Nuclear phosphoprotein phosphatase from calf liver.

Calf liver nuclear phosphoprotein phosphatase (phosphoprotein phosphohydrolase, EC 3.1.3.16) has been purified approx. 850-fold. The enzyme has a mol. wt. of 34 000 as determined by SDS-polyacrylamide gel electrophoresis. The purified enzyme has a pH optimum between 7.0 and 7.5 with phosphophosphorylase, phosphohistones f1 and f2b, and phosphoprotamine as substrates. The enzyme activity towards these substrates follows the order, phosphophosphorylase greater than phosphohistone f1 greater than phosphohistone f2b greater than phosphoprotamine. The Km values toward phosphophospharylase and phosphohistone f1 are 17 and 28 micron phosphate, respectively. Dephosphorylated histone f1 and orthophosphate are competitive inhibitors of the enzyme with respective Ki values of 11 micron and 4.1 mM. NaCl and divalent metal ions inhibit the enzyme but CaCl2 is slightly stimulatory. It appears that metal ion inhibition occurs at two sites, one on the enzyme and the other on the substrate. The enzyme is also inhibited by NaF and EDTA. Nucleotides bearing the pyrophosphate structure are potent inhibitors of the enzyme while mononucleotides are slightly inhibitory. DNA and other polyions also inhibit the enzyme. The enzyme appears to require free sulfhydryl groups for activity since it is inhibited by N-ethylmaleimide and p-hydroxymercuribenzoate; the latter inhibition can be reversed by mercaptoethanol and dithiothreitol.