Affinity chromatography of acetylcholine acetyl-hydrolase. EC 3.1.1.7. A new straight chain aliphatic inhibitor

Inhibition of acetylcholinesterase from Electrophorus electricus (L.) by tricyclic antidepressants

The effects of tricyclic antidepressants drugs (TCA) amitriptyline, imipramine and nortriptyline, on purified Electrophorus electricus (L.) acetylcholinesterase (AChE; acetylcholine hydrolase, EC 3.1.1.7) were studied using kinetic methods and specific fluorescent probe propidium. The antidepressants inhibited AChE activity by a non-competitive mechanism. Inhibition constants range from 200 to 400 microM. Dimethylated amitriptyline and imipramine were more potent inhibitors than the monomethylated nortriptyline. Fluorescence measurements using bis-quaternary ligand propidium were used to monitor ligand-binding properties of these cationic antidepressants to the AChE peripheral anionic site (PAS). This ligand exhibited an eight-fold fluorescence enhancement upon binding to the peripheral anionic site of AChE from E. electricus (L.) with K(D)=7 x 10(-7)M. It was observed that TCA drugs displaced propidium from the enzyme. On the basis of the displacement experiments antidepressant dissociation constants were determined. Similar values for the inhibition constants suggest that these drugs have similar affinity to the peripheral anionic site. The results also indicate that the catalytic active center of AChE does not participate in the interaction of enzyme with tricyclic antidepressants. These studies suggest that the binding site for tricyclic antidepressants is located at the peripheral anionic site of E. electricus (L.) acetylcholinesterase.

Reactivation of nerve agent inhibited human acetylcholinesterases by HI-6 and obidoxime

Acetylcholinesterase was purified from human caudate nucleus and skeletal muscle. The enzyme preparations were used to study aging and reactivation by HI-6 and obidoxime after inhibition by soman and its isomers. HI-6 was found to be the most potent reactivator. For both enzyme preparations a higher reactivatability and a higher rate of aging were observed after inhibition by C+-soman than after inhibition by C(-)-soman. Aging was retarded by propidium diiodide. Reactivation by the two oximes was also studied after inhibition by tabun, sarin and VX. Tissue homogenates were used for this part of the work. Our conclusion is that HI-6 is superior to obidoxime for human acetylcholinesterases inhibited by soman and sarin, while obidoxime is better towards tabun-inhibited enzyme.

Hen's egg yolk cholinesterase. Purification, characterization and comparison with hen's liver and blood plasma cholinesterase

The cholinesterase (acylcholine acylkhydrolase, EC 3.1.1.8) of chicken egg yolk was partly purified and characterized. It was compared to homologous enzymes of liver and blood plasma of laying hens. During gel filtration, yolk and liver cholinesterase were resolved into two fractions. Blood plasma cholinesterase showed one form only, identical with yolk and liver cholinesterase 1 *** (EC 3.1.1.8). This form has an Mr of 440 000 and may be a tetramer of a cholinesterase form present in yolk and liver (Mr 104 000). Substrate specificity, pH optima, Km values, the influence of effectors (ammonium derivatives, choline, eserine, fluoride), gel filtration, gel electrophoresis, isoelectric focusing and affinity chromatography, all point to a very close similarity, if not identity, of the corresponding forms.

Interactions of acetylcholine receptor and acetylcholinesterase with lipid monolayers

The interaction of acetylcholine receptor and acetylcholinesterase with lipid monolayers was followed by measuring changes in surface pressure. When injected into the subphase of a lipid monolayer, the proteins caused increases in surface pressure from 5 to 10 dynes/cm, indicating a penetration of protein into the monolayer. At pH values below the isoelectric point of the proteins the incorporation was improved. The same was observed when Ca2+ (2mM) was added. The presence of the enzyme in the mixed film could be demonstrated by using diiso [3H] propyl fluorophosphate-labelled acetylcholinesterase as well as by measuring enzyme activity. Acetylcholine receptor was shown to be present in the mixed film by using a complex made of the receptor and alpha-[3H]neurotoxin.

Purification of acetylcholinesterase from house fly brain by affinity chromatography

Acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) from the heads of house flies (Musca domestica L.) was purified by affinity chromatography. The enzyme was adsorbed from the crude extracts on an affinity column containing trimethyl(p-aminophenyl) ammonium chloride hydrochloride (Ki approximately 1.7 - 10(-4) M), covalently linked to Sepharose 4B, then eluted with a solution of a selective reversible inhibitor, 1,5-bis (4-allyl dimethyl ammoniumphenyl)-pentan-3-one dibromide (BW 284C51; Ki approximately 1 - 10(-7) M). The enzyme was purified 1223 times in one step and had a specific activity of 752 units/mg protein. Disc gel electrophoresis in polyacrylamide gel revealed five protein bands, four corresponding to the enzyme activity bands and one devoid of enzyme activity. On the basis of periodic acid-Schiff stain intensity, the slower moving isozyme I and the contaminating band appear to be rich in carbohydrate. The purity of the enzyme estimated by disc gel electrophoresis was 94%. Density gradient centrifugation in sucrose showed two major species each of which ran as a single band on disc gel electrophoresis. The average molecular weights were 306000 (+/- 11 150) for heavy (s20,w = 11.5 S) form and 143000 (+/- 4700) for light (s20,w = 6.9 S) form.

