Homocholine and acetylhomocholine: false transmitters in the cholinergic electromotor system of Torpedo

Homoarsenocholine - A novel arsenic compound detected for the first time in nature

The arsenic speciation was determined in macrofungi of the Ramaria genus with HPLC coupled to inductively coupled plasma mass spectrometry. Besides arsenic species that are already known for macrofungi, like arsenobetaine or arsenocholine, two compounds that were only known from marine samples so far (trimethylarsoniopropanate and dimethylarsinoylacetate) were found for the first time in a terrestrial sample. An unknown arsenical was isolated and identified as homoarsenocholine. This could be a key intermediate for further elucidation of the biotransformation mechanisms of arsenic.

Some currently neglected aspects of cholinergic function

At the request of the organizers of the XIII International Symposium on Cholinergic Mechanisms, I have selected for review three topics not well represented at the meeting: the synthesis, storage, and release of acetylcholine at cholinergic nerve endings; cholinergic-specific antigens; and neuropeptides as cholinergic co-transmitters. The first topic is illustrated by work with two model systems, the electromotor nerve terminals of the electric ray and the myenteric plexus of the guinea-pig ileum, the second by work with Chol-1, a group of cholinergic-specific gangliosides sialylated on their N-acetylgalactosamine residues, and the third by vasoactive intestinal polypeptide which is a co-transmitter at 70% of cholinergic nerve endings in brain and is also present in both of the model cholinergic systems.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Arcachon and cholinergic transmission

The cholinergic nature of transmission at the electromotor synapse of Torpedo marmorata was established at Arcachon in 1939 by Feldberg, Fessard and Nachmansohn (J. Physiol. (Lond.) 97 (1939/1940) 3P-4P) soon after transmission at the neuromuscular junction had been shown to be cholinergic. In 1964, after a quarter of a century of neglect, workers in Cambridge, then in Paris, Göttingen and elsewhere, began to use this system, 500-1000 times richer in cholinergic synapses than muscle, for intensive studies of cholinergic transmission at the cellular and molecular level.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine and ATP are coreleased from the electromotor nerve terminals of Narcine brasiliensis by an exocytotic mechanism

Although the exocytotic mechanism for quantal acetylcholine (ACh) release has been widely accepted for many years, it has repeatedly been challenged by reports that ACh released upon stimulation originates from the cytosol rather than synaptic vesicles. In this report, two independent experimental approaches were taken to establish the source of ACh released from the electromotor system of Narcine brasiliensis. Since ATP is colocalized with ACh in the cholinergic vesicle, the exocytotic theory predicts the corelease of these two components with a stoichiometry identical to that of the vesicle contents. The stimulated release of ATP from isolated synaptosomes could be accurately quantitated in the presence of the ATPase inhibitor adenosine 5'-[alpha, beta-methylene]triphosphate (500 microM), which prevented degradation of the released ATP. Various concentrations of elevated extracellular potassium (25-75 mM), veratridine (100 microM), and the calcium ionophore ionomycin (5 microM) all induced the corelease of ACh and ATP in a constant molar ratio of 5-6:1 (ACh/ATP), a stoichiometry consistent with that established for the vesicle content. In parallel to these stoichiometry studies, the compound 2-(4-phenylpiperidino)cyclohexanol (AH5183) was used to inhibit specifically the vesicular accumulation of newly synthesized (radiolabeled) ACh without affecting cytosolic levels of newly synthesized ACh in cholinergic nerve terminals. Treatment with AH5183 (10 microM) was shown to inhibit the release of newly synthesized ACh without markedly affecting total ACh release; thus, the entry of newly synthesized ACh into the synaptic vesicle is essential for its release. We conclude that ACh released upon stimulation originates exclusively from the vesicular pool and is coreleased stoichiometrically with other soluble vesicle contents.

Selective depletion of the acetylcholine and vasoactive intestinal polypeptide of the guinea-pig myenteric plexus by differential mobilization of distinct transmitter pools

