pH-dependent long-term radical scavenging activity of AA-2G and 6-Octa-AA-2G against 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation

Transformation mechanisms of tetracycline by horseradish peroxidase with/without redox mediator ABTS for variable water chemistry

The threat of antibiotics in the environment causing antibiotics resistance is a global health concern. Enzymes catalyze pollutant transformations, and how commercially available enzymes like horseradish peroxidase (HRP), with or without a redox mediator, may be used to degrade antibiotics in water treatment is of great interest. This work demonstrates tetracycline transformation by HRP, and how it is significantly enhanced by free radicals created from the mediator 2,2-Azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). Water temperature and pH strongly influence the tetracycline removal rate due to their correlation with the enzyme activity, abundance and stability of ABTS^•+. Four transformation products were identified in the pure HRP system using a liquid chromatography tandem mass spectrometry hybrid quadrupole-orbitrap mass spectrometer system. Addition of 25 μmol L^-1 ABTS not only accelerated the degradation of tetracycline, but also expanded the range of degradation pathways. Potential tetracycline transformation pathways are proposed based on these observations, which include a range of mechanisms such as hydroxylation, demethylation, dehydration, decarbonylation and secondary alcohol oxidation. Despite of decreased efficiency, the HRP/ABTS system was able to degrade tetracycline in a domestic wastewater treatment plant effluent matrix, which demonstrates the potential of the system to be utilized in wastewater treatment.

Flavonoids with Glutathione Antioxidant Synergy: Influence of Free Radicals Inflow

This report explores the antioxidant interaction of combinations of flavonoid–glutathione with different ratios. Two different 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid radical (ABTS^•+)-based approaches were applied for the elucidation of the antioxidant capacity of the combinations. Despite using the same radical, the two approaches employ different free radical inflow systems: An instant, great excess of radicals in the end-point decolorization assay, and a steady inflow of radicals in the lag-time assay. As expected, the flavonoid–glutathione pairs showed contrasting results in these two approaches. All the examined combinations showed additive or light subadditive antioxidant capacity effects in the decolorization assay. This effect showed slight dilution dependence and did not change when the initial ABTS^•+ concentration was two times as high or low. However, in the lag-time assay, different types of interaction were detected, from subadditivity to considerable synergy. Taxifolin–glutathione combinations demonstrated the greatest synergy, at up to 112%; quercetin and rutin, in combination with glutathione, revealed moderate synergy in the 30–70% range; while morin–glutathione appeared to be additive or subadditive. In general, this study demonstrated that, on the one hand, the effect of flavonoid–glutathione combinations depends both on the flavonoid structure and molar ratio; on the other hand, the manifestation of the synergy of the combination strongly depends on the mode of inflow of the free radicals.

ABTS/PP Decolorization Assay of Antioxidant Capacity Reaction Pathways

The 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•+) radical cation-based assays are among the most abundant antioxidant capacity assays, together with the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-based assays according to the Scopus citation rates. The main objective of this review was to elucidate the reaction pathways that underlie the ABTS/potassium persulfate decolorization assay of antioxidant capacity. Comparative analysis of the literature data showed that there are two principal reaction pathways. Some antioxidants, at least of phenolic nature, can form coupling adducts with ABTS^•+, whereas others can undergo oxidation without coupling, thus the coupling is a specific reaction for certain antioxidants. These coupling adducts can undergo further oxidative degradation, leading to hydrazindyilidene-like and/or imine-like adducts with 3-ethyl-2-oxo-1,3-benzothiazoline-6-sulfonate and 3-ethyl-2-imino-1,3-benzothiazoline-6-sulfonate as marker compounds, respectively. The extent to which the coupling reaction contributes to the total antioxidant capacity, as well as the specificity and relevance of oxidation products, requires further in-depth elucidation. Undoubtedly, there are questions as to the overall application of this assay and this review adds to them, as specific reactions such as coupling might bias a comparison between antioxidants. Nevertheless, ABTS-based assays can still be recommended with certain reservations, particularly for tracking changes in the same antioxidant system during storage and processing.

