Lipid-soluble Initiator Research Articles

The Location of Vitamin E in Model Membranes and its Effect on Oxidation

There are no proven health benefits to supplementing with Vitamin E, so why do we require it for healthy living? The whole notion that vitamin E is an in-vivo antioxidant is now being seriously questioned. We believe the debate in literature is due to much of the existing data being collected using techniques which require the presences of non-biological and invasive probes, and often in the wrong model systems. Using neutron diffraction, supported by solid state 2H NMR, we have correlated vitamin E's location in model membranes with its antioxidant activity. Experiments were conducted using phosphatidylcholine (PC) bilayers whose fatty acid chains varied in their degree of unsaturation. PC bilayers made up of mixed acyl chains (i.e., saturated and unsaturated) and different headgroup moieties were also studied. UV/Vis spectroscopy studies were conducted to examine vitamin E's oxidation at its various locations within the different model membranes. Both water soluble and lipid soluble initiators were used to start the oxidation process.We observe vitamin E up-right in all lipids examined, with its overall height in the bilayer lipid dependant. Interestingly we observe vitamin E's hydroxyl in the headgroup region of the bilayer for both the fully saturated and poly unsaturated lipids. Vitamin E was most effective at intercepting water borne oxidants than radical initiated within the bilayer core. However for lipids where vitamin E resides slightly lower(glycerol backbone) we observe comparable antioxidant activity against both water borne and hydrocarbon borne oxidants. Thus showing lipid species can modulate the location of vitamin E's activity.

Synergistic inhibition of lipid peroxidation by vitamin E and a dopamine agonist, cabergoline

We examined antioxidant activity of cabergoline, a dopamine agonist, during the aerobic oxidation of phosphatidylcholine liposomes at 37°C. Cabergoline retarded the oxidation initiated with a lipid-soluble initiator significantly better than that with a water-soluble initiator, suggesting that cabergoline locates in the lipid layer of liposomal membranes. Cabergoline inhibited the oxidation of liposomal membranes synergistically with endogenous antioxidants such as ascorbic acid, ubiquinol-10 and vitamin E, and vitamin E was the most efficient synergist. These results suggest that cabergoline may have a neuroprotective effect on the substantia nigra of Parkinsonian patients because of its synergistic antioxidant activity with vitamin E as well as its action on dopamine receptor.

Autoxidation of Binary Mixtures of Phosphatidyicholine and Phosphatidylethanolamine in Aqueous Dispersions Initiated by a Lipid-Soluble Initiator

The aerobic oxidation of binary mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in aqueous dispersions was carried out at 37°C initiated by a lipid-soluble azo initiator. The relative rates of the oxidation of PC and PE was found to depend on the number of bisallylic hydrogens in the phospholipids irrespective of the nitrogen moiety used. The increase in the mole ratios of PE to PC resulted in a decrease in the oxidizabilities of the lipids in aqueous dispersions, probably due to an increase in the microviscosity.

Mechanism of free radical-induced hemolysis of human erythrocytes. II. Hemolysis by lipid-soluble radical initiator.

Hemolysis of human erythrocytes induced by free radicals initiated from lipid-soluble 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) was examined under various conditions. From the ESR spectra of the spin-labeled erythrocytes, it was found that the fluidity of the membrane did not change during the radical-induced hemolysis. The curves of the time courses of the extent of oxidation and the conformational change of band 3 proteins were hyperbolic, though the hemolysis curves were sigmoidal. In spite of the necessity of lipid peroxidation, the peroxidation did not seem to relate directly to the hemolysis. It was observed that the hemolytic holes were formed by a lateral clustering of band 3, an anion exchange protein in erythrocytes. The competitive reaction model between lipid peroxidation and the redistribution of oxidized band 3 proteins, which was previously presented for the hemolysis initiated from water-soluble, 2,2'-azobis(amidinopropane)dihydrochloride (AAPH), could explain well the curves for the hemolysis by ADVN. Further, the rates of lipid peroxidation at various concentrations of ADVN and AAPH were calculated on the basis of the hemolysis curves, and they were compared with the experimental values estimated from the curves for the lipid peroxidation. The curves which showed a dependence of the calculated rate constant on the concentration of radical initiators were similar to those of the experimental values. These results indicate that the competitive reaction model is appropriately represents the hemolysis induced by free radicals which also originated from lipid-soluble initiators.

