Light-induced oxidation of unsaturated lipids as sensitized by flavins

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k approximately 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with DeltaH(double dagger) = 22.6 kJ mol(-1) and DeltaS(double dagger) = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k approximately 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (DeltaG(o)(HAT) = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) approximately -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that DeltaG(o)(ET) > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of (3)RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

Characterization of peroxides formed by riboflavin and light exposure of milk. Detection of urate hydroperoxide as a novel oxidation product

Characterization of peroxides by size exclusion chromatography (SEC) of milk following exposure to riboflavin and light showed that hydrogen peroxide was the most abundant peroxide formed since it could be removed by catalase. Formation of peroxides after separation by SEC showed that hydrogen peroxide formation was primarily increased in the presence of caseins and ascorbate, although whey proteins also were found to contribute. Caseins and beta-lactoglobulin also formed catalase-resistant peroxides, presumably protein hydroperoxides. A catalase-resistant and unstable peroxide was observed in fractions containing urate. Experiments performed with pure urate suggested that urate radicals reacted further with superoxide leading to a urate hydroperoxide. Electron paramagnetic resonance spectroscopy using spin-traps showed that the presence of oxygen was required for urate radical formation, which could be assigned as nitrogen-centered radicals. These results suggest a new route during light-induced oxidation sensitized by flavins, in effect making urate pro-oxidative.

Efficiency of hemoglobin from rainbow trout, cod, and herring in promotion of hydroperoxide-derived free radicals

Hemoglobin (Hb) from rainbow trout (Oncorhynchus mykiss), Atlantic cod (Gadus morhua), and herring (Clupea harengus) were evaluated in terms of capacity to generate hydroperoxide-derived free radicals in aqueous solution at pH 6.8. Cumene hydroperoxide (CumOOH) was used as a model for preformed lipid hydroperoxides, and free radicals were monitored by stabilizing with the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) and further detection by electron spin resonance spectroscopy. The generation of hydroperoxide-derived free radicals was found to be Hb concentration-dependent, and intermediate Hb concentrations corresponding to hydroperoxide/Hb molar ratios of roughly 200 were found most active. The results indicated the following relative activity in the formation of hydroperoxide-derived free radicals under low and medium hydroperoxide levels (hydroperoxide/Hb molar ratios < or =5): cod Hb approximately herring Hb > trout Hb. This free radical generating-activity was not affected by heating fish hemoglobins at 70 degrees C for 10 and 45 min in aqueous solution. Cod and herring Hb also exhibited faster autoxidation rates and stronger redox instability in the presence of hydroperoxides compared to trout Hb. According to their ability to generate hydroperoxide-derived free radicals, hemoglobin from cod and herring showed more activity than trout Hb as catalysts of lipid oxidation in lecithin liposomes. These data emphasize the central contribution of the free radicals formed through hydroperoxide-decomposition on the hemoglobin-mediated lipid oxidation.

Baicalin in radical scavenging and its synergistic effect with beta-carotene in antilipoxidation

The lipophilic flavonoid glycoside baicalin from the traditional oriental herb Scutellaria baicalensis Georgi (logP = 1.27, pK(a1) = 7.6, pK(a2) = 10.1 as determined at 25 degrees C in 0.1 M NaCl) is found to be as reducing (0.39 V vs NHE, reversible two-electron oxidation by CV at pH 7.4) as other catechol flavonoids but a poor radical scavenger (TEAC = 1.12, pH 7.4) and a poor antioxidant against free radical initiated lipid oxidation in liposomes. However, this compound is able to regenerate beta-carotene (beta-Car) from beta-Car(*+) with a second-order rate constant of (5.6 +/- 0.5) x 10(9) L mol(-1) s(-1) in the methanol/chloroform binary solvent (1:9, v/v) and, more importantly, to exhibit a prominent synergistic effect with beta-Car against the lipoxidation induced by AMVN-derived peroxyl radical in liposomal membrane. Thus, baicalin by itself is not an effective antioxidant, but it becomes one via interaction with beta-Car. The radical scavenging and antilipoxidation properties of baicalin are discussed in terms of its physicochemical properties and molecular structures.

Browning of freeze-dried probiotic bacteria cultures in relation to loss of viability during storage

Freeze-dried cultures of Lactobacillus acidophilus (La-5) showed visible brown discoloration even after a short storage at relatively mild conditions (a(w) = 0.22 and 30 degrees C), and the browning processes were found to coincide with bacteria inactivation. It was demonstrated, by using high-pressure treatment for obtaining bacteria samples with different ratios of live/dead bacteria, that death of bacteria is not a prerequisite for the browning processes. Furthermore, it was shown that hydroxymethylfurfural (HMF) (or condensation products of HMF) introduces accelerated viability loss when HMF is added to the freeze-drying medium. Discoloration of bacteria cultures containing only sucrose/maltodextrin or lactose/maltodextrin in the freeze-drying matrices is suggested to be related to various types of nonenzymatic browning reactions, including carbonyl-protein (or carbonyl-DNA) interactions and carbohydrate condensation/polymerization (without involvement of proteins), the latter proceeding at low a(w) following hydrolysis of the peptidoglycan layer in the bacteria cell wall. More than one single type of browning reaction is accordingly concluded to be related to bacteria death, and the loss of viability in freeze-dried bacteria seems to be influenced by oxidation reactions, browning reactions, and the physical instability of the bacteria membrane/cell wall.

