Fluorescent probes for monitoring myeloperoxidase-derived hypochlorous acid: a comparative study

A fluorescent NBD "turn-on" probe for the rapid and on-site analysis of fructose in food

High fructose intake is an important cause of metabolic disease. Due to the increasing prevalence of metabolic diseases worldwide, the development of an accurate and efficient tool for monitoring fructose in food is urgently needed to control the intake of fructose. Herein, a new fluorescent probe NBD-PQ-B with 7-nitrobenz-2-oxa-1, 3-diazole (NBD) as the fluorophore, piperazine (PQ) as the bridging group and phenylboronic acid (B) as the recognition receptor, was synthesized to detect fructose. The fluorescence of NBD-PQ-B increased linearly at 550 nm at an excitation wavelength of 497 nm with increasing fructose concentration from 0.1 to 20 mM. The limit of detection (LOD) of fructose was 40 μM. The pKa values of NBD-PQ-B and its fructose complexes were 4.1 and 10.0, respectively. In addition, NBD-PQ-B bound to fructose in a few seconds. The present technique was applied to determine the fructose content in beverages, honey, and watermelon with satisfactory results. Finally, the system could not only be applied in an aqueous solution with a spectrophotometer, but also be fabricated as a NBD-PQ-B/polyvinyl oxide (PEO) film by electrospinning for on-site food analysis simply with the assistance of a smartphone.

Quantitative Factors Introduced in the Feasibility Analysis of Reactive Oxygen Species (ROS)-Sensitive Triggers

Reactive oxygen species (ROS) are critical for cellular signaling. Various pathophysiological conditions are also associated with elevated levels of ROS. Hence, ROS-sensitive triggers have been extensively used for selective payload delivery. Such applications are predicated on two key functions: (1) a sufficient magnitude of concentration difference for the interested ROS between normal tissue/cells and intended sites and (2) appropriate reaction kinetics to ensure a sufficient level of selectivity for payload release. Further, ROS refers to a group of species with varying reactivity, which should not be viewed as a uniform group. In this review, we critically analyze data on the concentrations of different ROS species under various pathophysiological conditions and examine how reaction kinetics affect the success of ROS-sensitive linker chemistry. Further, we discuss different ROS linker chemistry in the context of their applications in drug delivery and imaging. This review brings new insights into research in ROS-triggered delivery, highlights factors to consider in maximizing the chance for success and discusses pitfalls to avoid.

Evaluation of a novel pyridinium cation-linked styryl-based boronate probe for the detection of selected inflammation-related oxidants

Reactive oxygen and nitrogen species (RONS) are a range of chemical individuals produced by living cells that contribute to the proper functioning of organisms. Cells under oxidative and nitrative stress show excessive production of RONS (including hydrogen peroxide, H2O2, hypochlorous acid, HOCl, and peroxynitrite, ONOO-) which may result in a damage proteins, lipids, and genetic material. Thus, the development of probes for in vivo detection of such oxidants is an active area of research, focusing on molecular redox sensors, including boronate-caged fluorophores. Here, we report a boronate-based styryl probe with a cationic pyridinium moiety (BANEP+) for the fluorescent detection of selected biological oxidants in vitro and in vivo. We compare the chemical reactivity of the BANEP+ probe toward H2O2, HOCl, and ONOO- and examine the influence of the major intracellular non-enzymatic antioxidant molecule, glutathione (GSH). We demonstrate that, at the physiologically relevant GSH concentration, the BANEP+ probe is efficiently oxidized by peroxynitrite, forming its phenolic derivative HNEP+. GSH does not affect the fluorescence properties of the BANEP+ and HNEP+ dyes. Finally, we report the identification of a novel type of molecular marker, with the boronate moiety replaced by the iodine atom, formed from the probe in the presence of HOCl and iodide anion. We conclude that the reported chemical reactivity and structural features of the BANEP+ probe may be a basis for the development of new red fluorescent probes for in vitro and in vivo detection of ONOO-.

Pro-fluorescent probe with morpholine moiety and its reactivity towards selected biological oxidants

Biological oxidants participate in many processes in the human body. Their excessive production causes organelle damage, which may result in the accumulation of cytotoxic mediators and cell degradation and may manifest itself in various diseases. Peroxynitrite (ONOO- ), hypochlorous acid (HOCl), hydrogen peroxide (H2 O2 ), and peroxymonocarbonate (HOOCO2 - ) are important oxidants in biology, toxicology, and various pathologies. Derivatives of coumarin, containing an oxidant-sensitive boronate group, have been recently developed for the fluorescent detection of inflammatory oxidants. Here, we report the synthesis and characterization of 4-[2-(morpholin-4-yl)-2-oxoethyl]-2-oxo-2H-chromen-7-yl boronic acid (MpC-BA) as a fluorescent probe for the detection of oxidants, with better solubility in water, high stability and fast response time toward peroxynitrite and hypochlorous acid. The effectiveness of the MpC-BA probe for the detection of peroxynitrite was measured by adding bolus ONOO- or using the co-generating superoxide and nitrogen oxide system. MpC-BA is oxidized by ONOO- to 7-hydroxy-4-[2-(morpholin-4-yl)-2-oxoethyl]-2H-chromen-2-one (MpC-OH). However, peroxynitrite-specific product (MpC-H) is formed in the minor reaction pathway. MpC-OH is also yielded in the reaction of MpC-BA with HOCl, and the subsequent formation of a chlorinated MpC-OH gives a specific product for HOCl (MpC-OHCl). H2 O2 slowly oxidizes MpC-BA. However, the addition of NaHCO3 increased the MpC-OH formation rate. We conclude that MpC-BA is potentially an improved fluorescent probe detecting peroxynitrite and hypochlorite in biological settings. Complementation of the fluorescence measurements by HPLC-based identification of chlorinated and reduced coumarin(s) will help identify the oxidants detected.

