Choline acetyltransferase--evidence for acetyl transfer by a histidine residue

Cholinergic left-right asymmetry in the habenulo-interpeduncular pathway

The habenulo-interpeduncular pathway, a highly conserved cholinergic system, has emerged as a valuable model to study left-right asymmetry in the brain. In larval zebrafish, the bilaterally paired dorsal habenular nuclei (dHb) exhibit prominent left-right differences in their organization, gene expression, and connectivity, but their cholinergic nature was unclear. Through the discovery of a duplicated cholinergic gene locus, we now show that choline acetyltransferase and vesicular acetylcholine transporter homologs are preferentially expressed in the right dHb of larval zebrafish. Genes encoding the nicotinic acetylcholine receptor subunits α2 and β4 are transcribed in the target interpeduncular nucleus (IPN), suggesting that the asymmetrical cholinergic pathway is functional. To confirm this, we activated channelrhodopsin-2 specifically in the larval dHb and performed whole-cell patch-clamp recording of IPN neurons. The response to optogenetic or electrical stimulation of the right dHb consisted of an initial fast glutamatergic excitatory postsynaptic current followed by a slow-rising cholinergic current. In adult zebrafish, the dHb are divided into discrete cholinergic and peptidergic subnuclei that differ in size between the left and right sides of the brain. After exposing adults to nicotine, fos expression was activated in subregions of the IPN enriched for specific nicotinic acetylcholine receptor subunits. Our studies of the newly identified cholinergic gene locus resolve the neurotransmitter identity of the zebrafish habenular nuclei and reveal functional asymmetry in a major cholinergic neuromodulatory pathway of the vertebrate brain.

Peripheral type of choline acetyltransferase: biological and evolutionary implications for novel mechanisms in cholinergic system

The peripheral type of choline acetyltransferase (pChAT) is an isoform of the well-studied common type of choline acetyltransferase (cChAT), the synthesizing enzyme of acetylcholine. Since pChAT arises by exons skipping, its amino acid sequence is similar to that of cChAT, except the lack of a continuous peptide sequence encoded by all the four exons from 6 to 9. While cChAT expression has been observed in both the central and peripheral nervous systems, pChAT is preferentially expressed in the peripheral nervous system. pChAT appears to be a reliable marker for the visualization of peripheral cholinergic neurons and their processes, whereas other conventional markers including cChAT have not been used successfully for it. In mammals like rodents, pChAT immunoreactivity has been observed in most, if not all, physiologically identified peripheral cholinergic structures such as all parasympathetic postganglionic neurons and most neurons of the enteric nervous system. In addition, pChAT has been found in many peripheral neurons that are derived from the neural crest. These include sensory neurons of the trigeminal ganglion and the dorsal root ganglion, and sympathetic postganglionic neurons. Recent studies moreover indicate that pChAT, as well as cChAT, appears ubiquitously expressed among various species not only of vertebrate mammals but also of invertebrate mollusks. This finding implies that the alternative splicing mechanism to generate pChAT and cChAT has been preserved during evolution, probably for some functional benefits.

Structural insights and functional implications of choline acetyltransferase

The biosynthetic enzyme for the neurotransmitter acetylcholine, choline acetyltransferase (ChAT) (E.C. 2.3.1.6), is essential for the development and neuronal activities of cholinergic systems involved in many fundamental brain functions. ChAT catalyzes the transfer of an acetyl group from acetyl-coenzyme A to choline to form the neurotransmitter acetylcholine. Since its discovery more than 60 years ago much research has been devoted to the kinetic studies of this enzyme. For the first time we report the crystal structure of rat ChAT (rChAT) to 1.55 A resolution. The structure of rChAT is a monomer and consists of two domains with an interfacial active site tunnel. This structure, with the modeled substrate binding, provides critical insights into the molecular basis for the production of acetylcholine and may further our understanding of disease causing mutations.

