Encapsulating gold nanoclusters into metal-organic frameworks to boost luminescence for sensitive detection of copper ions and organophosphorus pesticides

Gold nanoclusters (Au NCs) with luminescence property are emerging as promising candidates in fluorescent methods for monitoring contaminants, but low luminescence efficiency hampers their extensive applications. Herein, GSH-Au NCs@ZIF-8 was designed by encapsulating GSH-Au NCs with AIE effect into metal-organic frameworks, achieving high luminescence efficiency and good stability through the confinement effect of ZIF-8. Accordingly, a fluorescent sensing platform was constructed for the sensitive detection of copper ions (Cu²⁺) and organophosphorus pesticides (OPs). Firstly, the as-prepared GSH-Au NCs@ZIF-8 could strongly accumulate Cu²⁺ due to the adsorption property of MOFs, accompanied by a significant fluorescence quenching effect with a low detection limit of 0.016 μM for Cu²⁺. Besides, thiocholine (Tch), the hydrolysis product of acetylthiocholine (ATch) by acetylcholinesterase (AchE), could coordinate with Cu²⁺ by sulfhydryl groups (-SH), leading to a significant fluorescence recovery, which was further used for the quantification of OPs owing to its inhibition to AChE activity. Furthermore, a hydrogel sensor was explored to accomplish equipment-free, visual, and quantitative monitoring of Cu²⁺ and OPs by a smartphone sensing platform. Overall, this work provides an effective and universal strategy for enhancing the luminescence efficiency and stability of Au NCs, which would greatly promote their applications in contaminants monitoring.

A Sensitive and Selective Colorimetric Method Based on the Acetylcholinesterase-like Activity of Zeolitic Imidazolate Framework-8 and Its Applications

In this study, a simple colorimetric method was established to detect copper ion (Cu²⁺), sulfathiazole (ST), and glucose based on the acetylcholinesterase (AChE)-like activity of zeolitic imidazolate framework-8 (ZIF-8). The AChE-like activity of ZIF-8 can hydrolyze acetylthiocholine chloride (ATCh) to thiocholine (TCh), which will further react with 5,5′-dithiobis (2-nitrobenzoic acid) (DTNB) to generate 2-nitro-5-thiobenzoic acid (TNB) that has a maximum absorption peak at 405 nm. The effects of different reaction conditions (buffer pH, the volume of ZIF-8, reaction temperature and time, and ATCh concentration) were investigated. Under the optimized conditions, the value of the Michaelis-Menten constant (K_m) is measured to be 0.83 mM, which shows a high affinity toward the substrate (ATCh). Meanwhile, the ZIF-8 has good storage stability, which can maintain more than 80.0% of its initial activity after 30 days of storage at room temperature, and the relative standard deviation (RSD) of batch-to-batch (n = 3) is 5.1%. The linear dependences are obtained based on the AChE-like activity of ZIF-8 for the detection of Cu²⁺, ST, and glucose in the ranges of 0.021–1.34 and 5.38–689.66 µM, 43.10–517.24 µM, and 0.0054–1.40 mM, respectively. The limit of detections (LODs) are calculated to be 20.00 nM, 9.25 µM, and 5.24 µM, respectively. Moreover, the sample spiked recoveries of Cu²⁺ in lake water, ST in milk, and glucose in strawberry samples were measured, and the results are in the range of 98.4–115.4% with the RSD (n = 3) lower than 3.3%. In addition, the method shows high selectivity in the real sample analysis.

A Novel Paper-Based Electrochemical Biosensor Based on N,O-Rich Covalent Organic Frameworks for Carbaryl Detection

A new N,O-rich covalent organic framework (COF_DHNDA-BTH) was synthesized by an amine-aldehyde condensation reaction between 2,6-dialdehyde-1,5-dihydroxynaphthalene (DHNDA) and 1,3,5-phenyltriformylhydrazine (BTH) for carbaryl detection. The free NH, OH, and C=O groups of COF_DHNDA-BTH not only covalently couples with acetylcholinesterase (AChE) into the pores of COF_DHNDA-BTH, but also greatly improves the catalytic activity of AChE in the constrained environment of COF_DHNDA-BTH's pore. Under the catalysis of AChE, the acetylthiocholine (ATCl) was decomposed into positively charged thiocholine (TCl), which was captured on the COF_DHNDA-BTH modified electrode. The positive charges of TCl can attract anionic probe [Fe(CN)₆]^3-/4- on the COF_DHNDA-BTH-modified electrode to show a good oxidation peak at 0.25 V (versus a saturated calomel electrode). The carbaryl detection can inhibit the activity of AChE, resulting in the decrease in the oxidation peak. Therefore, a turn-off electrochemical carbaryl biosensor based on a flexible carbon paper electrode loaded with COF_DHNDA-BTH and AChE was constructed using the oxidation peak of an anionic probe [Fe(CN)₆]^3-/4- as the detection signal. The detection limit was 0.16 μM (S/N = 3), and the linear range was 0.48~35.0 μM. The sensor has good selectivity, repeatability, and stability, and has a good application prospect in pesticide detection.

