Citrate as the Precursor of the Acetyl Moiety of Acetylcholine

Abstract

Abstract— Rat brain cortex slices were incubated with glucose labeled with either 3H or 14C in the 6‐position. The 3H/14C ratios and the incorporation of radioactivity into lactate, citrate, malate and acetylcholine were determined. While the 3H/14C ratio of lactate was close to that of glucose, the ratios in the acetyl moiety of acetylcholine and the acetyl (C‐4,5) portion of citrate decreased in a similar proportion. This was interpreted as indirect evidence for the participation of citrate as a precursor to the acetyl moiety of acetylcholine. Two inhibitors of the citrate cleavage pathway: n‐butylmalonate, an inhibitor of citrate transport and (‐)‐hydroxycitrate, an inhibitor of ATP‐citrate lyase were studied for their effect on acetylcholine synthesis. N‐butylmalonate (10 mM) and (‐)‐hydroxycitrate (7.5 mM) led to a decrease in the per cent of 14C recovered as acetylcholine. In each instance the 3H/14C ratio in acetylcholine was higher in the presence of inhibitor while the corresponding ratios in lactate and citrate (C‐4.5) remained unchanged. From the results, it is suggested that citrate is involved in the transport mechanism of acetyl units from its site of synthesis in mitochondria to the site of acetylcholine synthesis in the cytosol.

Effect of Chronic Lead Ingestion by Rats on Glucose Metabolism and Acetylcholine Synthesis in Cerebral Cortex Slices

“…The effect of low dose chronic Pb2+ exposure on general metabolism and subsequent ACh synthesis in cerebral cortex slices was examined by using mixed-labeled substrates (See Sterling and O'Neill, 1978;Sterling et al, 1981 for details). The technique is based on the fact that one of the three hydrogens of the methyl group of acetyl-CoA is removed in its condensation with oxaloacetate in the formation of citrate, thereby lowering the "H/14C ratio in citrate to 0.67 (computed to a glucose ratio of I .OO).…”

“…1936;Browning and Shulman, 1968;Tutek and Cheng, 1974). cytosol and cleaved to AcCoA by ATP-citrate lyase (Sterling and O'Neill, 1978;Sterling et al, 1981). Several mechanisms for the transport of acetyl units across the mitochondrial membrane to the cytosol have been proposed (Jope, 1979;Doleial and Tutek, 1981;Tutek et al, 1981).…”

“…Immediately following incubation, samples were treated with perchloric acid, homogenized, and centrifuged as previously described (see Sterling and O'Neill, 1978 for details). The rats were given food (Michaelson's traceelement-free purified chow) and drinking water containing PbAc, 200 ppm (1 mM) or 600 ppm (3 mM) ad libitum for 20 days.…”

Carnitine: transport and physiological functions in the brain

“…AcetylCoA is synthesized in peroxisomes and mitochondria, the intramitochondrial acetylCoA can pass the inner mitochondrial membrane in the form of citrate, delivering acetylCoA to cytosol, due to the action of citrate lyase. It was, however, demonstrated that citrate pathway supplied not more than 30% of acetyl moieties for acetylcholine synthesis in mammalian brain (Gibson and Shimada, 1980;Sterling and O'Neill, 1978;Tu cek et al, 1981). The works of Tu cek and his collaborators demonstrated that acetylcarnitine can be a source of acetyl groups for acetylcholine synthesis Tu cek, 1985Tu cek, , 1990, moreover being a better precursor than citrate Cheng, 1970, 1974;Tu cek et al, 1981).…”

Expression of Pyruvate Carboxylase in Cultured Oligodendroglial, Microglial and Ependymal Cells

“…Hence, PC is considered to have an impact on a multitude of processes, which are directly or indirectly dependent on the availability of oxaloacetate and other members of the TCA cycle, i.e., gluconeogenesis [4] and glycogen synthesis [5,6], lipogenesis [7,8], biosynthesis of non-essential amino acids with impact on the cellular/tissue nitrogen balance [9,10], the metabolism and the mitochondriacytosol shuttling of acetyl-CoA [4], and the regeneration of NADPH [11]. Due to its anaplerotic role, PC in brain may contribute to maintaining a cytosolic level of acetyl-CoA that is required for the synthesis of the neurotransmitters acetylcholine [12], glutamate, and GABA [13,14].…”