Voltammetric assay of butyrylcholinesterase in plasma samples and its comparison to the standard spectrophotometric test

Rational design of a near-infrared fluorescent probe for monitoring butyrylcholinesterase activity and its application in development of inhibitors

Butyrylcholinesterase (BChE) is widely expressed in multiple tissues and has a vital role in several key human disorders, such as Alzheimer's disease and tumorigenesis. However, the role of BChE in human disorders has not been investigated. Thus, to quantitatively detect and visualize dynamical variations in BChE activity is essential for exploring the biological roles of BChE in the progression of a number of human disorders. Herein, based on the substrate characteristics of BChE, we customized and synthesized three near-infrared (NIR) fluorescent probe substrates with cyanine-skeleton, and finally selected a NIR fluorescence probe substrate named CYBA. The CYBA demonstrated a significant increase in fluorescence when interacting with BChE, but mainly avoided AChE. Upon the addition of BChE, CYBA could be specifically hydrolyzed to TBO, resulting in a significant NIR fluorescence signal enhancement at 710 nm. Systematic evaluation revealed that CYBA exhibited exceptional chemical stability in complex biosamples and possessed remarkable selectivity and sensitivity towards BChE. Moreover, CYBA was successfully applied for real-time imaging of endogenous BChE activity in two types of nerve-related living cells. Additionally, CYBA demonstrated exceptional stability in the detection of complex biological samples in plasma recovery studies (97.51%-104.01%). Furthermore, CYBA was used to construct a high-throughput screening (HTS) method for BChE inhibitors using human plasma as the enzyme source. We evaluated inhibitory effects of a series of natural products and four flavonoids were identified as potent inhibitors of BChE. Collectively, CYBA can serve as a practical tool to track the changes of BChE activity in complicated biological environments due to its excellent capabilities.

Real-time visualization of butyrylcholinesterase activity using a highly selective and sensitive chemiluminescent probe

Butyrylcholinesterase (BChE), one of the critical human cholinesterases, plays crucial roles in numerous physiological and pathological processes. Accordingly, it is a striking and at the same time challenging target for bioimaging studies. Herein, we developed the first ever example of a 1,2-dixoetane-based chemiluminescent probe (BCC) for monitoring BChE activity in native biological contexts such as living cells and animals. BCC was initially shown to exhibit a highly selective and sensitive turn-on response in its luminescence signal upon reacting with BChE in aqueous solutions. Later, BCC was utilized to image endogenous BChE activity in normal and cancer cell lines. It was also shown through inhibition experiments that BChE can detect fluctuations of BChE levels successfully. In vivo imaging ability of BCC was demonstrated in healthy and tumor-bearing mice models. BCC enabled us to visualize the BChE activity in different regions of the body. Furthermore, it was successfully employed to monitor tumors derived from neuroblastoma cells with a very high signal to noise ratio. Thus, BCC appears as a highly promising chemiluminescent probe, which can be used to further understand the contribution of BChE to regular cellular processes and the formation of diseased states.

Dual-Mode Ratiometric Electrochemical and Turn-On Fluorescent Detection of Butyrylcholinesterase Utilizing a Single Probe for the Diagnosis of Alzheimer's Disease

