THE PERIODONTOPATHOGEN PORPHYROMONAS-GINGIVALIS BINDS IRON PROTOPORPHYRIN-IX IN THE MU-OXO DIMERIC FORM - AN OXIDATIVE BUFFER AND POSSIBLE PATHOGENIC MECHANISM

Mossbauer spectroscopy was used to re-evaluate iron protoporphyrin IX, FePPIX, binding and the chemical nature of the black iron porphyrin p igment of Porphyromonas gingivalis. We demonstrate that FePPIX is boun d to the cell in the mu-oxo dimeric form, [Fe(III)PPIX](2)O, and that the iron porphyrin pigment is also composed of this material. P. gingi valis also assimilated monomeric Fe(II)- and Fe(III)PPIX into mu-oxo d imers in vitro. Scatchard analysis revealed a greater binding maximum of cells for mu-oxo dimers than for monomeric Fe(III)- or Fe(II)PPIX, although the relative affinity constant for the dimers was lower. Form ation of [Fe(III)PPIX](2)O via reactions of Fe(II)PPIX with oxygen, an d its toxic derivatives, would serve as an oxidative buffer and permit P. gingivalis and other black-pigmenting anaerobes to engender and ma intain a local anaerobic environment. Tying up of free oxygen species with iron protoporphyrin IX would also reduce and limit Fe(II)PPIX-med iated oxygen-radical cell damage. More importantly, formation of a cel l-surface mu-oxo dimer layer may function as a protective barrier agai nst assault by reactive oxidants generated by neutrophils. Selective i nterference with these mechanisms would offer the possibility of atten uating the pathogenicity of P. gingivalis and other iron protoporphyri n IX-binding pathogens whose virulence is regulated by this reactive m olecule.