Antioxidant activity of 3-methyl-1-phenyl-2-pyrazolin-5-one

The antioxidant activity of an anti-ischemic agent, 3-methyl-1-phenyl-2-pyrazolin-5-one (MCI-186), was examined. The pKa value of MCI-186 is 7.0 and the rate of oxidation of MCI-186 initiated with an azo compound increased with increasing pH, suggesting that the anionic form of MCI-186 is much more reactive than the non-ionic form. The major products were 3-methyl-1-phenyl-2-pyrazolin-4,5-dione (4,5-dione) and 2-oxo-3-(phenylhydrazono)-butanoic acid (OPB). Hydrolysis of 4,5-dione gave OPB. The minor intermediate product was 4-hydroxy-4-(3-methyl-1-phenyl-1H-pyrazolin-5-on-4-yl)-3-methyl-1-phenyl-1H-pyrazolin-5-one (BPOH). The nucleophilic attack of the anionic form of MCI-186 to 4,5-dione is likely to give BPOH. MCI-186 (50 μM) inhibited the aerobic oxidation at 37°C of 5.2 mM unilamellar soybean phosphatidylcholine (PC) liposomal membranes, initiated with a water-soluble initiator, as efficientlyas did ascorbate (100 μM). MCI-186 (50 μM) also inhibited the oxidation of the same PC liposomal membranes, this time initiated with a lipid-soluble initiator, almost as efficiently as did α-tocopherol (2 μM). Furthermore, the combination of MCI-186 with ascorbate or α-tocopherol showed almost complete inhibition of PC oxidation induced by both initiators. These data suggest that MCI-186 may work as a good antioxidant in cellular systems as well as in cell-free systems.

Characterisation of novel indenoindoles. Part II. Redox-recycling with ascorbate

In the accompanying paper it was shown that the new antioxidant, H 290/51 (cis-5,5a,6,10b-tetrahydro-9-methoxy-7-methylindeno [2,1-b]indole), is a powerful antioxidant in several pharmacological models of lipid peroxidation and could be useful as a therapeutic agent in pathophysiological situations where lipid peroxidation plays an important role. In the present study, we characterised H 290/51 as an inhibitor of peroxidation of pure methyl linoleate. H 290/51 almost completely inhibited peroxidation induced by a lipid-soluble initiator at 37 °C during the induction period, both in an aqueous solution of micelles in the presence of detergents and in a homogeneous ethanol solution. In both systems, the time of the induction period was linearly related to the concentration of H 290/51. In the ethanol solution, ascorbic acid had a sparing effect on H 290/51, indicating effective interference with radical chain propagation. In aqueous solution with micelles of methyl linoleate made with the nonionic detergents Triton X-100 or Lubrol PX, ascorbic acid did not inhibit peroxidation. However, in these micelles, H 290/51 showed a concentration-dependent extension of the induction period by ascorbic acid, suggesting recycling. In the presence of the zwitterionic detergent CHAPS, although a clear induction period is seen with H 290/51, no recycling by ascorbic acid was found. The ability of H 290/51 to recycle in aqueous solutions, thus depends on the micellar composition.

Oxidation of Human Plasma by Water- and Lipid-soluble Radical Initiators.

