Distinct Effect of Benzalkonium Chloride on the Esterase and Aryl Acylamidase Activities of Butyrylcholinesterase

Biosensors based on enzyme inhibition

The present chapter describes the use of biosensors based on enzyme inhibition as analytical tools. The parameters that affect biosensor sensitivity, such as the amount of immobilized enzyme, incubation time, and immobilization type, were critically evaluated, highlighting how the knowledge of enzymatic kinetics can help researchers optimize the biosensor in an easy and fast manner. The applications of these biosensors demonstrating their wide application have been reported. The objective of this survey is to give a critical description of biosensors based on enzyme inhibition, of their assembly, and their application in the environmental, food, and pharmaceutical fields.

Synthesis and biological activity of derivatives of tetrahydroacridine as acetylcholinesterase inhibitors

Current state of medical sciences does not allow to treatment neurodegenerative diseases such as Alzheimer's disease (AD). At present treatment of AD is severely restricted. The main class of medicines which are applied in AD is acetylcholinesterase inhibitors (AChEIs) like tacrine, donepezil, galantamine and rivastigmine that do not contribute to significant and long-term improvement in cognitive and behavioural functions. In this work, we report synthesis and biological evaluation of new hybrids of tacrine-6-hydrazinonicotinamide. The synthesis was based on the condensation reaction between tacrine derivatives and the hydrazine nicotinate moiety (HYNIC). All obtained compounds present affinity for both cholinesterases and are characterized by high selectivity in relation to butyrylcholinesterase (BChE).

Human butyrylcholinesterase components differ in aryl acylamidase activity

Apart from its esterase activity, butyrylcholinesterase (BuChE) displays aryl acylamidase (AAA) activity able to hydrolyze o-nitroacetanilide (ONA) and its trifluoro-derivative (F-ONA). We report here that, despite amidase and esterase sites residing in the same protein, in human samples depleted of acetylcholinesterase the ratio of amidase to esterase activity varied depending on the source of BuChE. The much faster degradation of ONA and F-ONA by BuChE monomers (G1) of colon and kidney than by the tetramers (G4) suggests aggregation-driven differences in the AAA site between single and polymerized subunits. The similar ratio of F-ONAto butyrylthiocholine hydrolysis by serum G1 and G4 forms support structural differences in the amidase site according to the source of BuChE. The changing ratios of amidase to esterase activities in the human sources probably arise from post-translational modifications in BuChE subunits, the specific proportion of monomers and oligomers and the variable capacity of the tetramers for degrading ONA and F-ONA. The elevated amidase activity of BuChE monomers and the scant activity of the tetramers justify the occurrence of single BuChE subunits in cells as a means to sustain the AAA activity of BuChE which otherwise could be lost by tetramerization.

Kinetic analysis of effector modulation of butyrylcholinesterase-catalysed hydrolysis of acetanilides and homologous esters

