title: Cadmium binding to human alpha 2-macroglobulin.
abstract: alpha 2-Macroglobulin (alpha 2M) is one of the major cadmium-binding proteins of human plasma. As determined with equilibrium dialysis, alpha 2M bound 4.6 (+/- 0.7) mol Cd2+ per mol protein with an apparent dissociation constant of (9.6 (+/- 5.0] X 10(-7) M. Methylamine-modified alpha 2M (alpha 2M-Me) had a similar affinity for Cd2+ (Kd,app = 5.3 X 10(-7) M), but fewer binding sites. Cadmium produced a small increase in the amidolytic activity of trypsin in the presence of alpha 2M and soybean trypsin inhibitor. Using the binding parameters determined from the equilibrium dialysis studies, the Cd2+ concentration which produced a half-maximal increase in amidolytic activity corresponded to saturation of all Cd2+-binding sites in one-half of the alpha 2M molecules. From these results, a model is proposed in which one Cd2+-binding site is present in each of the four polypeptide chains which compose alpha 2M.
mesh_terms: D002104:Cadmium / Q000097:blood* / Q000494:pharmacology; D019256:Cadmium Chloride; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D005779:Immunodiffusion; D007700:Kinetics; D011485:Protein Binding; D014357:Trypsin / Q000378:metabolism; D000511:alpha-Macroglobulins / Q000378:metabolism*
chemical_list: D000511:alpha-Macroglobulins; D002104:Cadmium; D014357:Trypsin; D019256:Cadmium Chloride