Protein Kinases in Myelin of Rat Brain: Solubilization and Characterization

Myclin from rat brain contained adenosine 3′, 5′‐monophosphate (cyclic AMP)‐dependent protein kinase activity, which was solubilized by 0.2% Triton X‐100 and required exogenous protein substrate for its activity. Also present was a protein kinase which catalysed the phosphorylation of the endogenous substrate and which was neither solubilized by Triton X‐100 nor stimulated by cyclic AMP. Sodium fluoride was required to maintain the activity of the endogenous phosphorylation, probably by inhibiting ATPase activity, but had no effect on the phosphorylation of histone by the solubilized enzyme. Protamine and myelin basic protein served as well as histone as a substrate for the solubilized enzyme. A protein kinase modulator had no effect on the endogenous phosphorylation, but inhibited histone phosphorylation by the solubilized enzyme. Cyclic AMP‐binding activity was observed in both the solubilized and non‐solubilized preparations. The concentration of cyclic AMP required to give half‐maximal binding activity of the preparations was about 2.5 nM. The results indicate that the cyclic AMP‐binding site of the protein kinase in myelin may partially be accessible, whereas the catalytic site may be integrated into the membrane structure of myelin.