Dephosphorylation of phosphorylated atrial natriuretic peptide by protein phosphatase 2A

Abstract

A phosphatase which exhibits strong activity toward phosphorylated atrial natriuretic peptide (ANP) was identified in the soluble fraction of rat brain homogenate. This ANP phosphatase has a neutral pH optimum, does not require divalent cations for activity, is inhibited by low concentrations of okadaic acid (50% inhibition at 1 nM) and preferentially dephosphorylates the α subunit of phosphorylase kinase. These properties are characteristic of serine/threonine protein phosphatase type 2A (PP2A). The apparent molecular mass of the ANP phosphatase (160 kDa), as estimated by gel filtration, is similar to that of the native heterotrimeric from of PP2A. In addition, phosphorylated ANP is an excellent substrate for the purified catalytic subunit of PP2A (K_m= 42 μM, V_max= 10.3 μmol × min⁻¹× mg⁻¹). In contrast, protein phosphatase 2B (PP2B) has only very low ANP phosphatase activity (K_m= 2.5 μM, V_max= 0.008 μmol × min⁻¹× mg⁻¹), and the catalytic subunit of protein phosphatase type 1 (PP1) as well as purified protein phosphatase type 2C (PP2C) are essentially inactive on ANP. These findings are consistent with the observation that PP2A-like activity accounts for virtually all ANP dephosphorylation in brain homogenate. While the phosphorylation of ANP in vitro by cAMP-dependent protein kinase is well documented, this is a first report on a phosphatase that efficiently can reverse this modification.