PROTEIN-KINASE ACTIVITY OF SIGA ANTIBODIES FROM HUMAN-MILK - CATALYTICALLY ACTIVE ANTIBODIES IN NORMAL HUMANS

A fraction of class A secreted immunoglobulins (sIgA) from the milk of healthy mothers was sequentially purified by chromatography on protei n A Sepharose (in the presence of 18 Triton X-100), Toyopear 1 HW-55, Sepharose 4B (to remove anti-polysaccharide antibodies), DEAE-cellulos e (to separate IgG and sIgA antibodies), and on affinity sorbent with immobilized ATP and casein. The material obtained exhibited the proper ties of sIgA antibodies and was free of protein impurities. Secreted i mmunoglobulins A were able to phosphorylate casein (but not histones) at serine residues in presence of gamma-[P-32]ATP. This activity was r esistant to acidic shock (pH = 2.3), bound tightly with immobilized an tibodies against the H chain of IgA, and was eluted in the same peak w ith antibodies. The sIgA-ATP complex was stable during gel filtration. Affinity modification of sIgA with ATP analogs (up to 2-3 moles per m ole protein) resulted in labeling of light chains. Under dissociating conditions only light chains were sorbed on ATP-Sepharose. Optimal con ditions for sIgA kinase activity substantially differed from those des cribed for protein kinases. It was concluded that kinase activity is t he first example of catalytic function of sIgA, and that normal humans may have natural abzymes which catalyze synthesis rather than hydroly sis.