Studies on the phosphoproteins of brain. 2. Partial purification of a phosphoprotein attached to subcellular particles

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Abstract
A lipophosphoprotein fraction has been prepared from easily sediment-ing particles of ox brain. The preparation contains 8% N,0.3-0.6% P and 40-66% lipids. Calculated in terms of the total P content of the particles, the preparation has been purified 10-14 times. Calculated in terms of the total N content of the particles it has been purified 4-5 times. The recovery is 25% of the starting material. The preparations are not pure and can be separated into at least two major components by salt fractionation. The lipid-free residue contains 11.2-12.0% N. This figure is not increased by exhaustive extraction of the residue. It contains 0.1-0.24% P of which 50-75% is labile in alkali. Qualitative analysis revealed an amino acid pattern suggestive of an acidic protein. The fraction does not contain cholesterol. Neuraminic acid and galactose from cerebrosides are present only in small amounts. The lipids contain a high proportion of glycerol esters thought to be triglycerides. When incubated either with a fortified cerebral dispersion or with rat-liver mitochondria in the presence of radioactive phosphate, the preparations are phosphorylated, radioactive phosphorylserine being detected in hydrolysates of the phosphoprotein at the end of incubation.