The Synthesis, Storage, and Release of Propionylcholine by the Electric Organ of Torpedo marmorata

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Abstract
Little is known about the specificity of the mechanisms involved in the synthesis and release of acetylcholine [ACh] for the acetyl moiety. To test this, blocks of tissue from the electric organ of Torpedo were incubated with either [1-14C]acetate or [1-14C]propionate, and the synthesis, storage and release of [1-4C]ACh and [14C]propionylcholine were compared. To obtain equivalent amounts of the 2 labeled choline esters, a 50-fold higher concentration of propionate than of acetate was needed. Following subcellular fractionation, similar proportions of [1-4C]ACh and [14C]propionylcholine were recovered with synaptosomes and with synaptic vesicles. Both labeled choline esters were protected to a similar extent from degradation during homogenization of tissue in physiological medium, indicating that the 2 choline esters were equally well incorporated into synaptic vesicles. Yet depolarization of tissue blocks by 50 mM KCl released much less [14C]propionylcholine than [14C]ACh. During field stimulation of the tissue blocks, the difference between the releasibility of the 2 choline esters was less marked, but ACh was still released in preference to propionylcholine. Evidence for specificity of the release mechanism was also obtained when the release of the 2 choline esters in response to field stimulation was compared in tissue blocks preincubated with both [3H]choline and [14C]propionate.