Oxidation of vitamin E during iron-catalyzed lipid peroxidation: evidence for electron-transfer reactions of the tocopheroxyl radical.

Incubation of phosphatidylcholine liposomes containing the biological antioxidant alpha-tocopherol (alpha-TH) with xanthine, xanthine oxidase, and FeCl2 caused alpha-TH oxidation to alpha-tocopherol quinone (alpha-TQ) and 8a-hydroperoxytocopherone (2). In addition, 4a,5-epoxy-8a-hydroperoxytocopherone (3), 7,8-epoxy-8a-hydroperoxytocopherone (4), and their respective hydrolysis products 2,3-epoxy-alpha-tocopherol quinone (6) and 5,6-epoxy-alpha-tocopherol quinone (7) also were formed. alpha-TQ was the major product at less than 20% alpha-TH oxidation, whereas epoxides were the predominant products when alpha-TH was more extensively oxidized. 8a-(Alkyldioxy)tocopherones 1, which are formed when peroxyl radicals oxidize alpha-TH in other systems and which are precursors to alpha-TQ, were not found. 8a-Hydroxytocopherone (5), rather than 8a-(alkyldioxy)tocopherones 1, appeared to be the precursor to alpha-TQ. Approximately 30% of the alpha-TH consumed was regenerated by treatment of samples with ascorbic acid or nordehydroguaiaretic acid (NDGA) at pH 3, but not at pH 7. The stability of the ascorbic acid- and NDGA-reducible species and pH dependence for regeneration matched those of 8a-hydroxytocopherone (5) and contrasted with the properties of the tocopheroxyl radical (alpha-T.). Incubation of liposomes containing alpha-TH with the diphenylpicrylhydrazyl (DPPH) radical, which oxidizes alpha-TH to alpha-T. in high yield, formed an ascorbic acid-reducible species with properties identical to those of compound 5. The results indicate that phospholipid peroxyl radicals oxidize alpha-T. to epoxides, 8a-hydroperoxytocopherone (2), and the tocopherone cation (alpha-T+), which hydrolyzes to 5, the immediate precursor to alpha-TQ.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrochemical selective determination of ascorbic acid at redox active polymer modified electrode derived from direct blue 71

A simple selective method for determination of ascorbic acid using polymerized direct blue 71 (DB71) is described. Anodic polymerization of the azo dye DB71 on glassy carbon (GC) electrode in 0.1M H(2)SO(4) acidic medium was found to yield thin and stable polymeric films. The poly(DB71) films were electroactive in wide pH range (1-13). A pair of symmetrical redox peaks at a formal redox potential, E('0)=-0.02V vs. Ag/AgCl (pH 7.0) was observed with a Nernstian slope -0.058V, is attributed to a 1:1 proton+electron involving polymer redox reactions at the modified electrode. Scanning electron microscope (SEM), atomic force microscope (AFM) and electrochemical impedance spectroscopy (EIS) measurements were used for surface studies of polymer modified electrode. Poly(DB71) modified GC electrode showed excellent electrocatalytic activity towards ascorbic acid in neutral buffer solution. Using amperometric method, linear range (1x10(-6)-2x10(-3)M), dynamic range (1x10(-6)-0.01M) and detection limit (1x10(-6)M, S/N=3) were estimated for measurement of ascorbic acid in pH 7.0 buffer solution. Major interferences such as dopamine and uric acid are tested at this modified electrode and found that selective detection of ascorbic acid can be achieved. This new method successfully applied for determination of ascorbic acid in commercial tablets with satisfactory results.     

