Pb2 and Hg2 binding to α‐lactalbumin

Interactions of human α‐lactalbumin with Pb2+ and Hg2+ were studied by intrinsic protein fluorescence. Lead ions bind to the strong Ca2+ binding site of α‐lactalbumin (association constant Kass≈2×106 M‐1) with concomitant spectral changes which are similar to those induced by the binding of Ca2+. Pb2+ also binds to the strong Zn2+ site with Kass≈105 M‐1 and some secondary binding site(s) (which probably contain histidine residues) with apparent Kass≈104 M‐1, causing pronounced aggregation of the protein. Mercury ions bind to α‐lactalbumin at the primary Zn2+ sites with Kass≈(1‐4)×104 M‐1, although the stoichiometry of the binding depends on the conformational state of the protein. Secondary Hg2+ binding sites were suggested to contain histidines, while the strong Hg2+ site contains carboxylates in the coordination sphere and seems to coincide with the strong Zn2+ site. The binding of both Pb2+ and Hg2+ decreases the thermal stability of the Ca2+‐loaded protein and in some conditions causes pronounced protein aggregation.