The effects of tyramine, serotonin and benzalkonium on the esterase and aryl acylamidase activities of wild-type human butyrylcholinesterase and its peripheral anionic site mutant, D70G, were investigated. The kinetic study was carried out under steady-state conditions with neutral and positively charged aryl acylamides [o-nitrophenylacetanilide, o-nitrotrifluorophenylacetanilide and m-(acetamido) N,N,N-trimethylanilinium] and homologous esters (o-nitrophenyl acetate and acetylthiocholine). Tyramine was an activator of hydrolysis for neutral substrates and an inhibitor of hydrolysis for positively charged substrates. The affinity of D70G for tyramine was lower than that of the wild-type enzyme. Tyramine activation of hydrolysis for neutral substrates by D70G was linear. Tyramine was found to be a pure competitive inhibitor of hydrolysis for positively charged substrates with both wild-type butyrylcholinesterase and D70G. Serotonin inhibited both esterase and aryl acylamidase activities for both positively charged and neutral substrates. Inhibition of wild-type butyrylcholinesterase was hyperbolic (i.e. partial) with neutral substrates and linear with positively charged substrates. Inhibition of D70G was linear with all substrates. A comparison of the effects of tyramine and serotonin on D70G versus the wild-type enzyme indicated that: (a) the peripheral anionic site is involved in the nonlinear activation and inhibition of the wild-type enzyme; and (b) in the presence of charged substrates, the ligand does not bind to the peripheral anionic site, so that ligand effects are linear, reflecting their sole interaction with the active site binding locus. Benzalkonium acted as an activator at low concentrations with neutral substrates. High concentrations of benzalkonium caused parabolic inhibition of the activity with neutral substrates for both wild-type butyrylcholinesterase and D70G, suggesting multiple binding sites. Benzalkonium caused linear, noncompetitive inhibition of the positively charged aryl acetanilide m-(acetamido) N,N,N-trimethylanilinium for D70G, and an unusual mixed-type inhibition/activation (alpha > beta > 1) for wild-type butyrylcholinesterase with this substrate. No fundamental difference was observed between the effects of ligands on the butyrylcholinesterase-catalysed hydrolysis of esters and amides. Thus, butyrylcholinesterase uses the same machinery, i.e. the catalytic triad S198/H448/E325, for the hydrolysis of both types of substrate. The differences in response to ligand binding depend on whether the substrates are neutral or positively charged, i.e. the differences depend on the function of the peripheral site in wild-type butyrylcholinesterase, or the absence of its function in the D70G mutant. The complex inhibition/activation effects of effectors, depending on the integrity of the peripheral anionic site, reflect the allosteric 'cross-talk' between the peripheral anionic site and the catalytic centre.

Detection of carbamic and organophosphorous pesticides in water samples using a cholinesterase biosensor based on Prussian Blue-modified screen-printed electrode

In the present paper, a comparative study using Co-phthalocyanine and Prussian Blue-modified screen-printed electrodes, has been performed. Both the electrodes have demonstrated an easiness of preparation together with high sensitivity towards thicoholine (LOD=5 x 10(-7) and 5 x 10(-6) M for Co-phthalocyanine and Prussian Blue, respectively) with high potentialities for pesticide measurement. Prussian Blue-modified screen-printed electrodes were then selected for successive enzyme immobilization due to their higher operative stability demonstrated in previous works. AChE and BChE enzymes were used and inhibition effect of different pesticides was studied with both the enzymes. AChE-based biosensors have demonstrated a higher sensitivity towards aldicarb (50% inhibition with 50 ppb) and carbaryl (50% inhibition with 85 ppb) while BChE biosensors have shown a higher affinity towards paraoxon (50% inhibition with 4 ppb) and chlorpyrifos-methyl oxon (50% inhibition with 1 ppb). Real samples were also tested in order to evaluate the matrix effect and recovery values comprised between 79 and 123% were obtained.

Homocysteine inhibits butyrylcholinesterase activity in rat serum

In the present work we investigated the in vitro effects of homocysteine (Hcy) and methionine (Met), metabolites accumulated in homocystinuria, on butyrylcholinesterase (BuChE) activity in rat serum. We also studied the kinetics of the inhibition of BuChE activity caused by Hcy. For determination of BuChE we used serum of 60-day-old Wistar rats, which was incubated in the absence (control) or presence of Hcy (0.01-0.5 mM) or Met (0.2-2.0 mM). The kinetics of the interaction of Hcy and BuChE was determined using the Lineweaver-Burk double reciprocal plot. Results showed that serum BuChE activity was not altered by Met, but it was significantly inhibited (37%) by 500 microM Hcy, a concentration similar to those found in blood of homocystinuric patients. The apparent Km values, in the absence and presence of 500 microM of Hcy, were 0.034 and 0.142 mM, respectively, and V(max) of BuChE for acetylcholine (ACh) as substrate was 1.25 micromol ACSCh/h/mg of protein. The Ki value obtained was 120 microM, and the inhibition was of the competitive type, suggesting a common binding site for Hcy and ACh. It is proposed that inhibition of cholinesterase activity may be one of the mechanisms involved in the neurological dysfunction observed in homocystinuria.