Studies on the phosphoproteins of brain. 3. Phosphorylserine sequences in brain phosphoprotein

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Abstract
By use of low-voltage paper electrophoresis of partial acid hydrolysis of phosphoproteins, it has been confirmed that casein and phosvitin contain sequences of phosphorylseryl residues of the type (Ser-P)n. The fractions separated by electrophoresis, though migrating as single components, were not pure and contained phosphoseryl peptides linked with other amino acies, probably glutamic acid and partially dephos-phorylated phosphoseryl peptides. Phosphoproteins of brain contain similar phosphoseryl sequences. This has been shown both by comparison of rates of migration of phosphopeptides separated from partial hydrolysates and by a technique involving isotopic dilution. The results are discussed in relation to the possible metabolic role of brain phosphoproteins in the intact tissue.