Binding of pyrene-1-butyric acid to serum albumin : species differences

Changes of the fluorescence spectra and quantum yield of pyrene-1-butyric acid (PYB) induced by rat (RSA), horse (HoSA), and pig serum albumins (PSA) and rat serum were compared at pH 7.4. RSA and rat serum caused marked quenching of the PYB fluorescence. On the contrary, HoSA and PSA increased the fluorescence intensity. In RSA and rat serum solutions, the PYB fluorescence intensity due to monomer emission was greatly diminished, and an additional emission due to excimer formation appeared at 480 nm. On the other hand, in the case of HoSA and PSA solutions, monomer emission gave rise to high fluorescence intensity and no excimer formation was observed. The binding parameters of PYB to RSA, HoSA, and PSA as well as human (HSA), bovine (BSA), rabbit (RbSA) and dog serum albumins (DSA) were compared by equilibrium dialysis method. The primary binding site affinity was in the order RSA>HSA>HoSA>RbSA>DSA≥BSA>PSA. However, for RSA and HSA, which showed reduced monomer emission and clear excimer fluorescence, the primary binding site number (n1) was 2.71 and 2.90, respectively, considerably greater than those values for the other serum albumins. These findings suggest that PYB binds to RSA and HSA as a dimer. On the other hand, PYB appears to bind with BSA, RbSA, DSA and HoSA as a monomer. The primary binding site was not clearly recognized in PSA, which showed little enhancement of fluorescence. The similar behavior of RSA and HSA in the PYB binding may be attributed to their resemblance in the hydrophobic environment and flexibility around their binding sites.