The release of iron from horse spleen ferritin by reduced flavins

Abstract

Ferritin-Fe(III) was rapidly and quantitatively reduced and liberated as Fe(II) by FMNH2, FADH2 and reduced riboflavin. Dithionite also released Fe(II) from ferritin but at less than 1% of the rate with FMNH2. Cysteine, glutathione and ascorbate gave a similar slower rate and yielded less than 20% of the total iron from ferritin within a few hours. The reduction of ferritin-Fe(III) by the three riboflavin compounds gave complex second-order kinetics with overlapping fast and slow reactions. The fast reaction appeared to be non-specific and may be due to a reduction of Fe(III) of a lower degree of polymerization, equilibrated with ferritin iron. The amount of this Fe3+ ion initially reduced was small, less than 0.3% of the total iron. Addition of FMN to the ferritin–dithionite system enhanced the reduction; this is due to the reduction of FMN by dithionite to form FMNH2 which then reduces ferritin-Fe(III). A comparison of the thermodynamic parameters of FMNH2–ferritin and dithionite–ferritin complex formation showed that FMNH2 required a lower activation energy and a negative entropy change, whereas dithionite required 50% more activation energy and showed a positive entropy change in ferritin reduction. The effectiveness of FMNH2 in ferritin–Fe(III) reduction may be due to a specific binding of the riboflavin moiety to the protein portion of the ferritin molecule.