NUCLEAR PHOSPHOPROTEIN PHOSPHATASE FROM CALF LIVER

Abstract

This chapter presents a study on nuclear phosphoprotein phosphatase from calf liver. Calf liver nuclear phosphoprotein phosphatase has been purified approximately 850-fold. The enzyme has a molecular weight of 34,000 daltons as determined by SDS polyacrylamide gel electrophoresis. The purified enzyme has a pH optimum between 7.0 and 7.5 with phosphophosphorylase, phosphohistones f 1 and f 2b , and phosphoprotamine as substrates. The enzyme activity toward these substrates follows the order: phosphophosphorylase > phosphohistone f 1 > phosphohistone f 2b > phosphoprotamine. The K m values toward phosphophosphorylase and phosphohistone f 1 are 17 and 28 μM phosphate, respectively. Dephosphorylated histone f 1 and orthophosphate are competitive inhibitors of the enzyme with respective K m values of 11 μM and 4.1 mM. Divatent metal ions and NaCl inhibit the enzyme, but CaC1 2 is slightly stimulatory. The results of Dixon plots suggested 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.