Binding of the Pepper Alkaloid Piperine to Bovine β-Lactoglobulin: Circular Dichroism Spectroscopy and Molecular Modeling Study

The pepper alkaloid piperine is a nontoxic, natural dietary compound with a broad range of physiological activity. The present work is the first demonstration of its interaction with a mammalian protein. Circular dichroism (CD) spectroscopy was used to reveal and analyze the binding of piperine to a lipocalin protein. Induced CD spectra measured in pH 7.7 phosphate buffer at 37 °C demonstrated reversible, non-covalent association of piperine with bovine β-lactoglobulin (BLG), the major whey protein in milk. The binding parameters (Ka ≈ 8 × 104 M-1, n = 0.8) determined from the CD titration data showed no significant differences between the piperine binding properties of the two main genetic variants of BLG (A and B). The vanishing extrinsic CD signal obtained upon acidification of the piperine−BLG sample solution (Tanford transition) suggested that the ligand binds in the central hydrophobic cavity of the β-barrel. The cavity binding concept was further supported by a CD displacement experiment using palm...