Antioxidant activity and Neurite outgrowth-enhancing activity of scorbamic acid and a red pigment derived from ascorbic acid

L-Ascorbic acid (AA), known as vitamin C, can form browning products by a non-enzymatic process during storage and the browning products cause deterioration of agricultural products. In the browning reaction, a red pigment, 2,2´-nitrilodi-2(2´)-deoxy-L-ascorbic acid ammonium salt (NDA), is generated from AA via L-scorbamic acid (SCA) as an intermediate. However, the biological activities of SCA and NDA have not yet been clarified. In this study, we assayed the antioxidant activities of SCA and NDA using ABTS radical cation and their neurite outgrowth-enhancing activities in PC12 cells. SCA showed stronger radical-scavenging activity than that of AA, while NDA hardly showed any activity. SCA and NDA enhanced the neurite outgrowth induced by dibutyryl cyclic AMP after their incorporation into cells in the same manner as that of AA. The results indicated that SCA has antioxidant activity and that SCA and NDA have neurite outgrowth-enhancing activity.

Isolation and Characterization of the 2,2'-Azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) Radical Cation-Scavenging Reaction Products of Arbutin

Arbutin, a glucoside of hydroquinone, has shown strong 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation-scavenging activity, especially in reaction stoichiometry. This study investigated the reaction mechanism of arbutin against ABTS radical cation that caused high stoichiometry of arbutin in an ABTS radical cation-scavenging assay. HPLC analysis of the reaction mixture of arbutin and ABTS radical cation indicated the existence of two reaction products. The two reaction products were purified and identified to be a covalent adduct of arbutin with an ABTS degradation fragment and 3-ethyl-6-sulfonate benzothiazolone. A time-course study of the radical-scavenging reactions of arbutin and the two reaction products suggested that one molecule of arbutin scavenges three ABTS radical cation molecules to generate an arbutin-ABTS fragment adduct as a final reaction product. The results suggest that one molecule of arbutin reduced two ABTS radical cation molecules to ABTS and then cleaved the third ABTS radical cation molecule to generate two products, an arbutin-ABTS fragment adduct and 3-ethyl-6-sulfonate benzothiazolone.

Hydrogen-bonded multilayers of tannic acid as mediators of T-cell immunity

Type 1 diabetes is an autoimmune-mediated disease resulting in the destruction of insulin-secreting pancreatic β-cells. Transplantation of insulin-producing islets is a viable treatment to restore euglycemia in Type 1 diabetics; however, the clinical application remains limited due to the use of toxic immunosuppressive therapies to prevent immune-mediated rejection. A nanothin polymer material with dual antioxidant and immunosuppressive properties capable of modulating both innate and adaptive immune responses crucial for transplantation outcome is presented. Through the use of hollow microparticles (capsules) composed of hydrogen-bonded multilayers of natural polyphenol (tannic acid) with poly(N-vinylpyrrolidone) (TA/PVPON) and with poly(N-vinylcaprolactam) (TA/PVCL), proinflammatory reactive oxygen and nitrogen species are efficiently dissipated and the production of interferon (IFN)-γ and tumor necrosis factor (TNF)-α proinflammatory cytokines is attenuated by cognate antigen-stimulated autoreactive CD4+ T cells. These results provide evidence that TA-containing capsules are efficacious in immunomodulation and may provide physical transplant protection and prevent diabetogenic autoreactive T-cell responses. Future studies will determine if xeno- and allotransplantation with (TA/PVPON)- or (TA/PVCL)-coated pancreatic islets will decrease the risk of graft rejection due to attenuation of oxidative stress and IFN-γ, and restore euglycemia in Type 1 diabetics.

