Choline transport is not coupled to acetylcholine synthesis

Abstract

IT has been proposed^1–3 that choline transport, particularly that performed by the so-called high-affinity system, is obligatorily coupled to acetylcholine synthesis and therefore only choline that has just entered by the transport mechanism is acetylated. If this were so it would imply that the enzyme choline acetyltransferase is bound close to the transport sites on the membrane. The problem is important because of its implications for the molecular organisation of the synapse and also because subcellular fractionation studies^4,5 show fairly clearly that, at the ionic strengths prevailing inside the cell, choline acetyltransferase is soluble and not bound to the plasma or synaptic vesicle membrane (as would be expected if the hypothesis of obligatory coupling is true). The evidence for this is indirect and relies on the finding that the half-maximal concentration for acetylation of choline and its analogues in situ approximates more nearly to the half-maximal concentration of their transport than of their acetylation by the isolated enzyme¹. However, this need only imply a coupling between transport and synthesis if the latter is rate controlling, and from a survey⁶ of the rates of release and synthesis of acetylcholine (ACh) it seems that the activity of choline acetyltransferase is in considerable excess of normal requirements. Certain exceptions to the obligatory coupling of transport and synthesis have been noted but these involve synthetic analogues of choline⁷ or unusual preparations^8,9. Reduction of the acetyl-CoA levels reduces ACh concentrations without a concomitant reduction in choline transport¹⁰ but neither this nor the previous studies exclude the possibility that the residual ACh synthesis remains coupled; that is, obligatorily dependent on choline that has just been transported across the membrane. We describe here an experiment which we believe demonstrates that choline that has been recently transported has no privileged access to choline acetyltransferase in the synaptic terminal. Instead it mixes with the pre-existing pool of choline and therefore there is no coupling between transport and synthesis.