Graphitic-phase C3N4 nanosheets combined with MnO2 nanosheets for sensitive fluorescence quenching detection of organophosphorus pesticides

In this study, we have developed a sensitive approach to measure organophosphorus pesticides (OPs) using graphitic-phase C₃N₄ nanosheets (g-C₃N₄) combined with a nanomaterial-based quencher, MnO₂ nanosheets (MnO₂ NS). Since MnO₂ NS can quench the fluorescence of g-C₃N₄ via the inner-filter effect (IFE), enzymatic hydrolysate (thiocholine, TCh) can efficiently trigger the decomposition of MnO₂ nanosheets in the presence of acetylcholinesterase (AChE) and acetylthiocholine (ATCh), resulting in the fluorescence recovery of g-C₃N₄. OPs, as inhibitors to AChE activity, can prevent the generation of TCh and decomposition of MnO₂ nanosheets while exhibiting fluorescence quenching. Therefore, the AChE-ATCh-MnO₂-g-C₃N₄ system can be utilized to quantitatively detect OPs based on g-C₃N₄ fluorescence. Under optimal conditions, the linear ranges for the determination of parathion-methyl (PM) and 2,2-dichlorovinyl dimethyl phosphate (DDVP) were found to be 0.1-2.1 ng/mL and 0.5-16 ng/mL, respectively, with limits of detection of 0.069 ng/mL and 0.20 ng/mL, respectively. The advantages of this assay are user-friendliness, ease of use, and cost effectiveness compared to other more sophisticated analytical instruments.

Red-to-blue paper-based colorimetric sensor integrated with smartphone for point-of-use analysis of cerebral AChE upon Cd2+ exposure

Herein, combined with a pervasive smartphone installed with a color recognition app, dual-responsive CDs@Eu/GMP ICPs were designed as a red-to-blue paper-based colorimetric sensor for the point-of-use analysis of cerebral acetylcholinesterase (AChE) upon Cd2+ exposure. Blue-emitting CDs with multi-functional groups as guests were encapsulated into the network of Eu/GMP ICPs to obtain CDs@Eu/GMP ICPs with the sensitized red fluorescence of Eu3+. With the presence of thiocholine (TCh), derived from acetylthiocholine (ATCh) hydrolyzed by AChE, the coordination environment of the CDs@Eu/GMP ICPs was interrupted, leading to the collapse of the CDs@Eu/GMP ICP network and the corresponding release of guest CDs into the surrounding environment. Consequently, the sensitized red fluorescence of Eu3+ decreased and the blue fluorescence of the CDs increased. This obvious red-to-blue fluorescent color changes of CDs@Eu/GMP ICPs on test paper could then be integrated with the smartphone for point-of-use analysis of cerebral AChE upon Cd2+ exposure, which not only offers a new analytical platform for a better understanding of the environmental risk of Alzheimer's Dementia (AD), but also holds great potential in the early diagnosis of AD even at the asymptomatic stage with the decrease in CSF AChE as an early biomarker.

Oxidase-mimicking activity of ultrathin MnO2 nanosheets in colorimetric assay of acetylcholinesterase activity

In the present study, a novel colorimetric sensing platform was constructed for quantitative detection of acetylcholinesterase (AChE) activity and its inhibitor. Manganese dioxide (MnO₂) nanosheets as an oxidase-mimicking nanomaterial could directly oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxTMB without the need for horseradish peroxidase and H₂O₂. When AChE was introduced, acetylthiocholine could be catalytically hydrolyzed to produce thiocholine, which easily triggers the decomposition of MnO₂ nanosheets, causing the decrease of solution absorbance. Owing to the inhibition effect of organophosphorus pesticides, the enzymatic activity was suppressed, preventing the decomposition of MnO₂ and resulting in the increase of absorbance. Under optimal conditions, the colorimetric platform shows sensitive responses to AChE and paraoxon in the range of 0.1-15 mU mL^-1 and 0.001-0.1 μg mL^-1, respectively. The detection limits of AChE and paraoxon reached 35 μU mL^-1 and 1.0 ng mL^-1, respectively. Furthermore, the MnO₂-TMB platform has been used to fabricate test strips for rapid and convenient visual detection of AChE and its inhibitor with highly promising performance.

