Deuteroporphyrin-albumin binding equilibrium. The effects of porphyrin self-aggregation studied for the human and the bovine proteins

The binding equilibrium of deuteroporphyrin IX to human serum albumin and to bovine serum albumin was studied, by monitoring protein-induced changes in the porphyrin fluorescence and taking into consideration the self-aggregation of the porphyrin. To have control over the latter, the range of porphyrin concentration was chosen to make dimers (non-covalent) the dominant aggregate. Each protein was found to have one high-affinity site for deuteroporphyrin IX monomers, the magnitudes of the equilibrium binding constants (25°C, neutral pH, phosphate-buffered saline) being 4.5(± 1.5) x 10⁷ M^-1 and 1.7(± 0.2) x 10⁶ M^-1 for human serum albumin and for bovine serum albumin respectively. Deuteroporphyrin IX dimers were found to bind directly to the protein, each protein binding one dimer, with high affinity. Two models are proposed for the protein-binding of porphyrin monomers and dimers in a porphyrin system having both species: a competitive model, where each protein molecule has only one binding site, which can be occupied by either a monomer or a dimer; a non-competitive model, where each protein molecule has two binding sites, one for monomers and one for dimers. On testing the fit of the data to the models, an argument can be made to favour the non-competitive model, the equilibrium binding constants of the dimers, for the non-competitive model (25°C, neutral pH, phosphate-buffered saline), being 8.0(± 1.8) x 10⁸ M^-1 and 1.2(± 0.6) x 10⁷ M^-1 for human serum albumin and bovine serum albumin respectively.