Molecular forms of purified human erythrocyte membrane acetylcholinesterase investigated by crosslinking with diimidates

The preparation and kinetic analysis of multiple forms of human erythrocyte acetylcholinesterase

A method for preparing various forms of acetylcholinesterase (AChE) from human erythrocyte has been established and they have been characterized in terms of kinetic parameters such as K(m), rate constant (k), turnover number (kcat), specificity constant (ksp), Vmax, half-life (t1/2), IC50 and Ki for tetrabutylammonium hydroxide, procaine and physostigmine in the present study. The solubility experiments show that, there is one major form of AChE i.e. membrane bound AChE (MBAChE) and one minor form i.e. water soluble form (WSAChE). The MBAChE shows several subforms, and on the basis of percentage activity only three MBAChE forms have been selected for complete characterization by various kinetic parameters and found that these three forms of MBAChE demonstrate significant differences in their kinetic properties except IC50. This study supports the recommendation of the use of these kinetic parameters as a tool for the analysis of the multiple forms of the various enzymes in the biological systems.

Molecular form of human lymphocyte membrane-bound acetylcholinesterase

The membrane-bound acetylcholinesterase (AchE) from human peripheral blood lymphocyte gives only one symmetrical peak on sucrose density gradient centrifugation in the presence of Triton X-100 detergent, with the calculated sedimentation coefficient of 6.5 S. However, this dimeric form of AchE was converted to a monomeric 3.8 S form when treated with 2-mercaptoethanol and iodoacetic acid. The results are consistent with studies which have shown by sodium dodecyl sulfate gel electrophoresis that the enzyme is built up of two identical monomers inter-linked by disulfide bond(s). Under reducing conditions, revealed a single species of 70,000 molecular weight, whereas under non-reducing conditions, another species of 140,000 molecular weight of the AchE was found. Polyacrylamide gel electrophoresis indicated a single band with AchE activity in the presence of Triton X-100. In contrast, in the absence of the same detergent multiple band pattern could be observed. These results suggest that membrane-bound AchE enzyme is present in homogenous dimeric form on human lymphocyte membrane.

Characterization of an active hydrophilic erythrocyte membrane acetylcholinesterase obtained by limited proteolysis of the purified enzyme

Purified human erythrocyte membrane acetylcholinesterase was subjected to limited proteolysis with papain. This treatment generated a hydrophilic form of the enzyme as determined by charge-shift crossed immunoelectrophoresis and by binding to phenyl-Sepharose. The hydrophilic enzyme was stable and its activity was independent of the presence of amphiphiles. Electroimmunochemical analysis showed no antigenic difference between the two enzyme forms. Although the proteolytic treatment only brought about a small change in molecular weight, marked differences in the hydrodynamic properties were encountered. The Stokes radius decreased from 8.2 to 5.9 nm and the sedimentation coefficient increased from 6.3 to 7.0 S. The results are consistent with the view that a short hydrophobic peptide responsible for the amphipatic character of acetylcholinesterase is removed by the treatment with papain.

[Electrophoretic study of aged butyrylcholinesterase after inhibition by soman]