Antioxidative properties of ascorbigen in using multiple antioxidant assays

The antioxidative properties of ascorbigen, one of the major indole-derived compounds of Brassica vegetables, were systematically evaluated using multiple assay systems with comparison to the well-known antioxidants ascorbic acid and Trolox. We first performed assays using model radicals, DPPH radical, galvinoxyl radical, and ABTS radical cation (ABTS(•+)). Ascorbigen showed stronger activity than that of ascorbic acid in the ABTS(•+)-scavenging assay but showed no activity in the DPPH radical- and galvinoxyl radical-scavenging assays. In the ABTS(•+)-scavenging assay, the indole moiety of ascorbigen contributed to scavenging of the radicals to produce indole-3-aldehyde as one of the final reaction products. The activity of ascorbigen was then evaluated by an oxygen radical absorbance capacity assay and an oxidative hemolysis inhibition assay using physiologically relevant peroxyl radicals, AAPH-derived radicals. Ascorbigen showed much stronger antioxidant activity than did ascorbic acid and Trolox. Therefore, antioxidant activity of ascorbigen might be more beneficial than has been thought for daily health care.

Protection of free radical-induced cytotoxicity by 2-O-α-D-glucopyranosyl-L-ascorbic acid in human dermal fibroblasts

The stable ascorbic acid (AA) derivative, 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G), exhibits vitamin C activity after enzymatic hydrolysis to AA. The biological activity of AA-2G per se has not been studied in detail, although AA-2G has been noted as a stable source for AA supply. The protective effect of AA-2G against the oxidative cell death of human dermal fibroblasts induced by incubating with 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) for 24 h was investigated in this study. AA-2G showed a significant protective effect against the oxidative stress in a concentration-dependent manner. AA-2G did not exert a protective effect during the initial 12 h of incubation, but had a significant protective effect in the later part of the incubation period. Experiments using a α-glucosidase inhibitor and comparative experiments using a stereoisomer of AA-2G confirmed that AA-2G had a protective effect against AAPH-induced cytotoxicity without being converted to AA. Our results provide an insight into the efficacy of AA-2G as a biologically interesting antioxidant and suggest the practical use of AA-2G even before being converted into AA as a beneficial antioxidant.

Truly nonionic polymer shells for the encapsulation of living cells

Engineering surfaces of living cells with natural or synthetic compounds can mediate intercellular communication and provide a protective barrier from hostile agents. We report on truly nonionic hydrogen-bonded LbL coatings for cell surface engineering. These ultrathin, highly permeable polymer membranes are constructed on living cells without the cationic component typically employed to increase the stability of LbL coatings. Without the cytotoxic cationic PEI pre-layer, the viability of encapsulated cells drastically increases to 94%, in contrast to 20% viability in electrostatically-bonded LbL shells. Moreover, the long-term growth of encapsulated cells is not affected, thus facilitating efficient function of protected cells in hostile environment.

Reassessment of antioxidant activity of arbutin: multifaceted evaluation using five antioxidant assay systems

Arbutin, a practically used skin-lightening agent, has been reported to possess a weak antioxidant activity compared to that of its precursor, hydroquinone. However, its antioxidant activity has not been systematically evaluated. Hence, this study reassessed its activity using five assay systems. Assays were first performed using model radicals, DPPH radical and ABTS(*+). Arbutin showed weak DPPH radical-scavenging activity compared to that of hydroquinone, but showed strong ABTS(*+)-scavenging activity. Its activity by ORAC assay was then evaluated using a physiologically relevant peroxyl radical. Arbutin exerted weak but long-lasting radical-scavenging activity and showed totally the same antioxidant activity as that of hydroquinone. Finally, it was shown that, in two cell-based antioxidant assays using erythrocytes and skin fibroblasts, arbutin exerted strong antioxidant activity comparable or even superior to that of hydroquinone. These findings indicate that the antioxidant activity of arbutin may have been under-estimated and suggest that it acts as a potent antioxidant in the skin.

