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Herein, a novel magnetic relaxation sensing strategy based on the change in Fe3+ content has been proposed by utilizing the conversion of Fe3+ ions to Prussian blue (PB) precipitates. Compared with the common detection approach based on the valence state change of Fe3+ ions, our strategy can cause a larger change in the relaxation time of water protons and higher detection sensitivity since PB precipitate can induce a larger change in the Fe3+ ion concentration and has a weaker effect on the relaxation process of water protons relative to Fe2+ ions. Then, we employ alkaline phosphatase (ALP) as a model target to verify the feasibility and detection performance of the as-proposed strategy. Actually, ascorbic acid (AA) generated from the ALP-catalyzed L-ascorbyl-2-phosphate hydrolysis reaction can reduce potassium ferricyanide into potassium ferrocyanide, and potassium ferrocyanide reacts with Fe3+ to form PB precipitates, leading to a higher relaxation time. Under optimum conditions, the method for ALP detection has a wide linear range from 5 to 230 mU/mL, and the detection limit is 0.28 mU/mL, sufficiently demonstrating the feasibility and satisfactory analysis performance of this strategy, which opens up a new path for the construction of magnetic relaxation sensors. Furthermore, this strategy has also been successfully applied to ascorbic acid oxidase detection, suggesting its expansibility in magnetic relaxation detection.

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In this study, the alkaline phosphatase (ALP)-like activity of zeolitic-imidazolate framework-8 (ZIF-8) is reported for the first time. Then, colorimetric sensors for the ascorbic acid oxidase (AAO) and copper ion (Cu2+) detection were developed based on the acetylcholinesterase (AChE)- and ALP-like activities of ZIF-8. The ZIF-8 has good mimetic enzyme activity and exhibits high affinity to the substrates. Its AChE- and ALP-like activities also have good reusability and storage stability. Good linear dependences are obtained in the range of 1.3-250.0 µM (AChE-like activity-based) and 4.5-454.5 µM (ALP-like activity based) for Cu2+ detection. Furthermore, good linear dependence is also obtained based on the ALP-like activity of ZIF-8 for AAO detection in the range of 2.3-454.5 U/L. Their limits of detection (LODs) are calculated to be 0.7 µM, 2.8 µM, and 1.8 U/L, respectively. Finally, the sample spiked recoveries of Cu2+ in tap water, Cu2+, and AAO in human serum and rabbit plasma were measured, and the results are in the range of 80.0-119.3%. In short, the preparation of ZIF-8 is simple, environmentally friendly, and harmless, and can realize highly selective detection of AAO and Cu2+ in an efficient and fast process.

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A sensitive photoelectrochemical (PEC) sensor based on hexagonal carbon nitride tubes (HCNT) as photoactive material was prepared for the detection of human epidermal growth factor receptor 2 (HER2). Magnetic Fe3O4 nanospheres (MNs) modified with anti-HER2 antibodies were employed for highly efficient capture of HER2 from serum sample, and Co3O4 nanoparticles (Co3O4 NPs) modified with ascorbic acid oxidase (AAO) as well as HER2 aptamer were used for signal amplification. When the aptamer-Co3O4-AAO probe was captured onto the electrode surface through the specific binding of the aptamer with HER2, the photocurrent intensity decreased. This was because Co3O4 NPs competed with HCNT for consumption of the excitation energy. As a consequence AAO catalyzed the oxidation of the electron donor (AA), and the aptamer-Co3O4-AAO probe increased the steric hindrance at the electrode surface, leading to significant photocurrent intensity decrease, thus realizing multiple signal amplification. Based on this signal amplification strategy, at 0 V (vs Ag/AgCl), the PEC sensor shows a wide linear response ranging from 1 pg mL-1 to 1 ng mL-1 with a low detection limit of 0.026 pg mL-1 for HER2. Importantly, the prepared PEC sensor was applied for detection of HER2 in human serum samples with recoveries between 98.8 and 101%. Sensitive photoelectrochemical sensor based on Co3O4 nanoparticles modified with ascorbic acid oxidase for signal amplification is reported.

