Molecular Species of Diacylglycerols and Phosphoglycerides and the Postmortem Changes in the Molecular Species of Diacylglycerols in Rat Brains

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Abstract
The molecular species of 1,2‐diacyl‐sn‐glycerol (DAG), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylinositol 4‐phosphate (PIP), and phosphatidylinositol 4,5‐bisphosphate (PIP2) from brains of adult rats (weighing 150 g) were determined. The DAG, isolated from brain lipid extracts by TLC, was benzoylated, and the molecular species of the purified benzoylated derivatives were separated from each other by reverse‐phase HPLC. The total amount and the concentration of each species were quantified by using 1,2‐distearoyl‐sn‐glycerol (18:0–18:0) as an internal standard. About 30 different molecular species containing different fatty acids at the sn‐1 and sn‐2 positions of DAG were identified in rat brains (1 min postmortem), and the predominant ones were 18:0–20:4 (35%), 16:0–18:1 (15%), 16:0–16:0 (9%), and 16:0–20:4 (8%). The molecular species of PC, PE, PS, and PI were determined by hydrolyzing the lipids with phospholipase C to DAG, which was then benzoylated and subjected to reverse‐phase HPLC, PIP and PIP2 were first dephosphorylated to PI with alkaline phosphatase before hydrolysis by phospholipase C. The molecular species composition of phosphoinositides showed predominantly the 18:0–20:4 species (50% in PI and ∼65% in PIP and PIP2). PS contained mainly the 18:0–22:6 (42%) and 18:0–18:1 (24%) species. PE was mainly composed of the 18:0–20:4 (22%), 18:0–22:6 (18%), 16:0–18:1 (15%), and 18:0–18:1 (15%) species. In PC the main molecular species were 16:0–18:1 (36%), 16:0–16:0 (19%), and 18:0–18:1 (14%). Studies on postmortem brains (30 s to 30 min) showed a rapid increase in the total amount (from 40–50 nmol/g in 0 min to 210–290 nmol/g in 30 min) and in all the molecular species of DAG. Comparatively larger increases (seven‐ to 10‐fold) were found for the 18:0–20:4 and 16:0–20:4 species. Comparison of DAG species with the molecular species of different glycerolipids indicated that the rapid postmortem increase in content of DAG was mainly due to the breakdown of phosphoinositides. However, a slow but continuous breakdown of PC to DAG was also observed.