Acetylcholinesterase in mouse neuroblastoma NB2A cells: analysis of production, secretion, and molecular forms

The mouse neuroblastoma cell line NB2A produces cellular and secreted acetylcholinesterase (AChE). After incubation of the cells for 4 days the ratio between AChE secreted into the medium and AChE in the cells was 1:1. The cell-associated enzyme could be subdivided into soluble AChE (25%) and detergent-soluble AChE (75%). Both extracts contained predominantly monomeric AChE (4.6S) and minor amounts of tetrameric AChE (10.6S), whereas the secreted AChE in the culture supernatant contained only the tetrameric form. All forms were partially purified by affinity chromatography. It could be demonstrated that the secretory and the intracellular soluble tetramers were hydrophilic, whereas the detergent-soluble tetramer was an amphiphilic protein. On the other hand the soluble and the detergent-soluble monomeric forms were amphiphilic and their activity depended on the presence of detergent. By digestion with proteinase K amphiphilic monomeric and tetrameric AChE could be converted to a hydrophilic form that no longer required detergent for catalytic activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of [3H]diisopropylfluorophosphate-labelled AChE gave one band at 64 kilodaltons (kD) under reducing conditions and two additional bands at 120 kD and 140 kD under nonreducing conditions.

Modes of attachment of acetylcholinesterase to the surface membrane

Acetylcholinesterase (AChE) occurs in multiple molecular forms differing in their quaternary structure and mode of anchoring to the surface membrane. Attachment is achieved by post-translational modification of the catalytic subunits. Two such mechanisms are described. One involves attachment to catalytic subunit tetramers, via disulfide bridges, of a collagen-like fibrous tail. This, in turn, interacts, primarily via ionic forces, with a heparin-like proteoglycan in the extracellular matrix. A second such modification involve the covalent attachment of a single phosphatidylinositol molecule at the carboxyl-terminus of each catalytic subunit polypeptide; the diacylglycerol moiety of the phospholipid serves to anchor the modified enzyme hydrophobically to the lipid bilayer of the plasma membrane. The detailed molecular structure of these two classes of acetylcholinesterase are discussed, as well as their biosynthesis and mode of anchoring.

Immunochemical differences among molecular forms of acetylcholinesterase in brain and blood

Molecular forms of acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) differ in their solubility properties as well as in the number of their catalytic subunits. We used monoclonal antibodies to investigate the structure of acetylcholinesterase forms in brain, erythrocytes and serum of rats, rabbits and other mammals. Two antibodies were found to bind tetrameric acetylcholinesterase in preference to the monomeric enzyme. These antibodies also displayed lower affinity for certain forms of 'soluble' brain acetylcholinesterase than for the 'membrane-associated' counterparts. Furthermore, one of them was virtually lacking in affinity for the membrane-associated enzyme of erythrocytes. The basis for the antibody specificity was not fully determined. However, the immunochemical results were supported by measurements of enzyme thermolability, which showed that the catalytic activity of 'soluble' acetylcholinesterase was comparatively heat-resistant. These observations point toward structural differences among the solubility classes of acetylcholinesterase.

Molecular forms of acetylcholinesterase from human caudate nucleus: comparison of salt-soluble and detergent-soluble tetrameric enzyme species

Extraction of human caudate nucleus under high-ionic-strength conditions solubilized 20-30% of total acetylcholinesterase (AChE) activity. Density gradient centrifugation revealed monomeric (5.0 S) and tetrameric (11.0 S) enzyme species. The purified, tetrameric salt-soluble (SS) AChE sedimented at 10.6 S and did not bind detergents. It showed an immunochemical reaction of identity with the detergent-soluble (DS) AChE species from human caudate nucleus and human erythrocytes, but did not cross-react with antibodies raised against human serum cholinesterase. The remaining activity was solubilized under low-ionic-strength conditions in the presence of 1.0% Triton X-100. The purified tetrameric, DS-AChE sedimented at 10.0 S as detergent-protein mixed micelle and on extensive removal of the detergent this enzyme formed defined aggregates by self-micellarization. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions revealed that the salt-soluble and detergent-soluble tetrameric enzyme species both contained a heavy and a light dimer; under reducing conditions mainly one band corresponding to the light subunit was seen. Molecular weights of 300,000 dalton and 280,000 dalton were calculated for SS-AChE and DS-AChE, respectively. Limited digestion of DS-AChE with proteinase K led to isolation of an enzyme that no longer bound detergents and lacked the intersubunit disulfide bridges.

Polymorphism and immunochemical cross-reactivity of acetylcholinesterases from the brains of human, dog, hog, bovine and horse

In the caudate nucleus of the species tested about 20% of the acetylcholinesterase was salt soluble and sedimented in sucrose density gradient centrifugation as monomeric 5 S and tetrameric 10 S enzyme. About 80% was solubilized by micellar concentrations of Triton X-100 and sedimented as a tetrameric 10 S species in the presence of detergent but formed aggregates in the absence thereof. All the enzyme displayed poor cross-reactivity with a precipitating assay (Ouchterlony) but in a solid phase non-precipitating assay the cross-reactivity could be quantified and ranged from 96 to less than 1% depending on the species.

