Modulation of the substrate specificity of the polycation‐stimulated protein phosphatase from Xenopus laevis oocytes

Abstract

A polycation‐stimulated (PCS) protein phosphatase was isolated in high yield (280 μg/100 g ovaries) from Xenopus laevis oocytes through a procedure involving a tyrosine‐agarose hydrophobic chromatography. The 220‐kDa enzyme contains a 35‐kDa and a 62‐kDa subunit. It was identified as the low‐M_r polycation‐stimulated (PCS_L) protein phosphatase. The labile p‐nitrophenyl phosphatase activity, copurifying with the phosphorylase phosphatase activity, can be increased severalfold by preincubating the purified enzyme with ATP, its analogues or PP_i. This activation is time‐dependent and accompanied by a parallel decrease of the phosphorylase phosphatase activity. Although the stimulation was antagonized by metal ions during the preincubation, the basal and ATP‐stimulated p‐nitrophenyl phosphatase requires Mg²⁺ or Mn²⁺ in the assay, with pH optima of 8.5–9 and 7.5 respectively.