The effect of electrical field stimulation on the release of acetylcholine (ACh) and vasoactive intestinal polypeptide (VIP) from superfused strips of myenteric plexus-longitudinal muscle (MPLM) of guinea-pig ileum and on the transmitter content of the tissue was investigated at different frequencies and in the presence and absence of choline hemicholinium-3 and colchicine. Low frequency electrical field stimulation released ACh by more than 4 times the basal release; the simultaneously detected VIP secretion was increased only slightly above the resting level. During high frequency stimulation (50 Hz) the release of VIP was greatly increased (to 5 times the resting release) whereas the release of ACh increased to only 150% of the basal output. When choline was present, the ACh content of the tissue itself was not altered by electrical stimulation indicating a rate of synthesis sufficient to maintain release. It was reduced in a frequency-dependent manner in the absence of exogenous choline or in the presence of 10 microM hemicholinium-3 (an inhibitor of choline uptake) by up to 54% of the original content. A similar but even larger reduction took place in the amount of ACh released. Neither the secretion of VIP nor the tissue VIP content was altered by these treatments. Long-lasting (greater than 60 min) high-frequency (50 Hz) stimulation resulted in the depletion of the VIP pool (by 25%) while the ACh content remained unaltered.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential release of [3H]acetylcholine from the rat phrenic nerve-hemidiaphragm preparation by electrical nerve stimulation and by high potassium

Neuronal transmitter stores of the phrenic nerve were labelled under different conditions. Subsequently, transmitter release evoked by electrical nerve stimulation and by a high potassium-low sodium solution was studied. Incubation of the end-plate preparation with [3H]choline at rest led to the synthesis of [3H]acetylcholine which could not be released by electrical nerve stimulation but it was released by high potassium-low sodium solution, independent of the presence of extracellular calcium. When the end-plate preparation was labelled during stimulation at 1 Hz, prolonged periods of electrical nerve stimulation released 83% of the total releasable [3H]transmitter pool in a completely calcium-dependent manner. After exhaustion of the electrically releasable pool, high potassium-low sodium solution still caused a significant outflow. Without a preceding exhaustion of the [3H]acetylcholine pool, high potassium-low sodium solution released a similar amount in the absence of extracellular calcium or after pretreatment with the intracellular calcium chelating substance, Quin-2. When evoked transmitter release was studied at different temperatures (36, 26 and 16 degrees C) Q 10 values of 1.6 and 1.0 were found for the release caused by electrical nerve stimulation and high potassium-low sodium solution (calcium-independent effect), respectively. After labelling during a short interval (2 min) but at a high stimulation rate (50 Hz), only 72% of the releasable [3H]transmitter could be released by electrical nerve stimulation, whereas the outflow due to the calcium-independent effect of high potassium-low sodium solution increased from 17 (labelling during stimulation at 1 Hz) to 28%. It is suggested that the calcium-independent effect of high potassium-low sodium solution reflects the release of acetylcholine from the cytoplasmic compartment, as this outflow occurred after labelling at rest and increased when cytoplasmic synthesis was enhanced by a high loading stimulation. In contrast to high potassium-low sodium solution, propagated nerve activity cannot release acetylcholine synthesized at rest (presumed to be cytoplasmic), but only [3H]acetylcholine synthesized during quantal release (presumed to be vesicular). The absolute requirement of extracellular calcium for electrically stimulated release suggests an exocytotic release mechanism. The low Q 10 value of 1.6 does not fit into the concept of a carrier- or channel-operated release mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

A kinetic study of stimulus-induced vesicle recycling in electromotor nerve terminals using labile and stable vesicle markers

The kinetics of recovery, by recycling electromotor synaptic vesicles, of the biophysical parameters of the reserve population has been studied in perfused blocks of electric organ of Torpedo marmorata prestimulated in vivo, followed by density gradient separation of the extracted vesicles in a zonal rotor using labile (acetylcholine and ATP) and stable (proteoglycan) vesicle markers. Stimulation in vivo at 0.15 Hz for 3.3 h depleted tissue acetylcholine much less than stimulation at 1 Hz for 1 h but nevertheless generated a much larger pool of recycled vesicles that recovered more slowly. At the lower rate of stimulation, recovery of the biophysical characteristics of the reserve population by the recycled vesicles, identified by their content of newly synthesized transmitter, was essentially complete by 8 h. The stable proteoglycan marker was immunochemically assayed and was bimodally distributed in the vesicle-containing portion of the density gradient even in experiments with unstimulated or recovered tissue. The second peak corresponded with that of newly synthesized transmitter and was thus identified as containing the recycled vesicles. Its normalized acetylcholine/proteoglycan ratio was lower than that of the first peak, which is consistent with earlier findings that recycled vesicles, before recovery, are only partially loaded with transmitter. However, as expected, the proportion of total vesicular proteoglycan and acetylcholine associated with the recycled vesicle fraction was very much lower in preparations derived from unstimulated or recovered tissue than in those from recently stimulated tissue.

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)].

