Protein phosphatase activity in cell nuclei of rat liver and its relationship to the protein phosphatase in the cytoplasm.

Nuclear protein phosphatase (phosphoprotein phosphohydrolase, EC. 3.1.3.16, abbreviated: NPPase) was extracted from rat liver cell nuclei and subnuclear fractions under different conditions. NPPase activity proved to be strongly bound to chromatin and its presence cannot be explained by an incomplete removal of the cytoplasm from nuclear preparations. The small extent of activation of NPPase after treatment with ethanol or mercaptoethanol suggests that NPPase is present in the nucleus in its activated form. On the other hand, cytoplasmic protein phosphatase (abbreviated: NPPase) from rat liver also showed only a small extent of activation after precipitation with ammonium sulphate or ethanol. Therefore, the pronounced activation of cytoplasmic PPase, which has been observed in rabbit liver and skeletal muscle, cannot be used for differentiation between cytoplasmic and nuclear PPases in rat liver.