Boronate-Based Oxidant-Responsive Derivatives of Acetaminophen as Proinhibitors of Myeloperoxidase

Myeloperoxidase (MPO) is an important component of the human innate immune system and the main source of a strong oxidizing and chlorinating species, hypochlorous acid (HOCl). Inadvertent, misplaced, or excessive generation of HOCl by MPO is associated with multiple human inflammatory diseases. Therefore, there is a considerable interest in the development of MPO inhibitors. Here, we report the synthesis and characterization of a boronobenzyl derivative of acetaminophen (AMBB), which can function as a proinhibitor of MPO and release acetaminophen, the inhibitor of chlorination cycle of MPO, in the presence of inflammatory oxidants, i.e., hydrogen peroxide, hypochlorous acid, or peroxynitrite. We demonstrate that the AMBB proinhibitor undergoes conversion to acetaminophen by all three oxidants, with the involvement of the primary phenolic product intermediate, with relatively long half-life at pH 7.4. The determined rate constants of the reaction of the AMBB proinhibitor with hydrogen peroxide, hypochlorous acid, or peroxynitrite are equal to 1.67, 1.6 × 104, and 1.0 × 106 M-1 s-1, respectively. AMBB showed lower MPO inhibitory activity (IC50 > 0.3 mM) than acetaminophen (IC50 = 0.14 mM) toward MPO-dependent HOCl generation. Finally, based on the determined reaction kinetics and the observed inhibitory effects of two plasma components, uric acid and albumin, on the extent of AMBB oxidation by ONOO- and HOCl, we conclude that ONOO- is the most likely potential activator of AMBB in human plasma.

A New and Fast-Response Fluorescent Probe for Monitoring Hypochlorous Acid Derived from Myeloperoxidase

Hypochlorous acid (HOCl) has been implicated in numerous pathologies associated with an inflammatory component, but its selective and sensitive detection in biological settings remains a challenge. In this report, imaging of HOCl was realized with a thiomorpholine-based probe as derivative of nitrobenzothiadiazole (NBD-S-TM). The fluorescence is based on photoinduced electron transfer by using nitrobenzothiadiazole core as a donor and thiomorpholine substituent as an acceptor. NBD-S-TM showed high sensitivity and a fast response to HOCl k = (2.6 ± 0.2) × 107 M-1s-1 with a 1:1 stoichiometry. The detection limit for HOCl was determined to be 60 nM. Furthermore, the desirable features of NBD-S-TM for the detection of HOCl in aqueous solutions, such as its reliability at physiological pH, rapid fluorescence response, and biocompatibility, enabled its application in the detection of HOCl in myeloperoxidase enzymatic system. Moreover, NBD-S-TM exhibited excellent selectivity and sensitivity for HOCl over other biologically relevant species, such as hydrogen peroxide and peroxynitrite. The fluorescent S-oxidized product (NBD-S-TSO) is only formed in the presence of HOCl. Probing with NBD-S-TM may be helpful to further the development of high throughput screening assays to monitor the activity of myeloperoxidase.

Factors Important in the Use of Fluorescent or Luminescent Probes and Other Chemical Reagents to Measure Oxidative and Radical Stress

Numerous chemical probes have been used to measure or image oxidative, nitrosative and related stress induced by free radicals in biology and biochemistry. In many instances, the chemical pathways involved are reasonably well understood. However, the rate constants for key reactions involved are often not yet characterized, and thus it is difficult to ensure the measurements reflect the flux of oxidant/radical species and are not influenced by competing factors. Key questions frequently unanswered are whether the reagents are used under 'saturating' conditions, how specific probes are for particular radicals or oxidants and the extent of the involvement of competing reactions (e.g., with thiols, ascorbate and other antioxidants). The commonest-used probe for 'reactive oxygen species' in biology actually generates superoxide radicals in producing the measured product in aerobic systems. This review emphasizes the need to understand reaction pathways and in particular to quantify the kinetic parameters of key reactions, as well as measure the intracellular levels and localization of probes, if such reagents are to be used with confidence.

A thiomorpholine-based fluorescent probe for the far-red hypochlorous acid monitoring

A simple thiomorpholine-based fluorescent probe was designed and synthesized by combining thiomorpholine (TM) and nitrobenzenoselenadiazoles fluorophore (NBD-Se). The thiomorpholine group quenches the fluorescence of NBD-Se efficiently through the photoinduced electron transfer (PET) effect. Hypochlorous acid (HOCl) oxidizes the NBD-Se-TM probe to its fluorescent S-oxide (NBD-Se-TSO) with a 1:1 stoichiometry. The desirable features of NBD-Se-TM for detecting HOCl in aqueous solutions, such as its high sensitivity and selectivity, reliability at physiological pH, and rapid fluorescence response, enabled its application in the detection of HOCl produced by myeloperoxidase. The results proved that NBD-Se-TM is a promising fluorescent probe that can be used in screening assays for MPO inhibitors. Its high reaction rate constant with HOCl (²k = 2.0 × 10⁷M^-1s^-1) indicates the possibility of application in more complex biological systems.