Binding of Zn(II) ions to alpha-lactalbumin.

The binding of Zn(II) ions to human and bovine alpha-lactalbumin has been studied by fluorescence, scanning microcalorimetry, and proteolytic digestion. The intrinsic tryptophan fluorescence spectrum of Ca(II)-loaded alpha-lactalbumin is insensitive to Zn(II) binding to the strong cation binding sites (Zn:protein ratios up to 20), yet the thermal denaturation transition, as detected by intrinsic fluorescence, is shifted toward lower temperatures. On the other hand, low concentrations of Zn(II) ([Zn]:[protein] less than 1) shift heat sorption curves toward lower temperatures. It was concluded that alpha-lactalbumin possess several relatively strong Zn(II) binding sites, which are filled sequentially, the process being accompanied by protein aggregation. The strongest Zn(II) binding (5 x 10(5) M-1) increases its susceptibility to tryptic and chymotryptic digestion, slightly decreases its affinity for the fluorescent probe, bis-ANS, and alters its interactions with UDP-galactose. Zn(II) binding to aggregated forms of alpha-lactalbumin increases its affinity to bis-ANS.