Accumulation, acetylation, and releasability of diethylhomocholine from a sympathetic ganglion

Superior cervical ganglia of the cat perfused with [14C]diethylhomocholine [( 14C]DEHCh) synthesized acetyldiethylhomocholine (ADEHCh), but rather little of this ester was released by subsequent preganglionic nerve stimulation. Stimulation evoked the release of an appreciable amount of unchanged DEHCh when ganglia had been exposed to the analogue in the absence of choline (Ch), but did not do so when exposed to both Ch and DEHCh. The release of DEHCh was Ca2+ dependent, and was not the result of the release and subsequent hydrolysis of ADEHCh. This is the first clear demonstration of the release of an unacetylated compound from mammalian tissue; therefore, the characteristics of the transmitter release mechanism are further defined. The effect of preganglionic nerve stimulation on the uptake and acetylation of DEHCh was also measured. Stimulated ganglia accumulated approximately 4 times more labeled analogue and synthesized 7.5 times more ADEHCh than did rested ganglia. Stimulated ganglia perfused with 2-(4-phenylpiperidino)cyclohexanol, a compound considered to inhibit acetylcholine (ACh) release by inhibiting its transport into synaptic vesicles, accumulated 3.4 times as much and acetylated 6 times as much DEHCh as did rested ganglia. When the concentration of Mg2+ in the perfusion medium was increased to block ACh release, accumulation of the labelled analogue was enhanced by stimulation, but its acetylation was increased much less than during perfusion with normal medium. It is concluded that the synthesis of ADEHCh is subject to the same regulation as is ACh synthesis and that the activation of ester synthesis during activity can be dissociated from ester release.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo acetylation of homocholine and beta-methylcholine in rat brain

A nitrogen phosphorus-gas chromatographic procedure was modified to determine the extent of in vivo acetylation of the choline analogs homocholine and beta-methylcholine. Infusion of homocholine (18 mumoles) for 2 hours into the lateral ventricle of the rat produced 2.3 nmoles/gram of acetylhomocholine which represented 0.035% of the detected homocholine. Infusion of the same quantity of beta-methylcholine produced 1.0 nmole/gram of acetyl-beta-methylcholine representing 0.025% of the detected beta-methylcholine. Although pretreatment with hemicholinium-3 reduced the amount of acetylated product formed from either analog, the reduction was significant only for acetyl-beta-methylcholine (p less than 0.01).

Separation of recycling and reserve synaptic vesicles from cholinergic nerve terminals of the myenteric plexus of guinea pig ileum

Acetylcholine-rich synaptic vesicles were isolated from myenteric plexus-longitudinal muscle strips derived from the guinea pig ileum by the method of Dowe, Kilbinger, and Whittaker [J. Neurochem. 35, 993-1003 (1980)] using either unstimulated preparations or preparations field-stimulated at 1 Hz for 10 min using pulses of 1 ms duration and 10 V . cm-1 intensity. The organ bath contained either tetradeuterated (d4) choline (50 microM) or [3H]acetate (2 muCi . ml-1); d4 acetylcholine was measured by gas chromatography-mass spectrometry. As with Torpedo electromotor cholinergic vesicle preparations made under similar conditions the distribution of newly synthesized (d4 or [3H]) acetylcholine in the zonal gradient from stimulated preparations was not identical with that of endogenous (d0, [1H]) acetylcholine, but corresponded to a subpopulation of denser vesicles (equivalent to the VP2 fraction from Torpedo) that had preferentially taken up newly synthesized transmitter. The density difference between the reserve (VP1) and recycling (VP2) vesicles was less than that observed in Torpedo but this smaller difference can be accounted for theoretically by the difference in size between the vesicles of the two tissues. At rest, a lesser incorporation of labelled acetylcholine into the vesicle fraction was observed, and the peaks of endogenous and newly synthesized acetylcholine coincided. Stimulation in the absence of label followed by addition of label did not lead to incorporation of labelled acetylcholine, suggesting that the synthesis and storage of acetylcholine in this preparation and its recovery from stimulation is much more rapid than in Torpedo.

Uptake, Metabolism, and Releasability of Ethyl Analogues of Homocholine by Rat Brain

