Phosphorylation of different human milk proteins by human catalytic secretory immunoglobulin A.

The ability of secretory immunoglobulin A (slgA) from milk of healthy mothers to phosphorylate various milk proteins in the presence of gamma-[32P]-ATP was shown to be a property of the antibodies. The polyclonal slgA was purified by sequential chromatography on Protein-A Sepharose and DEAE-cellulose, and then separated by chromatography on the ATP-Sepharose. The preparations containing all milk proteins except for protein kinases [integrated milk proteins (IMP)] were obtained by extraction of the kinase activities from the milk using their affinity to the insoluble crosslinked staphylococcus. Addition of slgA fractions (having a different affinity to ATP) to the IMP led to phosphorylation of casein and several other milk proteins with different efficiencies.