Synthesis and testing of trifluoromethyl-containing phosphonate-peptide conjugates as inhibitors of serine hydrolases

A modification of novel fluorinated organophosphorous compounds containing terminal alkyne group by different azidopeptides via Cu(I)-catalyzed click chemistry has been described. The inhibitor activity of trifluoromethyl-containing methylphosphonates and their peptide-conjugates towards acetylcholinesterase, butyrylcholinesterase, and carboxylesterase has been investigated. It was shown that the incorporation of peptide fragments significantly modulates the esterase profile of starting methylphosphonates.     

Acetylcholinesterase inhibition by pitofenone: a spasmolytic compound.

Pitofenone, a spasmolytic compound, inhibited the acetylcholinesterase activity from bovine erythrocytes and from electric eel. It is a potent inhibitor of this enzyme from the two sources, with Ki values of 36 and 45 microM, respectively. Of the five compounds structurally related to pitofenone, only those containing a piperidine moiety show acetylcholinesterase inhibition. All these inhibitions are reversible, linear, and noncompetitive in nature. A qualitative correlation between the anticholinesterase and the corresponding antimuscarinic activity for some of these compounds was apparent. Good separation of these two effects would be a desirable feature for newer muscarinic antagonists.     

Fluorinated aldehydes and ketones acting as quasi-substrate inhibitors of acetylcholinesterase.

1. The inhibition of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) by compounds containing trifluoromethyl-carbonyl groups was investigated and related to the effects observed with structurally similar, non-fluorinated chemicals. 2. Compounds that in aqueous solution readily form hydrates inhibit acetylcholinesterase in a time-dependent process. On the other hand non-hydrated, carbonyl-containing compounds showed rapid and reversible, time-independent enzyme inactivation when assayed under steady state conditions. 3. m-N,N,N-Trimethylammonium-acetophenone acts as a rapid and reversible, time-independent, linear competitive inhibitor of acetylcholinesterase (Ki = 5.0 . 10(-7) M). 4. The most potent enzyme inhibitor tested in this series was N,N,N,-trimethylammonium-m-trifluoroacetophenone. It gives time-dependent inhibition and the concentration which inactivates eel acetylcholinesterase to 50% of the original activity after 30 min exposure is 1.3 . 10(-8) M. The bimolecular rate constant for this reaction is 1.8 . 10(6) 1 . mol-1 . min-1. The enzyme-inhibitor complex is very stable as the inhibited enzyme after 8 days of dialysis is reactivated to 20% only. This compound represents a quasi-substrate inhibitor of acetylcholinesterase.     

The effect of pH and reversible inhibitors on decarbamylation of acetylcholinesterase

Decarbamylation rate of membrane-bound methyl- and dimethyl-carbamylated acetylcholinesterase of human erythrocytes and bovine brain is reliably 1.1-1.6 times lower than that of the soluble enzyme. Such reversible inhibitors as tacrine (of non-competition action), ambenonium (mixed action) and galanthamine (competitive type of action) decelerate the decarbamylation rate of acetylcholinesterase. At pH 6 tacrine inhibits the reduction rate of soluble acetylcholinesterase activity of human erythrocytes more intensively than that of membrane-bound acetylcholinesterase. No differences in decarbamylation rate were found for the both forms of the enzyme at pH 8. Tacrine, a non-competitive inhibitor in concentrations below the inhibition constant (Ki = 1.4 x 10(-7) M) exerts the most intensive effect on the decarbamylation rate of methyl- and dimethylcarbamylated acetylcholinesterase of the mouse brain, while ambenonium and galanthamine in concentrations much (tens times) exceeding their Ki (3.1 x 10(-10) M and 4.4 x 10(-7) M, respectively) provide a decrease of the decarbamylation rate.     