Vitamin C (ascorbic acid) induced hydroxyl radical formation in copper contaminated household drinking water: role of bicarbonate concentration

We have previously shown that Vitamin C (ascorbic acid) can trigger hydroxyl radical formation in copper contaminated household drinking water. We report here that the capacity of ascorbic acid to catalyze hydroxyl radical generation in the drinking water samples is strongly dependent on the bicarbonate concentration (buffer capacity and pH) of the samples. We found that at least 50 mg/l bicarbonate was required in the water samples to maintain the pH over 5.0 after ascorbic acid addition. At this pH, that is higher than the pKa

1

4.25 of ascorbic acid, a hydroxyl radical generating redox cycling reaction involving the mono-anion of vitamin C and copper could take place. The ascorbic acid induced hydroxyl radical generating reaction could easily be mimicked in Milli-Q water by supplementing the water with copper and bicarbonate. Our results demonstrate that ascorbic acid can induce a pH dependent hydroxyl radical generating reaction in copper contaminated household tap water that is buffered with bicarbonate. The impact of consuming ascorbic acid together with copper and bicarbonate containing drinking water on human health is discussed.     

Kinetics of the reaction of prostaglandin H synthase compound II with ascorbic acid

The reduction of prostaglandin H synthase compound II by ascorbic acid in the presence of diethyldithiocarbamate was studied in 0.1 M phosphate buffer (pH 8.0) at 4.0 +/- 0.5 degrees C, by rapid scan spectrometry and transient state kinetics. A saturation effect and nonzero intercept were observed in the plot of pseudo-first-order rate constant versus ascorbic acid concentration. The saturation behavior suggests formation of a complex between prostaglandin H synthase compound II and ascorbic acid, whereas the nonzero intercept is attributable to the reaction of compound II of prostaglandin H synthase with diethyldithiocarbamate present in the system as a stabilizing agent. A rate equation has been derived which includes all pathways for the conversion of prostaglandin H synthase compound II back to native enzyme. Kinetic parameters for the reduction of compound II by ascorbic acid were obtained. They are the second-order rate constant of (1.4 +/- 0.5) X 10(5) M-1, S-1, for the formation of the compound II-ascorbic acid complex, the first-order rate constant of (14 +/- 4) S-1 for the oxidation-reduction reaction of the complex and its dissociation, and a parameter, Km of 92 +/- 10 microM analogous to the Michaelis-Menten constant. Thus we demonstrate that a quantitative kinetic study on the prostaglandin H synthase reactions can be performed in the presence of diethyldithiocarbamate.     

Redox potentials of flavocytochromes c from the phototrophic bacteria, Chromatium vinosum and Chlorobium thiosulfatophilum.

The redox potentials of flavocytochromes c (FC) from Chromatium vinosum and Chlorobium thiosulfatophilum have been studied as a function of pH. Chlorobium FC has a single heme which has a redox potential of +98 mV at pH 7 (N = 1) that is independent of pH between 6 and 8. The average two-electron redox potential of the flavin extrapolated to pH 7 is +28 mV and decreases 35 mV/pH between pH 6 and 7. The anionic form of the flavin semiquinone is stabilized above pH 6. The redox potential of Chromatium FC is markedly lower than for Chlorobium. The two hemes in Chromatium FC appear to have a redox potential of 15 mV at pH 7 (N = 1), although they reside in very different structural environments. The hemes of Chromatium FC have a pH-dependent redox potential, which can be fit in the simplest case by a single ionization with pK = 7.05. The flavin in Chromatium FC has an average two-electron redox potential of -26 mV at pH 7 and decreases 30 mV/pH between pH 6 and 8. As with Chlorobium, the anionic form of the flavin semiquinone of Chromatium FC is stabilized above pH 6. The unusually high redox potential of the flavin, a stabilized anion radical, and sulfite binding to the flavin in both Chlorobium and Chromatium FCs are characteristics shared by the flavoprotein oxidases. By analogy with glycolate oxidase and lactate dehydrogenase for which there are three-dimensional structures, the properties of the FCs are likely to be due to a positively charged amino acid side chain in the vicinity of the N1 nitrogen of the flavin.     

