Comparison of the oxidizability of various glycerophospholipids in bilayers by the oxygen uptake method

The aims of this study were twofold: develop a convenient and rapid procedure for assessing the oxidizability of small quantities of glycerophospholipids in bilayers by the oxygen uptake method, and determine and compare the oxidizability of various glycerophospholipids in bilayers. Our purpose was to elucidate phospholipid oxidation characteristics in membranes. The quantitative autoxidation kinetics of dilinoleoyl phosphatidylcholine (DLPC) (18:2/18:2) was studied in large unilamellar vesicles (LUV) in aqueous dispersions with water-soluble initiator--2,2'-azobis(2-amidinopropane) dihydrochloride--and inhibitor 2-carboxy-2,5,7,8-tetramethyl-6-chromanol. The kinetic data indicated a high efficiency of free radical production, resulting in shortening of measuring time; the very low kinetic chain length, particularly in the induction period, suggested the possibility of including large errors in the kinetics data. Nevertheless, the autoxidation of DLPC obeyed the classic rate law: Rp = kp[LH]Ri(1/2)/(2kt)(1/2) (where Rp = rate of oxygen consumption, kp = rate constant for chain propagation, [LH] = substrate concentration; Ri(1/2) = square root of rate of chain initiation, and 2kt = rate constant for chain termination) in a mixed bilayer system with saturated dimyristoyl PC (14:0/14:0), which provided precise and reproducible data. Therefore, the system was used to assess the relative oxidizability of each glycerophospholipid DLPC (18:2/18:2), dilinolenoyl PC (18:3/18:3),1-palmitoyl-2-linoleoyl PC (16:0/18:2), 1-palmitoyl-2-arachidonoyl PC (16:0/20:4), 1-palmitoyl-2-docosahexaenoyl PC (16:0/22:6), and dilinoleoyl PE (18:2/18:2) in bilayers. The results suggested that the oxidizability of glycerophospholipid in bilayers is substantially influenced by the number of intramolecular oxidizable acyl chains and the content of bis-allylic hydrogen in a structured environment, and showed deviation of the rate law for autoxidation in PC and PE mixed LUV, which possibly was due to non-homogeneous phospholipid distribution in vesicles.