Protein phosphatase from rat liver nuclei.

Protein phosphatase, active on non-histone phosphoprotein substrate, was partially purified from rat liver cell nuclei by means of salt extraction, ammoniumsulfate precipitation, DEAE cellulose chromatography, gel filtration and preparative isoelectrofocusing. Rat liver nuclei contain a heterogenous population of different protein phosphatases. All the enzyme fractions eluted from DEAE cellulose are of low molecular weight between 12,000--31,000. The pH 5.5 peak fraction of preparative isoelectrofocusing was characterized in detail. It has a pH optimum of 6.8 using nuclear phosphoprotein substrate. It is inhibited by Na+ at 80 mM, and to a lesser extent by K+, activated by Mg2+ (5 mM) and Mn2+ (1 mM). However, the latter is inhibitory at 6 mM. The nuclear protein phosphatase is also active on labelled F1 and F2b histones and casein, however, its V is lower on histones and it contains component(s) active specifically on nuclear phosphoprotein substrate but not on casein.