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Herein, a sensitive fluorescence (FL) biosensor for the assay of ascorbic acid oxidase (AAO) was established based on the fluorescence resonance energy transfer (FRET) between MoS2 quantum dots (MQDs) and CoOOH nanoflakes. CoOOH nanoflakes as effective FL quencher could quench the FL signal of MQDs on the basis of FRET. When ascorbic acid (AA) was added to the MQDs/CoOOH nanoflakes system, the FL signal was restored due to the redox reaction between CoOOH nanoflakes and AA, in which CoOOH nanoflakes were reduced to Co2+ by AA. In the presence of AAO, the recovered FL signal of MQDs was quenched again because of the enzymatic catalytically reaction between AAO and AA, in which AA was oxidized to dehydroascorbic acid (DHA) and then prevented the decomposition of CoOOH nanoflakes. Under the optimal experimental conditions, this developed fluorescence method exhibited good linear ranges from 2 to 10 mU mL-1 and 10-40 mU mL-1 with a low detection of limit of 0.8 mU mL-1 for AAO detection. And the limit of quantification (LOQ) of 2.6 mU mL-1 was obtained. The proposed biosensor showed high sensitivity and selectivity, and was successfully applied for AAO determination in human serum samples.

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In this study, the mechanism activated by melatonin treatment at 100 µM for maintaining nutraceutical properties in pomegranate fruits during storage at 4 °C for 120 days was investigated. Our results showed that the higher G6PDH and 6PGDH activities in pomegranate fruits treated with melatonin may be responsible for sufficient supply of intracellular NADPH. Also, higher AA and GSH accumulation in pomegranate fruits treated with melatonin may ascribe to higher APX and GR activities coincided with lower AAO activity. In addition, pomegranate fruits treated with melatonin exhibited significantly higher PAL activity resulting in higher phenols and anthocyanins accumulation as well as higher DPPH scavenging capacity. Additionally, higher AOX gene expression in pomegranate fruits treated with melatonin may be beneficial for ROS scavenging molecules accumulation. Therefore, maintaining nutraceutical properties of pomegranate fruits treated with melatonin may ascribe to sufficient intracellular NADPH supply by promoting G6PDH and 6PGDH activities during cold storage.

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A simple enzyme-based nanohybrid material was fabricated via immobilizing ascorbic acid oxidase (AO) on the surface of flower-like electrodeposited gold nanoparticles (dpAu) and reduced graphene oxide (rGO) modified glassy carbon electrodes (GCEs). The composite material was used for stereoselective interaction with ascorbic acid (AA) and isoascorbic acid (IAA). Herein, AO was applied as a stereoselective selector, and the dpAu/rGO nanohybrid not only acted as a supporter for high loading of AO, but also served as the nanomaterial for signal amplification. The results showed obvious peak current differences between AA and IAA, indicating that this strategy could be employed to recognize AA and IAA. Under the optimum conditions, the sensor exhibited a good linear response to AA and IAA in a linear range of 1.0 × 10-4 - 5.0 × 10-3 M. This approach with the merits of simplicity and rapid response provided a promising perspective for identification of AA and IAA.

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Herein we reported Prussian blue nanoparticles (PBNPs) possess ascorbic acid oxidase (AAO)- and ascorbic acid peroxidase (APOD)-like activities, which suppressed the formation of harmful H2O2 and finally inhibited the anti-cancer efficiency of ascorbic acid (AA). This newly revealed correlation between iron and AA could provide new insight for the studies of nanozymes and free radical biology.

