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

Side effects based network construction and drug repositioning of ropinirole as a potential molecule for Alzheimer's disease: an in-silico, in-vitro, and in-vivo study

Alzheimer's disease (AD) is the leading cause of dementia in older adults. Drug repositioning is a process of finding new therapeutic applications for existing drugs. One of the methods in drug repositioning is to use the side-effect profile of a drug to identify a new therapeutic indication. The drugs with similar side-effects may act on similar biological targets and could affect the same biochemical process. In this study, we explored the Food and Drug Administration-approved drugs using PROMISCUOUS database to find those that have adverse effects profile comparable with the ligands being studied or used to treat AD. Here, we found that the ropinirole, a dopamine receptor agonist, shared a maximum number of side-effects with the drugs proven beneficial for treating AD. Furthermore, molecular modelling demonstrated that ropinirole exhibited strong binding affinity (-9.313 kcal/mol) and best ligand efficiency (0.49) with sigma-1 receptor. Here, we observed that the quaternary amino group of ropinirole is essential for binding with sigma-1 receptor. Molecular dynamic simulation indicated that the movement of the carboxy-terminal helices (α4/α5) could play a major role in the receptor's physiological functions. The neurotoxicity induced by Aβ25-35 in SH-SY5Y cells was reduced by ropinirole at concentrations 10, 30, and 50 µM. The effect on spatial learning and memory was examined in mice with Aβ25-35 induced memory deficit using the radial arm maze. Ropinirole (10 and 20 mg/kg) significantly improved the short and long-term memories in the radial arm maze test. Our results suggest that ropinirole has the potential to be repositioned for AD treatment.Communicated by Ramaswamy H. Sarma.

Acetylcholine esterase inhibitors in effluents from oil production platforms in the North Sea

Inhibition of acetylcholine esterase (AChE) activity is a biomarker for the exposure to neurotoxic compounds such as organophosphates and is intimately associated with the toxicity of several pesticides. In the present study, the AChE inhibiting potential of organic extracts of production water (produced water) from oil and gas production platforms in the Norwegian sector of the North Sea was determined in an in vitro bioassay based on commercially available purified AChE from the electric organ of Electrophorus electricus (L.). The results from the studies show that produced water contains a combination of AChE inhibiting compounds and compounds stimulating AChE enzymatic activity. The AChE inhibition was predominantly caused by unidentified aromatic compounds in the oil/particulate fraction of produced water, whereas polar compounds in both the water soluble and oil/particulate fraction of produced water caused an apparent stimulation of AChE activity. Substrate saturation studies with fixed concentrations of produced water extracts confirmed that the inhibition occurred in a non-destructive and competitive manner. The concentrations of AChE inhibitors (7.9-453 ng paraoxon-equivalents L⁻¹, 2.2-178 μg dichlorvos-equivalents L⁻¹) were in many cases found to be several orders of magnitude higher than background levels. The findings demonstrate that produced water contains potentially neurotoxic compounds and suggest that further laboratory studies with fish or field studies in the vicinity of oil production facilities are highly warranted.

Shoot-and-Trap: use of specific x-ray damage to study structural protein dynamics by temperature-controlled cryo-crystallography

Although x-ray crystallography is the most widely used method for macromolecular structure determination, it does not provide dynamical information, and either experimental tricks or complementary experiments must be used to overcome the inherently static nature of crystallographic structures. Here we used specific x-ray damage during temperature-controlled crystallographic experiments at a third-generation synchrotron source to trigger and monitor (Shoot-and-Trap) structural changes putatively involved in an enzymatic reaction. In particular, a nonhydrolyzable substrate analogue of acetylcholinesterase, the "off-switch" at cholinergic synapses, was radiocleaved within the buried enzymatic active site. Subsequent product clearance, observed at 150 K but not at 100 K, indicated exit from the active site possibly via a "backdoor." The simple strategy described here is, in principle, applicable to any enzyme whose structure in complex with a substrate analogue is available and, therefore, could serve as a standard procedure in kinetic crystallography studies.

Structural insights into substrate traffic and inhibition in acetylcholinesterase

Acetylcholinesterase (AChE) terminates nerve-impulse transmission at cholinergic synapses by rapid hydrolysis of the neurotransmitter, acetylcholine. Substrate traffic in AChE involves at least two binding sites, the catalytic and peripheral anionic sites, which have been suggested to be allosterically related and involved in substrate inhibition. Here, we present the crystal structures of Torpedo californica AChE complexed with the substrate acetylthiocholine, the product thiocholine and a nonhydrolysable substrate analogue. These structures provide a series of static snapshots of the substrate en route to the active site and identify, for the first time, binding of substrate and product at both the peripheral and active sites. Furthermore, they provide structural insight into substrate inhibition in AChE at two different substrate concentrations. Our structural data indicate that substrate inhibition at moderate substrate concentration is due to choline exit being hindered by a substrate molecule bound at the peripheral site. At the higher concentration, substrate inhibition arises from prevention of exit of acetate due to binding of two substrate molecules within the active-site gorge.