Layer-by-Layer-Coated Gelatin Nanoparticles as a Vehicle for Delivery of Natural Polyphenols

Natural polyphenols with previously demonstrated anticancer potential, epigallocatechin gallate (EGCG), tannic acid, curcumin, and theaflavin, were encased into gelatin-based 200 nm nanoparticles consisting of a soft gel-like interior with or without a surrounding LbL shell of polyelectrolytes (polystyrene sulfonate/polyallylamine hydrochloride, polyglutamic acid/poly-l-lysine, dextran sulfate/protamine sulfate, carboxymethyl cellulose/gelatin, type A) assembled using the layer-by-layer technique. The characteristics of polyphenol loading and factors affecting their release from the nanocapsules were investigated. Nanoparticle-encapsulated EGCG retained its biological activity and blocked hepatocyte growth factor (HGF)-induced intracellular signaling in the breast cancer cell line MBA-MD-231 as potently as free EGCG.

(-)-Epigallocatechin gallate/gelatin layer-by-layer assembled films and microcapsules

A new type of protein/polyphenol microcapsules on the basis of naturally occurring polyphenol (-)-epigallocatechin gallate (EGCG) and gelatin, type A, was obtained using the layer-by-layer (LbL) assembly method. The microcapsules show a more pronounced dependence of permeability on molecular weight of permeating substances than commonly used polyallylamine/polystyrene sulfonate capsules. The regularities of EGCG adsorption in alternation with type A and B gelatins have been investigated using quartz crystal microbalance and electrophoretic mobility measurements on microparticles and found to be dependent on gelatin properties. EGCG in the LbL assemblies retains its antioxidant activity. The kinetics of the reaction of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) cation-radicals with films consisting of 1-10 gelatin/EGCG bilayers is affected by film structure. The EGCG content in the protein/polyphenol film material is as high as 30% w/w. Encapsulation of EGCG via its alternated adsorption with gelatins can be a perspective way to new formulations containing the polyphenol for drug delivery applications.

Antioxidant properties of 2-O-beta-D-glucopyranosyl-L-ascorbic acid

The antioxidant activity of a provitamin C agent, 2-O-beta-D-glucopyranosyl-L-ascorbic acid (AA-2betaG), was compared to that of 2-O-alpha-D-glucopyranosyl-L-ascorbic acid (AA-2G) and ascorbic acid (AA) using four in vitro methods, 1,1-diphenyl-picrylhydrazyl (DPPH) radical-scavenging assay, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS(*+))-scavenging assay, oxygen radical absorbance capacity (ORAC) assay, and 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH)-induced erythrocyte hemolysis inhibition assay. AA-2betaG slowly and continuously scavenged DPPH radicals and ABTS(*+) in roughly the same reaction profiles as AA-2G, whereas AA quenched these radicals immediately. In the ORAC assay and the hemolysis inhibition assay, AA-2betaG showed similar overall activities to AA-2G and to AA, although the reactivity of AA-2betaG against the peroxyl radical generated in both assays was lower than that of AA-2G and AA. These data indicate that AA-2betaG had roughly the same radical-scavenging properties as AA-2G, and a comprehensive in vitro antioxidant activity of AA-2betaG appeared to be comparable not only to that of AA-2G but also to that of AA.

Ascorbic acid conjugates isolated from the phloem of Cucurbitaceae

Analysis of phloem exudates from the fruit of Cucurbitaceae revealed the presence of several compounds with UV-visible absorption spectra identical to that of l-ascorbic acid. In Cucurbita pepo L. (zucchini), the compounds could be isolated from phloem exudates collected from aerial parts of the plant but were not detected in whole tissue homogenates. The compounds isolated from the phloem exudates of C. pepo fruit were eluted from strong anion exchange resin in the same fraction as l-ascorbic acid and were oxidised by ascorbate oxidase (E.C. 1.10.3.3). The major compound purified from C. pepo fruit exudates demonstrated similar redox properties to l-ascorbic acid and synthetic 6-O-glucosyl-l-ascorbic acid (6-GlcAsA) but differed from those of 2-O-glucosyl-l-ascorbic acid (2-GlcAsA) isolated from the fruit of Lycium barbarum L. Parent and fragment ion masses of the compound were consistent with hexosyl-ascorbate in which the hexose moiety was attached to C5 or C6 of AsA. Acid hydrolysis of the major C. pepo compound resulted in the formation of l-ascorbic acid and glucose. The purified compound yielded a proton NMR spectrum that was almost identical to that of synthetic 6-GlcAsA. A series of l-ascorbic acid conjugates have, therefore, been identified in the phloem of Cucurbitaceae and the most abundant conjugate has been identified as 6-GlcAsA. The potential role of such conjugates in the long-distance transport of l-ascorbic acid is discussed.

