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[Do catalytically active antibodies exist in healthy people? (Protein kinase activity of sIgA antibodies from human milk)].

The fraction of secretory immunoglobulin A (sIgA) from the milk of healthy mothers was purified by sequential affinity chromatography on protein A-sepharose (in the presence of 1% triton x100), adsorbent Toyopearl HW-55 (gel-filtration), DEAE cellulose (separation of IgG and IgA antibodies), affinity sorbents with immobilized ATP and casein. The protein obtained corresponded to sIgA antibodies according to all known criteria and did not contain any protein contaminations. The ability of sIgA to phosphorylate selectively serine residues of casein (not histones) in the presence of [gamma-32P]ATP was shown. Purified kinase activity was stable at acid shock (pH 2.3), strongly interacted with immobilized antibodies against H-chain of sIgA and eluted from the sorbent with the peak corresponding to sIgA antibodies. The complex of sIgA and ATP was stable enough at the conditions of gel-filtration. Affinity modification of sIgA by chemically reactive analogs of ATP resulted in preferential modification of its light chain (2-3 mole reagent per mole of dimer form). In the condition of oligomer dissociation ATP-sepharose sorbed only the light chains of sIgA. sIgA have optimal conditions for phosphorylating activity different from those of known protein kinases. We suppose that sIgA antibodies with kinase activity are a first example of sIgA antibodies with catalytic activity. For the first time the possibility of existence of natural abzymes in a healthy human was shown. These abzymes catalyze the reaction of synthesis but not substrate degradation reaction.