STUDIES ON THE INTERACTION OF HEMATOPORPHYRIN WITH HEMOGLOBIN

Spectrophotometric and spectrofluorimetric studies reveal that an inte raction occurs between hemoglobin and hematoporphyrin, a photosensitiz ing drug used in photodynamic therapy. Two concentration ranges of hem atoporphyrin, 0.4-0.9 mu M and 1.8-3.6 mu M, representing significantl y monomeric and aggregated (dimeric) state, respectively, have been us ed in the binding studies. The binding affinity constant (K) decreases , while the possible number of binding sites (p) increases as the conc entration range of the porphyrin is increased. The nature of interacti on has been studied by fluorescence quenching titration method under d ifferent ionic strengths and temperature conditions. It appears to be predominantly electrostatic and enthalpy-driven in the lower range of porphyrin concentration. However, the interaction follows mostly hydro phobic and entropy-driven modality in the higher concentration range o f the ligand. The porphyrin-hemoglobin interaction results in release of oxygen from the protein. The extent of oxygen release depends on th e stoichiometric ratio of hematoporphyrin: hemoglobin. (C) 1997 Elsevi er Science S.A.