Thermodynamics of Binding Interactions between Bovine β-Lactoglobulin A and the Antihypertensive Peptide β-Lg f142-148

The binding capacity of bovine β-lactoglobulin variant A (β-Lg A) for six peptides derived from β-Lg was evaluated using an ultrafiltration method under the following conditions:  pH 6.8, 40 °C, and a β-Lg A/peptide molar ratio of 1:5. Only peptides β-Lg f102-105, f142-148, and f69-83 bound in significant amounts to β-Lg A corresponding to 1.5, 1.1, and 0.7 mol of peptide per mole of β-Lg A, respectively. The interaction between β-Lg A and the antihypertensive peptide β-Lg f142-148 was investigated further by isothermal titration calorimetry. The binding isotherms at pH 6.8 and 25 °C confirmed that β-Lg f142-148 bound to β-Lg A and that the interaction followed a sequential three-site binding model with constants of association of 2 × 10³, 1 × 10³, and 0.4 × 10³ M^-1 for the first, second, and third binding sites, respectively. The enthalpy of binding was exothermic for the first and second binding sites and endothermic for the third binding site. Binding of the peptide to all three sites was spontaneous as shown by the negative free energy values. These results show for the first time that β-Lg A can bind bioactive peptides. This potential could be exploited to transport bioactive peptides and protect them in the gastrointestinal tract following their oral administration as nutraceuticals.