The Dependence of Acetylcholinesterase Aggregation at Low Ionic Strength upon a Polyanionic Component

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Abstract
The low ionic strength aggregation of Electrophorus asymmetric forms of acetylcholinesterase has been found to depend upon a distinct aggregating agent, present in large excess in the electric organ extracts. The aggregates appeared, in electron micrographs, as bundles of a few molecules, the tails of which were linked in a side‐to‐side association, leaving the head groups at both ends. The proportion of aggregated enzyme, but not the sedimentation properties of the aggregates, depended in a complex manner upon ionic conditions and aggregating agent levels. The aggregates were not dissociated by dilution even at very low concentrations. Properties of the aggregating agent suggested that it might be a non‐proteic polymer, interacting with positively charged groups of the enzyme tail.Various polyanions were found to promote aggregation of acetylcholinesterase, but induced aggregates of different sedimentation coefficients. Polycarboxylates were effective only at high concentrations while sulfate and phosphate polyanions were very efficient. Aggregates induced by these latter polyanions possessed the same morphological features as those produced by the endogenous preparation.Analysis of the Electrophorus aggregating preparation demonstrated that its chondroitin sulfate content fully accounts for its aggregating capacity, and this was in agreement with its sensitivity to specific mucopolysaccharidases.Aggregates were also obtained in Electrophorus/Torpedo mixed systems but their characteristics were found to depend both on the nature of the enzyme and of the aggregating agent. Aggregation had no effect on catalytic efficiency, and we discuss its relevance to the synaptic localization of the enyme, through interactions between the tails of acetylcholinesterase and chondroitin‐sulfatetype glycosaminoglycan components.