Structural isomers of saligenin-based ß2-agonists: synthesis and insight into the reaction mechanism.

Salmeterol and albuterol are well-known ß2-adenoreceptor agonists widely used in the treatment of inflammatory respiratory diseases, such as bronchial asthma and chronic obstructive pulmonary disease. Here we report the preparation of structural isomers of salmeterol and albuterol, which can be obtained from the same starting material as the corresponding ß2-agonists, depending on the synthetic approach employed. Using 1D and various 2D NMR measurements, we determined that the structure of prepared isomers holds the ß-aryl-ß-aminoethanol moiety, in contrast to the α-aryl-ß-aminoethanol moiety found in salmeterol and albuterol. We investigated the reaction of ß-halohydrin and amines responsible for the formation of ß-aryl-ß-amino alcohol - both experimentally and using computational methods. The structure of ß-halohydrin with the methyl salicylate moiety imposes the course of the reaction. The solvent plays a relevant, yet ambiguous role in the direction of the reaction, while the strength of the base influences the reaction yield and isomer ratio in a more evident way. Using computational methods, we have shown that the most probable reaction intermediate responsible for the formation of the unexpected isomer is the corresponding para-quinone methide, which can be formed due to phenol present in the methyl salicylate moiety. After successful preparation of albuterol and salmeterol isomers, we tested their inhibition potency to human acetylcholinesterase (AChE) and usual and atypical butyrylcholinesterase (BChE). Kinetic studies revealed that both isomers are low-potency reversible inhibitors of human cholinesterases.

Interactions of Paraoxonase-1 with Pharmacologically Relevant Carbamates.

Mammalian paraoxonase-1 hydrolyses a very broad spectrum of esters such as certain drugs and xenobiotics. The aim of this study was to determine whether carbamates influence the activity of recombinant PON1 (rePON1). Carbamates were selected having a variety of applications: bambuterol and physostigmine are drugs, carbofuran is used as a pesticide, while Ro 02-0683 is diagnostic reagent. All the selected carbamates reduced the arylesterase activity of rePON1 towards the substrate S-phenyl thioacetate (PTA). Inhibition dissociation constants (Ki), evaluated by both discontinuous and continuous inhibition measurements (progress curves), were similar and in the mM range. The rePON1 displayed almost the same values of Ki constants for Ro 02-0683 and physostigmine while, for carbofuran and bambuterol, the values were approximately ten times lower and two times higher, respectively. The affinity of rePON1 towards the tested carbamates was about 3-40 times lower than that of PTA. Molecular modelling of rePON1-carbamate complexes suggested non-covalent interactions with residues of the rePON1 active site that could lead to competitive inhibition of its arylesterase activity. In conclusion, carbamates can reduce the level of PON1 activity, which should be kept in mind, especially in medical conditions characterized by reduced PON1 levels.

Resorcinol-, catechol- and saligenin-based bronchodilating ß2-agonists as inhibitors of human cholinesterase activity.

We investigated the influence of bronchodilating ß2-agonists on the activity of human acetylcholinesterase (AChE) and usual, atypical and fluoride-resistant butyrylcholinesterase (BChE). We determined the inhibition potency of racemate and enantiomers of fenoterol as a resorcinol derivative, isoetharine and epinephrine as catechol derivatives and salbutamol and salmeterol as saligenin derivatives. All of the tested compounds reversibly inhibited cholinesterases with Ki constants ranging from 9.4 µM to 6.4 mM and had the highest inhibition potency towards usual BChE, but generally none of the cholinesterases displayed any stereoselectivity. Kinetic and docking results revealed that the inhibition potency of the studied compounds could be related to the size of the hydroxyaminoethyl chain on the benzene ring. The additional π-π interaction of salmeterol's benzene ring and Trp286 and hydrogen bond with His447 probably enhanced inhibition by salmeterol which was singled out as the most potent inhibitor of all the cholinesterases.

