METABOLIC TURNOVER OF MOLECULAR-SPECIES OF PHOSPHATIDYLINOSITOL AND ITS PRECURSOR PHOSPHATIDIC-ACID IN GUINEA-PIG CEREBRAL HEMISPHERES

The molecular species composition of phosphatidylinositol from guinea pig cerebral hemispheres was studied and found similar to that of phosphatidylinositol from ox cerebral hemispheres. In both cases the tetraenoic species was predominant. Phosphatidic acid from guinea pig cerebral hemispheres contained 2 major molecular species; the monoenoic and hexaenoic (33.4 and 24 mol/100 mol, respectively). To study the metabolism of molecular species of phosphatidic acid and phosphatidylinositol in the cerebral hemispheres, guinea pigs were injected intracisternally with 32Pi and [U-14C]glucose. After 5 min of isotopic exchange, the specific radioactivity of 32P in phosphatidylinositol was nearly equal to that in phosphatidic acid, whereas specific radioactivity of 14C in the glycerol was 1.4 times and in the fatty acids nearly 0.5 times that in the phosphatidic acid, respectively, indicating metabolic heterogeneity of both phospholipids. The glycerol specific radioactivity was different in all the molecular species of phosphatidic acid, being greatest in the monoenoic and least in the tetragenoic species. When the molecular species were arranged in this way, the order was representative of their relative rates of synthesis by acylation of glycerol-3-phosphate. An almost opposite order was obtained when the molecular species were arranged according to their phosphate/glycerol radioactivity ratios, indicating the relative contribution of the diacylglycerol kinase pathway to their formation. When the specific radioactivity values and ratios of phosphatidylinositol were similarly considered, the orders of the molecular species were, on the whole, similar to that of phosphatidic acid. This indicated that synthesis de novo was operative in the formation of most of its molecular species, but due to other considerations it was concluded that part of the tetraenoic, and probably the whole of saturated phosphatidylinositol may be formed by trans-acylation reactions. The experimental limitations of previous and present techniques for the analysis of phospholipid molecular species were discussed.