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.