Peripheral site and acyl pocket define selective inhibition of mouse butyrylcholinesterase by two biscarbamates.

In this study we related metacarb (N-(2-(3,5-bis(dimethylcarbamoyloxy)phenyl)-2-hydroxyethyl)propan-2-aminium chloride) and isocarb (N-(2-(3,4-bis(dimethylcarbamoyloxy)phenyl)-2-hydroxyethyl)propan-2-aminium chloride) inhibition selectivity, as well as stereoselectivity of mouse acetylcholinesterase (AChE; 3.1.1.7) and butyrylcholinesterase (BChE; 3.1.1.8) to the active site residues by studying the progressive inhibition of AChE, BChE and six AChE mutants with racemic and (R)-enantiomers of metacarb and isocarb. Metacarb and isocarb proved to be very potent BChE inhibitors with inhibition rate constants in the range of 10(3)-10(4)M(-1)s(-1). For metacarb and isocarb, inhibition of BChE w.t. was 260 and 35 times, respectively, faster than inhibition of AChE w.t. For four mutants inhibition was faster than for AChE w.t. but none reached the inhibition rate of BChE. The highest increase in the inhibition rate (about 30 times for metacarb and 13 times for isocarb) was achieved with mutants F295L/Y337A and Y124Q meaning that selective inhibition of mouse BChE is dictated mainly by two amino acids from BChE: leucine 286 from the acyl pocket and glutamine 119 from the peripheral site. Wild type enzymes displayed pronounced stereoselectivity for (R)-enantiomers of metacarb and isocarb. Interestingly, the residues that define selective inhibition of mouse BChE by biscarbamates also affect the stereoselectivity of enzymes.

Evaluation of 4-aminoquinoline derivatives with an n-octylamino spacer as potential multi-targeting ligands for the treatment of Alzheimer's disease.

The most successful therapeutic strategy in the treatment of Alzheimer's disease (AD) is directed toward increasing levels of the neurotransmitter acetylcholine (ACh) by inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), the enzymes responsible for its hydrolysis. In this paper, we extended our study on 4-aminoquinolines as human cholinesterase inhibitors on twenty-six new 4-aminoquinolines containing an n-octylamino spacer on C(4) and different substituents on the terminal amino group. We evaluated the potency of new derivatives to act as multi-targeted ligands by determining their inhibition potency towards human AChE and BChE, ability to chelate biometals Fe, Cu and Zn, ability to inhibit the action of ß-secretase 1 (BACE1) and their antioxidant capacity. All of the tested derivatives were very potent inhibitors of human AChE and BChE with inhibition constants (Ki) ranging from 0.0023 to 1.6 µM. Most of the compounds were estimated to be able to cross the blood-brain barrier (BBB) by passive transport and were nontoxic to human neuronal, kidney and liver cells in concentrations in which they inhibit cholinesterases. Generally, newly synthesised compounds were weak reductants compared to standard antioxidants, but all possessed a certain amount of antioxidant activity compared to tacrine. Of the eleven most potent cholinesterase inhibitors, eight compounds also inhibited BACE1 activity at 10-18%. Based on our overall results, compounds 8 with 3-fluorobenzyl, 11 with 3-chlorobenzyl and 17 with 3-metoxy benzyl substituents on the terminal amino group stood out as the most promising for the treatment of AD; they strongly inhibited AChE and BChE, were non-toxic on HepG2, HEK293 and SH-SY5Y cells, had the potential to cross the BBB and possessed the ability to chelate biometals and/or inhibit the activity of BACE1 within a range close to the therapeutically desired degree of inhibition.

Synthesis, Biological Evaluation and Machine Learning Prediction Model for Fluorinated Cinchona Alkaloid-Based Derivatives as Cholinesterase Inhibitors.

