The effect of zinc and other divalent cations on the structure and function of human alpha 2-macroglobulin

Zinc binding to human alpha 2-macroglobulin was studied to assess its involvement in the structure and function alpha 2-macroglobulin. Equilibrium dialysis experiments indicated multiple classes of zinc-binding sites, the one of highest affinity having a site number of 20 and a Kd value of 8 X 10(-7) M. Native alpha 2-macroglobulin and alpha 2-macroglobulin-trypsin complexes bound comparable amount of zinc. The proteinase inhibitory activity of alpha 2-macroglobulin was not affected by zinc binding at physiological concentrations nor by the removal of zinc by EDTA. Above 25 microM zinc, alpha 2-macroglobulin activity decreased, although binding of [125I]trypsin was not affected. When nondenaturing gel electrophoresis was performed, the preparation of alpha 2-macroglobulin migrated as half-molecules at increasing zinc concentration. Experiments with other divalent cations correlated decreases in alpha 2-macroglobulin activity with apparent dissociation of the alpha 2-macroglobulin tetramer in the presence of copper and mercury, but not barium, cadmium or nickel. While zinc binding to alpha 2-macroglobulin does not function in proteinase inhibition, it might be involved in zinc transport in vivo. At nonphysiological concentrations, zinc and other divalent cations are useful as probes of protein quaternary structure.