Purification and Characterization of a Phosphoprotein Phosphatase that Dephosphorylates Myosin and the Isolated Light Chain from Chicken Gizzard Smooth Muscle1

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Abstract
A phosphatase that dephosphorylates myosin and the isolated light chain has been purified to near homogeneity from chicken gizzard smooth muscle. The molecular weight of the enzyme was estimated to be 100,000 and 35,000 under native and denatured conditions, respectively. It requires Mg2+ or Mn2+ The activity was measured quantitatively with a coupled enzyme system with the aid of myosin light chain kinase. The Vm and Km were determined to be 23.4 μmol/mg/min and 4.2 μM, respectively, with the isolated light chain as substrate under the optimal conditions (5 mM Mg2+ at pH 8.45). The specific activity with myosin as substrate at a concentration of 0.9 μM was found to be 1.25 μmol/mg/min, which was about one-fifth of the activity for the isolated light chain under the same conditions. The phosphatase seems to be specific to gizzard myosin. It may play an important role in the regulation of the myosin-actin interaction in smooth muscle.