Histone phosphokinase activity in nuclear and cytoplasmic cell fractions from normal and regenerating rat livers.

1. Liver cell fractions were prepared by non-aqueous procedures and nuclei were also obtained in a hyperosmotic sucrose medium. Histone phosphokinase activity, assayed with histone F1 as substrate, was present in the soluble fraction of the cytoplasm and also bound on to the chromatin fraction of the nucleus. 2. The activity of the enzyme increased sixfold in nuclei from regenerating livers 22h after partial hepatectomy. 3. The enzyme bound in the nucleus was only marginally activated by 1mum-3':5'-cyclic AMP which stimulated the cytoplasmic soluble enzyme fourfold. 4. Nuclei prepared by the non-aqueous technique were also able to phosphorylate histones F2a and F3 and showed histone phosphatase activity with histone F1 phosphate as substrate.