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.

ABTS/PP Decolorization Assay of Antioxidant Capacity Reaction Pathways.

Quercetin (QT) is one promising candidate for the treatment of various cancers with virtually no toxic side effects. However, its anticancer effect is severely restricted by its poor bioavailability, low water solubility, and chemical instability in the neutral and alkaline medium. Herein, zeolitic imidazolate framework-8 (ZIF-8) is first reported as the multifunctional nanoplatform to the codelivery of quercetin as an anticancer agent and CuS nanoparticles as a photothermal therapy (PTT) agent for synergistic combination of chemotherapy and PTT as well as overcoming the drawbacks of quercetin. Moreover, folic acid–bovine serum albumin (FA–BSA) conjugates are applied to stabilize the CuS@ZIF-8-QT to promote the bioavailability of quercetin and realize active-targeting drug delivery. Near-infrared (NIR) fluorescent imaging demonstrated the highly increased drug accumulations of FA–BSA/CuS@ZIF-8-QT in tumors, resulting from efficient internalization via FA-receptors-mediated endocytosis. The results of in v...

CuS@MOF-Based Well-Designed Quercetin Delivery System for Chemo-Photothermal Therapy.

http://digital.csic.es/bitstream/10261/111073/1/J%20Functional%20Foods-2015-Aceituno.pdf

Improved antioxidant capacity of quercetin and ferulic acid during in-vitro digestion through encapsulation within food-grade electrospun fibers

Solid dispersion of quercetin in cellulose derivative matrices influences both solubility and stability.

Binding, stability, and antioxidant activity of quercetin with soy protein isolate particles.

Oxidation of quercetin by air oxygen takes place in water and aqueous ethanol solutions under mild conditions, namely in moderately-basic media (pH approximately 8-10) at ambient temperature and in the absence of any radical initiators, without enzymatic catalysis or irradiation of the reaction media by light. The principal reaction products are typical of other oxidative degradation processes of quercetin, namely 3,4-dihydroxy-benzoic (proto-catechuic) and 2,4,6-trihydroxybenzoic (phloroglucinic) acids, as well as the decarboxylation product of the latter--1,3,5-trihydroxybenzene (phloroglucinol). In accordance with the literature data, this process involves the cleavage of the gamma-pyrone fragment (ring C) of the quercetin molecule by oxygen, with primary formation of 4,6-dihydroxy-2-(3,4-dihydroxybenzoyloxy)benzoic acid (depside). However under such mild conditions the accepted mechanism of this reaction (oxidative decarbonylation with formation of carbon monoxide, CO) should be reconsidered as preferably an oxidative decarboxylation with formation of carbon dioxide, CO2. Direct head-space analysis of the gaseous components formed during quercetin oxidation in aqueous solution at ambient temperature indicates that the ratio of carbon dioxide/carbon monoxide in the gas phase after acidification of the reaction media is ca. 96:4%. Oxidation under these mild conditions is typical for other flavonols having OH groups at C3 (e.g., kaempferol), but it is completely suppressed if this hydroxyl group is substituted by a glycoside fragment (as in rutin), or a methyl substituent. An alternative oxidation mechanism involving the direct cleavage of the C2-C3 bond in the diketo-tautomer of quercetin is proposed.

Identification of the products of oxidation of quercetin by air oxygen at ambient temperature.

Objective. To identify candidate biomarkers for inflammatory bowel disease (IBD) using non-targeted serum metabolomics in patients with ulcerative colitis (UC). Patients and methods. This pilot study included 9 male patients with active UC and 11 healthy male controls. Serum metabolomic analysis was carried out by gas chromatography-mass spectrometry (GC-MS). Compounds were identified using the NIST08 mass spectral library. Classification of samples and search for candidate biomarkers were performed using a support vector machine (SVM), projections to latent structures-discriminant analysis (PLS-DA), and a naive Bayes classifier (Naïve Bayes). When choosing a classifier, we were guided by the areas under the ROC curve. Results. Metabolomic analysis revealed 85 compounds, of which 79 were annotated. The normalized serum levels of 14 metabolites (2-hydroxybutyric acid, caprylic acid, erythronic acid, creatinine, β-glycerophosphate, α-glycerophosphate, 2-keto-D-gluconic acid, pentadecanoic acid, trans-palmitoleic acid, palmitic acid, margaric acid, palmitoleic acid, squalene, α-tocopherol) differed significantly between the groups. Serum levels of all these compounds (except α-tocopherol) were elevated in UC patients compared to healthy controls, which was accompanied by an increase in the concentration scatter. The largest area under the ROC curve corresponded to the Naïve Bayes classifier (AUC = 0.931; excellent model quality). Conclusion. The study identified 14 metabolites that can be used as candidate biomarkers for IBD after proper verification, both for diagnosis and for assessment of the efficacy of therapy. The identification of disease-associated metabolites will facilitate the development of novel biotherapeutics. Key words: inflammatory bowel disease, ulcerative colitis, metabolome, biomarkers, gas chromatography-mass spectrometry.

Viktor A Utsal

Papers

Identification of the products of oxidation of quercetin by air oxygen at ambient temperature.

Identification of candidate biomarkers for inflammatory bowel disease using non-targeted serum metabolomics

Determination of Perfluororganic Compounds in Whole Blood and Tissues of Laboratory Animals by Gas-Liquid Chromatography–Mass Spectrometry