PHOSPHORYLATION OF LIPIDS TIGHTLY BOUND TO SECRETORY IMMUNOGLOBULIN-AIN ANTIBODY FRACTIONS FROM HUMAN BREAST-MILK POSSESSING PROTEIN-KINASE ACTIVITY

The fractions of antibodies (Abs) containing only sIgA and IgG were pu rified from human breast milk by Protein A-Sepharose chromatography an d they catalyzed phosphorylation of casein in the presence of [gamma-P -32]ATP. Also, P-32-labeled low-molecular-weight non-protein products are formed which are visible as radioactive background on the polyacry lamide gel lanes during electrophoresis of Abs under denaturing condit ions. Separation of sIgA from IgG using a DEAE-sorbent with subsequent gel-filtration in 0.05 M NaOH indicates that the low-molecular-weight substances partially remain tightly bound to sIgA and are separated o nly by gel-filtration in a buffer containing 5% dioxane (non-denaturin g resolution) or by extraction of the sIgA pellet with the chloroform- methanol mixture (2:1). The P-32-labeled substances were separated by TLC in the system used for phospholipid chromatography forming two fra ctions (R-f 0.83 and 0.66) that were stained with iodine. The data sug gest that the substances co-isolated with sIgA are phospholipids. At 2 5 nM ATP, casein and lipids are P-32-labeled. At 1 mu M ATP, the sIgA polypeptides are also phosphorylated. Gentle removal of the lipids fro m Ab preparation enhanced P-32 incorporation into casein and sIgA poly peptides. Considering the heterogeneity of polyclonal sIgA in protein and ATP affinity, it is suggested that phosphorylation of casein and s IgA polypeptides is catalyzed by abzymes of different clonal origin.