RAPID AND PARALLEL FORMATION OF FE3-TYPE SUBUNIT FERRITIN MINERALIZATION( MULTIMERS, INCLUDING A TRIMER, DURING H)

Conversion of Fe ions in solution to the solid phase in ferritin conce ntrates iron required for cell function. The rate of the Fe phase tran sition in ferritin is tissue specific and reflects the differential ex pression of two classes of ferritin subunits (H and L). Early stages o f mineralization were probed by rapid freeze-quench Mossbauer, at stro ng fields (up to 8 T), and EPR spectroscopy in an H-type subunit, reco mbinant frog ferritin; small numbers of Fe (36 moles/mol of protein) w ere used to increase Fe3+ in mineral precursor forms, At 25 ms, four F e3+-oxy species (three Fe dimers and one Fe trimer) were identified, T hese Fe3+-oxy species were found to form at similar rates and decay su bsequently to a distinctive superparamagentic species designated the ' 'young core.'' The rate of oxidation of Fe2+ (1026 s(-1)) corresponded well to the formation constant for the Fe3+- tyrosinate complex (920 s(-1)) observed previously [Waldo, G. S., & Theil, E. C. (1993) Bioche mistry 32, 13261] and, coupled with EPR data, indicates that several o r possibly all of the Fe3+-oxy species involve tyrosine. The results, combined with previous Mossbauer studies of Y30F human H-type ferritin which showed decreases in several Fe3+ intermediates and stabilizatio n of Fe2+ [Bauminger, E. R., et al. (1993) Biochem, J. 296, 709], emph asize the involvement of tyrosyl residues in the mineralization of H-t ype ferritins. The subsequent decay of these multiple Fe3+-oxy species to the superparamagnetic mineral suggests that Fe3+ species in differ ent environments may be translocated as intact units from the protein shell into the ferritin cavity where the conversion to a solid mineral occurs.