Purification and some properties of protamine kinase from rabbit brain

A protein kinase of high specificity towards protamine and ATP was isolated from rabbit brain and purified about 200-fold. The enzyme does not catalyse phosphorylation of acidic proteins, e.g. casein and phosvitin, nor is it susceptible to cyclic AMP or protein inhibitors of the cAMP-dependent kinase. The enzyme shows the highest activity in the presence of 10 mM-Mg2+ and 2 mM-dithiothreitol at pH 8. The only phosphorylated amino acid found in protamine phosphorylated by the enzyme was phosphoserine.