KINETICS OF CONGO-RED BINDING BY HEMIN-LIMITED AND HEMIN-EXCESS CELLSOF PORPHYROMONAS-GINGIVALIS W50

The binding of Congo red to P. gingivalis W50 grown in a chemostat und er haemin-limitation and haemin-excess was quantified. Congo red bound to both haemin-excess and haemin-limited cells with similar capacity and affinity. Binding of Congo red was greater than for ferri- (haemin ) or ferroprotoporphyrin IX (haem), and was not influenced by redox po tential at low added Ligand concentrations. Both haemin-limited and ha emin-excess cells showed positive co-operativity towards Congo red bin ding. Preexposure of haemin-limited and haemin-excess cells to sub-sat urating concentrations of ferriprotoporphyrin IX did not affect Congo red binding, whereas pre-exposure of haemin-excess cells to ferroproto porphyrin IX increased binding. Iron protoporphyrin IX binding was enh anced after exposure of both haemin-excess and haemin-limited cells to Congo red, especially under reducing conditions. These results confir m that Congo red binding cannot be used as an indirect measure of haem in binding, nor can Congo red be used to inhibit haemin binding to P. gingivalis. (C) 1995 Academic Press