The formation of phosphatidylcholine hydroperoxide (PC-OOH), phosphatidylcholine hydroxide (PC-OH), cholesteryl ester hydroperoxide (CE-OOH), cholesteryl ester hydroxide (CE-OH), free fatty acid hydroperoxide (FFA-OOH), and free fatty acid hydroxide (FFA-OH), and changes in the levels of plasma antioxidants such as urate, ascorbate, bilirubin, ubiquinol-10, and α-tocopherol were measured during the oxidation of human plasma initiated with water- and lipid-soluble initiators in an aerobic condition at 37°C to assess the oxidation of plasma lipids and its inhibition by antioxidants. Rapid decrease in ascorbate level was induced by a water-soluble initiator and that in ubiquinol-10 and α-tocopherol levels by a lipid-soluble initiator indicate that the former initiator is active in water-phase while the latter in the lipid-phase. In addition to PC-OOH and CE-OOH, considerable amount of PC-OH was formed during plasma oxidation. PC-OH is likely to be produced by the action of peroxidase since the addition of PC-OOH to human plasma gave PC-OH as the major product. Considerable amount of CE-OH was also formed during plasma oxidation. Since CE-OOH was relatively stable in human plasma, CH-OH is likely to be derived from PC-OH by the action of lecithin : cholesterol acyltransferase. Little formation of FFA-OOH and FFA-OH suggests that phospholipase A 2 like activity and platelet-activating factor acetylhydrolase are not significant factors in the decomposition of PC-OOH and PC-DH. The differences between the initiators in plasma oxidation and between the oxidation of high and low density lipoproteins are discussed.

1992 Syntex Award Lecture Model biomembranes: quantitative studies of peroxidation, antioxidant action, partitioning, and oxidative stress

This review outlines a decade of research that employs quantitative kinetic methods of autooxidation to phospholipids aggregated into membranes. The classical rate law for autoxidation:[Formula: see text]was found to apply to heterogeneous systems of bilayers and micelles, where kp and 2kt. are the rate constants for chain propagation and termination, respectively, and Ri, the rate of chain initiation, is controlled by thermal initiators. The oxidizability of a typical lipid chain, linoleate 18:2, at 30 °C was similar (0.02–0.04 M−1/2 s−1/2) in different media (solution, micelles, bilayers) and for different procedures using water-soluble or lipid-soluble initiators and inhibitors. A reduction in the absolute rate constant for termination, 2kt by a hundredfold in bilayers of dilinoleoylphosphatidyl choline (DLPC) compared to that in tert-butyl alcohol solution, and a drop in the kp by a factor of five are interpreted in terms of diffusion of polarized peroxyl radicals from the hydrophobic bilayer phase to the aqueous surface, where peroxyls are strongly solvated by water. Such phenomena may also account for significant changes as observed by 31P NMR spectra in bilayer lamellar structure accompanying extensive peroxidation. Analysis of the hydroperoxides formed by peroxidation of mixed bilayers of DLPC + DPPC (16:0) initiated by a water-soluble initiator, azobis(2-amidinopropane•HCl) (ABAP), showed a linear trend between the ratio of cis, trans to trans, trans geometrical isomeric hydroperoxides and [DPLC] consistent with a peroxidation mechanism proposed in homogeneous solution.The antioxidant activities, kinh, of three classes of antioxidants: (a) polyalkyl-6-hydroxychromans (e.g., vitamin E), (b) polyalkyl-4-methoxyphenols, and (c) trialkylphenols, were measured in DLPC membranes. The results show an overall leveling and depression of kinh values in DLPC membranes in the series (a) (by several orders of magnitude), (b), and (c) compared to known values in solution in chlorobenzene. In aqueous bilayers it is proposed that kinh values are attenuated by hydrogen bonding by water at both the para ether oxygen and phenolic groups. Restricted diffusion (e.g., of α-tocopherol) may also reduce antioxidant activities in membranes. A synergistic effect between ascorbic acid and α-tocopherol was discovered under conditions of inhibited peroxidation of linoleate in SDS micelles. The natural peptide glutathione, GSH, however, acts as a co-antioxidant with vitamin E by trapping peroxyls in the aqueous phase.Solid cholesterol was found to partition directly into PC lipsomes by shaking, above or below the phase transition temperature, and membrane-bound cholesterol, unlike the solid, undergoes facile peroxidation. A water-soluble form of α-tocopherol complexed with bovine serum albumin (α-toc:BSA) is an effective antioxidant for autoxidations of linoleate in SDS micelles. In contrast, α-toc:BSA required a long equilibration time (e.g., 12 h) with liposomes (DLPC) before the α-tocopherol was transferred to the liposomes to provide effective antioxidant action.