A series of 46 Cinchona alkaloid derivatives that differ in positions of fluorine atom(s) in the molecule were synthesized and tested as human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitors. All tested compounds reversibly inhibited AChE and BChE in the nanomolar to micromolar range; for AChE, the determined enzyme-inhibitor dissociation constants (Ki) ranged from 3.9-80 µM, and 0.075-19 µM for BChE. The most potent AChE inhibitor was N-(para-fluorobenzyl)cinchoninium bromide, while N-(meta-fluorobenzyl)cinchonidinium bromide was the most potent BChE inhibitor with Ki constant in the nanomolar range. Generally, compounds were non-selective or BChE selective cholinesterase inhibitors, where N-(meta-fluorobenzyl)cinchonidinium bromide was the most selective showing 533 times higher preference for BChE. In silico study revealed that twenty-six compounds should be able to cross the blood-brain barrier by passive transport. An extensive machine learning procedure was utilized for the creation of multivariate linear regression models of AChE and BChE inhibition. The best possible models with predicted R2 (CD-derivatives) of 0.9932 and R2(CN-derivatives) of 0.9879 were calculated and cross-validated. From these data, a smart guided search for new potential leads can be performed. These results pointed out that quaternary Cinchona alkaloids are the promising structural base for further development as selective BChE inhibitors which can be used in the central nervous system.

Design, Synthesis and Biological Evaluation of Biscarbamates as Potential Selective Butyrylcholinesterase Inhibitors for the Treatment of Alzheimer's Disease.

As butyrylcholinesterase (BChE) plays a role in the progression of symptoms and pathophysiology of Alzheimer's disease (AD), selective inhibition of BChE over acetylcholinesterase (AChE) can represent a promising pathway in treating AD. The carbamate group was chosen as a pharmacophore because the carbamates currently or previously in use for the treatment of AD displayed significant positive effects on cognitive symptoms. Eighteen biscarbamates with different substituents at the carbamoyl and hydroxyaminoethyl chain were synthesized, and their inhibitory potential toward both cholinesterases and inhibition selectivity were determined. The ability of carbamates to cross the blood-brain barrier (BBB) by passive transport, their cytotoxic profile and their ability to chelate biometals were also evaluated. All biscarbamates displayed a time-dependent inhibition with inhibition rate constants within 10-3-10-6 M-1 min-1 range for both cholinesterases, with generally higher preference to BChE. For two biscarbamates, it was determined that they should be able to pass the BBB by passive transport, while for five biscarbamates, this ability was slightly limited. Fourteen biscarbamates did not exhibit a cytotoxic effect toward liver, kidney and neuronal cells. In conclusion, considering their high BChE selectivity, non-toxicity, ability to chelate biometals and pass the BBB, compounds 2 and 16 were pointed out as the most promising compounds for the treatment of middle and late stages of AD.

4-Aminoquinoline-Based Adamantanes as Potential Anticholinesterase Agents in Symptomatic Treatment of Alzheimer's Disease.

Considering that acetylcholinesterase (AChE) inhibition is the most important mode of action expected of a potential drug used for the treatment of symptoms of Alzheimer's disease (AD), our previous pilot study of 4-aminoquinolines as potential human cholinesterase inhibitors was extended to twenty-two new structurally distinct 4-aminoquinolines bearing an adamantane moiety. Inhibition studies revealed that all of the compounds were very potent inhibitors of AChE and butyrylcholinesterase (BChE), with inhibition constants (Ki) ranging between 0.075 and 25 µM. The tested compounds exhibited a modest selectivity between the two cholinesterases; the most selective for BChE was compound 14, which displayed a 10 times higher preference, while compound 19 was a 5.8 times more potent inhibitor of AChE. Most of the compounds were estimated to be able to cross the blood-brain barrier (BBB) by passive transport. Evaluation of druglikeness singled out fourteen compounds with possible oral route of administration. The tested compounds displayed modest but generally higher antioxidant activity than the structurally similar AD drug tacrine. Compound 19 showed the highest reducing power, comparable to those of standard antioxidants. Considering their simple structure, high inhibition of AChE and BChE, and ability to cross the BBB, 4-aminoquinoline-based adamantanes show promise as structural scaffolds for further design of novel central nervous system drugs. Among them, two compounds stand out: compound 5 as the most potent inhibitor of both cholinesterases with a Ki constant in low nano molar range and the potential to cross the BBB, and compound 8, which met all our requirements, including high cholinesterase inhibition, good oral bioavailability, and antioxidative effect. The QSAR model revealed that AChE and BChE inhibition was mainly influenced by the ring and topological descriptors MCD, Nnum, RP, and RSIpw3, which defined the shape, conformational flexibility, and surface properties of the molecules.