Optical Detection of Enzymatic Activity and Inhibitors on Non-Covalently Functionalized Fluorescent Graphene Oxide

It has been of great interest to measure the activity of acetylcholinesterase (AChE) and its inhibitor, as AChE is known to accelerate the aggregation of the amyloid beta peptides that underlie Alzheimer's disease. Herein, we report the development of graphene oxide (GO) fluorescence-based biosensors for the detection of AChE activity and AChE inhibitors. To this end, GO was non-covalently functionalized with phenoxy-modified dextran (PhO-dex-GO) through hydrophobic interaction; the resulting GO showed excellent colloidal stability and intense fluorescence in various aqueous solutions as compared to pristine GO and the GO covalently functionalized with dextran. The fluorescence of PhO-dex-GO remarkably increased as AChE catalyzed the hydrolysis of acetylthiocholine (ATCh) to give thiocholine and acetic acid. It was found that the turn-on fluorescence response of PhO-dex-GO to AChE activity was induced by protonation of carboxyl groups on it from the product of the enzymatic hydrolysis reaction, acetic acid. On the basis of its turn-on fluorescence response, PhO-dex-GO was able to report kinetic and thermodynamic parameters involving a maximum velocity, a Michaelis constant, and an inhibition dissociation constant for AChE activity and inhibition. These parameters enable us to determine the activity of AChE and the efficiency of the inhibitor.

Cholinesterase activities in the Antarctic scallop Adamussium colbecki: tissue expression and effect of ZnCl2 exposure

Biochemical characterization of cholinesterase activity (ChE) was carried out on the Antarctic scallop Adamussium colbecki collected in winter 2000 from Campo Icaro (Ross Sea, Antarctica) in order to increase its suitability as a sentinel organism for monitoring the Antarctic environment. The digestive gland, gills and adductor muscle were investigated for substrate specificity and inhibitors sensitivity using acetylthiocholine iodide (ASCh) and butyrylthiocholine iodide (BSCh) as substrates and tetra (monoisopropyl)pyrophosphor-tetramide (Iso-OMPA), 1,5-bis(4-allyldimethylammoniumphenyl)-penthan-3-one dibromide (BW284c51) and the insecticide chlorpyrifos as inhibitors. Effect of in vivo exposure to ZnCl(2) was also investigated. All the tissues expressed ChE activity (gill > adductor muscle > digestive gland) and low substrates specificity throughout the hydrolysis of both ASCh and BSCh substrates. Partial (25-29%) and total inhibition (100%) of ChE activity in gills was demonstrated following in vitro incubation with Iso-OMPA and BW284c51 (3 mM), respectively. Concentration-dependent inhibition was also evident with chlorpyrifos in the range 10(-4)-10(-10) M (IC(50) 10(-6)) while in vivo exposure to ZnCl(2) did not seem to affect ChE activity in the scallop. The potential use of ChE in the A. colbecki as biomarker for monitoring water contamination in the marine Antarctic environment is discussed.

High-resolution optical mapping of the right bundle branch in connexin40 knockout mice reveals slow conduction in the specialized conduction system

Connexin40 (Cx40) is a major gap junction protein that is expressed in the His-Purkinje system and thought to be a critical determinant of cell-to-cell communication and conduction of electrical impulses. Video maps of the ventricular epicardium and the proximal segment of the right bundle branch (RBB) were obtained using a high-speed CCD camera while simultaneously recording volume-conducted ECGs. In Cx40(-/-) mice, the PR interval was prolonged (47.4+/-1.4 in wild-type [WT] [n=6] and 57.5+/-2.8 in Cx40(-/-) [n=6]; P<0.01). WT ventricular epicardial activation was characterized by focused breakthroughs that originated first on the right ventricle (RV) and then the left ventricle (LV). In Cx40(-/-) hearts, the RV breakthrough occurred after the LV breakthrough. Additionally, Cx40(-/-) mice showed RV breakthrough times that were significantly delayed with respect to QRS complex onset (3.7+/-0.7 ms in WT [n=6] and 6.5+/-0.7 ms in Cx40(-/-) [n=6]; P<0.01), whereas LV breakthrough times did not change. Conduction velocity measurements from optical mapping of the RBB revealed slow conduction in Cx40(-/-) mice (74.5+/-3 cm/s in WT [n=7] and 43.7+/-6 cm/s in Cx40(-/-) [n=7]; P<0.01). In addition, simultaneous ECG records demonstrated significant delays in Cx40(-/-) RBB activation time with respect to P time (P-RBB time; 41.6+/-1.9 ms in WT [n=7] and 55.1+/-1.3 ms in [n=7]; P<0.01). These data represent the first direct demonstration of conduction defects in the specialized conduction system of Cx40(-/-) mice and provide new insight into the role of gap junctions in cardiac impulse propagation.