Functional analysis of conserved histidines in choline acetyltransferase by site-directed mutagenesis

The choline acetyltransferase (ChAT) reaction involves the transfer of the acetyl group of acetyl-CoA to choline, in which an active site histidine is believed to act as a general acid/base catalyst. A comparison of the deduced amino acid sequences of the enzyme from Drosophila, pig, rat, and Caernohabditis elegans revealed three conserved histidines: Drosophila His268, His393, and His426. Each of these histidines was replaced by a leucine and a glutamine, and the kinetic properties of each of the recombinant mutant enzymes were determined. The mutations yielded active His268Leu-ChAT, His268Gln-ChAT, and His393Gln-ChAT and inactive His393Leu-ChAT, His426Leu-ChAT, and His426Gln-ChAT. The kinetic constants Km(CoA), Km(acetylcholine), and Vmax were essentially the same for all of the active mutants. When the integrity of the CoASAc binding site was investigated in the inactive mutants, the data suggested that the binding site in His393Leu-ChAT is disrupted but conserved in His426Leu-ChAT and His426Gln-ChAT. These results suggest that His426 is an essential catalytic residue and could serve as an acid/base catalyst.

Kinetic and inactivation studies of recombinant Drosophila choline acetyltransferase

A cDNA for Drosophila choline acetyltransferase (ChAT) was expressed in E. coli and the recombinant enzyme partially purified. Kinetic analysis yielded the following constants for the recombinant enzyme; KmAcCoA = 29 microM, KmCoA = 25 microM, Kmcholine = 330 microM, and Kmacetylcholine = 2 mM. The recombinant Drosophila enzyme, like the enzyme from other species, exhibited an increase in activity as a function of increased salt concentration. Chemical modification studies using dithio-bis-nitro-2-carboxylate, butanedione, and diethylpyrocarbonate showed that the recombinant enzyme contains active site cysteine, arginine, and histidine residues. These studies demonstrate that the recombinant Drosophila ChAT possesses the same catalytic properties as the enzyme from a variety of other sources.

Filarial parasites exhibit unusually high levels of choline acetyltransferase activity

The presence of unusually high levels of choline acetyltransferase (ChAT, EC 2.3.1.6) in human and animal filarial parasites has been demonstrated. The levels of ChAT were highest in male worms of Brugia malayi and Brugia pahangi, with specific activities in crude extracts of about 2.27 and 1.26 mumol min-1 (mg protein)-1, respectively. The enzyme levels in these worms were over 10-20 times higher than in male worms of Litomosoides carinii. The ChAT levels were about 2-5 times higher in male than in female worms. The enzyme was also present in appreciably high levels in microfilariae of Brugia species, L. carinii and Wuchereria bancrofti. The levels of ChAT in male worms of Brugia species were several thousand-fold higher than in the intestinal nematodes Trichuris muris and Necator americanus, and were over three orders of magnitude higher than in mammalian brain. Unlike the mammalian ChAT, the parasite enzyme was extremely stable. The parasite enzyme was not inhibited by any of the antifilarial agents except suramin. The filarial ChAT was strongly inhibited by sulphydryl reagents and diethylpyrocarbonate. Ethacrynic acid (EA), a diuretic and a sulphydryl reagent, irreversibly inhibited the filarial ChAT activity at low concentrations. In contrast, EA inhibited the activity of mammalian brain ChAT at much higher concentrations. The motility of adult worms and microfilariae was irreversibly inhibited by low concentrations of EA. Furthermore, the inhibition of motility was paralleled by the inactivation of ChAT in these parasites. These studies indicate that ChAT activity appears to be vital for parasite's survival and that acetylcholine might play a key role in the control of worm motility.

Molecular genetic approach to the study of mammalian choline acetyltransferase

The enzyme choline acetyltransferase catalyses the biosynthesis of the neurotransmitter acetylcholine and constitutes a specific marker of cholinergic system. To date, there is very limited information about the structure of the mammalian enzyme. More detailed understanding of this enzyme is particularly desirable because of the importance of the cholinergic system in neurotransmission, as well as the possible involvement of this system in certain neurological disorders. In this article, recent studies concerning the isolation of a cDNA encoding the complete sequence of the porcine enzyme are reported and the potential applications of this probe are discussed.