Evaluation of activity of selected antioxidants on proteins in solution and in emulsions

Protection against protein oxidation by lipophilic and hydrophilic antioxidants in model systems using bovine serum albumin (BSA) in solution alone, or in an emulsion with linolenic acid methyl ester (LnMe) was found to be strongly dependent on the oxidation initiator. Tocopherol, Trolox, or the carotenoids astaxanthin and canthaxanthin were incubated with BSA or BSA/LnMe and oxidation was initiated either with the water-soluble azo-initiator 2,2' azo-bis-(2-amidinopropane) hydrochloride (AAPH), or FeCl3 and ascorbate, or the Fenton system using FeCl2/EDTA/H2O2, or with the singlet oxygen generating species anthracene-9,10-dipropionic acid disodium 1,4 endoperoxide (NDPO2). The results show that all the antioxidants tested were inefficient in the system with FeCl3/ascorbate. However, with the other initiating agents, the hydrophilic antioxidant, Trolox, was the most effective in preventing both protein and lipid oxidation. In contrast the lipophilic antioxidants were ineffective in preventing oxidation of BSA in aqueous solution, but did show some moderate antioxidative activity on protein and lipid in the BSA/LnMe system. Using the singlet oxygen generating system it was also demonstrated that Trolox always provided better protection of the protein than tocopherol and the carotenoids in both the BSA and the BSA/LnMe systems. In conclusion, prevention of protein oxidation using a water-soluble antioxidant has a protective effect on the lipid fraction and this approach deserves further attention in complex biological systems.

Galloylated polyphenols efficiently reduce alpha-tocopherol radicals in a phospholipid model system composed of sodium dodecyl sulfate (SDS) micelles

The ability of several polyphenolic fractions from grape ( Vitis vinifera ) pomace, pine ( Pinus pinaster ) bark, and witch hazel ( Hammamelis virginiana ) bark to repair alpha-tocopherol (alpha-TOH) through reduction of the alpha-tocopheroxyl radical was investigated in a homogeneous hexane system and a phospholipid-like system based on SDS micelles. These natural polyphenols were compared with pure related phenolics (epicatechin, gallic acid, epigallocatechin gallate, quercetin, and rutin) and ascorbic acid, which is a substance with a well-recognized capacity for regenerating alpha-TOH. alpha-Tocopheroxyl radicals were monitored and quantified by electron spin resonance (ESR) spectroscopy in the absence and presence of phenolics. Polyphenols from grape and pine bark were essentially catechin monomers and proanthocyanidins differing in the content of galloyl residues; those from pine bark had a negligible degree of galloylation. Polyphenolic fractions from witch hazel bark were composed of approximately 80% hydrolyzable tannins rich in galloyl moieties, together with a smaller amount of catechin monomers and proanthocyanidins. In the homogeneous hexane system, polyphenols from grape and pine bark exhibited similar activities, reducing the alpha-tocopheroxyl radicals by over 27-40%, whereas phenols from witch hazel were more highly effective, reducing 80% of alpha-TOH. In contrast, pine bark polyphenols were found to be significantly less active than the grape fractions in SDS micelles, reducing 30 and 70% of alpha-tocopheroxyl radicals, respectively. Polyphenolic fractions from witch hazel were also able to reduce the highest amount of alpha-TOH in SDS-micelles. The reducing capacity on alpha-tocopheroxyl radical of polyphenolic fractions was found to be pH-dependent and more effective at higher pH in the range of pH studied (5.8-7.8). These results stress the potential role of polyphenols, in particular those rich in galloyl groups, to maintain intact endogenous alpha-TOH in biological membranes through reduction of alpha-tocopheroxyl radicals.

Identification of free radical intermediates in oxidized wine using electron paramagnetic resonance spin trapping

Free radicals are thought to be key intermediates in the oxidation of wine, but their nature has not been established. Electron paramagnetic resonance spectroscopy was used to detect and identify several free radical species in wine under oxidative conditions with the aid of spin traps. The 1-hydroxylethyl radical was the sole radical species observed when α-(4-pyridyl-1-oxide)-N-tert-butylnitrone was used as a spin trap in a heated (55 °C), low-sulfite (15 mg L(-1)) red wine. This radical appears to arise from ethanol oxidation via the hydroxyl radical, and this latter species was confirmed by using a high concentration (1.5 M) of the 5,5-dimethylpyrroline-N-oxide spin trap, thus providing the first direct evidence of the Fenton reaction in wine. Hydroxyl radical formation in wine was corroborated by converting hydroxyl radicals to methyl radicals by its reaction with dimethyl sulfoxide. The novel spin trap 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide was also used in this study to identify sulfite radicals in wine for the first time. This spin trap has also been shown to trap hydroperoxyl radicals, the generation of which is predicted in wine; however, no evidence of this species was observed.