Bovine nucleus caudatus acetylcholinesterase: active site determination and investigation of a dimeric form obtained by selective proteolysis

The number of catalytic subunits of purified bovine nucleus caudatus acetylcholinesterase (E.C. 3.1.1.7) has been determined by active site labelling with [3H]diisopropyl fluorophosphate ([3H]DFP). The 10.5 S, 16 S, and 20 S forms were estimated to contain two, four, and six active sites, respectively, per molecule. A 4.8 S form, which showed a weak amphiphile-dependent activity behavior, was obtained by selective proteolytic digestion with pronase. The inability of the purified 4.8 S form to aggregate after detergent removal, and the molecular mass in the range of 130-165 kD under nondenaturating conditions, indicate that this form is a dimeric form, lacking those hydrophobic regions responsible for aggregation.

Evidence for and separation of globular hydrophobic and non-hydrophobic forms of acetylcholinesterase from bovine caudate nucleus

Almost complete removal of the detergent from a purified 10.7S complex consisting of acetylcholinesterase and [3H]Triton X-100 bound to an affinity gel was achieved by extensive washing and subsequent elution of the enzyme. Applying this procedure of detergent removal, the majority of the enzyme could be stabilized by self-aggregation forming soluble 16S and 20S aggregates, which contained small amounts of bound residual Triton X-100 in the range of 0.4-1.6 mol Triton X-100 per mol acetylcholinesterase. Besides these aggregates, a nearly detergent-free 10.5S form was observed, lacking the hydrophobic region responsible for detergent binding and self-aggregation.

An amphiphile-dependent form of human brain caudate nucleus acetylcholinesterase: purification and properties

Different forms of acetylcholinesterase (AChE), EC 3.1.1.7, were demonstrated in human brain caudate nucleus. One form was solubilized at high ionic strength, the other with Triton X-100. The detergent-extractable form was purified to homogeneity by affinity chromatography. This form of AChE is amphiphile-dependent; i.e., it was active only in the presence of amphiphiles (detergents or lipids). Further, the enzyme was shown to bind detergents and to interact hydrophobically with Phenyl-Sepharose. In the presence of detergents the enzyme is a tetramer (subunit molecular weight, 78,000) which aggregates on the removal of detergents. Human brain AChE showed a reaction of identity with human erythrocyte AChE in crossed-line immunoelectrophoresis. The high-salt-soluble brain enzyme did not cross-react with the erythrocyte enzyme. The two classes of AChE seem not to be related, as they show no common antigenic determinant.

Heterogeneity of rat brain acetylcholinesterase: a study by gel filtration and gradient centrifugation

According to their solubilization properties, two classes of acetylcholinesterases (AChE) can be detected in the adult rat brain: a "soluble" species (easily solubilized without detergent), and a membrane-bound species (solubilized only in the presence of detergent). The latter was found to be homogeneous by gel filtration (Stokes radius 8.05 +/- 0.35 nm) and sucrose gradient centrifugation (9.75 +/- 0.2 S) in the presence of Triton X-100. The "soluble" AChE gives three stable species in the presence of the same detergent with Stokes radii and sedimentation constants of 10.9 +/- 0.5 nm and 16 +/- 2S; 6.75 +/- 0.30 nm, and 10.7 +/- 0.4 S; 5.37 +/- 0.35 nm and 4.37 +/- 0.1 S. Co-chromatography and co-sedimentation or the reduction and alkylation of disulfide bridges show that all the soluble species are different from the membrane-bound AChE. The possibility that soluble and membrane-bound AChE are completely different molecules is discussed.

Purification and properties of the membrane-bound acetylcholinesterase from adult rat brain

The membrane-bound acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) from adult rat brain has been purified to homogeneity using sequential affinity chromatography on Con A-Sepharose and on dimethyl-aminoethylbenzoic acid-Sepharose 4B followed by DEAE-cellulose chromatography. The yield of the purified enzyme (specific activity: 3068 U/mg protein) is higher than 50%. Polyacrylamide gel electrophoresis in the presence of Triton X-100 gives only one band with acetylcholinesterase activity. With the exception of electrofocusing and pore gradient electrophoresis, where a multiple band pattern was detected (which seems to be artefactual), the enzyme appears to be homogeneous. Gel filtration and sucrose density gradient centrifugation in the presence of Triton X-100 give only one symmetrical peak, with a calculated molecular weight of 328 000. Since polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) and mercaptoethanol gives only one band with a molecular weight of 74 500, a tetrameric structure can be postulated for the membrane-bound acetylcholinesterase from rat brain.

Concerning the anchoring of acetylcholinesterase in biomembranes

Highly purified acetylcholinesterases from various sources do not contain hydroxyproline and hydroxylysine, indicating the absence of a collagenlike 'tail'. The enzymes examined are therefore bound to the membrane in a different way compared with acetylcholinesterase isolated from electric organ.