AF64A: an active site directed irreversible inhibitor of choline acetyltransferase

Ethylcholine mustard aziridinium ion (AF64A, MEChMAz) has been proposed as a cholinergic neuron-specific neurotoxin. We report that in further studies on its mechanism of action incubation of the cholinergic neuroblastoma X glioma cell line, NG-108-15, with 100 microM AF64A resulted in a rapid decrease in cellular choline acetyltransferase (ChAT) activity which preceded cytotoxicity. Thus, a 60-85% decrease in ChAT activity was measured within 5 h of AF64A exposure, whereas cell lysis (measured as the release of the cytosolic enzyme lactate dehydrogenase into the medium) did not become apparent until 18 h of AF64A exposure. This led us to examine the effects of AF64A on partially purified ChAT. We report a concentration- and time-dependent inhibition of partially purified ChAT by AF64A that could not be reversed by dialysis but could be prevented by coincubation of the enzyme and AF64A with choline but not with acetyl-coenzyme A. We present kinetic evidence that choline and AF64A compete for the same site on the enzyme. In addition, thiosulfate, which inactivates the aziridinium ion, eliminated AF64A's capacity to inhibit the enzyme. AF64A also irreversibly inhibited partially purified choline kinase and acetylcholinesterase but not lactate dehydrogenase, alcohol dehydrogenase, carboxypeptidase A, or chymotrypsinogen, enzymes that do not use choline as a substrate or product. Thus, the data suggest that AF64A acts as an irreversible active site directed inhibitor of ChAT and possibly other enzymes recognizing choline.

An endogenous inhibitory factor for choline acetyltransferase

Chromatography of partially purified choline acetyltransferase (CAT) over carboxymethyl cellulose may result in the loss of up to 95% of the enzyme activity. This loss of activity can be prevented by running the chromatographs at low protein concentration with a large gradient volume suggesting that interactions between CAT and other endogenous proteins are involved in the mechanism of inactivation. Further experiments showed that CM-cellulose chromatography separates an endogenous inhibitory factor(s) and an endogenous activating factor(s) which protects the enzyme from the action of the former. The inhibitory factor elutes with CAT and produces almost complete inactivation unless the protein concentration is maintained below 0.05 mg/ml. Mixing experiments demonstrated that the activating factor is capable of blocking the effect of the inhibitory factor. The low degree of temperature dependence of the inhibitory factor essentially rules out the possibility that the inhibitor is a proteolytic enzyme. The I50 was estimated to be 10(-7) M or less suggesting a possible physiological role of these factors in the regulation of CAT activity.

Choline acetyltransferase

Acetylcholine is essential to neural function. It synthesis is catalyzed by choline acetyltransferase, the enzyme responsible for the acetylation of choline by acetyl coenzye A, a reaction favored slightly thermodymodynamically and not at all kinetically. An analytically pure enzyme still has not been obtained; however, method of purification have been greatly improved recently. Numerous inhibitors of the enzyme have been synthesized and their structure-action relationships examained. Evidence has been accumulated showing the essential involvement of an imidazole group in the active site of choline acetyltransferase. The literature regarding the controversial role to thiol groups in choline acetyltransferase is reviewed. Recently, derivatives of coenzyme A have been introduced as inhibitors of this enzyme and the specificity of coenzyme A binding has been examined. Possible mechanisms responsible for the control fo acetylcholine synthesis are discussed.

Acetyl-coenzyme A and coenzyme A analogues. Their effects on rat brain choline acetyltransferase

Choline acetyltransferase has the same affinity for acetyl-CoA, propionyl-CoA and butyryl-CoA (Km=1.4 micron). Choline acetyltransferase may use the two latter compounds as substrate, but the longer the acyl chain the lower will be Vmax. CoA is an inhibitor (Ki=1.8 micron). The position of the 3'-phosphate is of primary importance. Desphospho-CoA is a weak inhibitor (Ki=500 micron). 5'-AMP is already an inhibitor (Ki=2500 micron). Phosphopantetheine is not an inhibitor. Dextran Blue is a potent inhibitor (Ki=0.05 micron). Choline acetyltransferase binds to hydrophobic affinity columns. Because of its affinity for nucleotides, affinity for Dextran Blue and hydrophobicity, it is proposed that it contains the 'nucleotide fold', which is a common structural domain present in several enzymes binding nucleotides.