Inhibition of free radical-induced erythrocyte hemolysis by 2-O-substituted ascorbic acid derivatives

Inhibitory effects of 2-O-substituted ascorbic acid derivatives, ascorbic acid 2-glucoside (AA-2G), ascorbic acid 2-phosphate (AA-2P), and ascorbic acid 2-sulfate (AA-2S), on 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH)-induced oxidative hemolysis of sheep erythrocytes were studied and were compared with those of ascorbic acid (AA) and other antioxidants. The order of the inhibition efficiency was AA-2S> or =Trolox=uric acid> or =AA-2P> or =AA-2G=AA>glutathione. Although the reactivity of the AA derivatives against AAPH-derived peroxyl radical (ROO(*)) was much lower than that of AA, the derivatives exerted equal or more potent protective effects on AAPH-induced hemolysis and membrane protein oxidation. In addition, the AA derivatives were found to react per se with ROO(*), not via AA as an intermediate. These findings suggest that secondary reactions between the AA derivative radical and ROO(*) play a part in hemolysis inhibition. Delayed addition of the AA derivatives after AAPH-induced oxidation of erythrocytes had already proceeded showed weaker inhibition of hemolysis compared to that of AA. These results suggest that the AA derivatives per se act as biologically effective antioxidants under moderate oxidative stress and that AA-2G and AA-2P may be able to act under severe oxidative stress after enzymatic conversion to AA in vivo.

2-O-alpha-D-glucopyranosyl-L-ascorbic acid scavenges 1,1-diphenyl-2-picrylhydrazyl radicals via a covalent adduct formation

The 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging mechanism of 2-O-alpha-D-glucopyranosyl-L-ascorbic acid (AA-2G) was studied. We found two undefined products, named X and Y, in the reaction mixture of AA-2G and the DPPH radical under acidic conditions by HPLC analysis. The reaction mixture was further subjected to LC-MS analysis. X was found to be a covalent adduct of AA-2G and the DPPH radical. On the other hand, Y could not be identified, probably because it was a mixture. A time-course study of the radical-scavenging reaction revealed that one molecule of AA-2G scavenged one molecule of DPPH radical to generate an AA-2G radical, which readily reacted with another molecule of the DPPH radical to form a covalent adduct (X). Subsequently, this adduct slowly quenched a third molecule of the DPPH radical, resulting in reaction products (Y). Therefore, one molecule of AA-2G has only one oxidizable -OH group, but can scavenge three molecules of the DPPH radical. The radical-scavenging mechanism of AA-2G elucidated in this study should be useful in understanding the biological roles of AA-2G per se in the food and cosmetic fields.

An isocratic HPLC method for the simultaneous determination of novel stable lipophilic ascorbic acid derivatives and their metabolites