Thermodynamic control of D-amino acid oxidase by benzoate binding

The redox properties of D-amino acid oxidase (D-amino-acid: O2 oxidoreductase (deaminating) EC1.4.3.3) have been measured at 18 degrees C in 20 mM sodium pyrophosphate, pH 8.5, and in 50 mM sodium phosphate, pH 7.0. Over the entire pH range, 2 eq are required per mol of FAD in D-amino acid oxidase for reduction to the anion dihydroquinone. The red anion semiquinone is thermodynamically stable as indicated by the separation of the electron potentials and the quantitative formation of the semiquinone species. The first electron potential is pH-independent at -0.098 +/- 0.004 V versus SHE while the second electron potential is pH-dependent exhibiting a 0.060 mV/pH unit slope. The redox behavior of D-amino acid oxidase is consistent with that observed for other oxidase enzymes. On the other hand, the behavior of the benzoate-bound enzyme under the same conditions is in marked contrast to the thermodynamics of free D-amino acid oxidase. Spectroelectrochemical experiments performed on inhibitor-bound (benzoate) D-amino acid oxidase show that benzoate binding regulates the redox properties of the enzyme, causing the energy levels of the benzoate-bound enzyme to be consistent with the two-electron transfer catalytic function of the enzyme. Our data are consistent with benzoate binding at the enzyme active site destroying the inductive effect of the positively charged arginine residue. Others have postulated that this positively charged group near the N(1)C(2) = O position of the flavin controls the enzyme properties. The data presented here are the clearest examples yet of enzyme regulation by substrate which may be a general characteristic of all flavoprotein oxidases.     

A new procedure for the purification of rat testis-specific histone TH2B involving affinity-related chromatography

A new procedure for the isolation of rat testis-specific histone TH2B has been devised. First, rat testis chromatin fragments were applied to a hydroxylapatite column in 0.5 m NaCl, 0.1 m potassium phosphate buffer, pH 6.7, and histones were selectively stripped off the bound DNA in groups (H1/TH1, H2A/H2B/TH2B, and H3/H4). The fraction containing H2A, H2B, and TH2B, but lacking H3, was reduced, desalted, and applied to a p-chloromercuribenzoyl-aminoethyl-Sepharose 4B-CL column in 8 m urea, 0.1 m Tris-HCl, pH 8.1. After washing with the same buffer to remove H2A and H2B, covalently bound TH2B was eluted out with 10 mm dithiothreitol in the same buffer. No contaminants were detectable in the purified TH2B either by polyacrylamide gel electrophoresis in 0.4% Triton X-100, 2.5 m urea, 0.9 n acetic acid, or by N-terminal analysis with dansyl chloride.     

Protective effect of seminal plasma proteins on the degradation of ascorbic acid

The objectives of this study were to determine ascorbic acid stability and its effect on antiproteinase activity of seminal plasma in the presence of an oxidant. Effect of seminal plasma, and additives: glutathione, albumin, hydrogen peroxide and Tris buffer, on ascorbic acid degradation was investigated by UV absorbance. Antiproteinase against trypsin amidase activity was measured spectrophotometrically using N-benzoyl-DL-arginine-p-nitroanilide (BAPNA) as substrate. Ascorbic acid was destroyed much more rapidly with the addition of hydrogen peroxide than in Tris buffer at pH 8.2 alone. Seminal plasma protected ascorbic acid more efficiently than glutathione and albumin alone. The protective effect of seminal plasma on ascorbic acid degradation may closely relate to the function of ascorbic acid in reproductive system of scurvy-prone animals including teleost fish. Within the range of 1–8 mM concentrations, ascorbic acid had a pro-oxidant action on seminal plasma antiproteinase activityin vitro when they were incubated with hydrogen peroxide.Abbreviations AA Ascorbic acid - BAPNA N-benzoyl-DL-arginine-p-nitroanilide - DMSO dimethyl sulfoxide - GSH glutathione - H₂O₂ hydrogen peroxide     

Destabilization of phosphatidylethanolamine liposomes at the hexagonal phase transition temperature

