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We present the first evidence that electrophoretically and immunologically homogeneous sIgAs purified from milk of healthy human mothers by chromatography on Protein A-Sepharose and FPLC gel filtration contain intrinsically bound metal ions (Ca  >  Mg ≥ Al > Fe approximately  Zn ≥ Ni ≥ Cu ≥ Mn), the removal of which by a dialysis against ethylenediamine tetraacetic acid (EDTA) leads to a significant decrease in the β-casein-hydrolyzing activity of these antibodies (Abs). An affinity chromatography of total sIgAs on benzamidine-Sepharose interacting with canonical serine proteases separates a small metalloprotease sIgA fraction (6.8 ± 2.4%) from the main part of these Abs with a serine protease-like β-casein-hydrolyzing activity. The relative activity of this metalloprotease sIgA fraction containing intrinsically bound metal ions increases ∼1.2-1.9-fold after addition of external metal ions (Mg(2+) > Fe(2+) > Cu(2+) ≥ Ca(2+) ≥ Mn(2+)) but decreases by 85 ± 7% after the removal of the intrinsically bound metals. The metalloprotease sIgA fraction free of intrinsic metal ions demonstrates a high β-casein-hydrolyzing activity in the presence of individual external metal ions (Fe(2+) > Ca(2+) > Co(2+) ≥ Ni(2+)) and especially several combinations of metals: Co(2+) + Ca(2+) < Mg(2+) + Ca(2+) < Ca(2+) + Zn(2+) < Fe(2+) + Zn(2+) < Fe(2+) + Co(2+) < Fe(2+) + Ca(2+). The patterns of hydrolysis of a 22-mer oligopeptide corresponding to one of sIgA-dependent specific cleavage sites in β-casein depend significantly on the metal used. Metal-dependent sIgAs demonstrate an extreme diversity in their affinity for casein-Sepharose and chelating Sepharose, and interact with Sepharoses bearing immobilized monoclonal mouse IgGs against λ- and κ-type light chains of human Abs. Possible ways of the production of metalloprotease abzymes (Abz) by human immune system are discussed., (2010 John Wiley & Sons, Ltd.)