Purification and properties of cyclic AMP-dependent protein kinase from rat epididymis.

Two protein kinases (I and II: EC 2.7.1.37) that show a high degree of substrate specificity for protamine rather than histones, phosvitin and casein were partly purified from rat epididymal tissue. The enzymes were present in the cytosol because greater than 80% of the enzymic activity was recovered in the soluble fraction. The kinases required Mg2+ for activity although Co2+ and Mn2+ were partial substitutes. Zn2+ (1 mM) inhibited nearly completely the activity of the enzymes. Both the kinases showed high affinity for activation with cyclic AMP compared to other cyclic nucleotides. Amino acid analysis of 32P-labelled protamine product revealed that the kinases transfer the terminal phosphate of ATP to serine residues of the protein. The isoenzymes I and II showed certain differences in relation to their hydroxyapatite-chromatography profiles, pH activation profiles, heat sensitivity and Km for ATP and cyclic AMP.