Inactivation of rat brain acetylcholinesterase by pyridoxal 5'-phosphate

Effects of pyridoxal 5'-phosphate on the activity of crude and purified acetylcholinesterase from cerebral hemispheres of adult rat brain were examined. Acetylcholinesterase was completely inactivated by incubation with 0.5 mM pyridoxal 5'-phosphate. The enzyme activity remained unaltered in the presence of analogs of pyridoxal 5'-phosphate, pyridoxal, pyridoxamine and pyridoxamine 5'-phosphate. The inhibition of acetylcholinesterase activity by pyridoxal 5'-phosphate appeared to be of a noncompetitive nature, as determined by Lineweaver-Burk analysis. The inhibitory effect of pyridoxal 5'-phosphate on acetylcholinesterase appeared to be a general one, as the activity of the enzyme from the brains of immature chick and egg-laying hen, and from different tissues of the adult male rats, exhibited a similar pattern in the presence of the inhibitor. The inhibitory effects of pyridoxal 5'-phosphate could be reversed upon exhaustive dialysis of the pyridoxal 5'-phosphate-treated acetylcholinesterase preparations. We propose that the effects of pyridoxal 5'-phosphate are due to its interaction with acetylcholinesterase, and that it can be employed as a useful tool for studying biochemical aspects of this important brain enzyme.     

Osmotic fragility of chromaffin granules prepared under isoosmotic or hyperosmotic conditions and localization of acetylcholinesterase

In chromaffin cells of the adrenal medulla, catecholamines are stored in secretory granules. Different methods have been described to purify chromaffin granules. In the present study, storage granules were prepared using isoosmotic self-generating Percoll gradients or hyperosmotic sucrose gradients, and a comparison of their physical properties in response to osmotic changes was made. Catecholamines, dopamine beta-hydroxylase activity and protein were detected both in the external medium and in the granule fraction according to the medium osmolality. Suspension turbidity was used as a measure of organelle integrity. Acetylcholinesterase activity was found to be associated with both isoosmotically and hyperosomotically prepared granules. The total acetylcholinesterase activity was determined after adding Triton X-100 to the assay medium. When adrenal medullary tissue was homogenized in buffers containing echothiopate, an inhibitor of acetylcholinesterase, only 15-20% of enzyme activity was inhibited, excluding the possibility that main granule acetylcholinesterase could be due to contamination by plasma membrane fragments, endoplasmic reticulum and Golgi membranes. When granules were suspended in hypoosmotic buffers, a soluble acetylcholinesterase form was released into the external medium, while an insoluble acetylcholinesterase form was still found associated with the membrane fraction. Soluble acetylcholinesterase was found to be released differently than soluble dopamine beta-hydroxylase, indicating that acetylcholinesterase may be associated with a more osmotically resistant granule population.     

Inactivation of rat brain acetylcholinesterase by pyridoxal 5′-phosphate

Effects of pyridoxal 5′-phosphate on the activity of crude and purified acetylcholinesterase from cerebral hemispheres of adult rat brain were examined. Acetylcholinesterase was completely inactivated by incubation with 0.5 mM pyridoxal 5′-phosphate. The enzyme activity remained unaltered in the presence of analogs of pyridoxal 5′-phosphate, pyridoxal, pyridoxamine and pyridoxamine 5′-phosphate. The inhibition of acetylcholinesterase activity by pyridoxal 5′-phosphate appeared to be of a noncompetitive nature, as determined by Lineweaver-Burk analysis. The inhibitory effect of pyridoxal 5′-phosphate on acetylcholinesterase appeared to be a general one, as the activity of the enzyme from the brains of immature chick and egg-laying hen, and from different tissues of the adult male rats, exhibited a similar pattern in the presence of the inhibitor. The inhibitory effects of pyridoxal 5′-phosphate could be reversed upon exhaustive dialysis of the pyridoxan 5′-phosphate-treated acetylcholinesterase preparations. We propose that the effects of pyridoxal 5′-phosphate are due to its interaction with acetylcholinesterase, and that it can be employed as a useful tool for studying biochemical aspects of this important brain enzyme.     

