Microcalorimetric study of the interaction of human and bovine serum albumins with tryptophan and tryptophan analogs.

The thermodynamic parameters for the protein-ligand binding have been obtained by microcalorimetry on three albumin samples (fatty acid free human serum albumin (I), fraction V human serum albumin (II), and fraction V bovine serum albumin (III)) bound with l-tryptophan (A) and three-ring tryptophan analogs (B and B*). The percentage, mu, of binding molecules (equivalent to the number of sites) is found to be 1.0 for I and 0.65 for II (in good agreement with dialysis results on the same systems) and 1.0 for III. The large negative delta H(bind) (-27.2 to -33.2 kJ . mol-1) constitutes the main contribution to delta G(bind) (-23.0 to -31.2 kJ . mol-1). The better binding of I-III towards B and B* compared with A is due to delta S(bind) being less negative. This is interpreted as a lesser loss of entropy for the three-ring ligands than for the normal tryptophan when they are bound. Data obtained on the proteins (heat of dilution; Huggins' constant, k') correlate well with mu or Qmax. This indicates that these physicochemical data could be used to characterize and compare rapidly some albumins of different sources. The unexpected finding that the parameters of I and III are nearer to each other than they are from II is discussed.