The Contribution of Citrate to the Synthesis of Acetyl Units in Synaptosomes of Developing Rat Brain

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Abstract
The activities of pyruvate dehydrogenase, citrate synthase, and choline acetyltransferase in rat brain synaptosomes increased during on‐togenesis by 3 and 14 times, respectively. Activity of ATP‐citrate lyase decreased by 26% during the same period. Pyruvate consumption by synapto‐somes from 1‐day‐old animals was 40% lower than that found in older rats; however, citrate efflux from intrasynaptosomal mitochondria in immature synaptosomes was over twice as high as that in mature ones. The rates of production of synaptoplasmic acetyl‐CoA, ATP‐citrate lyase were 1.03, 1.40, and 0.49 nmol/min/mg protein in 1‐, 10‐day‐old, and adult rats, respectively. 3‐Bromopyruvate (0.5 mM) inhibited pyruvate consumption by 70% and caused a complete block of citrate utilization by citrate lyase in every age group. Parameters of citrate metabolism in cerebellar synaptosomes were the same as those in cerebral ones. These data indicate that production of acetyl‐CoA. from citrate in synaptoplasm may be regulated either by adaptative, age‐dependent changes in permeability and carrier capacity of the mitochondrial membrane or by the inhibition of synthesis of intramitochondrial acetyl‐CoA. ATP‐citrate lyase activity is not a rate‐limiting factor in this process. Metabolic fluxes of pyruvate to cytoplasmic citrate and acetyl‐CoA. are presumably the same in both cholinergic and noncholinergic nerve endings. The significance of citrate release from intrasynaptosomal mitochondria as a regulatory step in acetylcholine synthesis is discussed.