Positional distribution and turnover of fatty acids in phosphatidic acid, phosphoinositides, phosphatidylcholine and phosphatidylethanolamine in rat brain in vivo

Abstract

1. 1. Experiments were designed to examine the possible relation between fatty acids of phosphatidic acid and the phospholipids derived from phosphatidic acid in rat brain in vivo. After intracerebral injection of radioactive fatty acids, distribution studies revealed that the enzymes for phosphatidic acid synthesis in brain were selective with respect to placement of the major saturated and polyunsaturated fatty acids. Thus, stearate and palmitate were incorporated preferentially into the 1-position and arachidonate almost exclusively into the 2-position. Oleate and linoleate were almost equally divided between both positions. 2. 2. The uptake of [ 3H]arachidonate by phosphatidylcholine and monophosphoinositide was very rapid and exceeded that by phosphatidic acid. Incorporation into phosphatidylethanolamine was less and appeared to be biphasic. Double-label ratios from simultaneously injected [ 3H]arachidonate and [ 14C]glycerol suggested that the rapid incorporation of arachidonate resulted from acyl-exchange reactions independent of de novo synthesis. Double-label ratios of di-and triphosphoinositides were much lower than monophosphoinositide, indicating there was a compartment of monophosphoinositide, highly labelled with arachidonate, that did not equilibrate with the polyphosphoinositides. 3. 3. A sequential transfer of stearoyl groups from phosphatidic acid to other phospholipids was indicated, since incorporation of [ 3H]stearate into phosphatidic acid preceded that into monophosphoinositide, phosphatidylcholine and phosphatidylethanolamine. However, double-label experiments with [ 14C]glycerol suggested that a fraction of the stearoyl groups may be turning over by acyl-exchange reactions. 4. 4. I-Acyl lysophosphatidylcholine, labelled with [ 3H]stearate and [ 14C]glycerol, injected intracerebrally, was incorporated intact into diacyl phosphatidylcholine, confirming that direct acylation of lysointermediates could occur in brain. I-Acyl lysomonophosphoinositide was degraded rapidly and there was no significant conversion to diacyl lipid.