THE SUBCELLULAR DISTRIBUTION OF CEREBRAL PHOSPHOPROTEINS.

1. The subcellular distribution of the phosphoprotein fraction of grey matter from guinea-pig brain has been determined. Phosphoproteins occurred in all fractions of the tissue, the mitochondrial fraction containing 29%, supernatant 26%, microsomal fraction 23% and nuclear fraction 15%. The quantity of phosphoproteins per unit quantity of protein was highest in the microsomal fraction and lowest (by a factor of nearly 50%) in the mitochondrial fraction. 2. Observations are reported on the nature of the nuclear subfraction believed from earlier work to contain a phosphoprotein sensitive to the effects of electrical pulses applied to cerebral slices in vitro. With respect to its content of phosphoprotein, cholesterol, ribonucleic acid and succinate-dehydrogenase activity, the subfraction resembles the nerve-ending fraction of the mitochondrial fraction, from which it appears to be derived by cross-contamination. Electron microscopy tended to confirm this impression. Reasons are adduced for discounting the earlier claim that the sensitive phosphoprotein is contained in a new subcellular structure distinct from nuclei, mitochondria or microsomes.