We have examined whether there is a relationship between the lamellar-hexagonal phase transition temperature, TH, and the initial kinetics of H+- and Ca2+-induced destabilization of phosphatidylethanolamine (PE) liposomes. The liposomes were composed of dioleoylphosphatidylethanolamine, egg phosphatidylethanolamine (EPE), or phosphatidylethanolamine prepared from egg phosphatidylcholine by transesterification (TPE). These lipids have well-spaced lamellar-hexagonal phase transition temperatures (approximately 12, approximately 45, and approximately 57 degrees C) in a temperature range that allows us to measure the initial kinetics of bilayer destabilization, both below and above TH. The liposomes were prepared at pH 9.5. The TH of EPE and TPE was measured by using differential scanning calorimetry, and it was found that the TH was essentially the same at low pH or at high pH in the presence of 20 mM Ca2+. At temperatures well below TH, either at pH 4.5 or at pH 9.5 in the presence of Ca2+, the liposomes aggregate, leak, and undergo lipid mixing and mixing of contents. We show that liposome/liposome contact is involved in the destabilization of the PE liposomes. The temperature dependence of leakage, lipid mixing, and mixing of contents shows that there is a massive enhancement in the rate of leakage when the temperature approaches the TH of the particular PE and that lipid mixing appears to be enhanced. However, the fusion (mixing of aqueous contents) is diminished or even abolished at temperatures above TH. At and above the TH, a new mechanism of liposome destabilization arises, evidently dependent upon the ability of the PE molecules to adapt new morphological structures at these temperatures. We propose that this destabilization demarks the first step in the pathway to the eventual formation of the HII phase. Thus, the polymorphism accessible to PE is a powerful agent for membrane destabilization, but additional factors are required for fusion.     

A fluorometric method for measurement of oxygen radical-scavenging activity of water-soluble antioxidants

The relative activities of the antioxidants Trolox, ascorbic acid, uric acid, quercetin, and rutin, and the activities of total antioxidants in serum samples were determined using a fluorometric assay based on the dye 6-carboxyfluoroscein (6C-Fl) as a fluorescent indicator; 2,2'-azobis-2-amidinopropane hydrochloride (AAPH) as a peroxyl radical generator; 6-hydroxy-2,5,7, 8-tetramethyl-1-chroman-2-carboxylic acid (Trolox) as a calibrator; and phosphate buffer (pH 7.0) as a solvent. Incubation of 6C-Fl in 0. 075 M phosphate buffer, in the presence of AAPH at 37 degrees C, resulted in loss of its fluorescence signal at 520 nm with excitation at 495 nm. The antioxidants Trolox, ascorbic acid, and uric acid provided protection of the fluorescence of 6C-Fl, and the relative antioxidant activities, determined by the net protection area under curve technique, were found to be 1:0.4:1, respectively. Trolox and ascorbic acid were used to validate this assay. A linear correlation of the net protection value with the concentration of serum, Trolox, ascorbic acid, and uric acid was demonstrated. Quercetin and rutin were shown to have strong antioxidant activities, nearly 10 times those of vitamin C. This assay is simple, reliable, and suitable for automation to handle many samples and requires few microliters of serum samples.     

Destabilization of phosphatidylethanolamine-containing liposomes: hexagonal phase and asymmetric membranes

