Changes in the Affinity of Haemin-binding by Porphyromonas gingivalis W50 Under Different Environmental Conditions

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Abstract
The binding affinities of haemin (ferriprotoporphyrin IX chloride) were measured for whole cells of Porphyromonas gingivalis W50 grown in a chemostat under haemin-limitation (HL) and haemin-excess (HE), using a spectropho-tometric assay under oxidising and reducing conditions. Plots of bound (B) versus free ligand (F), and 1/B versus 1/F revealed haemin-excess cells to have both higher binding capacities and affinities for both ferriprotoporphyrin IX (oxidised) and ferroprotoporphyrin IX (reduced), than haemin-limited cells. Rosenthal (modified Scatchard) plots of B/F versus B indicated a biphasic system of ferriprotoporphyrin binding to both HE and HL cells, with high and low affinity binding sites. For either of the high or low affinity sites, the binding maxima for both cell types were similar. However, the association binding constants (Ka) of both high and low affinity sites for the HE cells were greater than their HL counterparts. The binding of ferroprotoporphyrin to both HE and HL cells under reducing conditions yielded Rosenthal 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 acquire iron porphyrin in the partially anaerobic gingival sulcus or in a periodontal pocket at a much lower redox potential may enhance the virulence of P. gingivalis, and may be a significant factor in the development and progression of periodontal disease at a site.