Activation of the ATP.Mg-dependent type 1 protein phosphatase by the Fe2+/ascorbate system

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Abstract
The ATP.Mg-dependent type 1 protein phosphatase is inactive as isolated but can be activated in several different ways. In this report, we show that the phosphatase can also be activated by the Fe2+/ascorbate system. Activation of the phosphatase requires both Fe2+ ion and ascorbate and the level of activation is dependent on the concentrations of Fe2+ ion and ascorbate. In the presence of 20 mM ascorbate, the Fe2+ ion concentrations required for half-maximal and maximal activation are about 0.3 and 3mM, respectively. Several common divalent metal ions, including Co2+, Ni2+, Cu2+, Mg2+, and Ca2+ ions, cannot cooperate with ascorbate to activate the phosphatase, and SH-containing reducing agents such as 2-mercaptoethanol and dithiothreitol cannot cooperate with Fe2+ ion to activate the phosphatase, indicating that activation of the phosphatase by the Fe2+/ascorbate system is a specific process. Moreover, H2O2, a strong oxidizer, could significantly diminish the phosphatase activation by the Fe2+/ascorbate system, suggesting that reduction mechanism other than SH-SS interchange is a prerequisite for the Fe2+/ascorbate-mediated phosphatase activation. Taken together, the present study provides initial evidence for a new mode of type 1 protein phosphatase activation mechanism.