Choline pivaloyl esters improve in rats cognitive and memory performances impaired by scopolamine treatment or lesions of the nucleus basalis of Meynert

The effects of two choline pivaloyl esters, [2-(2,2-dimethylpropionyloxy)ethyl]trimethylammonium iodide (1) and [2-(2,2-dimethylpropionyloxy)ethyl]trimethylammonium 2,2-dimethylpropionate (2), on learning and memory impairments induced in rats by scopolamine or lesions of nucleus basalis magnocellularis (NBM) have been evaluated by object recognition and Morris water maze tests in comparison with Tacrine (THA). Both 1 and 2 restored discrimination in object recognition test for assessing working-episodic memory and improved spatial memory in scopolamine or NBM-lesioned rats as well. The positive effects produced by 1 and 2 on cognitive and memory deficits were well comparable with those evoked by THA, used as reference compound.

Molecular modeling and enzymatic studies of the interaction of a choline analogue and acetylcholinesterase

Pivaloyl-choline iodide 1 interactions with acetylcholinesterase (AChE) have been studied by theoretical and enzymatic methods. An integrated computational approach has clearly shown a substrate rather than inhibitory profile for 1. Enzymatic experiments have also supported the same theoretical conclusion indicating that AChE was able to hydrolyze 1 to choline.

Synthesis and properties of 18F-labeled potential myocardial blood flow tracers

PET centers without particle accelerators make clinical PET widely available at reduced cost. For myocardial perfusion tracers, these satellite PET centers are limited to generator- produced 82Rb(+) and 62Cu[PTSM]. Their limitations motivate a search for transportable alternatives. In search of new tracers we have synthesized several 18F-labeled amines and quaternary ammonium salts. Among them, 4-[18F]fluorotri-N-methylanilinium ([18F]FTMA) has flow-tracing properties. The compound is functional, but has properties that justify a continued search.

Structure-based alignment and comparative molecular field analysis of acetylcholinesterase inhibitors

The method of comparative molecular field analysis (CoMFA) was used to develop quantitative structure-activity relationships for physostigmine, 9-amino-1,2,3,4-tetrahydroacridine (THA), edrophonium (EDR), and other structurally diverse inhibitors of acetylcholinesterase (AChE). The availability of the crystal structures of enzyme/inhibitor complexes (EDR/AChE, THA/AChE, and decamethonium (DCM)/AChE) (Harel, M.; et al. Quaternary ligand binding to aromatic residues in the active-site gorge of acetylcholinesterase. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 9031-9035) provided information regarding not only the active conformation of the inhibitors but also the relative mutual orientation of the inhibitors in the active site of the enzyme. Crystallographic conformations of EDR and THA were used as templates onto which additional inhibitors were superimposed. The application of cross-validated R2 guided region selection method, recently developed in this laboratory (Cho, S.J.; Tropsha, A. Cross-Validated R2 Guided Region Selection for Comparative Molecular Field Analysis (CoMFA): A Simple Method to Achieve Consistent Results. J. Med. Chem. 1995, 38, 1060-1066), to 60 AChE inhibitors led to a highly predictive CoMFA model with the q2 of 0.734.

A contribution to the mechanism of action of SAD-128

SAD-128 was found to be an effective protector of acetylcholinesterase against inhibition by soman, due to its ability to function as a reversible inhibitor and allosteric modifier of the AChE active site. It also attenuated aging of the soman-inhibited enzyme. In order to study the connection between some of these effects of SAD-128 and structural changes in acetylcholinesterase and/or the membrane to which the enzyme is bound, the influences of SAD-128 on the EPR spectra of the spin labelled enzyme and of the membrane were studied under various conditions and the results correlated with the kinetic parameters. SAD-128 increases the fluidity of human erythrocyte membranes but not that of the Torpedo marmorata electric organ. Similarly, the binding properties of membrane acetylcholinesterase for SAD-128, expressed in terms of the Hill coefficient, differ for the two preparations. Some structural changes in the enzyme active site were also observed in the presence of SAD-128. The high protective effect of SAD-128 against AChE inhibition was confirmed by the EPR method regardless of the organophosphorus inhibitor tested. On the other hand, the effect of SAD-128 on the retardation of irreversible inhibition of the enzyme essentially depends on the inhibitor used. From present results it can be concluded that the protective effects of SAD-128 against inhibition of m-AChE are related to the structural changes of the active site and can be additionally moderated by the microviscosity changes of the membrane.