The synthesis and distribution of acetylcholine has been studied in the isolated synaptosome. Binding or contamination of [14C]acetylcholine in the synaptic vesicle fraction represents 1.5 percent of the total amount found in the synaptoplasm (synaptosomal cytoplasm). However, when the [ 14C]acetylcholine is derived solely by synthesis from labeled choline, then the labeled acetylcholine in the synaptic vesicle is 15 percent of the amount of acetylcholine synthesized in the synaptoplasm. The results suggest that the [14C]acetylcholine found in the vesicle fraction of synaptosomes incubated with labeled choline is due to vesicular synthesis.
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