The following states of purified tetrameric form (C4) of human plasma butyrylcholinesterase were studied by electrophoretic techniques: native, inhibited by soman and by methane sulfonyl fluoride and soman-aged. In order to detect a significant conformational change of the aged cholinesterase as compared to the non-inhibited (native) species, enzymes were treated with a set of bifunctional reagents (diimidates) of different chain lengths. After denaturation, the cross-link products were subjected to sodium dodecyl-sulfate polyacrylamide gel electrophoresis. The peak areas of the cross-linked species and the parameters of cross-linkability were calculated from densitometric data, versus the maximal effective reagent length. The effect of occupancy of the esteratic site by substituted phosphonyl group and by methyl-sulfonyl residue on the binding activity of the anionic site was studied by affinity electrophoresis at varying temperatures with immobilized-procaïnamide as ligand. Apparent dissociation-constants of the enzyme-ligand complexes were estimated from measurement of mobilities versus ligand concentration. Corresponding thermodynamic quantities were calculated from Van't Hoff plots and basic thermodynamic equations. The reactivity of aged-cholinesterase with diimidates was similar to that of the native enzyme. Affinity for immobilized-procaïnamide was slightly lowered in aged and inhibited enzymes as compared to the native and sulfonylated enzymes. As for the ligand-induced isomerization of anionic site (A----B), revealed by affinity electrophoresis, the ligand concentration at the midpoint of transition (A = 0,5) was slightly greater for the aged enzyme than for the native one. From these results, the following conclusions can be drawn: the dealkylation of soman-cholinesterase conjugate (aging) does not seem to induce structural changes detectable in the cross-linkability of lysyle residues at the subunit interfaces and on the surface of the tetrameric enzyme. On the other hand, the affinity of the anionic site and ligand-induced isomerization process are altered in soman-inhibited and aged enzymes. These data suggest the occurrence of a weak conformational change of the active center and/or the formation of non-covalent bonds between the methylphosphonyl residue and side chain groups as a result of the dealkylation reaction.

A monomeric form of human erythrocyte membrane acetylcholinesterase

Purified detergent solubilized dimeric human erythrocyte acetylcholinesterase (6.3 S form) was converted to a stable monomeric 3.9 S species when treated with 2-mercaptoethanol and iodoacetic acid. More than 60% of the enzymatic activity were recovered after this treatment. A decreased susceptibility to reduction and alkylation was observed with purified, detergent depleted acetylcholinesterase aggregates. When erythrocyte membranes (ghosts) were subjected to the same treatment, acetylcholinesterase could subsequently be solubilized as monomeric 3.9 S form and and more than 90% of the activity were recovered. Monomeric acetylcholinesterase was less reactive towards antibodies raised against (dimeric) human erythrocyte membrane acetylcholinesterase and towards antibodies against human erythrocyte membranes. The results suggest that acetylcholinesterase is present as dimeric species in human erythrocyte membranes despite the fact that fully active monomers can be obtained.

Reconstitution of human erythrocyte membrane acetylcholinesterase in phospholipid vesicles. Analysis of the molecular forms by cross-linking studies

Unilamellar lipid vesicles were formed upon removal of Triton X-100 with Amberlite XAD-2 from a mixture of egg phosphatidylcholine and Triton-solubilized pure human erythrocyte membrane acetylcholinesterase. A majority of large (230 nm diameter) vesicles together with a minor population of smaller (30 nm diameter) strictures were observed in freeze-fracture electron micrographs. Reconstitution experiments performed with [phenyl-3H(n)]-Triton X-100 showed that only one detergent molecule per 600 lipid molecules was present in the vesicles. Density gradient centrifugation showed co-sedimentation of acetylcholinesterase with the lipid vesicles. About 60% of the incorporated enzyme was directed towards the vesicle exterior and could be partially degraded by papain. Mainly dimeric acetylcholinesterase was found when the reconstituted or, alternatively, the lipid-free but Triton-solubilized enzyme were cross-linked with glutaraldehyde. Aggregates were observed when the detergent-depleted oligomeric forms of the enzyme were cross-linked. The results thus indicate that mainly the dimeric enzyme form is present in a phospholipid environment.

Lipid-protein interactions in human erythrocyte-membrane acetylcholinesterase. Modulation of enzyme activity by lipids

Purified human erythrocyte membrane acetylcholinesterase was incorporated into vesicles of various lipid compositions. The activities of the free and the lipid-associated enzyme were assayed at temperatures between 4 degrees C and 40 degrees C and the results were visualized as plots of log v versus 1/T (Arrhenius plots). For the purified, detergent-depleted enzyme a linear relation was obtained. If Triton X-100 was added to the assay medium a curved plot resulted. For acetylcholinesterase incorporated into dimyristoylphosphatidylcholine vesicles a clear break in the plot was observed at the phase transition temperature of the lipid. With lipids not experiencing a phase transition within the temperature range investigated, again a linear relation was obtained. These results show that the activity of human erythrocyte membrane acetylcholinesterase is strongly modulated by its hydrophobic environment.