Endogenous Basic Protein Phosphatases in the Brain Myelin

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Abstract
Direct treatment of brain myelin with freezing/thawing in 0.2 M2‐mercaptoethanol stimulated the endogenous myelin phosphatase activity manyfold when 32P‐la‐beled phosphorylase a was used as a substrate, a result indicating that an endogenous myelin phosphatase is a latent protein phosphatase. When myelin was treated with Triton X‐100, this endogenous latent phosphatase activity was further stimulated 2.5‐fold. Diethylaminoethyl‐cellulose and Sephadex G‐200 chromatography of solubilized myelin revealed a pronounced peak of protein phosphatase activity stimulated by freezing/thawing in 0.2 M2‐mercaptoethanol and with a molecular weight of 350, 000, which is characteristic of latent phosphatase 2, as previously reported. Moreover, endogenous phosphorylation of myelin basic protein (MBP) in brain myelin was completely reversed by a homogeneous preparation of exogenous latent phosphatase 2. By contrast, under the same conditions, endogenous phosphorylation of brain myelin was entirely unaffected by ATP. Mg‐dependent phosphatase and latent phosphatase 1, although both enzymes are potent MBP phosphatases. Together, these findings clearly indicate that a high‐molecular‐weight latent phosphatase, termed latent phosphatase 2, is the most predominant phosphatase responsible for dephosphorylation of brain myelin.