CHANGES IN THE AFFINITY OF HEMIN-BINDING BY PORPHYROMONAS-GINGIVALIS W50 UNDER DIFFERENT ENVIRONMENTAL-CONDITIONS

The binding affinities of haemin (ferriprotoporphyrin IX chloride) wer e measured for whole cells of Porphyromonas gingivalis W50 grown in a chemostat under haemin-limitation (HL) and haemin-excess (HE), using a spectrophotometric assay under oxidising and reducing conditions. Plo ts of bound (B) versus free ligand (F), and 1/B versus 1/F revealed ha emin-excess cells to have both higher binding capacities and affinitie s for both ferriprotoporphyrin IX (oxidised) and ferroprotoporphyrin I X (reduced), than haemin-limited cells. Rosenthal (modified Scatchard) plots of B/F versus B indicated a biphasic system of ferriprotoporphy rin binding to both HE and HL cells, with high and low affinity bindin g sites. For either of the high or low affinity sites, the binding max ima for both cell types were similar. However, the association binding constants (K-3) of both high and low affinity sites for the HE cells were greater than their HL counterparts. The binding of ferroprotoporp hyrin to both HE and HL cells under reducing conditions yielded Rosent hal plots which were indicative of cooperativity. These data indicate that P. gingivalis may bind iron protoporphyrin IX under conditions of both high and low redox potential in vivo. However, the ability to ac quire iron porphyrin in the partially anaerobic gingival sulcus or in a periodontal pocket at a much lower redox potential may enhance the v irulence of P. gingivalis, and may be a significant factor in the deve lopment and progression of periodontal disease at a site.