Quinuclidine-Based Carbamates as Potential CNS Active Compounds.

The treatment of central nervous system (CNS) diseases related to the decrease of neurotransmitter acetylcholine in neurons is based on compounds that prevent or disrupt the action of acetylcholinesterase and butyrylcholinesterase. A series of thirteen quinuclidine carbamates were designed using quinuclidine as the structural base and a carbamate group to ensure the covalent binding to the cholinesterase, which were synthesized and tested as potential human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitors. The synthesized compounds differed in the substituents on the amino and carbamoyl parts of the molecule. All of the prepared carbamates displayed a time-dependent inhibition with overall inhibition rate constants in the 103 M-1 min-1 range. None of the compounds showed pronounced selectivity for any of the cholinesterases. The in silico determined ability of compounds to cross the blood-brain barrier (BBB) revealed that six compounds should be able to pass the BBB by passive transport. In addition, the compounds did not show toxicity toward cells that represented the main models of individual organs. By machine learning, the most optimal regression models for the prediction of bioactivity were established and validated. Models for AChE and BChE described 89 and 90% of the total variations among the data, respectively. These models facilitated the prediction and design of new and more potent inhibitors. Altogether, our study confirmed that quinuclidinium carbamates are promising candidates for further development as CNS-active drugs, particularly for Alzheimer's disease treatment.

Carbamate group as structural motif in drugs: a review of carbamate derivatives used as therapeutic agents.

Due to their very good chemical and proteolytic stability, ability to penetrate cell membranes, and resemblance to a peptide bond, carbamate derivatives have received much attention in recent years and got an important role in modern drug discovery and medicinal chemistry. Today, carbamates make structural and/or functional part of many drugs and prodrugs approved and marketed for the treatment of various diseases such as cancer, epilepsy, hepatitis C, HIV infection, and Alzheimer's disease. In drugs they can play a role in drug-target interaction or improve the biological activity of parent molecules. In prodrugs they are mainly used to delay first-pass metabolism and enhance the bioavailability and effectiveness of compounds. This brief review takes a look at the properties and use of carbamates in various fields of medicine and provides quick insights into the mechanisms of action for some of them.

Structural aspects of 4-aminoquinolines as reversible inhibitors of human acetylcholinesterase and butyrylcholinesterase.

Eight derivatives of 4-aminoquinolines differing in the substituents attached to the C(4)-amino group and C(7) were synthesised and tested as inhibitors of human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Both enzymes were inhibited by all of the compounds with inhibition constants (Ki) ranging from 0.50 to 50 µM exhibiting slight selectivity toward AChE over BChE. The most potent inhibitors of AChE were compounds with an n-octylamino chain or adamantyl group. The shortening of the chain length resulted in a decrease in AChE inhibition by 5-20 times. Docking studies revealed that the quinoline group within the AChE active site was positioned in the choline binding site, while the C(4)-amino group substituents, depending on their lipophilicity, could establish hydrogen bonds or π-interactions with residues of the peripheral anionic site. The most potent inhibitors of BChE were compounds with the most voluminous substituent on C(4)-amino group (adamantyl) or those with a stronger electron withdrawing substituent on C(7) (trifluormethyl group). Based on AChE inhibition, compounds with an n-octylamino chain or adamantyl substituent were shown to possess the capacity for further development as potential drugs for treatment of neurodegenerative diseases.

