Lactoperoxidase belongs to a family of mammalian peroxidases that catalyze the oxidation of halides and small organic molecules in the presence of H2O2. We have used photoacoustic calorimetry to characterize thermodynamic parameters associated with ligand dissociation from bovine milk lactoperoxidase. Upon CO photorelease, a prompt (tau < 50 ns) exothermic volume contraction (Delta H = -20 +/- 7 kcal mol(-1) and Delta V = -2 +/- 1 mL mol(-1)) was measured at pH 7.0 and 4.0, whereas an endothermic expansion (Delta H = 30 +/- 13 kcal mol(-1) and Delta V = 9 +/- 2 mL mol(-1)) was observed at pH 10.0 and 7.0 in the presence of 500 mM NaCl. We attribute the observed volume and enthalpy changes to electrostriction arising from changes in the charge distribution associated with a reorganization of the heme binding pocket upon ligand dissociation. It is likely that cleavage of the Fe-CO bond is accompanied by distortion of a salt bridge between Arg557 and the heme propionate group, resulting in the observed electrostriction due to changes in charge distribution.