A micro flow injection electrochemical biosensor for organophosphorus pesticides

We describe a disposable, amperometric micro flow injection electrochemical biosensor that can be applied to the identification and quantification of highly toxic organophosphorus (OP) compounds in the environment, on the spot and in a short time. The system traces very small quantities of OP by monitoring the enzymatic reaction of acetylcholine esterase (AChE) and its inhibition. The sensor is sensitive, rapid, small, inexpensive, disposable and can be operated by non-professional technicians. The electrochemical cell consists of screen-printed electrodes covered with an enzymatic membrane and placed in a home-made flow cell. The electrodes are connected to a computer-controlled potentiostat. We quantitatively detected the OP compound, dimethyl 2,2-dichlorovinyl phosphate (DDVP), by monitoring the OP induced decrease in enzymatic degradation of the substrate, acetylthiocholine chloride (ATCh), to thiocholine and acetic acid. Thiocholine reacts with hexacyanoferrate ion in the working solution and the reduction of [Fe(CN)6](-3) to [Fe(CN)6](-4) and its subsequent reoxidization by the electrode generates very sharp, rapid and reproducible electric signals. The ability to detect low quantities is extremely important when dealing with hazardous environmental pollutants.

The cytological localization of intracellular neuronal acetylcholinesterase

Sections of cat ciliary ganglia were stained for acetylcholinesterase activity by several modifications of the acetylthiocholine method in order to achieve optimal accuracy of cytological localization of the enzyme. These were compared by ordinary light and phase contrast microscopy with similar sections stained by standard techniques for Nissl substance, the Golgi apparatus, and the neurofibrillae, and by intravital methylene blue. The pattern of cytoplasmic distribution of acetylcholinesterase corresponded most closely with that of the Nissl substance. Following total inactivation of the ganglionic acetylcholinesterase by intravenously administered di-isopropyl fluorophosphate, the reappearance of the enzyme in vivo occurred at the same cytoplasmic sites prior to its reappearance at the cell membrane or preganglionic axonal terminations. These observations, and reports cited from the literature, provide support for the hypothesis that acetylcholinesterase is synthesized within the endoplasmic reticulum, then transported via its canaliculi to the surface of the cell and its processes, where its functional sites are oriented externally to the lipoidal membrane.

Complete staining of nerve fiber and myoneural junctions with acetylthiocholine and silver

We describe a combined stain for simultaneous demonstration of the preterminal axons and cholinesterase activity at myoneural junctions of mammalian muscles. This technique employs acetylthiocholine iodide as the substrate for cholinesterase activity and silver nitrate impregnation of preterminal axons. The procedure is rapid, simple and uses fresh muscles. Intramuscular nerves, preterminal axons and myoneural junctions are stained simultaneously brown or black with minimal background staining of connective tissue and muscle fibers.

Comparison of two substrates in determination of plasma cholinesterase activity & genetic variability

Two substrates, acetyl thiocholine iodide ATCI and benzoylcholine chloride BCC were compared for the determination of plasma cholinesterase ChE levels and after incorporation of dibucaine and sodium fluoride in the assay, their usefulness in determining plasma ChE genetic variability was assessed in 64 healthy subjects. With both substrates, plasma ChE levels were found to be in the reference range. However, ATCI could detect only two variants with the usual phenotype UU in 60 of 64 [93.75%] subjects whereas with BCC 6 different groups could be determined. Though both substrates are of equal value in estimation of plasma ChE levels, BCC is definitely superior in determining its genetic variants.

Mercuric chloride effects on the kinetic parameters of human erythrocyte membrane bound acetylcholinesterase

Kinetic analysis of the interaction of mercuric chloride with human erythrocyte acetylcholinesterase was investigated in the present study. It was found that mercuric chloride reversibly inhibited the AChE activity in a concentration dependent manner, the IC50 being 16 microM while the IC100 was 47 microM. The Km for the hydrolysis of acetylthiocholine iodide by AChE was found to be 97 microM in the control system, and the value increased by 16-144% in the mercuric chloride treated systems. The Vmax was 1.72 mumol/min/mg protein for the control as well as the mercuric chloride treated systems. Dixon as well as Lineweaver-Burke plots and their secondary replots indicated that the nature of the inhibition is of the reversible competitive type. The K(i) value was estimated as 6.26 microM. The K(i) value increased with an increase in substrate concentration.

Rapid histochemical identification of motor and sensory fascicles: preparation of solutions

We report a rapid and inexpensive staining method for identification of motor and sensory fibers within 1 hour (intraoperatively). This method should be easy to use in any hospital where frozen histologic sectioning is available.

Sensory and motor fiber differentiation with Karnovsky staining

We examined four acetylthiocholine methods based on Karnovsky's procedure--two fast-acting requiring 1 hour and two slow-acting requiring 24 hours. We compared these with our modification, which requires less than an hour and is simple to use. Rabbit sciatic nerves and spinal cords were used to compare methods. Our modification showed clearer differentiation than other fast-acting methods and staining identical to slow-acting methods. In blind examination of radial nerve specimens stained with our method, motor and sensory fascicles were correctly identified, showing sensitivity and specificity of 100%. In 12 clinical cases, our method produced staining in the proximal stump as long as 16 months after injury and in the distal stump as long as 5 days after injury. In 10 of 12 patients, this staining helped in aligning motor fascicles to motor fascicles and sensory fascicles to sensory fascicles.

