Rate Constants for Peroxidation of Polyunsaturated Fatty Acids and Sterols in Solution and in Liposomes

Abstract

Rate constants for autoxidation propagation of several unsaturated lipids in benzene solution at 37°C and in phosphatidylcholine liposomes were determined by a linoleate radical clock. This radical clock is based on competition between hydrogen atom abstraction by an intermediate peroxyl radical derived from linoleic acid that leads to a trans,cis-conjugated hydroxyoctadecadienoic product and β–fragmentation of the same peroxyl that gives the trans,trans-product hydroxyoctadecadienoic acid. Rate constants determined by this approach in solution relative to linoleic acid (kp = 62 M−1s−1) were: arachidonic acid (kp = 197 ± 13 M−1s−1), eicosapentaenoic acid (kp = 249 ± 16 M−1s−1), docosahexaenoic acid (kp = 334 ± 37 M−1s−1), cholesterol (kp = 11 ± 2 M−1s−1), and 7-dehydrocholesterol (kp = 2,260 ± 40 M−1s−1). Free radical oxidations of multilamellar and unilamellar liposomes of various mixtures of glycerophosphatidylcholine molecular species were also carried out. In some experiments, cholesterol or 7-dehydrocholesterol was incorporated into the lipid mixture undergoing oxidation. A phosphatidylcholine bearing a linoleate ester at sn-2 was a component of each liposome peroxidation reaction and the ratio of trans,cis/trans,trans (t,c/t,t)-conjugated diene oxidation products formed from this phospholipid was determined for each oxidation reaction. This t,c/t,t-product ratio from linoleate was used to “clock” liposome constituents as hydrogen atom donors in the lipid bilayer. Application of this lipid bilayer radical clock gives relative autoxidation propagation rate constants of arachidonate (20:4), eicosapentaenoate (20:5), docosahexaenoate (22:6), and 7-dehydrocholesterol to be 115 ± 7, 145 ± 8, 172 ± 13, and 832 ± 86, respectively, a reactivity trend that parallels the one in solution. We also conclude from the liposome oxidations that linoleate peroxyl radicals at different positions on the eighteen-carbon chain (at C-9 and C-13) have different kinetic properties. This is in contrast to the results of solution oxidations of linoleate in which the C-9 and C-13 peroxyl radicals have similar reactivities. We suggest that peroxyl radical β–scission depends on solvent polarity and the polarity of the local environment of peroxyl radicals in liposomal oxidations depends on the position of the peroxyl radical on the eighteen-carbon chain.

Effects of Different Phases of Cigarette Smoke on Lipid Peroxidation and Membrane Structure in Liposomes

“…The LPO process in bulk phospholipids depends on their unsaturation. [54][55][56][57] Thus, it has been hypothesised that the membrane response to cigarette smoke should differ, depending on initial unsaturation of the PC material being used for liposome preparations. There exists only a few reports on cell membranes and liposomes exposed to cigarette smoke.…”

Lipid Oxidation in Homogeneous and Micro-Heterogeneous Media in Presence of Prooxidants, Antioxidants and Surfactants

“…PL are the basic lipid components of plasmatic cell membranes and membranes of subcellular organelles of animals, plants, and microorganisms. Phosphatidylcholines (1,2-diacyl-sn-glycero-3-phosphocholines, lecithins) are the most widely used; they are present in large amounts in myocardium, liver, kidneys, and egg yolk [56]. In lecithin molecules, anionic phosphate and cationic choline (tetraalkylammonium) groups are connected via a zwitterionic bond to form a neutral polar head.…”

“…Lecithins are easily dissolved in organic solvents to yield compact reverse micelles [60,61]. In aqueous solutions, lecithin forms multilamellar liposomes or vesicles under the action of ultrasound dispersion [12,56,62]. Using the DLS method, it was found that, at egg lecithin concentrations 10-90 mg/mL, the size of microaggregates observed are equal to 5-6 nm in organic solvents and in water, liposomes are formed with a wide size distribution of 60-1000 nm.…”

Small amounts of isotope-reinforced polyunsaturated fatty acids suppress lipid autoxidation

“…1.4% 2-propanol and 0.1% acetic acid in hexanes) with selective reaction monitoring [24,25]. Among these HPLC-MS analyses, the reactions that have large KODEs (corresponding ketones from HODEs) were also analysed [25] in reactions with D 2 -Lin: Lin ratios varying from 0 to 9.…”

“…To determine the effect of the isotope reinforcement on the propagation rate constants of Lin autoxidation as compared to that of 11,11-D 2 -Lin, we performed parallel clocking experiments utilizing a linoleate peroxyl radical clock [6,23,24], as well as competition experiments by co-oxidizing 11,11-D 2 -Lin and Lin in solution.…”