The molecular characterisation and antimicrobial properties of amidated bovine β-lactoglobulin

Amidation of bovine β-lactoglobulin (β-Lg) imparted antimicrobial properties to this protein. Amidated β-Lg was strongly bactericidal against resting cells of Pseudomonas fluorescens, Pseudomonas fragi and Bacillus subtilis, but had a much weaker effect against Escherichia coli, Enterococcus faecalis, Salmonella typhimurium and Listeria monocytogenes. Neither native nor amidated β-Lg was effective against the yeast Saccharomyces cerevisiae and the mould Penicillium candidum. Mass spectrometric analysis demonstrated that amidation of β-Lg converted aspartyl and glutamyl residues to asparaginyl and glutaminyl residues, respectively, and that the amidation reaction did not occur to the same extent on every β-Lg molecule. The charge change was also confirmed by SDS–PAGE, ion exchange chromatography and the change in isoionic point of β-Lg. Reverse-phase chromatography showed that amidation also led to alterations in hydrophobicity of the β-Lg molecule. The antibacterial properties of the amidated β-Lg appear to be dependent on the net positive charge and charge distribution on the molecule.