Interaction between ferric ions,phospholipid hydroperoxides,and the lipid phosphate moiety at physiological pH

Iron-catalyzed lipid peroxidation was examined using ¹H NMR in a biphasic aqueous-chloroform system. At physiological pH (7.4), mole ratios of phospholipids/Fe³⁺ as low as 1300∶1 catalyzed the rapid disappearance of endogenous lipid hydroperoxides with a loss of two of the four double bonds in PC containing palmitic (16∶0) and arachidonic (20∶4) acids in the sn-1 and sn-2 positions, respectively. The predominant phospholipid products after 1 h at 20°C were a 9-carbon monounsaturated carbonyl and a phospholipid with an 11-carbon Δ^5,8 FA in the sn-2 position. PC with linoleic acid (18∶2) in the sn-2 position lost one double bond and formed a phospholipid with a 9-carbon FA. Cardiolipin (linoleic acid-rich) also lost about 40% of its double bonds. No detectable loss was seen for PC containing oleic acid (18∶1) or neutral lipids with PUFA. At arachidonyl PC/Fe³⁺ ratios less than 20∶1, significant broadening of the choline methyl proton peak was evident, indicating that Fe³⁺ may form a complex with the adjacent phosphate group and that the complex involves both the phosphate and the hydroperoxide adjacent to the Δ¹¹ double bond. The results demonstrate that, at physiological pH, Fe³⁺-catalyzed peroxidation in polyunsaturated phospholipids occurs selectively adjacent to specific double bonds (Δ⁹ or Δ¹¹). These PC-derived products have been shown to activate components of the inflammatory system. This suggests that the episodic release of ferric ions may play a significant role in generating inflammatory mediators.