Active-site peptides of acetylcholinesterase of Electrophorus electricus: labelling of His-440 by 1-bromo-[2-14C]pinacolone and Ser-200 by tritiated diisopropyl fluorophosphate

Design, Synthesis, and Pharmacological Evaluation of Embelin-Aryl/alkyl Amine Hybrids as Orally Bioavailable Blood-Brain Barrier Permeable Multitargeted Agents with Therapeutic Potential in Alzheimer's Disease: Discovery of SB-1448

The complex and multifaceted nature of Alzheimer's disease has brought about a pressing demand to develop ligands targeting multiple pathways to combat its outrageous prevalence. Embelin is a major secondary metabolite of Embelia ribes Burm f., one of the oldest herbs in Indian traditional medicine. It is a micromolar inhibitor of cholinesterases (ChEs) and β-site amyloid precursor protein cleaving enzyme 1 (BACE-1) with poor absorption, distribution, metabolism, and excretion (ADME) properties. Herein, we synthesize a series of embelin-aryl/alkyl amine hybrids to improve its physicochemical properties and therapeutic potency against targeted enzymes. The most active derivative, 9j (SB-1448), inhibits human acetylcholinesterase (hAChE), human butyrylcholinesterase (hBChE), and human BACE-1 (hBACE-1) with IC₅₀ values of 0.15, 1.6, and 0.6 μM, respectively. It inhibits both ChEs noncompetitively with k_i values of 0.21 and 1.3 μM, respectively. It is orally bioavailable, crosses blood-brain barrier (BBB), inhibits Aβ self-aggregation, possesses good ADME properties, and protects neuronal cells from scopolamine-induced cell death. The oral administration of 9j at 30 mg/kg attenuates the scopolamine-induced cognitive impairments in C57BL/6J mice.

Design, synthesis, and structure-activity relationship of caffeine-based triazoles as dual AChE and BACE-1 inhibitors

Natural products have significantly contributed to drug discovery for neurodegenerative diseases. Caffeine is one of the well-known central nervous system(CNS)-active natural products. Besides its CNS stimulant properties, it is a mild inhibitor of acetylcholinesterase (AChE) and possesses memory-enhancing properties. The present work aimed to improve the AChE inhibition activity of the caffeine. The rationally designed caffeine-based triazoles were synthesized and evaluated in vitro for cholinesterase and β-site amyloid precursor protein cleaving enzyme-1 (BACE-1) inhibitory activities. The attachment of triazole to the caffeine enhances its AChE inhibition activity from half-maximal inhibitory concentration (IC₅₀ ) of 129 µM to 0.49 µM (derivative, 6l). The caffeine core interacts with the peripheral anionic site, whereas the benzyl triazole occupies the catalytic anionic site located at the bottom of the active site gorge. The structure-activity relationship revealed that the four-atom ester linker is superior to shorter linkers for connecting the caffeine core to the triazole. The 2,6-difluorobenzyl triazole-linked caffeine derivative, 6d, exhibits dual inhibition of AChE and BACE-1 with IC₅₀ values of 1.43 and 10.9 µM, respectively. The derivative 6d inhibits AChE via a mixed-type mode with an inhibition rate constant (K_i ) value of 2.35 μM, which was corroborated by docking studies. The triazole 6d has an acceptable stability profile in human liver microsomes (t_1/2  = 54 min) and was found to possess CNS permeability when evaluated using the parallel artificial membrane permeability blood-brain barrier assay. The results presented herein warrant investigating caffeine-based triazoles in preclinical models of Alzheimer's disease.

Comparison of the reactivation rates of acetylcholinesterase modified by structurally different organophosphates using novel pyridinium oximes

A novel panel of oximes were synthesized, which have displayed varying degree of reactivation ability towards different organophosphorus (OP) modified cholinesterases. In the present article, we report a comparative reactivation profile of a series of quaternary pyridinium-oximes for electric eel acetylcholinesterase (EEAChE) inhibited by the organophosphorus (OP) inhibitors methyl paraoxon (MePOX), ethyl paraoxon (POX; paraoxon) and diisopropyl fluorophosphate (DFP) that are distinguishable as dimethoxyphosphoryl, diethoxyphosphoryl and diisopropoxyphosphoryl AChE-OP-adducts. Most of the 59-oximes tested led to faster and more extensive reactivation of MePOX- and POX-inhibited EEAChE as compared to DFP-modified EEAChE. All were effective reactivators of three OP-modified EEAChE conjugates showing 18-21% reactivation for DFP-inhibited AChE and ≥45% reactivation for MePOX- and POX-inhibited EEAChE. Oximes 7 and 8 showed k_r values better than pralidoxime (1) for DFP-inhibited EEAChE. Reactivation rates determined at different inhibition times showed no significant change in k_r values during 0-90 min incubation with three OPs. However, a 34-72% decrease in k_r for MePOX and POX and > 95% decrease in k_r for DFP-inhibited EEAChE was observed after 24 h of OP-exposure (aging).

