1-palmitoyl-2-linoleoylphosphatidylcholine and 2-2--azobis(2-amidinopropane)

Lipid peroxidation is believed to play an important role in the pathogenesis of many diseases. Much research has therefore been devoted to peroxidation of different lipids in biomembranes and in model systems (liposomes) of different compositions. Yet, in spite of the relative simplicity of the liposomes, the existing literature is insufficient to reach definite conclusions regarding basic questions including the susceptibility of cholesterol to oxidation, its effect on the peroxidation of polyunsaturated phospholipids such as palmitoyllinoleoylphosphatidylcholine (PLPC) and how cholesterol influences the effect of water-soluble antioxidants such as urate on the peroxidation. The aim of the present study was to clarify these issues. Its major findings are that: (i) AAPH-induced peroxidation of cholesterol is slow and independent of the peroxidation of PLPC. In turn, AAPH-induced peroxidation of PLPC is not affected by cholesterol, independent of the presence of urate in the system. (ii) Cholesterol is not susceptible to copper-induced oxidation, but its inclusion in PLPC liposomes affects the peroxidation of PLPC, slowing down the initial stage of oxidation but promoting later stages. (iii) Addition of urate accelerates copper-induced peroxidation of PLPC in the absence of cholesterol, whereas in cholesterol-containing liposomes it inhibits PLPC oxidation. We attribute the complexity of the observed kinetics to the known cholesterol-induced rigidization of liquid crystalline bilayers.

Topics: Amidines; Cholesterol; Lipid Bilayers; Lipid Peroxidation; Liposomes; Oxidation-Reduction; Phosphatidylcholines; Phospholipids

alpha-Tocopherol was reacted with the phosphatidylcholines (PCs), 1-palmitoyl-2-linoleoyl-3-sn-PC (PLPC), 1-palmitoyl-2-linolenoyl-3-sn-PC, 1-palmitoyl-2-arachidonoyl-3-sn-PC (PAPC) and 1-stearoyl-2-arachidonoyl-3-sn-PC, in the presence of the free radical initiator, 2,2'-azobis (2,4-dimethylvaleronitrile), at 37 degrees C. The addition products of alpha-tocopherol with the PC peroxyl radicals were isolated and identified as 8a-(PC-dioxy)-alpha-tocopherones, in which the peroxyl radicals derived from each PC molecule attacked the 8a-position of the alpha-tocopheroxyl radical. The antioxidative efficiency of alpha-tocopherol against the peroxidation of PLPC and PAPC in liposomes was assessed by the formation of the reaction products of alpha-tocopherol. When alpha-tocopherol was oxidized in the presence of the water-soluble free radical initiator, 2,2'-azobis (2-amidinopropane) dihydrochloride, epoxy-alpha-tocopherylquinones were mainly produced together with 8a-(PC-dioxy)-alpha-tocopherones and alpha-tocopherylquinone. The yield of alpha-tocopherylquinone was increased by treating each sample with dilute acid which indicates the presence of tocopherone precursors other than the 8a-(PC-dioxy)-alpha-tocopherones. The same products were also detected from iron-dependent peroxidation, although the yields were very low.

Topics: Amidines; Free Radicals; Lipid Peroxidation; Liposomes; Oxidants; Phosphatidylcholines; Phospholipid Ethers; Vitamin E

Phenolic antioxidants of the hydroxychroman class, alpha-tocopherol (alpha-TOC) and 2,2,5,6,7-pentamethyl-6-hydroxychroman (PMHC), and the hindered phenols 2,3-dihydro-5-hydroxy-2,2,4-trimethylnaphtho[1,2-b]furan (NFUR), 2,6-di-tert-butyl-4-methoxyphenol (DBHA), and 2,6-di-tert-butyl-4-methyl phenol (BHT), were delivered into oxidizable (ACCEPTOR) liposomes of dilinoleoylphosphatidylcholine (DLPC) or 1-palmitoyl-2-linoleoyl-phosphatidylcholine (PLPC) from saturated DONOR liposomes of dimyristoylphosphatidylcholine (DMPC) by liposomal transfer. The antioxidant activities, k(inh), by the inhibited oxygen uptake method were compared with the k(inh)s determined when the antioxidants were introduced into the liposomes by coevaporation from organic solvents. The peroxidations were initiated using either thermal initiators, water-soluble azo-bis-amidinopropane hydrochloride (ABAP), lipid-soluble azo-bis-2,4-dimethylvaleronitrile (ADVN) and di-tert-butylhyponitrite (DBHN), or the photoinitiator benzophenone. The antioxidants PMHC, NFUR, DBHA, and BHT transferred rapidly between liposomes, but several hours of incubation were needed to transfer alpha-TOC. The average k(inh)s in liposomes, in the relative order NFUR approximately DBHA > PMHC > BHT approximately alpha-TOC, were markedly lower than known values in organic solvent. k(inh) values in liposomes appear to be controlled by effects of hydrogen bonding with water and by restricted diffusion of antioxidants, especially in the case of alpha-TOC. Product studies of the hydroperoxides formed during inhibited oxygen consumption were carried out. The cis,trans/trans,trans (c,t/t,t) product ratios of the 9- and 13-hydroperoxides formed from PLPC during inhibited peroxidation by PMHC were similar for both the coevaporated and liposomal transfer procedures. The c,t/t,t ratio for the same concentration of alpha-TOC, 1.52, compares to a value of 1.69 for PMHC at the start of the inhibition period. The higher c,t/t,t ratio observed for NFUR in DLPC, which varied between values of 7.0 at the start of the inhibition to about 1.8 after the break in the induction period, is a reflection of the increased hydrogen atom donating ability of the antioxidant plus the increased concentration of oxidizable lipid provided by DLPC.

Topics: Amidines; Antioxidants; Azo Compounds; Butylated Hydroxytoluene; Chromans; Dimyristoylphosphatidylcholine; Furans; Lipid Peroxidation; Liposomes; Models, Chemical; Nitriles; Oxygen Consumption; Phosphatidylcholines; Vitamin E