CHOLINESTERASES IN THE BLOOD OF MAN

Influence of a stationary magnetic field on acetylcholinesterase in murine bone marrow cells

A thirty-minute exposure of mice to a homogeneous stationary magnetic field (SMF) of 1.4 Tesla at either 27 degrees C or 37 degrees C body temperature causes an inhibition of about 20 per cent of acetylcholinesterase (AChE, E.C. 3.11.7) in murine bone marrow cells (BMC) after 3.5 and 2 h, respectively, at the two aforementioned body temperatures. The extent of enzyme inhibition is independent of ambient temperature, but dependent on the time after exposure. This initial inhibition of AChE activity is followed by a limited recovery which is dependent upon the temperature during exposure to the SMF and remains incomplete even 15 h afterwards. We describe here certain enzymologic properties of AChE in BMC as well as inhibition studies with diisopropylfluorophosphate (DFP) to differentiate between AChE and nonspecific cholinesterases.

Membrane-bound acetylcholinesterase: an early differentiation marker for skeletal myoblasts

Cell-bound acetylcholinesterase (AChE) was found to be an early differentiation marker on embryonic chick skeletal myoblasts in mixed primary cell cultures. AChE biosynthesis was detected and characterized by (a) a sensitive microtiter assay, (b) use of selective inhibitors, and (c) with mono- and polyclonal antibodies. Both secreted and cell-bound AChE appeared on the first day in culture, at a time when no muscle cell fusion was observed. Characterization of this enzyme revealed that true AChE was bound and secreted by myoblasts. BW284c51, which permeates cell membranes poorly, inhibited all the cell-associated AChE activity on myoblasts, suggesting that the activity measured was on the outer cell surface. On the other hand, fibroblasts appeared to have no or very little bound enzyme and the low level of secreted enzyme activity had the characteristics of pseudo-, or butyrylcholinesterase. Polyclonal anti-Torpedo californica electroplax AChE antibody and several monoclonal antibodies were found to bind specifically to chick myoblasts. Since the cells had not been made permeable before antibody binding, a membrane-bound form of the enzyme was most likely being detected. The cell-bound true AChE was present in identifiable quantities from the first day of culture. Membrane-bound AChE can thus serve as an early differentiation marker for embryonic chick myoblasts in mixed primary cultures.

Cholinesterases during development of the avian nervous system

1. Long before onset of synaptogenesis in the chicken neural tube, the closely related enzymes butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are expressed in a mutually exclusive manner. Accordingly, neuroblasts on the ventricular side of the neural tube transiently express BChE before they abruptly accumulate AChE while approaching the outer brain surface. 2. By exploiting AChE as a sensitive and early histochemical differentiation marker, we have demonstrated complex polycentric waves of differentiation spreading upon the cranial part of the chicken neural tube but a smooth rostrocaudal wave along the spinal cord. Shortly after expression of AChE, these cells extend long projecting neurites. In particular, segmented spinal motor axons originate from AChE-positive motoneurones; they navigate through a BChE-active zone within the rostral half of the sclerotomes before contacting BChE/AChE-positive myotome cells. At synaptogenetic stages, cholinesterases additionally are detectable in neurofibrillar laminae foreshadowing the establishment of cholinergic synapses. 3. In order to elucidate the functional significance of cholinesterases at early stages, we have investigated specific cholinesterase molecules and their mechanisms of action in vivo and in vitro. A developmental shift from the low molecular weight forms to the tetramers of both enzymes has been determined. In vitro, the addition of a selective BChE inhibitor leads to a reduction of AChE gene expression. Thus, in vivo and in vitro data suggest roles of cholinesterases in the regulation of cell proliferation and neurite growth. 4. Future research has to show whether neurogenetic functioning of cholinesterases can help to understand their reported alterations in neural tube defects, mental retardations, dementias and in some tumours.

