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

The interaction of tetrasulfonated aluminum phthalocyanines (AlPcS4) with h uman serum albumin (HSA) has been studied. From binding isotherms based on spectrophotometric titration, the existence of two binding sites was inferr ed. An association constant for the primary binding site was (1.2 +/- 0.2) x 10(7) M-1 at 24 degreesC and decreased to (3.3 +/- 0.3) x 10(5) M-1 at 37 degreesC. By using the competitive binding of well-studied HSA ligands, th is site was found to be situated on domain I of HSA and to overlap the bind ing region of hemin. An association constant for the secondary binding site was calculated to be (6 +/- 2)x 10(4) M-1 at 24 degreesC. This site was sh own to be out of the main binding regions of the subdomains IIA and IIIA. T he AlPcS4 binding to HSA is predominantly electrostatic since it is highly sensitive to ionic strength. A high-binding affinity and an abundance of th e protein in blood serum indicate that HSA should be considered as the main endogenous carrier of AlPcS4. (C) 2001 Elsevier Science B.V. All rights re served.