Interaction of Choline Transport with Acetylcholine Release and Synthesis
It has been reported from several laboratories (2,9,10) that choline (Ch) uptake in the synaptic region is stimulated by acetylcholine (ACh) release. This would seem to be a possible control mechanism for ACh synthesis because Ch uptake and extra mitochondrial AcCoA concentrations are known to be rate limiting. The mechanism of this control has been supposed (10) to lie in a close association of the Ch transport process with ACh synthesis. This association had been previously invoked and defined by Barker and Mittag (1) to explain their results on the transport and acetylation of various analogues of Ch. A simple explanation of the equivalence of half saturating concentration for transport and acetylation that they observed would be that the transport process is rate controlling but they proposed that “transport and acetylation are coupled,” i.e. Ch transported into the synaptosome has no “free” existence and is immediately acetylated by choline acetyltransferase (ChAT). The eoupling hypothesis of Barker and Mittag (1) might seem a useful basis for a control mechanism but it flies in the face of evidence (3,7) that at the high ionic strengths prevalent inside the cell ChAT is a soluble enzyme not associated with any particulate or membranous material.
KeywordsAcetylcholine Release Coupling Ratio Choline Transport Choline Acetyl Half Saturate Concentration
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