Phosphatidylcholine is a quantitatively more important source of increased 1,2-diacylglycerol than is phosphatidylinositol in mast cells.

The widely accepted hypothesis that the increased 1,2-diacylglycerol (DAG) in stimulated cells is derived from phosphoinositides was tested by comparing the pattern of molecular species of phosphatidylinositol (PI) to that of DAG in mast cells. For any glycerol-based lipid, molecular species are defined by unique combinations of the two fatty acids esterified to glycerol. The quantitative frequency distribution of these molecular species represent a "fingerprint" that provides a sensitive approach to assessing precursor/product relationships. Based on mass, the molecular species fingerprints PI, phosphatidylcholine (PC), phosphatidylethanolamine, and phosphatidylserine were determined in unstimulated mast cells and compared to that of the DAG found after stimulation by IgE R bridging, compound 48/80 and the Ca+2 ionophore A23187. The molecular species fingerprint of DAG before stimulation was quite different from that of PI.IgE R cross-linking caused a 1.5 to 2-fold increase in DAG mass 1 to 3 min after stimulation with a concomitant shift in the pattern of DAG molecular species to one that bore only a partial resemblance to that of PI suggesting that considerably less than half of the incremental DAG is likely derived from PI. Ten to 20 min after Ag challenge, DAG levels became maximal (3.2- and 2.9-fold, respectively), but its molecular species pattern returned toward that seen in unstimulated cells suggesting that only perhaps 25% of the incremental DAG might be derived from PI. The molecular species fingerprint of DAG much more closely resembled that PC suggesting that as much as 75% of the incremental DAG might be derived from PC. Similar observations were made when 48/80 and A23187 were used as secretory agonists. These experiments indicate that the DAG participating in the "phosphoinositide cycle" represents a quantitatively modest fraction of the DAG accumulating in stimulated mast cells and suggest that mechanisms other than PI hydrolysis, including perhaps a "PC cycle," are more important than previously assumed in causing the rise in DAG during activation.