Inability of murine peritoneal macrophages to convert linoleic acid into arachidonic acid. Evidence of chain elongation

Various murine macrophage populations synthesize and secrete large amounts of arachidonic acid (20:4n-6) derived eicosanoids (cyclo-oxygenase and lipoxygenase products). These metabolites are known to possess a wide variety of functions with regard to the initiation and regulation of inflammation and tumorigenesis. Because the dietary intake of 20:4n-6 is usually low, tissues are largely dependent upon dietary linoleic acid (18:2n-6) as an initial unsaturated precursor for the biosynthesis of 20:4n-6. The purpose of these experiments was to determine whether resident or responsive murine macrophages possess desaturase and elongase activities capable of in vitro conversion of 18:2n-6 into 20:4n-6. Peritoneal exudate macrophages were purified by adherence and incubated in serum-free medium containing fatty acid-free BSA with [1-¹⁴C]18:2n-6. Approximately 90 to 98% of the [¹⁴C]18:2n-6 at 4 and 16 h was recoverd in phosphatidylcholine and phosphatidylethanolamine. The metabolism of [¹⁴C]18:2n-6 was determined after transesterification and separation of the ¹⁴C-fatty acid methyl esters by argentation TLC, reverse phase HPLC, and electron impact gas chromatography/mass spectrometry. Resident and responsive macrophages lacked the capacity to transform [¹⁴C]18:2n-6 into 20:4n-6. In addition, prelabeled macrophages incubated with soluble, calcium ionophore A23187 or phorbol myristate, or particulate, zymosan, membrane perturbing agents also lacked Δ⁶ desaturase activity. All macrophages tested were capable of elongating [¹⁴C]18:2n-6 into [¹⁴C]20:2n-6. These observations suggest that 20:4n-6, present in macrophage phospholipids, is biosynthesized elsewhere and transported to the macrophage for esterification into the phospholipids. In addition, these findings demonstrate that elongase activity is present in both the resident and responsible peritoneal macrophage.