Abstract
It was found that the cytosol fraction of mammary gland epithelial cell contained 85%-95% of the total activity of the protein kinase (EC 2.7.1.37) which catalyzed the phosphorylation of casein. Both specifid and total activity of the soluble casein kinase increased 2-3 times during pregnancy and lactation compared to those from virgin rats. The cytosol casein kinase was purified more than 100-fold. Effects of various nucleotides and metal ions were investigated on the casein kinase, which phosporylated preferentially native bovine whole casein, α_s-casein, β-casein but did histone to a lesser extent. Discelectrophoreses of native caseins phosphorylated by casein kinase revealed that ^<32>P was incorporated into each protein band. The main amino acid residures of casein which accepted ^<32>P were found to be serine and threonine. Apparent phosphorylation rates of dephosphorylated bovine whole casein, αs-casein, β-casein and rat casein by casein kinase were markedly decrased compared to those of their native casein. Cytosol casein kinase is investigaten in this paper, which is different from the casein kinase from Golgi apparatus (1) in activation effects by metal ions and the affinity for the phosphorylation of dephosphorylated casein. Mammary cytosol casein kinase seems to belong to one of the phosvitin kinase which was reported to occure in yeast (11), calf brain (10).