Autoxidation of model membranes. The kinetics and mechanism of autoxidation of mixed phospholipid bilayers

Kinetics of autoxidation of bilayer mixtures of oxidizable dilinoleoylphosphatidylcholine (DLPC) with saturated phospholipids as solvents (dimyristoyl-, DMPC, and dipalmitoyl-phosphatidylcholine, DPPC) follow the classical rate law of autoxidation: −d[O2]/dt = kp/2kt1/2[RH]Ri1/2. The kinetic order in substrate [RH] was found to be unity for peroxidation initiated by a lipid-soluble initiator, azobis-2,4-dimethylvaleronitrile (ADVN), and by the water-soluble azobis(2-amidinopropane•HCl) (ABAP). The kinetic order in rate of chain initiation, Ri, was found to be one-half for both initiation by ADVN, photochemically decomposed, and by ABAP. The oxidizability of unilamellar DLPC liposomes (kp/2kt1/2 = 0.232 M−1/2 s−2) is twice that of multilamellar DLPC (kp/2kt1/2 = 0.116 M−2 s−2). Analysis of the hydroperoxides formed during ABAP-initiated autoxidations of mixed DLPC + DPPC liposomes showed a linear trend between the ratio of cis,trans to trans,trans geometrical isomeric hydroperoxides and [DLPC], consistent with the peroxidation mechanism proposed for homogeneous systems.31P nmr spectra of mixed bilayers were used to distinguish between heterogeneous and homogeneous mixtures of DLPC + DPPC. Such spectra taken at various stages of oxidation indicate that the bilayer structure of DLPC is preserved at least to the 10% extent of oxidation used in kinetic studies. At much higher oxidative conversion, the spectra indicate changes in lamellar structures.

The antioxidant activity of alpha-tocopherol-bovine serum albumin complex in micellar and liposome autoxidations.

A comparison is made of the antioxidant activity of a water-soluble form of alpha-tocopherol complexed with bovine serum albumin (alpha-T X BSA) with that of micellar alpha-tocopherol and aqueous 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylate (Trolox) to inhibit autoxidation of linoleic acid in sodium dodecyl sulfate micelles. The peroxyl radical trapping ability of alpha-T X BSA compares favorably with that of alpha-tocopherol and Trolox, and all three can be used in quantitative measurements of the susceptibility of the micellar substrate to undergo autoxidation: the oxidizability, for reactions initiated in the micellar phase by di-tertbutylhyponitrite (DBHN) or in the aqueous phase by azobisamidinopropane hydrochloride (ABAP). alpha-Tocopherol and Trolox are also effective antioxidants to inhibit DBHN- or ABAP-initiated autoxidations of dilinoleoylphosphatidylcholine (DLPC) liposomes prepared as multilamellar or unilamellar bilayers characterized by 31P NMR spectra. The oxidizability of DLPC liposomes is determined by various combinations of water-soluble and lipid-soluble initiators and the antioxidants, alpha-tocopherol and Trolox. In contrast, alpha-T X BSA does not effectively trap peroxyl radicals when it is added after initiation of autoxidation in the lipid phase (DBHN) or in the aqueous phase (ABAP). The radical trapping ability of alpha-T X BSA becomes evident if it is mixed with the DLPC for some hours before initiation. This result is interpreted in terms of diffusion of alpha-tocopherol from the bound alpha-T X BSA form to the liposome before it exhibits antioxidant activity.

Autoxidation of micelles and model membranes. Quantitative kinetic measurements can be made by using either water-soluble or lipid-soluble initiators with water-soluble or lipid-soluble chain-breaking antioxidants

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAutoxidation of micelles and model membranes. Quantitative kinetic measurements can be made by using either water-soluble or lipid-soluble initiators with water-soluble or lipid-soluble chain-breaking antioxidantsL. R. C. Barclay, S. J. Locke, J. M. MacNeil, J. VanKessel, G. W. Burton, and K. U. IngoldCite this: J. Am. Chem. Soc. 1984, 106, 8, 2479–2481Publication Date (Print):April 1, 1984Publication History Published online1 May 2002Published inissue 1 April 1984https://doi.org/10.1021/ja00320a066RIGHTS & PERMISSIONSArticle Views438Altmetric-Citations167LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (427 KB) Get e-Alerts Get e-Alerts