Spirocyclohexadienones as an Uncommon Scaffold for Acetylcholinesterase Inhibitory Activity

BACKGROUND

The most important cause of dementia affecting elderly people is the Alzheimer's disease (AD). Patients affected by this progressive and neurodegenerative disease have severe memory and cognitive function impairments. Some medicines used for treating this disease in the early stages are based on inhibition of acetylcholinesterase. Population aging should contribute to increase the cases of patients suffering from Alzheimer's disease, thus requiring the development of new therapeutic entities for the treatment of this disease.

METHODS

The objective of this work is to identify new substances that have spatial structural similarity with donepezil, an efficient commercial drug used for the treatment of Alzheimer's disease, and to evaluate the capacity of inhibition of these new substances against the enzyme acetylcholinesterase.

RESULTS

Based on a previous results of our group, we prepared a set of 11 spirocyclohexadienones with different substitutions patterns in three steps and overall yield of up to 59%. These compounds were evaluated in vitro against acetylcholinesterase. We found that eight of them are able to inhibit the acetylcholinesterase activity, with IC50 values ranging from 0.12 to 12.67 µM. Molecular docking study indicated that the spirocyclohexadienone, 9e (IC50 = 0.12 µM), a mixedtype AChE inhibitor, showed a good interaction at active site of the enzyme, including the cationic (CAS) and the peripheral site (PAS).

CONCLUSION

We described the first study aimed at investigating the biological properties of spirocyclohexadienones as acetylcholinesterase inhibitors. Thus, we have identified an inhibitor, which provided valuable insights for further studies aimed at the discovery of more potent acetylcholinesterase inhibitors.

Identification of embelin, a 3-undecyl-1,4-benzoquinone from Embelia ribes as a multitargeted anti-Alzheimer agent

Beta-secreatse (BACE-1) and cholinesterases are clinically validated targets of Alzheimer's disease (AD), for which natural products have provided immense contribution. The multifaceted nature of AD signifies the need of multitargeted agents to tackle this disease. In the search of new natural products as dual BACE-1/cholinesterase inhibitors, a library of pure natural products was screened for inhibition of acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and BACE-1. The screening efforts have identified 1,4-benzoquinone "embelin," a natural product derived from Embelia ribes displaying inhibition of all three enzymes, with IC₅₀ values of 2.5, 5.4, and 2.1 μM, respectively. This screen has also identified isoquinoline alkaloids papaverine and L-tetrahydropalmatine as AChE inhibitors. Kinetic study has shown that embelin inhibits EeAChE and EqBChE with ki values of 4.59 and 0.57 μM, in an uncompetitive and noncompetitive manner, respectively. The interactions of embelin with allosteric peripheral anionic site of cholinesterases, has further supported the results of kinetic study. Embelin has also enhanced the activity of P-gp in LS-180 cells, the efflux pump which is involved in the clearance of amyloid-β from AD brain. Further, the cell viability study in neuronal cell line has indicated the excellent therapeutic window of embelin. These results are indicative of the fact that embelin is a multitargeted agent playing role in stopping the formation of amyloid-β oligomers (via inhibition of BACE-1), improves cholinergic-transmission (via inhibition of AChE/BChE) and increases amyloid-β clearance (via P-gp induction).

Capillary electrophoresis versus differential scanning calorimetry for the analysis of free enzyme versus enzyme-ligand complexes: in the search of the ligand-free status of cholinesterases

Cholinesterases (ChEs) are highly efficient biocatalysts whose active site is buried in a deep, narrow gorge. The talent of CE to discover inhibitors in the gorge of highly purified preparations has fairly altered the meaning of a ChE ligand-free status. To attempt at a description of this one, we investigated the stability of Bungarus fasciatus acetylcholinesterase (AChE), alone or complexed with different inhibitors. Determination of mid-transition temperature for thermal denaturation, using differential scanning calorimetry (DSC) and CE, provided conflicting results. Discrepancies strongly question the reality of a ligand-free AChE state. DSC allowed estimation of the stability of AChE-ligands complexes, and to rank the stabilizing effect of different inhibitors. CE acted as a detector of hidden ligands, provided that they were charged, reversibly bound, and thus dissociable upon action of electric fields. Then, CE allowed quantification of the stability of ligand-free AChE. CE and DSC providing each fractional and nonredundant information, cautious attention must be paid for actual estimation of the conformational stability of ChEs. Because inhibitors used in purification of ChEs by affinity chromatography are charged, CE remains a leading method to estimate enzyme stability and detect the presence of bound hidden ligands.

