A spectroscopic study of the interaction of tetrasulfonated aluminum phthalocyanine with human serum albumin

Abstract

The interaction of tetrasulfonated aluminum phthalocyanines (AlPcS4) with human serum albumin (HSA) has been studied. From binding isotherms based on spectrophotometric titration, the existence of two binding sites was inferred. An association constant for the primary binding site was (1.2±0.2)×107M−1 at 24°C and decreased to (3.3±0.3)×105M−1 at 37°C. By using the competitive binding of well-studied HSA ligands, this site was found to be situated on domain I of HSA and to overlap the binding region of hemin. An association constant for the secondary binding site was calculated to be (6±2)×104M−1 at 24°C. This site was shown to be out of the main binding regions of the subdomains IIA and IIIA. The AlPcS4 binding to HSA is predominantly electrostatic since it is highly sensitive to ionic strength. A high-binding affinity and an abundance of the protein in blood serum indicate that HSA should be considered as the main endogenous carrier of AlPcS4.