Mobilization of Different Arachidonate Pools and Their Roles in the Generation of Leukotrienes and Free Arachidonic Acid during Immunologic Activation of Mast Cells

Immunologic activation of mast cells leads to the mobilization of arachidonic acid (AA) from membrane phospholipids and the subsequent conversion of this AA to bioactive products. The objective of our study was to determine if segregated pools of AA-containing phospholipids within mast cells serve as independent sources of AA. Initial studies indicated that the appearance of free AA occurred rapidly (maximal level formed within 1 min) within supernatant fluids of Ag-stimulated mast cells and was kinetically different from the formation of leukotriene (LT) B₄ or LTC₄. To examine whether free AA and leukotrienes were mobilized from different sources, AA-containing phospholipids of mast cells were labeled with [¹⁴C] and [³H] AAsuch that all major subclasses (1-acyl-, 1-alkyl-1-alk-1′-enyl) of phospholipids contained different ratios of [³H] to [¹⁴C] (sp. act. ratios (SAR)). Mast cells were then stimulated with Ag and the SAR of cellular AA, extracellular AA and extracellular LTC₄, LTB₄, 6-trans LTB₄, were determined. The SAR were uniform in all LT mimicked the ratio found in cellular AA. By contrast, the SAR of AA released into supernatant fluids was twofold lower than that of LT. This indicated that AA released as free fatty acid clearly was derived from a different lipid pool than AA that formed LT. Although it was apparent that the pools which gave rise to AA and LT were different, defining phospholipid(s) that constitute these distinct pools proved more difficult. The SAR of LT suggested that their cellular precursor AA could have been derived from several phospholipid subclasses; however, the SAR in phosphatidylcholine and phosphatidylinositol most closely matched the LT. The SAR of AA in supernatant fluids implied that it was derived, in part, from phosphatidylethanolamine subclasses. Taken together, these data suggest that there are at least two different pools of AA that are mobilized in mast cells during Ag activation. Journal of Immunology, 1993, 150: 563.