Carboxy-terminal fragment of osteogenic growth peptide regulates myeloid differentiation through RhoA

The carboxy-terminal fragment of osteogenic growth peptide, OGP(10-14), is a pentapeptide with bone anabolic effects and hematopoietic activity. The latter activity appears to be largely enhanced by specific growth factors. To study the direct activity of OGP(10-14) on myeloid cells, we tested the pentapeptide proliferating/differentiating effects in HL60 cell line. In this cell line, OGP(10-14) significantly inhibited cell proliferation, and enhanced myeloperoxidase (MPO) activity and nitroblue tetrazolium reducing ability. Moreover, it induced cytoskeleton remodeling and small GTP-binding protein RhoA activation. RhoA, which is known to be involved in HL60 differentiation, mediated these effects as shown by using its specific inhibitor, C3. Treatment with GM-CSF had a comparable OGP(10-14) activity on proliferation, MPO expression, and RhoA activation. Further studies on cell proliferation and RhoA activation proved enhanced activity by association of the two factors. These results strongly suggest that OGP(10-14) acts directly on HL60 cells by activating RhoA signaling although other possibilities cannot be ruled out.     

Isolation and characterization of N98-1272 A, B and C, selective acetylcholinesterase inhibitors from metabolites of an actinomycete strain

A high throughput screening was carried out in order to search for inhibitors of acetylcholinesterase (AChE) from microorganism metabolites. An actinomycete strain was found to produce active compounds named N98-1272 A, B and C with IC50 of 15.0, 11.5, 12.5 microM, respectively. Structural studies revealed that the three compounds are identical to the known antibiotics, Manumycin C, B and A. Kinetic analyses showed that N98-1272 C (Manumycin A) acted as a reversible noncompetitive inhibitor of acetylcholinesterase, with a Ki value of 7.2 microM. The cyclohexenone epoxide part of the structure plays a crucial role in the inhibitory activity against AChE. Compared with Tacrine, N98-1272 A, B, and C exhibit much better selectivity toward AChE over BuChE.     

Human plasmin enzymatic activity is inhibited by chemically modified dextrans

Some synthetic dextran derivatives that mimic the action of heparin/heparan sulfate were shown to promote in vivo tissue repair when added alone to wounds. These biofunctional mimetics were therefore designated as "regenerating agents" in regard to their in vivo properties. In vitro, these biopolymers were able to protect various heparin-binding growth factors against proteolytic degradation as well as to inhibit the enzymatic activity of neutrophil elastase. In the present work, different dextran derivatives were tested for their capacity to inhibit the enzymatic activity of human plasmin. We show that dextran containing carboxymethyl, sulfate as well as benzylamide groups (RG1192 compound), was the most efficient inhibitor of plasmin amidolytic activity. The inhibition of plasmin by RG1192 can be classified as tight binding hyperbolic noncompetitive. One molecule of RG1192 bound 20 molecules of plasmin with a K(i) of 2.8 x 10(-8) m. Analysis with an optical biosensor confirmed the high affinity of RG1192 for plasmin and revealed that this polymer equally binds plasminogen with a similar affinity (K(d) = 3 x 10(-8) m). Competitive experiments carried out with 6-aminohexanoic acid and kringle proteolytic fragments identified the lysine-binding site domains of plasmin as the RG1192 binding sites. In addition, RG1192 blocked the generation of plasmin from Glu-plasminogen and inhibited the plasmin-mediated proteolysis of fibronectin and laminin. Data from the present in vitro investigation thus indicated that specific dextran derivatives can contribute to the regulation of plasmin activity by impeding the plasmin generation, as a result of their binding to plasminogen and also by directly affecting the catalytic activity of the enzyme.     