Interaction of quaternary ammonium compounds with acetylcholinesterase: characterization of the active site

The relation of the structure of 31 quaternary ammonium compounds (28 inhibitors; 3 substrate analogues) with their effects on the activity of acetylcholinesterase (EC 3.1.1.7; AChE) was studied. The compounds were structurally related to the natural substrate acetylcholine (ACh). All bear a trimethylammonium moiety as cationic head. The inhibitors include a variety of functional groups instead of an electrophilic ester group, making these substances suitable to probe the esteratic subsite. The inhibition constants and Km values were determined in kinetic experiments under steady state conditions (pH-stat method). Most of the substances acted as reversible, competitive inhibitors with KI in the range of 10(-6)-10(-3) M. The substrate analogues had Km values between (1.2-2.2) X 10(-4) M. The data allow the following main conclusions: (1) The quaternary trimethylammonium group of ACh is of high importance for substrate binding to AChE. It mediates association at the anionic site. (2) A poorer contribution to binding (two orders of magnitude lower) is attributable to the apolar methylene chain in ACh. It can be related to a hydrophobic interaction of the hydrocarbon chain at a region neighbouring the anionic site. (3) The ester group (both C = O and O) does not contribute to substrate binding. It is only responsible for reactivity.

An enzymatic method for erythrocyte acetylcholinesterase

The acetylcholinesterase (EC 3.1.1.7) in 50 microL of a 61-fold dilution of erythrocytes in water hydrolyzes acetylcholine during a timed 20-min reaction at 37 degrees C. The resulting choline is measured by use of choline oxidase coupled to peroxidase, with phenol and aminoantipyrene to give a pink product that absorbs maximally at 500 nm. For calibration, a choline iodide standard is included in each batch of up to 19 samples. Accuracy was assessed by using specific inhibitors and measuring choline in the presence of excess erythrocyte solution. The standard curve for the assay is linear to threefold the normal enzyme activity. Between-batch precision was 0.40 kU/L at a mean of 11.5 kU/L (CV 3.5%), and comparison with an acetylthiocholine procedure (x) gave a good correlation: y = 1.02x - 0.27 kU/L (r = 0.991). Long-term precision (10 months), assessed from three sets of assays of samples from 17 individuals, was 0.71 kU/L at a mean of 11.7 kU/L (CV 6.1%).

Reversible and reproducible in vivo staining of motor endplates by use of the acetylthiocholine-iodide method

In vivo staining of motor endplates was carried out in the anterior tibial muscle of the rat, using the acetylthiocholine-iodide staining method of Koelle and Friedenwald. A strong and distinct endplate staining was obtained after incubation in the staining solution for only one minute if the epimysium had been removed from the muscle. Staining with intact epimysium gave a less satisfactory result if the incubation time was 3 minutes, but it was excellent after incubation for 10 minutes. At re-examination 5 or 30 days later, the initial staining had disappeared in all animals, and a second staining at this time gave just as good results as on the first occasion. The inflammatory reaction in the muscle was more severe with removed epimysium than with intact epimysium, and the staining solution had a slight local toxic effect compared with 0.9% NaCl. Staining with intact epimysium should therefore be preferred if possible.

Autonomic neurons supplying the rat eye and the intraorbital distribution of vasoactive intestinal polypeptide (VIP)-like immunoreactivity

We traced the origin and path of autonomic nerves to the rat eye using, as an aid to dissection, a modified thiocholine method for the histochemical demonstration of cholinesterase. When applied to whole nerves and ganglia supplying the rat eye, this procedure is not specific for cholinergic neurons; instead it stains both sympathetic and parasympathetic nerves, many of which are otherwise too fine to identify in dissection. We found that nerves from the superior cervical and pterygopalatine ganglia form a plexus at the orbital apex corresponding to the retro-orbital plexus found in rabbit, monkey and man. In the rat, nerves from the retro-orbital plexus travel peripherally to the superior surface of the optic-nerve sheath. Here, they fuse with long ciliary nerves and the post-ganglionic nerves from the ciliary ganglion to form another dense nerve-fiber plexus that ultimately supplies the eye. Importantly, the plexus on the optic nerve contains many isolated or aggregated ganglion cells. These are comparable in number to those in the ciliary ganglion itself and are assumed to be accessory ciliary neurons. Using immunohistochemistry, we also sought evidence for vasoactive intestinal polypeptide in these ganglia and nerves. As previously known, many pterygopalatine ganglion cells are immunoreactive for this peptide. Vasoactive intestinal polypeptide (VIP)-like immunoreactive nerve fibers are present in nerves from the retro-orbital plexus to the optic-nerve sheath plexus, in most nerves of the latter plexus, and in most nerves entering the eye. Furthermore, a small proportion of nerve cells in the main and accessory ciliary ganglia also are immunoreactive for VIP. We conclude that in addition to the pterygopalatine ganglion, the ciliary ganglion and its accessory ganglia are sources of VIP-like immunoreactive nerves in the rat eye.

