Peroxidation of liposomal palmitoyllinoleoylphosphatidylcholine (PLPC), effects of surface charge on the oxidizability and on the potency of antioxidants

Peroxidation of membrane phospholipids is an important determinant of membrane function. Previously we studied the kinetics of peroxidation of the polyunsaturated fatty acid (PUFA) residues in model membranes (liposomes) made by sonication of palmitoyllinoleoylphosphatidylcholine (PLPC). Since most biomembranes are negatively-charged, we have now studied the effect of negative surface charge on the kinetics of peroxidation of liposomes made of PLPC and 9% of one of the negatively-charged phospholipids phosphatidylserine (PS) or phosphatidic acid (PA). Peroxidation was initiated by either CuCl2 or AAPH and continuously monitored spectrophotometrically. The following results were obtained: (i) The negative charge had only a slight effect on AAPH-induced peroxidation, but accelerated markedly copper-induced peroxidation of the liposomes, probably by increasing the binding of copper to the membrane surface. (ii) Ascorbic acid (AA) inhibited AAPH-induced but promoted copper-induced peroxidation in all the studied liposomes, probably by enhancing the production of free radicals upon reduction of Cu(II) to Cu(I). (iii) alpha-tocopherol (Toc) inhibited AAPH-induced peroxidation in all the studied liposomes, whereas the effect of tocopherol on copper-induced peroxidation varied from being pro-oxidative in PA-containing liposomes, to being extremely anti-oxidative in PS-containing liposomes, even at very low tocopherol concentrations. The significance of the latter unusual protective effect, which we attribute to recycling of tocopherol by a PS-Cu complex, requires further investigation.     

Adaptation of plasminogen activator sequences to known protease structures

The sequences of urokinase (UK) and tissue-type plasminogen activator (TPA) were aligned with those of chymotrypsin, trypsin, and elastase according to their 'structurally conserved regions'. In spite of its trypsin-like specificity UK was model-built on the basis of the chymotrypsin structure because of a corresponding disulfide pattern. The extra disulfide bond falls to cysteines 50 and 111d. Insertions can easily be accommodated at the surface. As they occur similarly in both, UK and TPA, a role in plasminogen recognition may be possible. Of the functional positions known to be involved in substrate or inhibitor binding, Asp 97, Lys 143 and Arg 217 (Leu in TPA) may contribute to plasminogen activating specificity. PTI binding may in part be impaired by structural differences at the edge of the binding pocket.     

Antioxidant effect of bovine serum albumin on membrane lipid peroxidation induced by iron chelate and superoxide

Albumin is supposed to be the major antioxidant circulating in blood. This study examined the prevention of membrane lipid peroxidation by bovine serum albumin (BSA). Lipid peroxidation was induced by the exposing of enzymatically generated superoxide radicals to egg yolk phosphatidylcholine liposomes incorporating lipids with different charges in the presence of chelated iron catalysts. We used three kinds of Fe³⁺-chelates, which initiated reactions that were dependent on membrane charge: Fe³⁺-EDTA and Fe³⁺-EGTA catalyzed peroxidation in positively and negatively charged liposomes, respectively, and Fe³⁺-NTA, a renal carcinogen, catalyzed the reaction in liposomes of either charge. Fe³⁺-chelates initiated more lipid peroxidation in liposomes with increased zeta potentials, followed by an increase of their availability for the initiation of the reaction at the membrane surface. BSA inhibits lipid peroxidation by preventing the interaction of iron chelate with membranes, followed by a decrease of its availability in a charge-dependent manner depending on the iron-chelate concentration: one is accompanied and the other is unaccompanied by a change in the membrane charge. The inhibitory effect of BSA in the former at high concentrations of iron chelate would be attributed to its electrostatic binding with oppositely charged membranes. The inhibitory effect in the latter at low concentrations of iron chelate would be caused by BSA binding with iron chelates and keeping them away from membrane surface where lipid peroxidation is initiated. Although these results warrant further in vivo investigation, it was concluded that BSA inhibits membrane lipid peroxidation by decreasing the availability of iron for the initiation of membrane lipid peroxidation, in addition to trapping active oxygens and free radicals.     