Active-site catalytic efficiency of acetylcholinesterase molecular forms in Electrophorus, torpedo, rat and chicken

The active sites of acetylcholinesterase multiple forms from four widely different zoological species (Electrophorus, Torpedo, rat and chicken) were titrated using a stable, irreversible phosphorylating inhibitor (O-ethyl-S2-diisopropylaminoethyl methyl-phosphonothionate). In all cases, we found that within a given species, the molecular forms we examined were equivalent in their catalytic activity per active site. As pure preparations of the molecular forms of Electrophorus acetylcholinesterase were available, we were able to establish that one inhibitor molecule binds per monomer unit for each of them. This had already been shown by several authors for the tetrameric globular form, but not for the tailed molecules. Analysis of the phosphorylation reaction showed that they are equally reactive. Under our experimental conditions, their turnover number per site was 4.4 x 10(7) mol of acetylthiocholine hydrolysed . h-1 at 28 degrees C, pH 7.0. The corresponding value was less than half for Torpedo (1.64 x 10(7) mol . h-1), and again lower for rat (1.32 x 10(7) mol . h-1) and chicken (1.05 x 10(7) mol . h-1). In the case of rat acetylcholinesterase, the activity per active site of solubilized (with or without Triton X-100) and membrane-bound enzyme were identical. We discuss the implications of these findings with respect to the quaternary structure of acetylcholinesterase, and to the physico-chemical state and physiological properties of its molecular forms.     

Semax and selank inhibit the enkephalin-degrading enzymes from human serum]]

A dose-dependent effect of synthetic heptapeptides Semax (Met-Glu-His-Phe-Pro-Gly-Pro) and Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro) on the enkephalin-degrading enzymes of human serum was demonstrated. The inhibitory effects of Semax (IC50 10 microM) and Selank (IC50 20 microM) are more pronounced than those of puromycin (IC50 10 mM), bacitracin, and some other inhibitors of peptidases. Beside the heptapeptides, their pentapeptide fragments also possessed an inhibitory effect; tri-, tetra-, and hexapeptide fragments did not display such an effect. As the above enzymes take part in degradation of not only enkephalins but also other regulatory peptides, it can be assumed that one of the mechanisms of biological activity of Semax and Selank is related to this inhibitory activity of theirs.     

Differential perturbation of erythrocyte membrane-associated transport and enzyme activities by structurally related lipophilic compounds

The alteration of two erythrocyte plasma membrane functions, acetylcholine hydrolysis and glucose exchange, by a series of structurally related small lipophilic compounds which exhibit similar antihemolytic behavior was studied. 2-Methyldimethylaminoazobenzene is a more potent inhibitor of acetylcholinesterase than the 3′-methyl analogue, while the unsubstituted compound fails to inhibit. Esterase inhibition by the 2-methyl compound is noncompetitive and dependent on the anion composition of the assay buffer. The temperature dependence of acetylcholinesterase activity in the presence of the 2-methyl compound suggests that interaction with inhibitor is influenced by the state of lipids tightly bound to the enzyme. Glucose exchange is inhibited to the same extent by both methyl derivatives but not by the unsubstituted dye, and the temperature dependence in the presence of inhibitor is not grossly altered. The lack of correlation between inhibition of membrane function and stabilization of erythrocytes against osmotic hemolysis is discussed.     

New Benzodiazepines Alter Acetylcholinesterase and ATPDase Activities

This study examines the effect of new 1,5 benzodiazepines on acetylcholinesterase (AChE) and ATPDase (apyrase) activities from cerebral cortex of adult rats. Simultaneously, the effects of the classical 1,4-benzodiazepine on these enzymes were also studied for comparative purpose. The compounds 2-trichloromethyl-4-phenyl-3H-1,5-benzodiazepin and 2-trichloromethyl-4-(p-methyl-phenyl)-3H-1,5-benzodiazepin significantly inhibited acetylcholinesterase activity (p < 0.01) when tested in the range of 0.18–0.35 mM. The inhibition caused by these two new benzodiazepines was noncompetitive in nature. Similarly, at concentrations ranging from 0.063 to 0.25 mM, the 1,5 benzodiazepines inhibited ATP and ADP hydrolysis by synaptosomes from cerebral cortex (p < 0.01). However, the inhibition of nucleotide hydrolysis was uncompetitive in nature. Our results suggest that, although diazepam and the new benzodiazepines have chemical differences, they both presented an inhibitory effect on acetylcholinesterase and ATPDase activities.     

