ability to catalyze the release of iron from ferritin. The results show that full reduction of sulfur into sulfide is needed to deplete iron from ferritin. These reactions suggest links between sulfur metabolism and intracellular iron homeostasis.