The thermodynamics of the interaction of mucus proteinase inhibitor (MPI) with heparin has been studied. The apparent association constant K’ and the enthalpy of reaction have been determined in buffered aqueous solutions (Hepes + NaCl) using flow microcalorimetry. The measurements have been performed as a function of pH, temperature, and NaCl concentration, and the standard transformed Gibbs energy, enthalpy, entropy, and heat capacity of association have been calculated. The following values have been found in 0.05 M Hepes and 0.1 M NaCl at pH 7.4 and 25 degrees C : K’ = (4.9 +/- 1.3) x 10(6); Delta(r)G’(d)egrees = -38.2 +/- 0.6 kJ mol(-1); Delta(r)H’(d)egrees = -64.9 +/- 1.3 kJ mol(-1); T Delta(r)S’(d)egrees = -26.7 +/- 1.9 kJ mol(-1); Delta rC’(d)(p)egrees = -180 +/- 20 J K-1 mol(-1). The pH dependency of K’ shows that one proton of the medium is transferred to the MPI-heparin complex upon association. This proton uptake is controlled by a group on the protein whose pK(a) is lower than 9. NaCl strongly decreases K’, an effect that indicates that heparin binds to MPI through five or six ionic interactions. NaCl also increases Delta(r)H’(d)egrees and Delta(r)S’(d)egrees, but the association process is enthalpy-driven over the whole range of NaCl concentrations. The small effect of temperature on Delta(r)H’(d)egrees and Delta(r)S’(d)egrees suggests that hydrophobic interactions are not involved in the association process. Our thermodynamic data also suggest that heparin behaves as a Ligand of the agonist type for the "receptor" MPI which may undergo a structural transition following the binding of the ligand.