Kinetics and structure-activity relationship studies on pregnane-type steroidal alkaloids that inhibit cholinesterases

The mechanism of inhibition of acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BChE, EC 3.1.1.8) enzymes by 23 pregnane-type alkaloids isolated from the Sarcococca saligna was investigated. Lineweaver-Burk and Dixon plots and their secondary replots showed that the majority of these compounds, that is 1, 4, 5, 6, 9, 10, 12, 13, 15-19, and 21 were found to be noncompetitive inhibitors of both enzymes. Compounds 8, 20, 22, and 23 were determined to be uncompetitive inhibitors of BChE, while compounds 11 and 14 were found to be uncompetitive and linear mixed inhibitors of AChE, respectively. Ki values were found to be in the range of 2.65-250.0 microM against AChE and 1.63-30.0 microM against BChE. The structure-activity relationship (SAR) studies suggested that the major interaction of the enzyme-inhibitor complexes are due to hydrophobic and cation-pi interactions inside the aromatic gorge of these cholinesterases. The effects of various substituents on the activity of these compounds are also discussed in details.     

甲基毒死蜱对红细胞膜乙酰胆碱酯酶的抑制作用及其与膜脂相互关系

以人红细胞膜为材料,研究了甲基毒死蜱与膜上乙酰胆碱酯酶(AChE)的相互作用及其与膜脂的关系。结果显示,甲基毒死蜱对人红细胞膜AChE有明显的抑制作用,与膜温育30min,其半数抑制浓度约为0.10 mmol/L。动力学分析表明,其抑制作用为非竞争性。0.2％Triton X-100并不改变AChE对甲基毒死蜱的敏感性,亦即AChE上甲基毒死蜱的作用部位与其所处的脂质微环境无关。     

N-hydroxy-2-acetylaminofluorene as effective inhibitor of aldehyde oxidase

N-Hydroxy-2-acetylaminofluorene has been found to be an effective inhibitor of aldehyde oxidase. At concentrations of 1 X 10(-6) M and 1 X 10(-5) M, 38% and 88% inhibition was observed on the oxidase activity towards N1-methylnicotinamide. The inhibition was of noncompetitive type and had a Ki value of 4.4 X 10(-6) M. In contrast, little inhibition of the enzyme was observed with 2-aminofluorene, 2-acetylaminofluorene and acetohydroxamic acid even at a concentration of 1 X 10(-4) M.     

Potent and selective small molecule NS3 serine protease inhibitors of Hepatitis C virus with dichlorocyclopropylproline as P2 residue

Starting from a pentapeptide Hepatitis C virus NS3 protease inhibitor, a number of alpha-ketoamide inhibitors based on novel dichlorocyclopropylproline P2 core were synthesized and investigated for their HCV NS3 serine protease activity. The key intermediate 3,4-dichlorocyclopropylproline was obtained through a dichloro carbene insertion to 3,4-dehydroproline. The size of the molecules was reduced significantly through a series of truncations of the initial pentapeptide. By varying P1 side chain in length and size, potency and selectivity were improved. A variety of aliphatic carbamate and urea capping groups were examined. In general, compounds with urea cappings were more potent and selective than their carbamate counterparts. The most potent compound was a tert-butyl urea analog. Variations at P3 position were also investigated. Among the three residues incorporated, tert-leucine was clearly superior, leading to compounds that had excellent enzyme potency and selectivity. The most potent compound achieved cell-based replicon assay EC50 of 40 nM. The most promising compound of all had excellent potency in both enzyme (Ki* = 9 nM) and replicon assays (EC50 = 100 nM). Its bioavailabilities were above 10% in all three animal species (rats, monkeys, and dogs). It has provided a lead for future investigations.     

