PB2+ AND HG2+ BINDING TO ALPHA-LACTALBUMIN

Interactions of human alpha-lactalbumin with Pb2+ and Hg2+ were studie d by intrinsic protein fluorescence. Lead ions bind to the strong Ca2 binding site of alpha-lactalbumin (association constant K(ass)approxi mate to 2x10(6) M(-1)) with concomitant spectral changes which are sim ilar to those induced by the binding of Ca2+. Pb2+ also binds to the s trong Zn2+ site with K(ass)approximate to 10(5) M(-1) and some seconda ry binding site(s) (which probably contain histidine residues) with ap parent K(ass)approximate to 10(4) M(-1), causing pronounced aggregatio n of the protein. Mercury ions bind to alpha-lactalbumin at the primar y Zn2+ sites with K-ass(1-4)x10(4) M(-1), although the stoichiometry o f the binding depends on the conformational state of the protein. Seco ndary Hg2+ binding sites were suggested to contain histidines, while t he strong Hg2+ site contains carboxylates in the coordination sphere a nd seems to coincide with the strong Zn2+ site. The binding of both Pb 2+ and Hg2+ decreases the thermal stability of the Ca2+-loaded protein and in some conditions causes pronounced protein aggregation.