Phosphoacetylcholinesterase: toxicity of phosphorus oxychloride to mammals and insects that can be attributed to selective phosphorylation of acetylcholinesterase by phosphorodichloridic acid

Phosphorus oxychloride (POCl(3)) is an intermediate in the synthesis of many organophosphorus insecticides and chemical warfare nerve gases that are toxic to insects and mammals by inhibition of acetylcholinesterase (AChE) activity. It was therefore surprising to observe that POCl(3), which is hydrolytically unstable, also itself gives poisoning signs in ip-treated mice and fumigant-exposed houseflies similar to those produced by the organophosphorus ester insecticides and chemical warfare agents. In mice, POCl(3) inhibits serum butyrylcholinesterase (BuChE) at a sublethal dose and muscle but not brain AChE at a lethal dose. In houseflies, POCl(3)-induced brain AChE inhibition is correlated with poisoning and the probable cause thereof. POCl(3) in vitro is selective for AChE (IC(50) = 12-36 microM) compared with several other serine hydrolases (BuChE, carboxylesterase, elastase, alpha-chymotrypsin, and thrombin) (IC(50) = 88-2000 microM). With electric eel AChE, methylcarbamoylation of the active site with eserine reversibly protects against subsequent irreversible inhibition by POCl(3). Most importantly, POCl(3)-induced electric eel AChE inhibition prevents postlabeling with [(3)H]diisopropyl phosphorofluoridate; i.e., both compounds phosphorylate at Ser-200 in the catalytic triad. Pyridine-2-aldoxime methiodide does not reactivate POCl(3)-inhibited AChE, consistent with an anionic phosphoserine residue at the esteratic site. The actual phosphorylating agent is formed within seconds from POCl(3) in water, has a half-life of approximately 2 min, and is identified as phosphorodichloridic acid [HOP(O)Cl(2)] by (31)P NMR and derivatization with dimethylamine to HOP(O)(NMe(2))(2). POCl(3) on reaction with water and HOP(O)Cl(2) have the same potency for inhibition of AChE from either electric eel or housefly head as well as the same toxicity for mice. In summary, the acute toxicity of POCl(3) is attributable to hydrolytic activation to HOP(O)Cl(2) that phosphorylates AChE at the active site to form enzymatically inactive [O-phosphoserine]AChE.

Electrophorus electricus as a model system for the study of membrane excitability

The stunning sensations produced by electric fish, particularly the electric eel, Electrophorus electricus, have fascinated scientists for centuries. Within the last 50 years, however, electric cells of Electrophorus have provided a unique model system that is both specialized and appropriate for the study of excitable cell membrane electrophysiology and biochemistry. Electric tissue generates whole animal electrical discharges by means of membrane potentials that are remarkably similar to those of mammalian neurons, myocytes and secretory cells. Electrocytes express ion channels, ATPases and signal transduction proteins common to these other excitable cells. Action potentials of electrocytes represent the specialized end function of electric tissue whereas other excitable cells use membrane potential changes to trigger sophisticated cellular processes, such as myofilament cross-bridging for contraction, or exocytosis for secretion. Because electric tissue lacks these functions and the proteins associated with them, it provides a highly specialized membrane model system. This review examines the basic mechanisms involved in the generation of the electrical discharge of the electric eel and the membrane proteins involved. The valuable contributions that electric tissue continues to make toward the understanding of excitable cell physiology and biochemistry are summarized, particularly those studies using electrocytes as a model system for the study of the regulation of membrane excitability by second messengers and signal transduction pathways.

Cloning and expression of acetylcholinesterase from Electrophorus. Splicing pattern of the 3' exons in vivo and in transfected mammalian cells

We cloned and expressed a cDNA encoding acetylcholinesterase (AChE) of type T from Electrophorus electricus organs. When expressed in COS, HEK, and Chinese hamster ovary cells, the AChET subunits generated dimers and tetramers. The cells produced more activity at 27 than at 37 degrees C. The kinetic parameters of a recombinant enzyme, produced in the yeast Pichia pastoris, were close to those of the natural AChE. Analysis of genomic clones showed that the coding sequence is interrupted by an intron that does not exist in Torpedo and differs in its location from that observed in the mouse. This intron is preceded by a sequence encoding a non-conserved 29-amino acid peptide, which does not exist in Torpedo or mammalian AChEs. According to a three-dimensional model, this non-conserved peptide is located at the surface of the protein, opposite from the entry of the catalytic gorge; its deletion did not modify the catalytic parameters. Sequence analyses and expression of various constructs showed that the gene does not contain any H exon. We also found that splicing of transcripts in mammalian cells reveals cryptic donor sites in exons and acceptor sites in introns, which do not appear to be used in vivo.