Biomarkers detection in blood with high accuracy is crucial for the diagnosis and treatment of many diseases. In this study, the proof-of-concept fabrication of a dual-mode sensor based on a single probe (Re-BChE) using a dual-signaling electrochemical ratiometric strategy and a "turn-on" fluorescent method is presented. The probe Re-BChE was synthesized in a single step and demonstrated dual mode response toward butyrylcholinesterase (BChE), a promising biomarker of Alzheimer's disease (AD). Due to the specific hydrolysis reaction, the probe Re-BChE demonstrated a turn-on current response for BChE at -0.28 V, followed by a turn-off current response at -0.18 V, while the fluorescence spectrum demonstrated a turn-on response with an emission wavelength of 600 nm. The developed ratiometric electrochemical sensor and fluorescence detection demonstrated high sensitivity with BChE concentrations with a low detection limit of 0.08 μg mL^-1 and 0.05 μg mL^-1, respectively. Importantly, the dual-mode sensor presents the following advantages: (1) dual-mode readout can correct the impact of systematic or background error, thereby achieving more accurate results; (2) the responses of dual-mode readout originate from two distinct mechanisms and relatively independent signal transduction, in which there is no interference between two signaling routes. Additionally, compared with the reported single-signal electrochemical assays for BChE, both redox potential signals were detected in the absence of biological interference within a negative potential window. Furthermore, it was discovered that the outcomes of direct dual-mode electrochemical and fluorescence quantifications of the level of BChE in serum were in agreement with those obtained from the use of commercially available assay kits for BChE sensing. This method has the potential to serve as a useful point-of-care tool for the early detection of AD.

Redox-regulated synthesis of fluorescent polydopamine nanoparticles for detection of butyrylcholinesterase activity

Butyrylcholinesterase (BChE) is an enzyme which is relevant to a variety of diseases, and often serve as a common biomarker of health. In this work, a novel fluorescence sensor based on redox-regulated synthesis of polydopamine nanoparticles (PDANPs) has been developed for simple and sensitive sensing BChE activity. A facile and rapid one-step approach for the preparation of fluorescent PDANPs uses potassium permanganate to oxidize dopamine. We demonstrated that the fluorescence intensity of PDANPs is dependent on the dose of potassium permanganate. Butyrylcholinesterase catalyzes the hydrolysis of butyrylthiocholine iodide (BTCh) to produce thiolcholine (TCh) which in a redox reaction with potassium permanganate prevents the formation of fluorescent PDANP. As a result, the activity of BChE can be determined in line with changes in the fluorescence of PDANPs. Based on this finding, a convenient and label-free fluorescence sensor for BChE activity was established via redox-control of the fluorescence intensity of PDANPs. A dynamic response range for BChE is acquired within 0.5 ∼ 200 U/L along with a detection limit of 0.047 U/L. Importantly, the proposed method achieves practical application toward BChE in human sera. Moreover, its satisfying performance for screening of inhibitors was also proved. Hence, the proposed sensor holds great potential for cholinesterase-related biomedical investigation.

A Bioorthogonally Synthesized and Disulfide-Containing Fluorescence Turn-On Chemical Probe for Measurements of Butyrylcholinesterase Activity and Inhibition in the Presence of Physiological Glutathione

Butyrylcholinesterase (BChE) is a biomarker in human blood. Aberrant BChE activity has been associated with human diseases. Here we developed a fluorescence resonance energy transfer (FRET) chemical probe to specifically quantify BChE activity in serum, while simultaneously discriminating against glutathione (GSH). The FRET chemical probe 11 was synthesized from a key trifunctional bicyclononyne exo-6 and derivatives of 5-(2-aminoethylamino)-1-naphthalenesulfonic acid (EDANS) and 4-[4-(dimethylamino)phenylazo]benzoic acid (DABCYL). EDANS fluorescence visualization and kinetic analysis of 11 in the presence of diverse compounds confirmed the outstanding reactivity and specificity of 11 with thiols. The thiol-dependent fluorescence turn-on property of 11 was attributed to a general base-catalyzed SN2 nucleophilic substitution mechanism and independent of metal ions. Moreover, all thiols, except GSH, reacted swiftly with 11. Kinetic studies of 11 in the presence of covalently modified GSH derivatives corroborated that the steric hindrance of 11 imposing on GSH was the likely cause of the distinguished reactivity. Since GSH commonly interferes in assays measuring BChE activity in blood samples, the 11-based fluorescent assay was employed to directly quantify BChE activity without GSH interference, and delivered a linear range of 4.3–182.2 U L−1 for BChE activity with detection limit of 4.3 U L−1, and accurately quantified serum BChE activity in the presence of 10 μM GSH. Finally, the 11-based assay was exploited to determine Ki of 5 nM for tacrine inhibition on BChE catalysis. We are harnessing the modulated characteristics of 6 to synthesize advanced chemical probes able to more sensitively screen for BChE inhibitors and quantify BChE activity in serum.