We have measured the temperature of the L alpha-HII phase transition, TH, for several types of phosphatidylethanolamine (PE), their binary mixtures, and several PE/cholesteryl hemisuccinate (CHEMS) mixtures. We have shown for liposomes composed of pure PE and in mixtures with CHEMS that there is an aggregation-mediated destabilization which is greatly enhanced at and above TH. We now ask the question: How well can a dioleoylphosphatidylethanolamine/CHEMS liposome, for example, destabilize TPE (transesterified from egg phosphatidylcholine)/CHEMS liposome and vice versa? We use Ca2+ and H+ to induce aggregation and to provide different values of TH: the TH of the PE/CHEMS mixture is much lower at low pH than with Ca2+. We find that if the temperature is above the TH of one lipid mixture, e.g., A, and below the TH of the other lipid mixture, e.g., B, then the destabilization sequence [measured by the fluorescent 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylylenebis(pyridinium bromide) leakage assay] is AA greater than AB much greater than BB. That is, the bilayer of the lipid A (which on its own would end up in the HII phase) destabilizes itself better than it destabilizes the bilayer of lipid B (which on its own would remain in the L alpha phase). The BB contact is the least unstable. From these experiments, we conclude that the enhanced destabilization of membranes provided by the polymorphism accessible to these lipids above TH is effective even if only one of the apposed outer monolayers is HII phase competent. The surprising result is that if the temperature is above the TH of both lipid mixtures, then the destabilization sequence is AB greater than AA, BB. That is, the mixed bilayers are destabilized more by contact than either of the pure pairs. We believe that this is due to specific differences in the kinetics of aggregation or close approach of the membranes. Similar results were obtained with pure PE liposomes induced to aggregate by Ca2+ at pH 9.5. We also found that the kinetics of low-pH-induced leakage from PE/CHEMS liposomes were initially faster when the CHEMS on both sides of the bilayer is fully protonated. However, in a citrate buffer, which cannot cross intact membranes, the leakage was eventually faster. Flip-flop of the protonated CHEMS to the inner monolayer can explain this observation.     

Simultaneous determination of ascorbic acid and dopamine in the presence of uric acid on ruthenium oxide modified electrode

RuOx x nH2O film was electrochemically synthesized conveniently using cyclic voltammetric technique. The film formation was ascertained by the Electrochemical quartz crystal microbalance (EQCM) method and 45 ng of deposit per cycle was obtained. Stoichiometric ratio of the ruthenium and ruthenium oxide have been studied with different pH of phosphate buffer. The stability of the modified electrode in the presence of different cations and anions with different concentrations and pH were examined. Electrochemical studies have shown that the ascorbic acid (AA) and dopamine (DA) catalytic oxidation on ruthenium oxide modified electrode (RME) with a span of 300 mV separation even in the presence of uric acid (UA) with a large decrease in their respective over potential compared with bare glassy carbon electrode (GC). Accidentally, the reversible redox properties of the AA have been expediently studied on the RME using cyclic voltammetry and this peculiarity was interrogated through rotating ring disc electrode (RRDE) experiments. RRDE experiment results are conformed to the CV studies result and thus reversible redox property of AA have been reiterated. Amperometric detection under stirred condition up to approximately 0.8mM of AA and DA was carried out at free of electrode fouling. Interestingly, the regeneration of used RME electrode even after many consequent analysis, 100% was obtained.     

Hydroxylation of deoxyguanosine at the C-8 position by ascorbic acid and other reducing agents

The C-8 position of deoxyguanosine (dGuo) was hydroxylated by ascorbic acid in the presence of oxygen (O2) in 0.1 M phosphate buffer (pH 6.8) at 37 degrees C. Addition of hydrogen peroxide (H2O2) remarkably enhanced this hydroxylation. The Udenfriend system [ascorbic acid, FeII, ethylenediaminetetraacetic acid (EDTA) and O2] was also effective for hydroxylation of dGuo in high yield. Guanine residues in DNA were also hydroxylated by ascorbic acid. Other reducing agents, such as hydroxylamine, hydrazine, dihydroxymaleic acid, sodium bisulfite and acetol, were also effective for the hydroxylation reaction, as were metal-EDTA complexes (FeII-, SnII-, TiIII-, CuI-EDTA). An OH radical seemed to be involved in this hydroxylation reaction in most of the above hydroxylating systems, but another reaction mechanism may also be involved, particularly when dGuo is hydroxylated by ascorbic acid alone or ascorbic acid plus H2O2. The possible biological significance of the hydroxylation of guanine residues in DNA in relation to mutagenesis and carcinogenesis is discussed.     