Amino acid residues involved in stereoselective inhibition of cholinesterases with bambuterol.

Bambuterol is a chiral carbamate known as selective inhibitor of butyrylcholinesterase (BChE). In order to relate bambuterol selectivity and stereoselectivity of cholinesterases to the active site residues, we studied the inhibition of recombinant mouse BChE, acetylcholinesterase (AChE) and six AChE mutants, employed to mimic BChE active site residues, by bambuterol enantiomers. Both enantiomers selectively inhibited BChE about 8000 times faster than AChE. The largest inhibition rate increase in comparison to AChE w.t. was observed with the F295L/Y337A mutant, showing that leucine 295 and alanine 337 are crucial residues in BChE for high bambuterol selectivity. All studied enzymes preferred inhibition by the R- over the S-bambuterol. The enlargement of the AChE choline binding site and of the acyl pocket by single or double mutations (Y337A, F295L/Y337A and F297I/Y337A) increased, in comparison to w.t. enzymes, inhibition rate constants of R- bambuterol more than that of S- bambuterol resulting in four times higher stereoselectivity. Peripheral site mutations (Y124Q and Y72N/Y124Q/Y337A) increased inhibition rate by S- more than R-bambuterol and consequently diminished the stereoselectivity.

Interactions of butane, but-2-ene or xylene-like linked bispyridinium para-aldoximes with native and tabun-inhibited human cholinesterases.

Kinetic parameters were evaluated for inhibition of native and reactivation of tabun-inhibited human erythrocyte acetylcholinesterase (AChE, EC 3.1.1.7) and human plasma butyrylcholinesterase (BChE, EC 3.1.1.8) by three bispyridinium para-aldoximes with butane (K074), but-2-ene (K075) or xylene-like linker (K114). Tested aldoximes reversibly inhibited both cholinesterases with the preference for binding to the native AChE. Both cholinesterases showed the highest affinity for K114 (K(i) was 0.01 mM for AChE and 0.06 mM for BChE). The reactivation of tabun-inhibited AChE was efficient by K074 and K075. Their overall reactivation rate constants were around 2000 min(-1)M(-1), which is seven times higher than for the classical bispyridinium para-aldoxime TMB-4. The reactivation of tabun-inhibited AChE assisted by K114 was slow and reached 90% after 20 h. Since the aldoxime binding affinity of tabun-inhibited AChE was similar for all tested aldoximes (and corresponded to their K(i)), the rate of the nucleophilic displacement of the phosphoryl-moiety from the active site serine was the limiting factor for AChE reactivation. On the other hand, none of the aldoximes displayed a significant reactivation of tabun-inhibited BChE. Even after 20 h, the reactivation maximum was 60% for 1 mM K074 and K075, and only 20% for 1 mM K114. However, lower BChE affinities for K074 and K075 compared to AChE suggest that the fast tabun-inhibited AChE reactivation by these compounds would not be obstructed by their interactions with BChE in vivo.

Stereoselective inhibition of human, mouse, and horse cholinesterases by bambuterol enantiomers.

Bambuterol is a chiral carbamate and a selective inhibitor of butyrylcholinesterase (BChE, EC 3.1.1.8). In order to relate bambuterol selectivity and stereoselectivity of BChE and acetylcholinesterase (AChE, EC 3.1.1.7) of different species, we studied the inhibition of human, mouse, and horse BChE, as well as AChE of human and mouse by (R)- and (S)-bambuterol. AChE and BChE of all studied species were progressively inhibited by both bambuterol enantiomers, with a preference for the (R)-bambuterol whose inhibition rate constants were about five times higher than that of (S)-bambuterol. We observed no significant difference between human and mouse in bambuterol enantiomer BChE inhibition. However, (R)-bambuterol inhibited horse BChE about 14 times slower than human and mouse BChE, and the inhibition rate for (S)-bambuterol was about 18 times slower. Although the primary structure of horse BChE differs from the other two species in 15 amino acids, we presumed that differences in inhibition rates could be attributed to threonine at position 69 located close to the peripheral site of BChE. Since BChE inhibition by bambuterol enantiomers was at least 8000 times faster than that of AChE, both bambuterol enantiomers proved to be selective BChE inhibitors, as was previously shown for racemate.