Redox-Controlled Fluorescent Nanoswitch Based on Reversible Disulfide and Its Application in Butyrylcholinesterase Activity Assay

Butyrylcholinesterase (BChE) mainly contributing to plasma cholinesterase activity is an important indicator for routinely diagnosing liver function and organophosphorus poisoning in clinical diagnosis, but its current assays are scarce and frequently suffer from some significant interference and instability. Herein, we report a redox-controlled fluorescence nanoswtich based on reversible disulfide bonds, and further develop a fluorometric assay of BChE via thiol-triggered disaggregation-induced emission. Thiol-functionalized carbon quantum dots (thiol-CQDs) with intense fluorescence is found to be responsive to hydrogen peroxide, and their redox reaction transforms thiol-CQDs to nonfluorescent thiol-CQD assembly. The thiols inverse this process by a thiol-exchange reaction to turn on the fluorescence. The fluorescence can be reversibly switched by the formation and breaking of disulfide bonds caused by external redox stimuli. The specific thiol-triggered disaggregation-induced emission enables us to assay BChE activity in a fluorescence turn-on and real-time way using butyrylthiocholine iodide as the substrate. As-established BChE assay achieves sufficient sensitivity for practical determination in human serum, and is capable of avoiding the interference from micromolar glutathione and discriminatively quantifying BChE from its sister enzyme acetylcholinesterase. The first design of reversible redox-controlled nanosiwtch based on disulfide expands the application of disulfide chemistry in sensing and clinical diagnostics, and this novel BChE assay enriches the detection methods for cholinesterase activity.

Construction of an Acetylcholinesterase Sensor Based on Synthesized Paramagnetic Nanoparticles, a Simple Tool for Neurotoxic Compounds Assay

Magnetic particles (MPs) have been widely used in biological applications in recent years as a carrier for various molecules. Their big advantage is in repeated use of immobilized molecules including enzymes. Acetylcholinesterase (AChE) is an enzyme playing crucial role in neurotransmission and the enzyme is targeted by various molecules like Alzheimer's drugs, pesticides and warfare agents. In this work, an electrochemical biosensor having AChE immobilized onto MPs and stabilized through glutaraldehyde (GA) molecule was proposed for assay of the neurotoxic compounds. The prepared nanoparticles were modified by pure AChE and they were used for the measurement anti-Alzheimer's drug galantamine and carbamate pesticide carbofuran with limit of detection 1.5 µM and 20 nM, respectively. All measurements were carried out using screen-printed sensor with carbon working, silver reference, and carbon auxiliary electrode. Standard Ellman's assay was used for validation measurement of both inhibitors. Part of this work was the elimination of reversible inhibitors represented by galantamine from the active site of AChE. For this purpose, we used a lower pH to get the original activity of AChE after inhibition by galantamine. We also observed decarbamylation of the AChE-carbofuran adduct. Influence of organic solvents to AChE as well as repeatability of measurement with MPs with AChE was also established.

Acetylcholinesterase Inhibitors Assay Using Colorimetric pH Sensitive Strips and Image Analysis by a Smartphone

Smartphones are widely spread and their usage does not require any trained personnel. Recently, smartphones were successfully used in analytical chemistry as a simple detection tool in some applications. This paper focuses on immobilization of acetylcholinesterase (AChE) onto commercially available pH strips with stabilization in the gelatin membrane. AChE degrades acetylcholine into choline and acetic acid which causes color change of acid-base indicator. Smartphone served as a tool for measurement of indicator color change from red to orange while inhibitors blocked this process. AChE inhibitors were measured with limits of detection, 149 nM and 22.3 nM for galanthamine and donepezil, respectively. Organic solvents were measured for method interferences. Measurement procedure was performed on 3D printed holder and digital photography was evaluated using red-green-blue (RGB) channels. The invented assay was validated to the standard Ellman's test and verified on murine plasma samples spiked with inhibitors. We consider that the assay is fully suitable for practical performance.