A microfluorometric assay for cholinesterases, suitable for multiple kinetic determinations of picomoles of released thiocholine

A highly sensitive microfluorometric assay for cholinesterases has been developed. Enzymatic activity is measured by monitoring the thiocholine produced by specific hydrolysis of acetylthiocholine. This is carried out by reacting the thiocholine formed with the fluorogenic compound N-(4(7 diethylamino-4-methylcoumarin-3-yl)phenyl)maleimide to yield an intensely fluorescent product. The assay is linear over a range extending from a few picomoles to nanomoles of thiocholine. The specificity and accuracy of this microfluorometric assay were examined using microgram quantities of rat brain tissue as a source for cholinesterases. The specific activities and the Km values determined by this new method for both cholinesterase activities present in the brain (acetylcholine hydrolase, EC 3.1.1.7, and "nonspecific" cholinesterase-acylcholine acylhydrolase, EC 3.1.1.8) were identical to those reported earlier using the less sensitive spectrophotometric and radiometric methods. The background emission caused by nonenzymatic hydrolysis of the substrate is relatively low, and does not exceed background values encountered in other methods. The assay may be used for monitoring the kinetics of enzymatic activities in microscale reaction mixtures, providing a linear determination of the thiocholine produced over a period of at least 30 h at room temperature. The method can also be adapted for use in other enzymatic assays where reagents containing thiol groups can be produced or consumed.

A potential systematic error in using lysivane as inhibitor in the measurement of amniotic fluid acetylcholinesterase by the Ellman method

The measurement of low levels of cholinesterase or acetylcholinesterase by the Ellman method requires correction for a non-enzymatic increase in absorption at 412 millimicron that is due both to non-enzymatic hydrolysis of the acetylthiocholine substrate and to modification of the colour reagent. The rate of increase in absorption is dependent on temperature and pH. Addition of an acidic solution of lysivane to the assay solution for selective measurement of amniotic fluid acetylcholinesterase gives rise to a shift in pH; the use of methanol is suggested as an easier method of dissolving the inhibitor and does not affect the pH of the assay, obviating any need to redetermine the background absorption. There is, however, no improvement in ability of the method to predict pregnancies associated with neural-tube defects.

Determination of acetylcholinesterase and pseudocholinesterase in gastrointestinal biopsy tissue

An optimized micromethod for the determination of soluble acetylcholinesterase and pseudocholinesterase in gastrointestinal biopsy tissue is introduced. Enzyme activities were obtained from measurements of total activity and activity in the presence of an inhibitor of pseudocholinesterase. Optimal conditions of measurement were established for enzymes in serum or erythrocytes. In intestinal tissue, measurements were precise and reproducible. Interference by blood contained in the biopsy specimens were found to be neglegible. Also drugs used for premedication had no significant influence. Finally, results from normal stomach, duodenum and colon are presented.

Thiocholine methods for the demonstration of acetylcholinesterase in neuromuscular junctions

The extensor digitorum longus muscles of rat were stained for the localization of acetylcholinesterase activity at the neuromuscular junctions. The modified methods of Koelle-Friedenwald and Karnovsky-Roots were used with acetylthiocholine iodide as the substrate. The merits and demerits of both these methods are discussed. TEM and SEM X-ray dispersive analyses of the muscle fibres treated histochemically by both the methods were also made in order to elucidate further the nature of the reaction products. Denervated muscles were subjected to similar treatment.

Nature of the cysteinyl residues in lipophilin from human myelin

Ellman's reagent was used to investigate the status and exposure of the cysteinyl residues in lipophilin, a proteolipid apoprotein from human myelin. The hydrolyzed protein contained 3.5 to 4.5 cysteines per molecule, which increased to 11 after complete reduction. The native protein was thought to contain three disulfide bonds and five free sulfhydryl groups, which undergo partial oxidation during purification. Exposure of -SH groups in the aqueous protein was minimal, even in the presence of 6 M guanidinium chloride, suggesting a location in hydrophobic domains not disrupted by this reagent. In the helicogenic solvent 2-chloroethanol, the full complement of -SH groups could not be revealed, even with the addition of sodium dodecyl sulfate; a difference of two sulfhydryls between intact and hydrolyzed protein was consistently observed. Similar sulfhydryl reactivity toward iodoacetamide was also established in this solvent. Sulfhydryl assays on whole myelin in 2-chloroethanol indicated that the occurrence of -SH groups in the proteolipid component was at least as high as in the purified apoprotein. Lipophilin was reduced and alkylated with 4-vinylpyridine at 10 of its cysteinyl residues. The modified protein adopted a beta structure under conditions where lipophilin is normally highly alpha-helical, and was also less helical than usual in 2-chloroethanol; however, it was still abnormally resistant to denaturation by guanidinium chloride. Modified lipophilin contained as many ester groups as the intact protein; thus, it appeared unlikely that the long chain fatty acids associated with the protein were attached to cysteine residues.

