Evidence for extramitochondrial pyruvate dehydrogenase involved in acetylcholine synthesis in nerve endings

Abstract

THE ultimate step of acetylcholine (ACh) synthesis in cholinergic nerve terminals is the transfer of an acetyl group from one molecule of acetyl coenzyme A (acetyl CoA) to one molecule of choline. This reaction is catalysed by choline acetyltransferase (CAT; EC 2.3.1.6), a cytoplasmic enzyme¹. As revealed by isotopic studies, glucose as well as pyruvate can be used as precursor of the acetyl moiety of ACh in mammalian brain tissue^2–5. From the equal efficiency of (1-¹⁴C) and (6-¹⁴C)glucose in labelling ACh, it was concluded that glucose is transformed into pyruvate through the Embden–Meyerhof pathway. As (2-¹⁴C) and (3-¹⁴C)pyruvate but not (1-¹⁴C)pyruvate give rise to (¹⁴C)ACh, pyruvate must be decarboxylated by a pyruvate dehydrogenase complex (PDHc; EC 1.2.4.1) to generate acetyl CoA, one of the two substrates of CAT. It is usually assumed that PDHc is exclusively located in mitochondria, and so several attempts have been made to understand how the cytoplasmic CAT is supplied in acetyl CoA. In spite of the great diversity of approaches used, none of the various mechanisms contributing normally to the transfer of acetyl groups through the inner mitochondrial membrane⁶ seem to be involved in the synthesis of ACh in brain cholinergic nerve endings^3–5,7–10. An alternative hypothesis was thus proposed which assumed the existence of two populations of PDHc within cholinergic nerve endings. Apart from the ubiquitous intramitochondrial PDHc providing the Krebs cycle with activated acetyl groups, an extramitochondrially located PDHc could exist which would be specifically involved in providing acetyl CoA to the cytoplasmic enzyme, CAT. Acetyl CoA used for the neurotransmitter synthesis would thus be generated in the same subcellular compartment in which ACh is formed¹¹. Experiments are described here which confirm the validity of this hypothesis.