Color Change of Phenol Red by Integrated Smart Phone Camera as a Tool for the Determination of Neurotoxic Compounds

The use of a cell phone as a detection system is easy, simple and does not require trained personnel, which is in contrast to standard laboratory instruments. This paper deals with immobilization of acetylcholinesterase (AChE) in a gelatin matrix, and phenol red, as an indicator of AChE activity, is used in order to establish a method that is easily compatible with a camera device. AChE splits acetylcholine into choline and acetic acid, which changes the pH of a medium, resulting in a phenol red color change. The coloration changed in presence of an AChE inhibitor. Measurements were performed on 3D-printed, tube-shaped holder, and digital photography, with subsequent analysis of red-green-blue (RGB), served for assay purposes. Calibration of AChE inhibitors, tacrine and galantamine, was performed, with limit of detection equal to 1.1 nM and 1.28 µM, respectively. Interferences were also measured, resulting in a proof-of-method stability. The method was further successfully validated for the standard Ellman’s assay, and verified on murine plasma samples spiked with inhibitors.

Perturbation of acyl ghrelin profile after liver transplantation

BACKGROUND

A significant problem to be solved for patients after liver transplantation (LT) is malnutrition with anorexia in the early posttransplant period. We hypothesized that this problem was due to the change in ghrelin metabolism during LT. The aim of this study was to examine the balance of acyl ghrelin (AG) and desacyl ghrelin and the dependence of the regulation mechanism on hepatic-related enzymes in patients during LT.

MATERIALS AND METHODS

AG, desacyl ghrelin, and acyl/total ghrelin (A/T) concentrations in blood samples were measured in 15 patients with liver failure (LF), 15 patients after LT, and 10 controls. The correlations between the participants' ghrelin profiles and hepatic function-related data, including liver enzymes, were evaluated. In vitro assays using synthetic AG for assessment of deacylation activity in serum were performed.

RESULTS

AG and A/T ratio were significantly higher in the LF patients than the patients after LT and controls (AG: 25.9 ± 12.6 versus 16.4 ± 12.6 and 9.8 ± 7.6 fmol/mL, P < 0.05; A/T ratio: 17.4 ± 4.1 versus 12.2 ± 5.5 and 11.8% ± 5.9%, P < 0.05). The serum cholinesterase level was inversely correlated with AG and A/T ratio (P < 0.01). In vitro assays showed that deacylation activity was significantly lower in patients with LF than controls (10.5% versus 42.4%, 90 min; P < 0.01). Degradation of AG was partially suppressed by a cholinesterase inhibitor.

CONCLUSIONS

Deacylation activity was lower in LF patients, which could cause elevation of AG levels. Serum cholinesterase may be responsible for deacylation in humans.

Biosensors containing acetylcholinesterase and butyrylcholinesterase as recognition tools for detection of various compounds

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are enzymes expressed in the human body under physiological conditions. AChE is an important part of the cholinergic nerves where it hydrolyses neurotransmitter acetylcholine. Both cholinesterases are sensitive to inhibitors acting as neurotoxic compounds. In analytical applications, the enzymes can serve as a biorecognition element in biosensors as well as simple disposable sensors (dipsticks) and be used for assaying the neurotoxic compounds. In the present review, the mechanism of AChE and BChE inhibition by disparate compounds is explained and methods for assaying the enzymes activity are shown. Optical, electrochemical, and piezoelectric biosensors are described. Attention is also given to the application of sol-gel techniques and quantum dots in the biosensors’ construction. Examples of the biosensors are provided and the pros and cons are discussed.