Synthesis of N-hydroxysuccinimide esters of phosphatidylethanolamine and some properties of liposomes containing these derivatives

The N-hydroxysuccinimide (NHS) ester of N-suberyl-dimyristoylphosphatidylethanolamine (sub-DMPE) was synthesized by reaction of DMPE with disuccinimidyl suberate, and isolated by preparative plate chromatography. Liposomes, which contain NHS-sub-DMPE, can covalently bind compounds that possess a free amino group such as ε-dinitrophenyl-lysine. The extent of DNP-lysine binding is influenced by the time and temperature of incubation, the amount of NHS-sub-DMPE incorporated into the liposomes, and the initial concentration of DNP-lysine. Binding occurs as a consequence of the formation of a new dinitrophenylated compound which has been characterized. Although NHS-sub-DMPE is stable to storage in organic solvents, preformed liposomes rapidly lose their ability to bind DNP-lysine due to hydrolysis of the N-hydroxysuccinimide ester bond. These findings bear on the future applicability of liposomes, containing N-hydroxysuccinimide esters of PE, as illustrated by the preparation of immunogenic liposomes.     

The hapten structure of a developmentally regulated glycolipid antigen (SSEA-1) isolated from human erythrocytes and adenocarcinoma: a preliminary note

Glycolipid antigen reacting to the monoclonal antibody directed to the developmentally regulated antigen SSEA-1 was isolated from human erythrocytes and colonic adenocarcinoma. The antigens have the Le^x (Galβl→4[Fucα]→3]GlcNAcβl→R) or Le^y (Fucαl→2Galβl→4[Fucαl→3]GlcNAcβl→R) structure at the termini of the branched polylactosaminolipid. In addition, a novel polyfucosyl structure locating exclusively at the internal GlcNAc was detected in the tumor antigen. The antibody reacts with a simple monovalent Le^x glycolipid (Galβl→4[Fucαl→3]GlcNAcβl→3Galβl→4Glcβl→Cer) previously isolated from colonic carcinoma when presented at a high density on liposomes. The antibody therefore may react to the bivalent or multivalent Le^x or Le^y structure.     

Dynamic fluorescence investigations of the effect of osmotic shocks on the microenvironments of charged and uncharged dipalmitoyl-D,L-α-phosphatidylcholine liposomes

Neutral methylanthracene (MA), anionic trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate, (pyranine), and cationic 3,6-diamino-10-methylacridinium chloride (acriflavine), have been used as fluorescence probes to investigate effects of osmotic shrinkage on neutral, cationic and anionic dipalmitoyl-D,L-α-phosphatidylcholine liposomes. The determined fluorescence polarizations in the liposomes and in solvents of known viscosities afforded the estimation of the microviscosities of the environments of these probes. The viscosity reported by pyranine for anionic and that by acriflavine for cationic single compartment liposomes, ～1.0 cP, indicate the aqueous environments of these probes. Increased viscosities following osmotic shrinkages have been rationalized in terms of changing the nature of the liposome entrapped water. Following the release of free water, some bound water is also released as the result of osmotic shrinkage. The determined shrinkage rates support this postulate. The viscosity of the environment of pyranine in cationic, 9.6 ± 0.3 cP, and that of acriflavine in anionic single compartment liposomes, 74 ± 5 cP, indicate electrostatic attractions of the probes to the charged liposome surface. Osmotic shrinkage results in lowering the viscosity of the environments of the probes presumably because the more concentrated sodium chloride replaces them from their sites. The high viscosities reported by MA, ～ 1000 cP, suggest the intercalation of this probe in the phospholipid bilayers. Osmotic shrinkage does not alter the environment of MA. However, in the presence of cholesterol, the viscosities reported by MA are greater than in its absence. These data contradict previous NMR, ESR and X-ray results as well as those obtained in the present work from osmotic shrinkage rates. The need for care in interpreting data obtained by the use of fluorescence probes is emphasized.