The synthesis, storage, and release of propionylcholine by the electric organ of Torpedo marmorata

Is propionylcholine present in or synthesized by electric organ?

When small blocks comprising four columns of electrocytes were excised from electric organs of Torpedo marmorata after stimulation in vivo via the electric lobe at 1 Hz for 1 h and allowed to recover at 20-22 degrees C for several hours in medium containing 100 microM d4 choline and 500 microM propionate, small quantities of propionylcholine amounting to no more than 1% of the endogenous acetylcholine of the tissue could be detected in tissue extracts by gas chromatography-mass spectrometry (GCMS). Kinetic studies demonstrated that there was no nonexchangeable propionylcholine in the tissue and in the absence of added propionate, propionylcholine levels were less than 0.2% of tissue acetylcholine. Vesicular propionylcholine amounted to less than 0.5% of vesicular acetylcholine and the distribution of d0 and d4 propionylcholine suggested that an appreciable proportion (up to one-third) of this could be an artifact of preparation for GCMS determinations. Propionylcholine formation during extraction and demethylation of an artificial mixture of acetylcholine, choline, and propionate was indeed detected. It is concluded that propionylcholine has no significance as an endogenous or as a false transmitter at this terminal, in conformity with the work of Sheridan et al. [Z. Zellforsch. 74, 281-307 (1966)] but in contrast to the report of O'Regan [J. Neurochem. 39, 764-772 (1982)].

Reconstitution of carrier-mediated choline transport in proteoliposomes prepared from presynaptic membranes of Torpedo electric organ, and its internal and external ionic requirements

Proteoliposomes made by a butanol-sonication technique from electric organ presynaptic membranes showed choline transport activity. In contrast to intact nerve terminals, the uptake of choline was dissociated from its conversion to acetylcholine in this preparation. The kinetics of choline uptake by proteoliposomes was best described by two Michaelis-Menten components. At a low concentration of choline, uptake was inhibited by hemicholinium-3 and required external Na+ and, thus, closely resembled high-affinity choline uptake by intact cholinergic nerve terminals. Choline transport could be driven by the Na+ gradient and by the transmembrane potential (inside negative) but did not directly require ATP. External Cl-, but not a Cl- gradient, was needed for choline transport activity. It is suggested that internal K+ plays a role in the retention of choline inside the proteoliposome. Proteoliposomes should prove a useful tool for both biochemical and functional studies of the high-affinity choline carrier.

Uptake of acetate and propionate by isolated nerve endings from the electric organ of Torpedo marmorata and their incorporation into choline esters

The uptake and incorporation into choline esters of acetate and propionate by electric organ synaptosomes were compared, with the aim of better understanding the basis for the selectivity of choline ester synthesis shown by this tissue for acetate. It was found that propionate uptake, like acetate uptake, was a temperature-dependent, saturable process. Both uptake mechanisms had similar affinities for their substrates, but the maximal velocity of propionate uptake was considerably lower than that of acetate uptake; and less of the accumulated propionate was used for choline ester synthesis than of the accumulated acetate. While acetate was a good inhibitor of propionate uptake, propionate was a very poor inhibitor of acetate uptake. This finding, in addition to the observation that the two uptakes were not affected in the same way by changes in pH, led to the suggestion that acetate uptake and propionate uptake reflect different processes. In both cases, however, the pH dependence of uptake indicated that these substrates cross the membrane as the charged species. Acetate uptake and acetylcholine synthesis remained closely associated under various experimental conditions, while propionate uptake could be dissociated from the synthesis of propionylcholine. Hence, it appears that acetate is taken up by a specific, high-velocity mechanism linked to acetylcholine synthesis, whereas propionate uptake may represent a less specific mechanism.