Rapid Incorporation In Vivo of Intracerebrally Injected 32Pi into Polyphosphoinositides of Three Subfractions of Rat Brain Myelin

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Abstract
At intervals ranging from 1 to 10 min after injection of 32Pi into rat brain, myelin was prepared and separated into three subfractions: heavy, medium, and light. The radioactivity of total phospholipids and polyphospho‐inositides (PPI) was then determined. There was rapid incorporation of 32Pi into PPI, which contained 50–70% of the radioactivity among total brain lipids and more than 70% among myelin lipids. The myelin fraction had incorporated 32Pi into total recovered PPI in the order of medium > heavy > light fraction: however, the order of relative specific radioactivities was heavy > light > medium. Labeling of the PPI precursors, phosphatidic acid (PA) and phos‐phatidylinositol (PI), was considerably lower in the purified myelin than in total brain. The di‐ (DPI) and triphosphoinositides (TPI) in heavy myelin exchanged 32Pi at rates 2 to 3 times faster than those in medium and light myelin. DPI of all subfractions of myelin exchanged much faster than TPI. The results show that the most active phosphate turnover of myelin PPI occurs in the heavy myelin fraction (probably largely consisting of myelin appurtenant regions). However, medium and light myelin (most probably representing the closely packed layers of myelin sheaths) also showed rapid turnover of PPI.