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BACKGROUND: Red-skin root disease has seriously decreased the quality and production of Panax ginseng (ginseng). METHODS: To explore the disease's origin, comparative analysis was performed in different parts of the plant, particularly the epidermis, cortex, and/or fibrous roots of 5-yr-old healthy and diseased red-skin ginseng. The inorganic element composition, phenolic compound concentration, reactive oxidation system, antioxidant concentrations such as ascorbate and glutathione, activities of enzymes related to phenolic metabolism and oxidation, and antioxidative system particularly the ascorbate-glutathione cycle were examined using conventional methods. RESULTS: Aluminum (Al), iron (Fe), magnesium, and phosphorus were increased, whereas manganese was unchanged and calcium was decreased in the epidermis and fibrous root of red-skin ginseng, which also contained higher levels of phenolic compounds, higher activities of the phenolic compound-synthesizing enzyme phenylalanine ammonia-lyase and the phenolic compound oxidation-related enzymes guaiacol peroxidase and polyphenoloxidase. As the substrate of guaiacol peroxidase, higher levels of H2O2 and correspondingly higher activities of superoxide dismutase and catalase were found in red-skin ginseng. Increased levels of ascorbate and glutathione; increased activities of l-galactose 1-dehydrogenase, ascorbate peroxidase, ascorbic acid oxidase, and glutathione reductase; and lower activities of dehydroascorbate reductase, monodehydroascorbate reductase, and glutathione peroxidase were found in red-skin ginseng. Glutathione-S-transferase activity remained constant. CONCLUSION: Hence, higher element accumulation, particularly Al and Fe, activated multiple enzymes related to accumulation of phenolic compounds and their oxidation. This might contribute to red-skin symptoms in ginseng. It is proposed that antioxidant and antioxidative enzymes, especially those involved in ascorbate-glutathione cycles, are activated to protect against phenolic compound oxidation.

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The objectives of this work were to evaluate infrared (IR) dry blanching in comparison with conventional water blanching prior to hot air drying of mango to inactivate polyphenol oxidase (PPO) and ascorbic acid oxidase (AAO) enzymes, and to study its effect on color change and retention of vitamin C and ß-carotene. Mango cylinders were blanched under similar temperature-time conditions either by IR heating or by immersion in a water bath during 2 min at 90 °C (high-temperature-short-time-HTST) or for 10 min at 65 °C (low-temperature-long-time-LTLT). After blanching mango was hot air dried at 70 °C. PPO was completely inactivated during the blanching treatments, but AAO had a moderate remaining activity after LTLT treatment (∼30%) and a low remaining activity after HTST treatment (9% to 15%). A higher retention of vitamin C was observed in mango subjected to IR dry blanching, 88.3 ± 1.0% (HTST) and 69.2 ± 2.9% (LTLT), compared with water blanching, 61.4 ± 5.3% (HTST) and 50.7 ± 9.6% (LTLT). All-trans-ß-carotene retention was significantly higher in water blanched dried mango, 93.2 ± 5.2% (LTLT) and 91.4 ± 5.1% (HTST), compared with IR dry blanched, 73.6 ± 3.6% (LTLT) and 76.9 ± 2.9% (HTST). Increased levels of 13-cis-ß-carotene isomer were detected only in IR dry blanched mango, and the corresponding dried mango also had a slightly darker color. IR blanching of mango prior to drying can improve the retention of vitamin C, but not the retention of carotenoids, which showed to be more dependent on the temperature than the blanching process. A reduction of drying time was observed in LTLT IR-blanching mango.

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The objective of this research was to study the enzyme kinetics and thermostability of endogenous ascorbic acid oxidase (AAO) in carrot purée (Daucus carota cv. Nantes) after being treated with pulsed electric field (PEF) processing. Various PEF treatments using electric field strength between 0.2 and 1.2kV/cm and pulsed electrical energy between 1 and 520kJ/kg were conducted. The enzyme kinetics and the kinetics of AAO thermal inactivation (55-70°C) were described using Michaelis-Menten model and first order reaction model, respectively. Overall, the estimated Vmax and KM values were situated in the same order of magnitude as the untreated carrot purée after being exposed to pulsed electrical energy between 1 and 400kJ/kg, but slightly changed at pulsed electrical energy above 500kJ/kg. However, AAO presented different thermostability depending on the electric field strength applied. After PEF treatment at the electric field strength between 0.2 and 0.5kV/cm, AAO became thermolabile (i.e. increase in inactivation rate (k value) at reference temperature) but the temperature dependence of k value (Ea value) for AAO inactivation in carrot purée decreased, indicating that the changes in k values were less temperature dependent. It is obvious that PEF treatment affects the temperature stability of endogenous AAO. The changes in enzyme kinetics and thermostability of AAO in carrot purée could be related to the resulting carrot purée composition, alteration in intracellular environment and the effective concentration of AAO released after being subjected to PEF treatment.

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