Hematoporphyrin binding sites on human serum albumin

Introduction: Photodynamic therapy is a minimally invasive clinical treatment modality for a variety of premalignant and malignant conditions combining a photosensitizing drug, oxygen and light irradiation. Hematoporphyrin is an organic photosensitizer, which mediates inhibition of endothelial cell proliferation and induces apoptosis. Human serum albumin is an endogenous drug carrier for hematoporphyrin. The present study aimed to investigate the hematoporphyrin binding to human serum albumin, which is its transport protein. Material and methods: The chemical reagents were hematoporphyrin (Hp), human serum albumin (HSA) and bovine serum albumin (BSA). In the experiment two techniques were used: spectrofluorimetry and UV-Vis absorption spectrophotometry. Results: The binding sites for Hp were identified in the tertiary structure of HSA by fluorescence quenching technique. The experiment with BSA delivered additional data on Hp-albumin interactions close to Trp135. The participation of tyrosyl residues apart from tryptophanyl ones was discussed. A decrease of the polarity in the binding sites, testifying to possible hydrogen bonding in the binding sites, was also described. The binding and quenching constants Hp-HSA and Hp-BSA were determined as well as the number of binding sites. Conclusions: Hp locates in subdomain IIA in the tertiary structure of HSA. The location in subdomain I close to Trp135 is also possible. Hp is also able to interact within tyrosyl residues.