Studies on the phosphoproteins of brain: the intracellular localization in brain of a phosphoprotein involved in the metabolic response of cortical slices to electrical stimulation

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Abstract
When dispersed in isotonic sucrose solution cerebral tissues have been found to contain 2 major phosphoprotein fractions, one soluble in sucrose solution, the other insoluble. In vivo, the incorporation of intracisternally-injected radioactive phosphate was greatest in the soluble phosphoprotein fraction. With slices of brain in vitro, incorporation of radioactive phosphate was greatest in the insoluble phosphoprotein fraction. These differences are due to a loss of most of the soluble phosphoprotein from the slice to the incubating medium and to a decreased rate of incorporation of phosphate into the remainder. Incubation in medium had little or no effect upon the total quantity of the insoluble phosphoprotein. After incubation in medium containing radioactive phosphate, tissue slices have been dispersed in isosmotic sucrose and fractionated by differential centrifugation. The bulk of the phosphoprotein was present in the nuclear fraction, the mitochondrial, microsomal and supernatant fractions containing relatively small amounts. Application of electrical pulses to the tissue slices before fractionation showed that the increased incorporation of radioactive phosphate took place largely in the phosphoprotein of the nuclear fraction. The nuclear fraction has been further subfractionated by centrifuging in 2 sucrose density gradients. The phosphoprotein was concentrated in particles which possess little or no succinic dehydrogenase activity and are not considered to be mitochondria. The acid and alkaline phosphatase activity of the particles has been measured.