Microcalorimetric assay of acetylcholinesterase

Comparative assays were made in a spectrophotometer and a microcalorimeter for the reaction between acetylcholinesterase (EC 3.1.1.7) and acetylthiocholine. The rate of light absorbance change and the rate of heat flow were measured from similar and simultaneous reactions in spectrophotometer and microcalorimeter, respectively. At the enzyme activity levels studied, i.e., 0.05-0.15 I.U. in calorimetry and 1-4 I.U. in spectrophotometry, the reaction rates were linear and showed first-order kinetics A highly significant positive correlation was seen between the two methods (r = 0.997). More importantly, spectrophotometric assay with acetylthiocholine (which utilized a secondary reaction with chromagen, dithiobisnitrobenzoic acid) stood in highly significant positive correlation with calorimetric assays (which did not require a chromagen) either with the same substrate (r = 0.976) or with acetylcholine (r = 0.900). It appears that microcalorimetry can be used in preference to spectrophotometry for enzyme kinetic studies to overcome the complexity of reaction mixture and interference problems and with the advantage of using natural substrates.

Cholinesterases isoenzymes: a comparative study in the skin and plasma

Acetylcholinesterase (Ache) and pseudocholinesterase (BUche) activities were studied quantitatively in healthy skin by spectrophotometric methods and qualitatively by polyacrylamide gel electrophoresis. The results were compared to those obtained in plasma. The substrates used to reveal enzyme activities were acetylthiocholine (ATC) iodide and butyrylthiocholine (BTC) iodide, respectively. A linear relationship exists between the values of BUche and Ache activities in plasma and those in skin. Six isoenzymes of different electrophoretic mobility were observed in the skin. One of them, which is never found in plasma extracts, appears to be specific to the skin. On gradient gel electrophoresis, with both substrates (ATC and BTC), a single band of enzyme activity, corresponding to a molecular weight of 600,000 was observed. These results suggest that in the skin there is only one enzyme, most probably butyryl cholinesterase, which cleaves BTC somewhat faster than ATC. This methodology, when applied to the study of dermatoses in which abnormalities of cutaneous nerve terminals are suspected, should furnish precise functional pathophysiological details.

Formation of an unstable covalent intermediate during the inhibition of electric-eel acetylcholinesterase with 1,3,2-dioxaphosphorinane 2-oxides

The kinetics of interaction of eel acetylcholinesterase (EC 3.1.1.7) with 1,3,2-dioxaphosphorinane 2-oxides were investigated. It was demonstrated that the rate of spontaneous re-activation as well as the re-activation profile in the presence of 2-pyridine aldoxime methiodide of the inhibited enzyme are irrespective of the leaving group of three inhibitors and exhibit the same values. The dissociation constant of the corresponding Michaelis complex was evaluated by two independent methods and the results were found to be in close agreement. It was shown that the active site is essential for interaction between the enzyme and the various dioxaphosphorinanes. The mixed anhydride of diethyl phosphoric acid and 2-hydroxy-1,3,2-dioxaphosphorinane 2-oxide behaves exactly as would be predicted from a typical diethyl phosphate inhibitor. Enxyme that was incubated with the cyclic acid or the corresponding methyl ester recovered immediately upon extensive dilution. Inhibition of enzyme in the presence of high concentratasions of the corresponding 2-chloro and 2-fluoro derivatives decreased the regeneration rates as well as the maximal amount of the re-activated enzyme. This observation could not be explained in terms of a classical aging process. On the basis of the kinetics observations it is suggested that an unstable covalent phospho-enzyme intermediate is formed during the reaction between acetylcholinesterase and 1,3,2-dioxaphosphorinane 2-oxides.

Diagnostic value of rectal mucosal acetylcholinesterase levels in Hirschsprung's disease

Acetylcholinesterase (AChE) activity was measured in rectal biopsy specimens obtained from 68 children aged between 2 days and 14 1/2 years in whom Hirschsprung's disease was suspected. The diagnosis was subsequently established in 12; in these, the mean AChE activity was found to be 30.5 X 10(-7) units/g tissue (range 16.9 to 63.0). The 56 non-Hirschsprung cases had a mean of 5.0 X 10(-7) units/g tissue (S.D. 2.2), the highest value in this group being 10.9. The results were unaffected by age, sex, nature of biopsy procedure, or the presence of blood. It is suggested that the assay of AChE activity in rectal biopsy material is a simple and quick procedure that is useful in the diagnosis of Hirschsprung's disease.