Ethyl analogues of homocholine were synthesized and used to describe further the specificities of the processes involved in choline uptake and acetylation and acetylcholine storage and release. Monoethylhomocholine, diethylhomocholine, and triethylhomocholine decreased the transport of choline into rat brain synaptosomes. The mono- and diethyl compounds were taken up into synaptosomes with similar affinity for the transport system as choline (5.8, 8.5, and 5.5 microM, respectively) but at a somewhat slower rate (11.3, 8.5, and 37.3 nmol/g original tissue/h, respectively); the triethyl analogue was not transported at the concentrations tested, which further defines the structural specificity of the transport system. L-Carnitine did not affect the transport of the analogues. The in situ acetylation of mono- and diethylhomocholine by slices of rat cerebral cortex was measurable, but the in vitro acetylation by choline acetyltransferase solubilized from rat forebrain was not. Acetylation of the diethyl analogue by slices of cerebellar cortex was less than 20% of that by slices of cerebral cortex. Subcellular fractionation of cerebral slices showed that acetyldiethylhomocholine localized preferentially to the cytosolic rather than vesicular stores, indicating specificity of the mechanism responsible for the incorporation of acetylated product into the vesicles. The release of acetyldiethylhomocholine and of acetylcholine was tested from sliced brain that had been incubated with the precursors. Both esters were released spontaneously but stimulation with increased K+ concentration enhanced the release of acetylcholine without changing the release of acetyldiethylhomocholine, suggesting that evoked transmitter release occurred from a vesicular store.

Possible mechanisms involved in the presynaptic cholinotoxicity due to ethylcholine aziridinium (AF64A) in vivo

AF64A is a toxin which can diminish irreversibly cholinergic transmission in vivo (1, 2). Disruption of neurotransmitter function in vivo is specific to the cholinergic system when AF64A is administered in nanomolar quantities (3, 4). The mechanisms involved appear to be mediated presynaptically (g). The neurochemical and behavioral consequences of AF64A administration are reminiscent of similar measures in patients with Alzheimer's disease (5, 6). Consequently, we have suggested tentatively that the AF64A treated animal may be explored as a potential animal model of this debilitating disease state (7). In this report we provide a brief overview of our recent findings using this compound in vivo, attempt to correlate these findings with those of others with similar aziridinium agnts in vitro, and propose a possible mechanism of action of AF64A in vivo, based on recent observations made in our laboratories.

Incorrect interpretations of radioactive labelling experiments with false transmitters

In a number of recent studies attempts have been made to determine the subcellular origin of the released transmitter by comparison of the ratio of the radioactivity (14C/3H) of true and false transmitters between those released by stimulation and the subcellular fractions. It is shown that use of the isotopic or pseudo-molar ratios will lead to incorrect conclusions because varying degrees of isotopic dilution render the ratio meaningless. The only correct basis for comparison is the true molar ratio.

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

Little is known about the specificity of the mechanisms involved in the synthesis and release of acetylcholine for the acetyl moiety. To test this, blocks of tissue from the electric organ of Torpedo were incubated with either [1-14C]acetate or [1-14C]propionate, and the synthesis, storage, and release of [14C]acetylcholine and [14C]propionylcholine were compared. To obtain equivalent amounts of the two labeled choline esters, a 50-fold higher concentration of propionate than of acetate was needed. Following subcellular fractionation, similar proportions of [14C]acetylcholine and [14C]propionylcholine were recovered with synaptosomes and with synaptic vesicles. Furthermore, both labeled choline esters were protected to a similar extent from degradation during homogenization of tissue in physiological medium, indicating that the two choline esters were equally well incorporated into synaptic vesicles. Yet depolarization of tissue blocks by 50 mM KCl released much less [14C]propionylcholine than [14C]acetylcholine. During field stimulation of the tissue blocks, the difference between the releasibility of the two choline esters was less marked, but acetylcholine was still released in preference to propionylcholine. Evidence for specificity of the release mechanism was also obtained when the release of the two choline esters in response to field stimulation was compared in tissue blocks preincubated with both [3H]choline and [14C]propionate.

Homocholine and short-chain N-alkyl choline analogues as substrates for Torpedo choline acetyltransferase

The kinetic parameters, Km and Vmax, for the acetylation of choline and several close analogues were determined by using (a) purified choline acetyltransferase and (b) a hypotonically lysed synaptosomal extract prepared from the electric organ of Torpedo marmorata. Whereas the Km for choline was similar in both cases (0.51 and 0.42 mM), the crude enzyme showed a three- to fivefold greater affinity for its analogues than the purified enzyme, the activity decreasing rapidly with increased N-alkyl substitution. Homocholine was a poor substrate, but was clearly acetylated by both preparations. The effect of salt on analogue acetylation by the crude enzyme was studied by increasing NaCl concentration from zero to 150 mM. There was an increase in both Km and Vmax for all substrates: choline, N,N,N-dimethylmonothylaminoethanol, -monomethyldiethylaminoethanol and -dimethylmonobutylaminoethanol showed the greatest changes, whilst N,N,N-triethylaminoethanol and -dimethylmonopropylaminoethanol and homocholine were much less affected However, in all cases, the kinetic parameter Vmax/Km remained unchanged. The maximal velocities of the different substrates varied more under conditions of high than of low salt. Sodium chloride up to 300 mM had no effect on the amount of enzyme which was bound to membranes in the synaptosomal extract. It is concluded that choline acetyltransferase has a high degree of substrate specificity, which is slightly altered by purification. The effects of salt cannot be explained as a consequence of nonspecific ionic association with membranes.