Oximes: Reactivators of phosphorylated acetylcholinesterase and antidotes in therapy against tabun poisoning.

One of the therapeutic approaches to organophosphate poisoning is to reactivate AChE with site-directed nucleophiles such as oximes. However, pyridinium oximes 2-PAM, HI-6, TMB-4 and obidoxime, found as the most effective reactivators, have limiting reactivating potency in tabun poisoning. We tested oximes varying in the type of ring (pyridinium and/or imidazolium), the length and type of the linker between rings, and in the position of the oxime group on the ring to find more effective oximes to reactivate tabun-inhibited human erythrocyte AChE. Three of our tested pyridinium oximes K027, K048, K074, along with TMB-4, were the most promising for AChE reactivation. Promising oximes were further tested in vivo on tabun poisoned mice not only as antidotes in combination with atropine but also as pretreatment drug. Herein, we showed that a promising treatment in tabun poisoning by selected oximes and atropine could be improved if oximes are also used in pretreatment. Since the reactivating efficacy of the oximes in vitro corresponded to their therapeutic efficacy in vivo, it seems that pharmacological effect of these oximes is indeed primarily related to the reactivation of tabun-phosphorylated AChE.

Differentiation of EDTA-sensitive from EDTA-insensitive human serum esterases hydrolysing phenylacetate.

The aim of this study was to differentiate the EDTA-sensitive from the EDTA-insensitive human serum esterases by evaluating their catalytic constants, K(M) and V(m), for the hydrolysis of phenylacetate (PA). Measurements were done at 37 degrees C in 0.1 M Tris/HCl buffer pH 7.4 and 8.4. The K(M,sen) and K(M,ins) constants were significantly different, 0.97 and 2.7 mM respectively, confirming that two esterases hydrolyse PA. The pH of the medium had no effect on K(M) values, and also no effect on V(m,sen) while V(m,ins) was two fold higher at pH 8.4 than at 7.4 further confirming the existence of two different enzymes. The stability of the esterases in aqueous media was also studied. EDTA-sensitive activity in buffer without CaCl(2) was extremely unstable; the time-course of inactivation followed a two-phase reaction kinetics, indicating that two EDTA-sensitive esterases hydrolyse PA. The EDTA-insensitive activity remained constant in aqueous media under the same experimental conditions.

Design and evaluation of selective butyrylcholinesterase inhibitors based on Cinchona alkaloid scaffold.

This paper describes the synthesis and anticholinesterase potency of Cinchona-based alkaloids; ten quaternary derivatives of cinchonines and their corresponding pseudo-enantiomeric cinchonidines. The quaternization of quinuclidine moiety of each compound was carried out with groups diverse in their size: methyl, benzyl and differently meta- and para-substituted benzyl groups. All of the prepared compounds reversibly inhibited human butyrylcholinesterase and acetylcholinesterase with Ki constants within nanomolar to micromolar range. Five cinchonidine derivatives displayed 95-510 times higher inhibition selectivity to butyrylcholinesterase over acetylcholinesterase and four were potent butyrylcholinesterase inhibitors with Ki constants up to 100 nM, of which N-para-bromobenzyl cinchonidinium bromide can be considered a lead for further modifications and optimizations for possible use in the treatment of neurodegenerative diseases.

Oxime-assisted reactivation of tabun-inhibited acetylcholinesterase analysed by active site mutations.