Histochemically distinct compartments in the striatum of human, monkeys, and cat demonstrated by acetylthiocholinesterase staining

We here report observations on the distribution of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) in the striatum of the adult human, the rhesus monkey, and the cat. By the histochemical staining methods of Geneser-Jensen and Blackstad and of Karnovsky and Roots, compartments of low cholinesterase activity were identified in parts of the striatum in all three species. In frontal sections, these enzyme-poor zones appeared as a variable number of weakly stained approximately 0.5-mm-wide zones embedded in a darkly stained background. The zones varied in cross-sectional shape from round to elongated and were sometimes branched. They were most prominent in the head of the caudate nucleus. Three-dimensional reconstructions of serial sections through the caudate nucleus in the human and cat suggest that over distances of at least several millimeters, the zones of low enzyme activity form nearly continuous labyrinths.

Pralidoxime as an insignificant reactivator in severe anticholinesterase (organophosphate insecticide) poisoning

In acute severe anticholinesterase poisoning by organophosphate compounds, pralidoxime (P-2-AM, pyridine-2-aldoxime methiodide) used in the recommended doses, intravenously, has not been shown to reactivate the inhibited cholinesterase, as evidenced both clinically and biochemically. In vitro studies using pralidoxime iodide up to ten times the recommended concentrations, produced insignificant reactivation of cholinesterases inhibited by the organophosphate insecticide Bidrin (di-methyl-3-hydroxyl-N, N-dimethyl-crotonamide phosphate). This was even so despite prolonged exposure of the inhibited cholinesterases to the oxime. The value of pralidoxime as a reactivator of phosphorylated cholinesterases is therefore in doubt, and should not be used in preference to large doses of atropine and other supportive treatment in poisoning by organophosphate insecticides.

Cholinesterases from plant tissues. VI. Preliminary characterization of enzymes from Solanum melongena L. and Zea mays L

Enzymes capable of hydrolyzing esters of thiocholine have been assayed in extracts of Solanum melongena L. (eggplant) and Zea Mays L. (corn). The enzymes from both species are inhibited by the anti-cholinesterases neostigmine, physostigmine, and 284c51 and by AMO-1618, a plant growth retardant and they both have pH optima near pH 8.0. The enzyme from eggplant is maximally active at a substrate concentration of 0.15 mM acetylthiocholine and is inhibited at higher substrate concentrations. On the basis of this last property, the magnitude of inhibition by the various inhibitors, and the substrate specificity, we conclude that the enzyme from eggplant, but not that from corn, is a cholinesterase.

Immobilized electric eel acetylcholinesterasemii. II. Flow kinetics of acetylcholinesterase chemically attached to nylon tubing

Acetylcholinesterase has been attached covalently to the inner surface of nylon tubing. An experimental study has been carried out on the flow kinetics; solutions of acetylthiocholine at various concentrations were passed through tubing at various flow rates, and measurements made of the rates of formation of product. The results were analyzed in the light of the theoretical treatment of Kobayashi and Laidler, four different methods of analysis being employed. It is found that at lower substrate concentrations and flow rates the reactions are largely diffusion controlled. The Km(app) values are substantially higher than the Km value for diffusion-free conditions, but approach it as the flow rate is increased, when the diffusion layer becomes less important. The results are entirely consistent with the Kobayaski-Laidler theory, and provide guidelines for the design of open tubular heterogeneous enzyme reactors, both for industrial and analytic purposes.

Cholinesterase reactivation: in vitro studies

In vitro studies confirm the clinical impression that P–2–AM does not reverse the cholinesterase inhibition caused by organophosphorus insecticides.

Distribution and properties of cholinesterases in subcellular fractions from rat jejunum and heart tissue

Homogenates of rat jejunum and heart tissue in 0.3 M sucrose have been separated into different subcellular fractions by centrifugation. The distribution of cholinesterases in these two tissues is different. Acetylthiocholine (Ac), propionylthiocholine (Pc), and butyrylthiocholine (Bc) iodides were used as substrates. Using 1 mmol/1 Ac in the presence of hexafluorenium, a local anaesthetic drug, inhibition percentages of these subcellular cholinesterases were found to be different. The differences in the effect of hexafluorenium were most marked at a concentration of 5 mumol/1. Four electrophoretically distinct subcomponents of cholinesterase have been identified in different subcellular fractions from both jejunum and heart tissues. The electrophoretic profile, as determined by polyacrylamide disc electrophoresis, of different subcellular cholinesterases in these two tissues also appears to be different.