Interchange reaction of disulfides and denaturation of oxytocin by copper(II)/ascorbic acid/O2 system

The interchange reaction of disulfides was caused by the copper(II)/ascorbic acid/O2 system. The incubation of two symmetric disulfides, L-cystinyl-bis-L-phenylalanine (PP) and L-cystinyl-bis-L-tyrosine (TT), with L-ascorbic acid and CuSO4 in potassium phosphate buffer (pH 7.2, 50 mM) resulted in the formation of an asymmetric disulfide, L-cystinyl-L-phenylalanine-L-tyrosine (PT), and the final ratio of PP:PT:TT was 1:2:1. As the reaction was inhibited by catalase and DMSO only at the initial time, hydroxyl radical generated by the copper(II)/ascorbic acid/O2 system seemed to be responsible for the initiation of the reaction. Oxytocin and insulin were denatured by this system, and catalase and DMSO similarly inhibited these denaturations. As the composition of amino acids was unchanged after the reaction, hydroxyl radical was thought to cause the cleavage and/or interchange reaction of disulfides to denature the peptides.     

Inhibition of ascorbate autoxidation by a dialyzed,heat-denatured extract of plant tissues

Preparations obtained from various plant sources were analyzed for their effect on the autoxidation of ascorbic acid and norepinephrine. The former reaction was followed by spectro-photometric detection of ascorbic acid at 265 nm, the latter one by measuring the formation of noradrenochrome at 480 nm. Extracts were prepared from $\text{Philodendron}$ leaves and the edible portion and seeds of green peppers ($\text{Capsicum Annuum}$). The tissues were minced, homogenized in 10 volumes of 16 mM Na-phosphate buffer pH 7.4 and centrifuged at 35,000g for 30 min. The supernatant was dialyzed in 12,000 m.w. cut-off tubing, denatured in boiling water and centrifuged at 10,000g for 10min. Aliquots (5–50 ul) of the supernatant were assayed in 5 ml 16 mM Na-phosphate buffer pH 7.4 containing 100 uM ascorbate or norepinephrine. The denatured extracts had marked dose-dependent inhibitory effect on the autoxidation of ascorbic acid, with negligible influence on the formation of noradrenochrome. EDTA inhibited both reactions. The selectiveness of the extract toward the autoxidation of ascorbic acid makes it unlikely that the inhibitory effect is based on sequestering metal-ions.     

Electron-transfer reaction between copper macrocyclic complex and ascorbate anion. Application of the Marcus theory

The kinetics of the reduction of a copper(II) complex with a tetrabenzo(b, f, j, n) (1, 5, 9, 13)tetraazacyclohexadecine (TAAB) by ascorbic acid has been investigated in aqueous buffer solutions of pH 2.8 up to 5.0. The polarographic half-wave potential of Cu(TAAB)²⁺ reduction in aqueous media and acetonitrile and the acid equilibrium constant of Cu(TAAB)²⁺ have been determined. The rate constant of the outer-sphere electron-transfer reaction is discussed in terms of the Marcus theory. The previously reported self-exchange kinetic parameters per one-electron couples HA^0/- and Cu(TAAB)^2+/+ have been confirmed, and estimates of some other parameters of the electron self-exchange reactions and the cross-reactions are reported here.     

Photostability and Interaction of Ascorbic Acid in Cream Formulations

The kinetics of photolysis of ascorbic acid in cream formulations on UV irradiation has been studied using a specific spectrophotometric method with a reproducibility of ±5%. The apparent first-order rate constants (k _obs) for the photolysis of ascorbic acid in creams have been determined. The photoproducts formed in the cream formulations include dehydroascorbic acid and 2,3-diketogulonic acid. The photolysis of ascorbic acid appears to be affected by the concentration of active ingredient, pH, and viscosity of the medium and formulation characteristics. The study indicates that the ionized state and redox potentials of ascorbic acid are important factors in the photostability of the vitamin in cream formulations. The viscosity of the humectant present in the creams appears to influence the photostability of ascorbic acid. The results show that the physical stability of the creams is an important factor in the stabilization of the vitamin. In the cream formulations stored in the dark, ascorbic acid undergoes aerobic oxidation and the degradation is affected by similar factors as indicated in the photolysis reactions. The rate of oxidative degradation in the dark is about seventy times slower than that observed in the presence of light.     