Molecular biological search for human genes encoding cholinesterases

Cholinesterases (ChEs) are highly polymorphic proteins, capable of rapidly hydrolyzing the neurotransmitter acetylcholine and involved in terminating neurotransmission in neuromuscular junctions and cholinergic synapses. In an attempt to delineate the structure and detailed properties of the human protein(s) and the gene(s) coding for the acetylcholine hydrolyzing enzymes, a human cDNA coding for ChE was isolated by use of oligodeoxynucleotide screening of cDNA libraries. For this purpose, a method for increasing the effectiveness of oligonucleotide screening by introducing deoxyinosine in sites of codon ambiguity and using tetramethyl-ammonium salt washes to remove false-positive hybrids was employed. The resulting isolated 2.4-kilobase (kb) cholinesterase cDNA sequences encode for the entire mature secretory protein, preceded by an N-terminal signal peptide. The human ChE primary sequence shows almost no homology to other serine hydrolases, with the exception of a hexapeptide at the active site. In contrast, it displays extensive homology with acetylcholinesterase form Torpedo californica and Drosophila melanogaster as well as with bovine thyroglobulin. These extensive homologies probably suggest the need of the entire coding sequence for the physiological function(s) fulfilled by the enzyme and further suggest a common, unique, ancestral gene for these cDNAs. In turn, the cDNA was used as a probe to isolate genomic DNA sequences for the 5'-region of the human ChE gene. The genomic DNA fragment encoding part of the 5'-region of ChEcDNA was detected by DNA blot hybridization, enriched 70-fold by gel electrophoresis and electroelution, cloned in lambda phage and isolated. Sequencing of the cloned DNA revealed that it did indeed include part of the 5'-region of ChEcDNA, starting at an adjacent 5'-position to the nucleotides coding for the initiator methionine, and ending with an EcoRI restriction site inherent to the ChEcDNA sequence. The isolated fragment of the human cholinesterase gene is currently employed to complete the structural characterization of this and related genes.

Cholinesterase phenotyping: clinical aspects and laboratory applications

The measurement of cholinesterase activity in human serum is an important investigation, especially in patients suspected of poisoning with organophosphate insecticides, or those experiencing prolonged paralysis following treatment with the short acting muscle relaxant succinylcholine. Appropriate clinical management of affected individuals can only be instituted if enzyme activity is measured using a method capable of clear interpretation and phenotypic ascription of cholinesterase, ascertained by use of selected enzyme inhibitors, is reliable. This review considers factors which lead to significant cholinesterase abnormalities and advises on the techniques most appropriate to their investigation. It is concluded that no one method is suitable both for measurement of activity and determination of genotype. The use of propionyl thiocholine is recommended for the first procedure and benzoyl choline for the latter. Those laboratories in which a request for the assessment of cholinesterase status is an unusual event should make greater use of assistance from colleagues whose experience with the problem is greater.

Activation of acetylcholine receptors causes the partition of hydrophobic cations into postsynaptic membrane vesicles

In the continued presence of cholinergic ligands, the acetylcholine receptor-channel complex (AChR) in postsynaptic membranes undergoes a sequence of conformational changes. On addition of the ligand, the receptor rapidly changes from a closed channel to an open channel conformation, then slowly changes to a nonconducting state termed desensitization. The lifetime of the open channel conformation and the rate of desensitization are both dependent on the magnitude of the membrane potential, suggesting that the ligand-induced conformational changes in AChR may involve the movement of electrical charges within the membrane. Measurements of charge redistribution in AChR-containing membranes following ligand binding have not been reported. Recently, measurements of changes in the membrane partition coefficient of hydrophobic ions have been used to detect electrostatic changes in both biological and model membranes. We report here that cholinergic ligands induce changes in the partition coefficient of the hydrophobic cation tetraphenylphosphonium (TPP) into AChR-enriched membranes. The extent and time course of these changes in TPP partition coefficient are accounted for in a kinetic model. We conclude that TPP movement is a monitor of a molecular event which may be associated with the slow component of AChR desensitization.