2-O-alpha-D-glucopyranosyl-6-O-hexadecanoyl-L-ascorbic acid (6-sPalm-AA-2G), a novel stable lipophilic ascorbic acid derivative, was hydrolyzed to 2-O-alpha-D-glucopyranosyl-L-ascorbic acid (AA-2G), ascorbyl 6-palmitate (6-sPalm-AA) and ascorbic acid (AA) with alpha-glucosidase and lipase. An HPLC method for the simultaneous determination of AA, AA-2G, 6-sPalm-AA and 6-sPalm-AA-2G was developed using a cyanopropyl column with an isocratic solution of methanol-phosphate buffer (pH 2.1) (65:35, v/v) containing 20mg/l of dithiothreitol at a detection wavelength of 240 nm. The calibration curves were found to be linear in the range of 10-200 microM. Linear regression analysis of the data demonstrated the efficacy of the method in terms of precision and accuracy. This method was satisfactorily applied to the determination of 6-sPalm-AA-2G and its three metabolites in a 6-sPalm-AA-2G solution treated with purified enzymes or a small intestine post-mitochondrial supernatant and to the separation of novel stable lipophilic AA derivatives other than 6-sPalm-AA-2G and their metabolites. AA, AA-2G and other well-known stable AA derivatives, ascorbic acid 2-phosphate and ascorbic acid 2-sulfate, were also separated under the same conditions. The results show that the procedure is rapid and simple and that it can be employed for in vitro metabolic analysis of various AA derivatives.

Characterization of the Radical-Scavenging Reaction of 2-O-Substituted Ascorbic Acid Derivatives, AA-2G, AA-2P, and AA-2S: A Kinetic and Stoichiometric Study

The aim of this study was to characterize the antioxidant activity of three ascorbic acid (AA) derivatives O-substituted at the C-2 position of AA: ascorbic acid 2-glucoside (AA-2G), ascorbic acid 2-phosphate (AA-2P), and ascorbic acid 2-sulfate (AA-2S). The radical-scavenging activities of these AA derivatives and some common low molecular-weight antioxidants such as uric acid or glutathione against 1,1-diphenyl-picrylhydrazyl (DPPH) radical, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS+), or galvinoxyl radical were kinetically and stoichiometrically evaluated under pH-controlled conditions. Those AA derivatives slowly and continuously reacted with DPPH radical and ABTS+, but not with galvinoxyl radical. They effectively reacted with DPPH radical under acidic conditions and with ABTS+ under neutral conditions. In contrast, AA immediately quenched all species of radicals tested at all pH values investigated. The reactivity of Trolox, a water-soluble vitamin E analogue, was comparable to that of AA in terms of kinetics and stoichiometrics. Uric acid and glutathione exhibited long-lasting radical-scavenging activity against these radicals under certain pH conditions. The radical-scavenging profiles of AA derivatives were closer to those of uric acid and glutathione rather than to that of AA. The number of radicals scavenged by one molecule of AA derivatives, uric acid, or glutathione was equal to or greater than that by AA or Trolox under the appropriate conditions. These data suggest the potential usage of AA derivatives as radical scavengers.

Permeation and metabolism of a series of novel lipophilic ascorbic acid derivatives, 6-O-acyl-2-O-alpha-D-glucopyranosyl-L-ascorbic acids with a branched-acyl chain, in a human living skin equivalent model

A series of novel lipophilic vitamin C derivatives, 6-O-acyl-2-O-alpha-D-glucopyranosyl-L-ascorbic acids possessing a branched-acyl chain of varying length from C(8) to C(16) (6-bAcyl-AA-2G), were evaluated as topical prodrugs of ascorbic acid (AA) with transdermal activity in a human living skin equivalent model. The permeability of 6-bAcyl-AA-2G was compared with those of the derivatives having a straight-acyl chain (6-sAcyl-AA-2G). Out of 10 derivatives of 6-sAcyl-AA-2G and 6-bAcyl-AA-2G, 6-sDode-AA-2G and 6-bDode-AA-2G exhibited most excellent permeability in this model. Measurement of the metabolites permeated from the skin model suggested that 6-bDode-AA-2G was mainly hydrolyzed via 6-O-acyl AA to AA by tissue enzymes, while 6-sDode-AA-2G was hydrolyzed via 2-O-alpha-D-glucopyranosyl-L-ascorbic acid to AA. The former metabolic pathway seems to be advantageous for a readily available source of AA, because 6-O-acyl AA, as well as AA, is able to show vitamin C activity.