Distinct Dimerization Between α-Lactalbumin and Lysozyme Exhibiting Novel Antimicrobial Activity against Gram-positive and Gram-negative Bacteria

Hisham   R.   Ibrahim; Norihiko      Taniyama; Takayoshi      Aoki

Abstract

Alpha-lactalbumin (α-LA) and c-type lysozyme (cLZ) are two proteins structurally similar but largely different in their biochemical properties. Despite c-type lysozyme is widely distributed in nature from bacteria to human, α-lactalbumin is the only of mammalian products. These two proteins uniquely co-exist in mammalian milk but the significance of their distinct presence in milk is completely unknown. In this study, hen egg white lysozyme and bovine α-lactalbumin were investigated to elucidate the nature of their interaction and antibacterial synergy. A complex of cLZ and α-LA at one molar ratio exhibited greatly enhanced antibacterial activity against the Gram-positive Staphylococcus aureus and Gram-negative E. coli K-12. The catalytic activity of cLZ was unexpectedly increased with an increase of the molar ratio of α-LA to cLZ. Under physiological pH, α-LA was associated with cLZ in a dosedependent manner, as revealed by turbidity and intrinsic tryptophan fluorescence spectra measurements. Chemical cross-linking experiments accompanied with immunoblotting indicated that the complex existed mainly as cLZ-αLA heterodimer with cLZ homodimer. It is the first demonstration of distinct association between α-LA and cLZ with promising antimicrobial synergism. Our results strongly suggest the important role of cLZ and α-LA in the host defense system of the newborn against gastric microbial infections, and thus offer a great opportunity for the design of potential antimicrobial drug in the treatment of infectious diseases.

Keywords: lysozyme, lactalbumin, dimerization, muramidase activity