1. Partial acid hydrolysates of proteins derived from cortical slices of guinea-pig brain were divided into two parts and fractionated by ion-exchange chromatography and high-voltage electrophoresis. 2. The apparent yield of protein-bound phosphorylserine by the ion-exchange method was about three times that obtained by electrophoresis. 3. The specific radioactivity of phosphorylserine isolated from 32P-labelled slices by electrophoresis was twice that isolated by chromatography. 4. The discrepancies were found to be due to the presence of unlabelled phosphates of unknown composition in the ‘phosphorylserine’ fraction obtained by the ion-exchange method. 5. Electrical stimulation of slices respiring in the presence of [32P]phosphate increased the specific radioactivity of the total phosphate in the chromatographic ‘phosphorylserine’ fraction by 53±11%, as compared with only 19±5% for the phosphorylserine isolated by electrophoresis.

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