The antidotal property of oximes is attributed to their ability to reactivate acetylcholinesterase (AChE) inhibited by organophosphorus compounds (OP) such as pesticides and nerve warfare agents. Understanding their interactions within the active site of phosphylated AChE is of great significance for the search for more efficient reactivators, especially in the case of the most resistant OP to reactivation, tabun. Therefore, herein we studied the interactions and reactivation of tabun-inhibited AChE by site-directed mutagenesis and a series of bispyridinium oximes. Our results indicated that the replacement of aromatic residues with aliphatic ones at the acyl pocket and choline binding site mostly interfered with the stabilisation of the oxime's pyridinium ring(s) within the active site gorge needed to obtain the proper orientation of the oxime group toward the phosphorylated active site serine. However, in the case of W286A, the mutation in the peripheral binding site by preventing a π-π interaction with one of the oxime's pyridinium rings allowed a more favourable position of the oxime for a nucleophilic attack on the phosphorylated catalytic serine. The mutation resulted in a 2-5 fold increase in the reactivation rates when compared to the AChE wild type. Therefore, it seems that aromatic amino acids at the peripheral binding site presented a limitation in bispyridinium oxime reactivation efficiency of tabun-phosphorylated AChE. Moreover, this is further corroborated by the reactivation by mono-pyridinium oxime 2-PAM, in which mutations at the peripheral site did not influence either the affinity or reactivation of tabun-inhibited AChE.

Structure-activity approach in the reactivation of tabun-phosphorylated human acetylcholinesterase with bispyridinium para-aldoximes.

We investigated interactions of bispyridinium para-aldoximes N,N'-(propano)bis(4-hydroxyiminomethyl) pyridinium bromide (TMB-4), N,N'-(ethano)bis(4-hydroxyiminomethyl)pyridinium methanosulphonate (DMB-4), and N,N'-(methano)bis(4-hydroxyiminomethyl)pyridinium chloride (MMB-4) with human erythrocyte acetylcholinesterase phosphorylated by tabun. We analysed aldoxime conformations to determine the flexibility of aldoxime as an important feature for binding to the acetylcholinesterase active site. Tabun-inhibited human erythrocyte acetylcholinesterase was completely reactivated only by the most flexible bispyridinium aldoxime - TMB-4 with a propylene chain between two rings. Shorter linkers than propylene (methylene or ethylene) as in MMB-4 and DMB-4 did not allow appropriate orientation in the active site, and MMB-4 and DMB-4 were not efficient reactivators of tabun-phosphorylated acetylcholinesterase. Since aldoximes are also reversible inhibitors of native acetylcholinesterase, we determined dissociation constants and their protective index against acetylcholinesterase inactivation by tabun.

[Organophosphorus compounds: classification and enzyme reactions]. / Organofosforni spojevi: klasifikacija i reakcije s enzimima.

Organophosphorus compounds are derivatives of phosphoric, phosphonic or phosphinic acids whose oxygen atoms bound directly to the phosphorus atom can be substituted by sulphur or nitrogen atoms. These compounds represent a large group of organic compounds used primarily as pesticides. Some are used as drugs and the most toxic compounds as nerve agents. Acute toxicity of organophosphorus compounds is due to the inhibition of acetylcholinesterase, the critical enzyme in neurotransmission. Organophosphorus compounds whose sulphur atom creates a coordinative covalent bond with the phosphor atom are not acetylcholinesterase inhibitors. To become biologically active these compounds must transform into their oxo analogues, passing through spontaneous or biotransformation reactions. Biotransformation reactions of organophosphorus compounds involve a large number of enzymatic reactions that can make them more or less toxic, or even non-toxic for acetylcholinesterase. The classification of organophosphorus compounds in this paper considers the nature of groups bound directly to the central phosphorus atom. The paper describes the enzymes taking part in biotransformation of organophosphorus compounds and gives examples of their reactions.