Incorporation of injected [32P] phosphate into the phosphoinositides of subcellular fractions from young rat brain.

1. Six subcellular fractions were prepared from the brain of 3-week-old rats after the injection of [(32)P]orthophosphate into the subarachnoidal space. The fractions were characterized by chemical and enzymic determinations and by electron microscopy. 2. The highest concentration of phosphoinositides was found in the microsomal fraction. 3. After an exchange period of 4hr. in vivo, the specific radioactivity of phosphatidylinositol was high in the nuclear, mitochondrial and supernatant fractions, and that of diphosphoinositide was high in the nuclear, microsomal and supernatant fractions. Triphosphoinositide specific activity was highest in the myelin fraction. 4. Specific activities (counts/100sec./mug. of P) were in the following order for all fractions except myelin: diphosphoinositide>phosphatidylinositol>triphosphoinositide. For myelin the order was: diphosphoinositide>triphosphoinositide>phosphatidylinositol. 5. Phosphatidylinositol extracted from a tissue fraction by acidified chloroform-methanol had roughly twice the specific activity of that extracted by neutral solvents. The same applied to diphosphoinositide.