Binding of carprofen to human and bovine serum albumins

The binding of carprofen (CP) to human serum albumin (HSA) and bovine serum albumin (BSA) was compared using equilibrium dialysis method. The affinity of CP for the primary binding site was BSA > HSA. However, the number of primary binding sites (n1) was 1.94 on HSA, considerably greater than that on BSA (0.79). The displacement of the binding of CP to HSA and BSA was studied in the presence of phenylbutazone (PB, site I marker), ibuprofen (IB, site II marker) or 2,3,5-triiodobenzoic acid (TIB, both site I and II marker). The binding of CP to HSA was altered by PB, IB and TIB, while the binding of CP to BSA was not changed by PB, although it was reduced by IB and TIB. These results suggested that, for the binding of CP, HSA has two major sites (site I and site II), whereas BSA has a single primary site (site II). The binding characteristics of 2-anthracenecarboxylic acid with HSA and BSA were found quite similar to those of CP. Thus, it seemed that long, planar compounds with a carboxyl group at one end can bind to site I and site II on HSA, but only to site II on BSA. The difference between HSA and BSA in the affinity of site I may be due to the difference in the basic and hydrophobic amino acid residues.