The aryl acylamidases and their relationship to cholinesterases in human serum, erythrocyte and liver

Human serum aryl acylamidase associated with serum cholinesterase was purified to homogeneity. Evidence for the identity of the two enzymes was based on co-elution profiles, co-purification in the different steps including affinity chromatography with constant ratios of specific activity and percentage recoveries, co-migration on gel electrophoresis, parallel inhibition by typical cholinesterase inhibitors and co-precipitation by antibody raised against the purified enzyme. Human liver aryl acylamidase was partially purified. Based on the elution profiles, purification data, inhibitory characteristics and gel electrophoresis it was concluded that aryl acrylamidase of liver was not associated with liver cholinesterase. More conclusive evidence for the non-association of the liver aryl acylamidase and cholinesterase came from their clear-cut separation on procainamide-Sepharose affinity chromatography. Both the serum and liver aryl acylamidase were compared with the purified erythrocyte aryl acylamidase associated with acetylcholinesterase. While the erythrocyte and serum aryl acylamidases showed some similarities in their sensitivities to amines like serotonin or tryptamine and choline derivatives, the liver enzyme was unaffected by any of these compounds. A notable observation was the activation by tyramine of the serum aryl acylamidase but not the erythrocyte and liver aryl acylamidases. The liver aryl acylamidase also differed from the other two in its relative insensitivity to inhibition by eserine, neostygmine and other cholinesterase inhibitors. Immunodiffusion and immunoprecipitation studies showed that the aryl acylamidases from the liver and erythrocytes were immunologically non-identical with the serum enzyme.

Pseudocholinesterase in obesity: hypercaloric diet induced changes in experimental obese mice

Pseudocholinesterase activity is significantly higher in liver and serum, but lower in adipose tissue of genetically obese, diabetic and gold thioglucose treated mice. Similar enzyme changes were also observed in lean mice on a high carbohydrate diet. A marked reduction (40%) in PChE activity occurred in the liver of genetically diabetic mice when starved for 24 h. These observations suggest that pseudocholinesterase induction in the liver and repression in the adipose tissue is affected by excessive caloric intake in obesity. This provides a model to study the biological function of PChE in health and disease.

Radiolabeled benzoylcholine derivatives as possible myocardial-imaging agents

Two radioiodinated analogs of benzoylcholine were investigated as possible myocardial-imaging agents. O-([2-125I]Iodobenzoyl)-choline and N-([2-125I]iodobenzoyl)cholamine were prepared by nucleophilic substitution of sodium [125I]iodide for stable iodine in O-(N,N]dimethylaminoethyl)-2-iodobenzoate and N-(N',N'-dimethylaminoethyl)-2-iodobenzamide, respectively, and by methylation with methyl iodide. The in vivo distribution of each compound in mice was determined as a function of time. Favorable heart-to-blood and heart-to-lung ratios were obtained with N-([2-125I]iodobenzoyl)cholamine.

In vitro actions of NN-dimethyl-2-aminoindane and related compounds

The actions of 2-aminoindane, 2-amino-1,2,3,4-tetrahydronaphthalene and amphetamine and their N-methyl and NN-dimethyl derivatives on the guinea-pig ileum were compared. All, except the tertiary 2-aminoindane, inhibited the responses to electrical stimulation and it is suggested that this is not fully explained by their sympathomimetic properties. NN-Diemthyl-2-aminoindane increased these responses and caused contractions of the unstimulated ileum due to a nicotine-like action. The indane series was more effective in producing contractions of the ileum than the other compounds. The three tertiary derivatives antagonized competitively the actions of histamine on the ileum and those of 5-hydroxy-tryptamine on the rat uterus.

Cholinesterases from plant tissues. VI. Preliminary characterization of enzymes from Solanum melongena L. and Zea mays L

Enzymes capable of hydrolyzing esters of thiocholine have been assayed in extracts of Solanum melongena L. (eggplant) and Zea Mays L. (corn). The enzymes from both species are inhibited by the anti-cholinesterases neostigmine, physostigmine, and 284c51 and by AMO-1618, a plant growth retardant and they both have pH optima near pH 8.0. The enzyme from eggplant is maximally active at a substrate concentration of 0.15 mM acetylthiocholine and is inhibited at higher substrate concentrations. On the basis of this last property, the magnitude of inhibition by the various inhibitors, and the substrate specificity, we conclude that the enzyme from eggplant, but not that from corn, is a cholinesterase.