Inhibition of substance P degradation in rat brain preparations by peptide hydroxamic acids

A peptidase activity of rat diencephalon membranes, which acts on the C-terminal hexapeptide sequence of substance P, was characterized using the radiolabeled substrate N alpha-[( 125I]iododesaminotyrosyl)-substance P (6-11)-hexapeptide. This activity presents certain characteristics similar to those of the substance-P-degrading enzyme purified from human brain by Lee et al. [Eur. J. Biochem. 114, 315-327 (1981)]. It is inhibited by metal chelators and some thiol reagents, but is insensitive to inhibitors of serine proteases and aminopeptidases. The activity is different from angiotensin-converting enzyme and enkephalinase, since it is not affected by specific inhibitors of these enzymes. Substance P and substance P C-terminal fragments longer than the pentapeptide inhibited the degradation of the radiolabeled substrate with inhibition constants around 200 microM. Short fragments of the substance P sequence, such as Boc-Phe-Phe-OMe and Boc-Phe-Phe-Gly-OEt, were also found to inhibit the degradation of the substrate. When the metal-chelating hydroxamic acid moiety was attached to the carboxyl terminus of these short peptides, potent inhibitors of the substance-P-degrading activity were obtained, with inhibition constants in the micromolar range. The most potent of these compounds, iododesaminotyrosyl-Phe-Phe-Gly-NHOH (IBH-Phe-Phe-Gly-NHOH), is a competitive inhibitor, with a Ki value of 1.9 microM. The degradation of substance P by rat diencephalon slices was inhibited to the same extent (40-50%) by IBH-Phe-Phe-Gly-NHOH (20 microM) and by phosphoramidon (1 microM). A combination of both reagents reduced the degradation rate by 75-80%, suggesting that both enkephalinase and the substance-P-degrading activity are involved in the metabolism of substance P in this preparation. IBH-Phe-Phe-Gly-NHOH seems to be quite specific for the latter enzyme, since at a high concentration (0.1 mM) it did not affect the degradation of the radiolabeled substrate by alpha-chymotrypsin, papain, or thermolysin.     

Role of divalent metals in the kinetic mechanism of insulin receptor tyrosine kinase

The kinetic and thermodynamic interrelationships of peptide substrate (Val5-angiotensin 11), metal-ATP, and divalent metal cations with rat liver insulin receptor tyrosine kinase (IRTK) were investigated. Results of the initial rate studies with varying peptide and MnATP substrates indicates that the kinetic mechanism for IRTK is of the sequential type and therefore rules out a ping pong Bi Bi pathway. Hence, peptide substrate and metal-ATP bind to the kinase prior to the release of products. MnADP was a linear competitive inhibitor of MnATP and a noncompetitive inhibitor of peptide substrate. A synthetic tyrosine-containing pentapeptide, Glu-Glu-Phe-Tyr-Phe (EEFYF), was a linear competitive inhibitor of peptide substrate and a noncompetitive inhibitor of MnATP. Accordingly, the data show that phosphorylation of peptide substrate occurs via a rapid random equilibrium Bi Bi mechanism in which the kinase has the potential to react initially with either of the two substrates. In contrast, divalent metal cations and metal-ATP were found to interact with the kinase in a mutually inclusive manner, with metal binding to the kinase prior to MnATP. It was also found that divalent metals increase the affinity of the kinase for metal-ATP but do not affect the affinity of IRTK for metal-ADP product. Hence, divalent metals, during the reaction of association of enzyme with one of its substrates to form the binary complex, increase the relative concentration of E-ATP complex versus E-peptide complex, thus introducing a thermodynamic-dependent ordering for the interaction of substrates with the enzyme. To investigate the thermodynamics of this system, we assumed that under initial conditions the kinetic data we obtained reflected the association constants of reactants with the enzyme.