The independency of choline transport and acetylcholine synthesis

The coupling of choline transport to acetylcholine synthesis has been investigated by measurement of the isotopic dilution of a pulse of [3H]choline during its incorporation into the recently synthesised acetylcholine of cerebral cortex synaptosomes. Recently synthesised acetylcholine was identified as that containing 14C-labelled precursors introduced by a preincubation before the pulse. When [14C]glucose was used to label acetyl-CoA coupling ratios (calculated as the inverse of the dilution of extracellular [3H]choline during its incorporation into [3H]acetylcholine) of about 0.05-0.2 were found at a choline concentration of 1 microM, rising to 0.5 at choline concentrations of 10-50 microM. Experiments using [14C]choline as a precursor gave similar results, and it was shown that the isotopic dilution did not occur extrasynaptosomally and was not affected by low glucose concentrations. Coupling ratios were always less than unity and rose as the choline concentration increased. It is concluded that choline transported into the nerve terminal has no privileged access to choline acetyltransferase. The results can be explained by a rate-controlling transport of choline into the terminal followed by its rapid acetylation rather than any linkage or coupling of the two processes.