Interactions between oxotremorine and inhibitors of cholinesterase enzymes

Interactions between oxotremorine and physostigmine or dyflos have been investigated in mice using doses giving approximately 50% inhibition of mouse brain cholinesterase. Oxotremorine LD50 was unaffected by pretreatment of mice with the anticholinesterases. Its ED50 (tremor) was decreased by physostigmine and neostigmine but not by dyflos. Dyflos antagonized oxotremorine hypothermia, physostigmine was without effect. Oxotremorine did not alter the inhibitory activity of the anticholinesterases. The lack of potentiation of oxotremorine by cholinesterase inhibitors suggests it acts directly in these tests by a mechanism independent of its ability to increase tissue acetylcholine concentration.

The influence of some aliphatic alcohols on the enzymic hydrolysis of methyl hippurate

The hydrolysis of methyl hippurate under the influence of α-chymotrypsin and some aliphatic alcohols has been investigated. Methanol, ethanol, n-propanol, n-butanol and n-pentanol were found to inhibit the rate of hydrolysis. The dissociation constants KI for an assumed enzyme-inhibitor complex and KSI for an assumed (enzyme-substrate)-inhibitor complex have been evaluated. In all cases KSI > KI with a ratio of 1·1 to 1 for methanol and 2 to 1 for other alcohols. From the relation between the pK values it is calculated that the free energy change accompanying loss of bound water from each -CH2- group was -2mD1 kJ mol−1.

The effects of eserine and neostigmine on the guinea-pig ileum and on ileal longitudinal muscle strips

Eserine and neostigmine produced spasms of the guinea-pig isolated ileum that were concentration-related (4 ng-16 μg/ml). Hexamethonium (100 μg/ml) and tetrodotoxin (100 ng/ml) reduced the spasmogenic activity of both drugs. Neostigmine, but not eserine, overcame the hexamethonium antagonism. Pretreatment of the ileum with dyflos (1 μg/ml for 10 min) did not affect the action of either eserine or neostigmine, therefore the spasm was not due to inhibition of cholinesterase. The residual response to eserine or neostigmine after tetrodotoxin was abolished by hyoscine (4 ng/ml). Isolated longitudinal muscle strips were so prepared to be innervated or denervated. Both types of strip gave similar bradykinin maxima but the response of the denervated strip to eserine and neostigmine was reduced to 40% of the innervated strip and the remaining response was blocked by hyoscine (2 ng/ml). It is concluded that eserine and neostigmine have an indirect action on the smooth muscle of the ileum that is the result of acetylcholine release from cholinergic nerves. Inhibition of cholinesterase would not seem to be important for this action. A second component of the action of these two drugs appears to be a direct effect on the muscarinic receptors of the smooth muscle.

The antinociceptive activities of oxotremorine, physostigmine and dyflos

Oxotremorine and physostigmine caused dose-related increases in the hot plate reaction time (HPRT) of mice, but dyflos did not. Physostigmine did not potentiate a low dose of oxotremorine (0.025 mg/kg) in the HPRT test, but some potentiation of higher doses of oxotremorine (0.05 and 0.1 mg/kg) may have occurred. Dyflos did not potentiate any dose of oxotremorine. Physostigmine and dyflos inhibited mouse brain cholinesterase but oxotremorine was without effect. The relation between change of HPRT and cholinesterase inhibition was different for physostigmine and dyflos.

The anti-acetylcholinesterase activities of the alkaloids of Physostigma venenosum seeds

Anti-acetylcholinesterase activities of physostigmine, physovenine, Na-norphysostigmine, geneserine and eseramine have been investigated in vitro using erythrocyte acetylcholinesterase. The former three bases show comparable activities whereas geneserine and eseramine are inactive under the same experimental conditions. The biologically active form of these bases is discussed in light of these enzyme inhibitory studies.