Comparison of flavonoids and isoflavonoids as antioxidants

The isoflavonoid genistein was found to be a better antioxidant than the isomeric flavonoid apigenin in phosphatidyl liposomes at pH 7.4. The higher antioxidation activity of genistein compared with apigenin is in agreement with its lower oxidation potential (0.73 vs 0.86 V as determined by cyclic voltammetry in aqueous solution of pH= 7.4), lower dissociation enthalpy (87.03 vs 87.88 kcal mol(-1) as calculated for the more reducing 4'-hydroxyl group), and higher radical scavenging capacity in the TEAC assay. On the basis of quantum mechanical calculations for genistein and apigenin in comparison with the flavonoid naringenin and the isoflavonoids puerarin, daidzein, and equol, a lower dipole moment and a larger deviation for the A-to-B dihedral angle from coplanarity (39.3 degrees for genistein, 18.5 degrees for apigenin) are suggested to be important for the increased antioxidant efficiency at water/lipid interfaces among (iso) flavonoids with an equal number of phenolic groups.

Characterization of major radical scavenger species in bovine milk through size exclusion chromatography and functional assays

Radical scavenging activities of bovine milk components were quantified following size exclusion chromatography (SEC) with postcolumn characterization of fractions using the scavenging of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) radicals (ABTS*(+)) in the Trolox equivalent antioxidant capacity (TEAC) assay and peroxyl radicals formed from cleavage of 2,2'-azobis(2-amidinopropane) (AAPH) in the oxygen radical absorbance capacity (ORAC) fluorometric assay. Caseins were quantitatively the major radical scavenger species in both assays, whereas beta-lactoglobulin (beta-lg) and alpha-lactalbumin (alpha-la) were much less active and only in the peroxyl radical assay. The radical scavenging activity of the caseins could be quantitatively accounted for by their constituent amino acids, as there were no effects of denaturing agents or complete digestion with proteases. In contrast, the activities of the whey proteins were dependent on denaturation or partial hydrolysis and dominated by the free thiol in beta-lg. A component in milk serum with a molecular mass of approximately 100 kDa contributed significantly to both ABTS*(+) and peroxyl radical scavenging but was absent in whey. This radical scavenger was identified as beta-casein. The only significant low molecular weight radical scavenger species were identified as ascorbate and urate in both assays.

Caffeic acid as antioxidant in fish muscle: mechanism of synergism with endogenous ascorbic acid and alpha-tocopherol

In an emulsion of corn oil in water with the addition of caffeic acid (Caf-OH) and alpha-tocopherol (alpha-TOH), Caf-OH was found to be very active in delaying lipid oxidation without affecting significantly the kinetics for alpha-TOH degradation. In contrast, Caf-OH addition to fish muscle retarded both the degradation of endogenous alpha-TOH and the propagation of lipid oxidation, measured by peroxide value (PV) and thiobarbituric acid reactive substances (TBARS), with increasing effect with increasing Caf-OH addition (55.5-555.1 micromol/kg). Electron spin resonance (ESR) spectroscopy confirmed a higher capacity of Caf-OH to regenerate alpha-TOH via reduction of the alpha-tocopheroxyl radical compared to other cinnamic acid derivatives (o-coumaric, ferulic, and chlorogenic acids). Degradation of endogenous ascorbate (AscH(-)) was accelerated at higher concentration of Caf-OH in fish tissue, suggesting a role of AscH(-) in the regeneration of Caf-OH. These results indicate that the antioxidant mechanism of Caf-OH implies the protection of endogenous alpha-TOH localized in tissue membranes where lipid oxidation is initiated and, at the same time, Caf-OH regeneration by the endogenous AscH(-). These combined effects result in a stronger antioxidant protection against lipid oxidation by favoring, as a final point, the protection of alpha-TOH, which is suggested as the last defense of fish muscle against lipid oxidation.

Acrylamide-asparagine relationship in baked/toasted wheat and rye breads

Acrylamide in baked and toasted wheat and rye bread was studied in relation to levels of asparagine in flour, dough, bread and toasts. Asparagine was consumed during bread preparation resulting in reduced acrylamide content in the products. In wheat bread, 12% of the asparagine initially present in the flour (0.14 g kg(-1)) remained after yeast fermentation and baking; for rye bread, 82% of asparagine remained after sourdough fermentation and baking. Asparagine present in untoasted wheat bread had totally reacted after hard toasting. Toasted wheat and rye bread slices contained 11-161 and 27-205 microg kg(-1) acrylamide, respectively, compared to untoasted wheat and rye bread with <5 and 7-23 microg kg(-1) acrylamide, respectively. The dietary intake of acrylamide from bread (untoasted) of 2 microg day(-1) is relatively low; however, acrylamide exposure from bread increases several fold for people eating toasted bread.