On the specificity of uptake by isolated Torpedo synaptic vesicles

Synaptic vesicles were isolated from the electric organ of Torpedo marmorata in highly purified form. Their uptake properties were examined using a large number of small organic molecules as substrates. Following incubation at 26 degree C for 1 h, it was found that concentrative accumulation, indicated by a vesicle:medium concentration ratio greater than unity, was achieved by all the choline analogues used and by four biogenic amines, but not by a variety of purine and pyrimidine bases and nucleosides. Amino acids penetrated poorly, as did sugars, and of organic anions, acetate but not citrate or thiocyanate, was almost excluded. Thus Torpedo vesicles are relatively impermeable to compounds which cannot utilize the ACh or ATP carriers, but show a very high rate of amine uptake, which may be linked to a pH gradient.

The Na+ and K+ content of isolated Torpedo synaptosomes and its effect on choline uptake

The Na+ and K+ concentrations in isolated Torpedo marmorata synaptosomes were determined. Synaptosomes made according to the method of Israël et al. have high internal Na+ (290 mM) and low internal K+ (30 mM) concentrations. Modification of the homogenisation media permitted the isolation of synaptosomes which could maintain transmembrane ion gradients (internal Na+, 96 mM; K+, 81 mM); 0.1 mM-ouabain abolished these gradients. The trans-membrane Na+ gradient started to dissipate after 15 min at 20 degrees C. Inclusion of ATP in the homogenisation medium enabled the synaptosomes to maintain the Na+ gradient for about 90 min. The presence of these transmembrane ion gradients stimulated choline uptake sevenfold. It is concluded that (a) by selecting the isolation media, Torpedo synaptosomes can be prepared with transmembrane ion gradients; (b) these gradients are ouabain-sensitive and stimulate choline uptake; (c) the synaptosomes require additional ATP to maintain the ion gradients.

Acetylation of choline and homocholine by membrane-bound choline-O-acetyltransferase in mouse forebrain nerve endings

The choline analog homocholine is not acetylated in vitro by choline-O-acetyltransferase (ChAT, EC 2.3.1.6), which is solubilized by 100 mM-sodium phosphate buffer washes of a crude vesicular fraction of mouse forebrain. However, both homocholine and choline are acetylated by a form of ChAT which is nonionically associated with a subcellular fraction of mouse forebrain containing membrane-associated organelles and occluded acetylcholine (P4). Acetylation of homocholine by membrane-associated ChAT is saturable. 4-(1-Naphthylvinyl)pyridine (NVP) inhibits the acetylation of both choline (60%) and homocholine (40%) by membrane-associated ChAT but reduces the acetylation of choline alone by soluble ChAT (76%). Choline and homocholine serve as competitive alternative substrates for the same membrane-associated ChAT, whereas homocholine acts only as a competitive inhibitor of choline acetylation by soluble ChAT. Acetylhomocholine competitively inhibits the acetylation of choline by both soluble and membrane-associated ChAT more dramatically than does the natural end product, acetylcholine.

Specific stimulated uptake of acetylcholine by Torpedo electric organ synaptic vesicles

The specificity of acetylcholine uptake by synaptic vesicles isolated from the electric organ of Torpedo californica was studied. In the absence of cofactors, [3H]acetylcholine was taken up identically to[14C]choline in the same solution (passive uptake), and the equilibrium concentration achieved inside the vesicles was equal to the concentration outside. In the presence of MgATP, [3H]acetylcholine and [14C]choline in the same solution were taken up identically, except only about half as much of each was taken up (suppressed uptake). [3H]Acetylcholine uptake was stimulated by MgATP and HCO3- about 4-fold relative to suppressed uptake, for a net concentrative uptake of about 2:1 (stimulated uptake). Uptake of [14C]choline in the same solution remained at the suppressed level. [3H]Acetylcholine taken up under stimulated conditions migrated with vesicles containing [14C]mannitol on analytical glycerol density gradients during centrifugation. Vesicle were treated with nine protein modification reagents under mild conditions. Two reagents had no effect on, dithiothreitol potentiated, and six reagents strongly inhibited subsequent stimulated uptake of [3H]acetylcholine. The results indicate that uptake of acetylcholine is conditionally specific for the transported substrate, is carried out by the synaptic vesicles rather than a contaminant of the preparation, and requires a functional protein system containing a critical sulfhydryl group.