Disruption of the tryptophan binding site in the human serum albumin dimer

Monomer and dimer fractions of human serum albumin (HSA) obtained from charcoal-treated Fraction V HSA have very similar fluorescence and circular dichroism spectra, but the dimer neither binds l-tryptophan nor reacts rapidly with p-nitrophenyl acetate. The latter reaction presumably occurs in a major binding site of the monomer as many strongly bound ligands including l-tryptophan, small fatty acid anions (e.g., S.-W. M. Koh and G. E. Means, 1979, Arch. Biochem. Biophys.192, 73–79), and several drugs (e.g, N. P. Sollenne and G. E. Means, 1979, Mol. Pharmacol.15, 754–757) all decrease rates of reaction in direct proportion to their concentration. Binding data for those ligands indicate the presence of less than one binding site per molecule of charcoal-treated Fraction V HSA, and thus appear to reflect only its content of albumin monomer. The absence of that binding site in the dimer may reflect its inclusion within the dimer interface.