Protein kinases in the lactating mammary gland.

Protein kinases in the cytosol of whole bovine lactating mammary gland were separated by phosphocellulose chromatography. Five protein kinases (identified as histone kinase and casein kinases A, B, C, and D) were characterized and compared with kinases from other tissues. The histone kinase activity was identified as cyclic AMP-dependent protein kinase. The casein kinases differed in their activities toward a variety of proteins (alpha s1-casein, native and dephosphorylated beta-casein, and alpha-lactalbumin) and peptides. Based on substrate specificity studies and the inhibitory effects of heparin, 2,3-diphosphoglycerate, and guanosine triphosphate, kinases B, C, and D were tentatively identified as glycogen synthase kinase-3, casein kinase I, and casein kinase II, respectively.