Affinity chromatography of acetylcholinesterase. The importance of hydrophobic interactions

An easily prepared affinity column for acetylcholinesterase is described, which may be operated at ionic strength high enough to prevent aggregation of the asymmetric forms of the enzyme. Specific elution by tetraethylammonium or decamethonium was quantitative. The performance of this column is comparable to that of the column described by Dudai and Silman. It is shown that the hexyl 'spacer arm' strongly participates in the enzyme binding and that its replacement with the more hydrophilic spermine chain lowers the affinity. The hexyl chain itself is shown to bind acetylcholinesterase, although with lower affinity and capacity than the complete column. This binding is also partly reversed by inhibitors. Such hydrophobic columns bind the native asymmetric forms of the enzyme more strongly than the lytic globular ones. The aromatic quaternary ligang inhibits Electrophorus but not Torpedo acetylcholinesterase; therefore the column does not retain the Torpedo enzyme. Differences in Km between acetylcholinesterases of the two species also point to differences in their active sites.

Torpedo marmorata acetylcholinesterase; a comparison with the Electrophorus electricus enzyme. Molecular forms, subunits, electron microscopy, immunological relationship

Electron microscopy, sequential degradation by hydrolytic enzymes and the physical-chemical properties of the molecular forms of Torpedo acetylcholinesterase indicate that these molecules are structurally related to each other in the same way as the molecular forms of Electrophorus acetylcholinesterase: all are derived from a complex structure in which three tetrameric groups of subunits are associated with a rod-like 'tail'. In aged preparations the catalytic subunits are split into fragments in a manner similar to those of Electrophorus acetylcholinesterase. Immunological cross-reaction between both enzymes demonstrates the occurrence of common antigenic sites. The enzymes from the two sources, however, are different in their molecular weights and susceptibility to hydrolytic enzymes. Also, Torpedo acetylcholinesterase does not precipitate with either isologous or heterologous antibodies.

Effect of p-nitrophenyl diazonium fluoroborate on cholinergic mechanisms

Electrophysiological experiments were done to investigate the effect o p-nitrophenyl diazonium fluoroborate (p-NPD) on motor endplates of the frog's m. cutaneus pectoris. The compound has no direct depolarizing effect on the postsynaptic membrane and stabilizes it irreversibly when added to the bath. Longtime iontophoretical applications of p-NPD produce a biphasic effect: initially a potentiation of the depolarizations due to acetylcholine (ACh) (both iontophoretically applied and presynaptically liberated), and subsequently an inhibition of the response to ACh. When the acetylcholinesterase (AChE) is inactivated previously, only the inhibiting effect of the compound is demonstrable. The association constant of p-NPD to purified AChE and to membrane fragments of electroplax was determined by biochemical methods. The compound's affinity to the AChE was found to be about 20 times greater than to the acetylcholine receptor (AChR). Iontophoretical application of p-NPD to cholinergic neurons in the hippocampal cortex of the cat also produced the characteristic biphasic effect on ACh-induced activity of these investigated neurons. The results suggest that the biphasic effect depends on the capacity of p-NPD to combine with both the AChE and the AChR. The AChE is first inhibited with low concentrations thereby potentiating the ACh response. At higher concentrations the AChR's are progressively inhibited too, thereby diminishing the excitability of the postsynaptic membrane up to a complete block.

Multiple molecular forms of purified human erythrocyte acetylcholinesterase

1. Human erythrocyte acetylcholinesterase was solubilized by Triton X-100 and purified by affinity chromatography to a specific activity of 3800 IU/mg of protein. The yield of the purified enzyme was 25--45%. 2. Gel filtration on Sepharose 4-B in the presence of Triton X-100 revealed one peak of enzyme activity with a Stokes' radius of 8.7 nm. Density gradient centrifugation in 0.1% Triton X-100 showed one peak of enzyme activity with an S4 value of 6.3S. 3. Isoelectric focusing in Triton X-100 resolved the enzyme into five molecular forms with isoelectric points of 4.55, 4.68, 4.81, 4.98 and 5.18. Upon incubation with neuraminidase the enzyme activity in the first four forms was decreased with a concommitant increase in activity in the form with the higher isoelectric point. 4. After removal of excess Triton X-100 on Bio-Gel HTP, polyacrylamide gel electrophoresis showed seven bands of protein and corresponding bands of enzyme activity. Density gradient centrifugation of the detergent-depleted enzyme at high ionic strength revealed five multiple molecular forms with S4 values of 6.3 S, 10.2 S, 12.2 S, 14.2 S and 16.3 S. At low ionic strength, higher aggregates were observed in addition to the other forms. Dodecylsulfate-polyacrylamide gel electrophoresis gave one subunit only with an apparent molecular weight of 80 000. 5. These results suggest that human erythrocyte acetylcholinesterase, solubilized by Triton X-100, exists in various forms differing in net charge but of apparently similar molecular dimensions. After removal of the detergent, forms with different molecular sizes are observed.