Chelating and antiradical effect of rutin during peroxidation of lipids from microsomes and liposomes

The antioxidative effect of rutin (vitamin P) on Fe2+-induced lipid peroxidation (LPO) in bovine heart microsomes and lecithin liposomes was studied. It was shown that the LPO-induced inhibition of microsomes and liposomes in the presence of rutin occurs via two mechanisms, i.e., association of Fe2+ ions to form an inactive complex and a direct interaction between rutin and free radicals. The contribution of these mechanisms depends on the composition of the reaction mixture. In bovine heart microsomes and liposomes, ascorbic acid has a dual activity towards LPO. At high concentrations of Fe2+ necessary for LPO induction (approximately 1 x 10(-3) M), ascorbic acid blocks LPO, whereas at low Fe2+ concentrations (less than 1 x 10(-4) M) it has a prooxidative effect. A combined use of ascorbic acid and rutin results in an additive antioxidative effect at high Fe2+ concentrations (approximately 1.10(-3) M). However, at low Fe2+ concentrations rutin acts as an antagonist of the prooxidative effect of ascorbic acid.     

Formation of alpha-tocopherol radical and recycling of alpha-tocopherol by ascorbate during peroxidation of phosphatidylcholine liposomes. An electron paramagnetic resonance study

The events accompanying the inhibitory effect of alpha-tocopherol and/or ascorbate on the peroxidation of soybean L-alpha-phosphatidylcholine liposomes, which are an accepted model of biological membranes, were investigated by electron paramagnetic resonance, optical and polarographic methods. The presence of alpha-tocopherol radical in the concentration range 10(-8)-10(-7) M was detected from its EPR spectrum during the peroxidation of liposomes, catalysed by the Fe3+-triethylenetatramine complex. The alpha-tocopherol radical, generated in the phosphatidylcholine bilayer, is accessible to ascorbic acid, present in the aqueous phase at physiological concentrations. Ascorbic acid regenerates from it the alpha-tocopherol itself. A kinetic rate constant of about 2 X 10(5) M-1 X s-1 was estimated from the reaction as it occurs under the adopted experimental conditions. The scavenging effect of alpha-tocopherol on lipid peroxidation is maintained as long a ascorbic acid is present.     

Temperature jump as a new technique to study the kinetics of fast transport of protons across membranes

Application of a temperature jump (2.5 degrees C) to a suspension of liposomes, having phosphate (delta pK/delta T approximately 0.005) as the internal buffer and tris(hydroxymethyl)aminomethane (delta pK/delta T approximately 0.031) as the external buffer, created a delta pH (pHin - pHout) of positive sign in ca. 5 microseconds. Decay of this delta pH was monitored by using the fluorescent pH indicator 8-hydroxy-1,3,6-pyrenetrisulfonic acid entrapped inside the liposome. This technique is useful to study transmembrane proton movement in the time range 5 microseconds-10 s at physiological pH values. The kinetics of proton transport aided by ion carriers such as nigericin, monensin, carbonyl cyanide m-chlorophenylhydrazone (CCCP), and valinomycin were studied by our method. The electrogenic nature of transport by CCCP and valinomycin and electroneutral ion transport by nigericin and monensin were shown. From the kinetics of proton transport aided by gramicidin, the time-averaged single-channel conductance of gramicidin channels was estimated to be (2.1 +/- 0.5) X 10(-16) S for H+ at pH 7.5.