Acetylcholine mustard labels the binding site aspartate in muscarinic acetylcholine receptors

Determination of the amino acid residue involved in [3H]beta-funaltrexamine covalent binding in the cloned rat mu-opioid receptor

We previously demonstrated that [3H]beta-funaltrexamine ([3H]beta-FNA) labeled the rat mu opioid receptor expressed in Chinese hamster ovary cells with high specificity, and [3H]beta-FNA-labeled receptors migrated as one broad band with a mass of 80 kDa. In this study, we determined the region and then the amino acid residue of the mu receptor involved in the covalent binding of [3H]beta-FNA. [3H]beta-FNA-labeled receptors were solubilized and purified to approximately 10% purity by immunoaffinity chromatography with antibodies against a C-terminal domain peptide. The site of covalent bond formation was determined to be within Ala206-Met243 by CNBr cleavage of partially purified labeled mu receptors and determinations of sizes of labeled receptor fragments. The amino acid residue of beta-FNA covalent incorporation was then determined by site-directed mutagenesis studies within this region. Mutation of Lys233 to Ala, Arg, His, and Leu completely eliminated covalent binding of [3H]beta-FNA, although these mutants bound beta-FNA with high affinity. Mutations of other amino acid residues did not affect covalent binding of [3H]beta-FNA. These results indicate that [3H]beta-FNA binds covalently to Lys233. Since [3H]beta-FNA is a rigid molecule, the information will be very useful for molecular modeling of interaction between morphinans and the mu receptor.

Stereoselective interaction of uncharged esters at four muscarinic receptor subtypes

We investigated the binding and pharmacological properties of the esters of 3,3-dimethylbutan-1-ol (the carbon analogue of choline) with either diphenylglycolic acid, (R)-phenylcyclohexylglycolic acid, or (S)-phenylcyclohexylglycolic acid [BS-6181, (R)-BS-7826 and (S)-BS-7826, respectively] at muscarinic M1, M2, M3 (Hm3) and M4 receptors. The three uncharged compounds were muscarinic receptor antagonists, with pA2 or pKi values between 7.9 and 5.6. The achiral ester BS-6181 displayed highest affinity for M1, M3 (Hm3) and M4 receptors (pA2 or pKi = 7.2-7.6) and lower affinity for M2 receptors (pA2 or pKi = 6.7 and 6.8). The four muscarinic receptor subtypes were able to distinguish between the two enantiomers of the cyclohexyl derivative of BS-6181 [(R)- and (S)-BS-7826], with a preference for the (R)-isomer (up to 79-fold). Interestingly, the (S)-enantiomer of BS-7826, being the distomer, was found to be M4 selective (pKi/M4 = 6.9; pA2 or pKi/M1-M3 (Hm3) = 5.6-6.2). These results indicate that uncharged compounds may (stereo)selectively bind to muscarinic receptors via hydrophobic interactions. Thus, an ionic bond between muscarinic ligands and an anionic site of the receptor is not absolutely necessary for recognition of muscarinic receptors.

Pharmacological basis for functional selectivity of partial muscarinic receptor agonists

Muscarinic receptor agonists activate phosphoinositide hydrolysis and adenylate cyclase in Chinese hamster ovary cells transfected with cDNAs encoding the human muscarinic ml and m3 receptors. Whereas carbachol activates similarly both receptor subtypes, 4-[3-chlorophenyl-carbamoyloxy]-2-butynyltrimethyl ammonium chloride (McN-A-343) preferentially activates the m1 subtype over m3, in regard to both phosphoinositide hydrolysis and adenylate cyclase activity. On the other hand, oxotremorine activates phosphoinositide hydrolysis to a similar extent in both cell lines, but it activates preferentially adenylate cyclase in m1 versus m3 receptor expressing cells. Relative to carbachol, both McN-A-343 and oxotremorine activate preferentially phosphoinositide hydrolysis over adenylate cyclase in both cell lines. Prolonged incubation of cells with either carbachol, McN-A-343, or oxotremorine down-regulated the m1 receptors. This was accompanied by a parallel decrease in adenylate cyclase activity, whereas phosphoinositide hydrolysis remained relatively high. Inactivation of the receptors by alkylation with acetylethylcholine mustard, or by blocking with atropine, reduced carbachol-stimulated adenylate cyclase activity more effectively than carbachol-induced phosphoinositide hydrolysis in both m1 and m3 receptor expressing cells. These findings imply that the receptor reserve in these cell lines is greater for phosphoinositide hydrolysis response than for adenylate cyclase response. Yet, the receptor reserve for each of these responses is similar in both m1 and m3 receptor expressing cells. Since the binding affinities of McN-A-343 and of oxotremorine to m1 and m3 receptors are very similar, and since both cell lines contain similar amounts of spare receptors, we propose that the preferential activation of muscarinic m1 over m3 receptor by partial agonists is related to differences in the abilities of the two receptor subtypes to undergo conformational changes following agonist binding. This hypothesis is supported by results showing that the muscarinic m1 but not m3 receptor exhibits two affinity states in a competition binding assay.

Acetylcholinesterase inhibition by fasciculin: crystal structure of the complex

The crystal structure of the snake toxin fasciculin, bound to mouse acetylcholinesterase (mAChE), at 3.2 A resolution reveals a synergistic three-point anchorage consistent with the picomolar dissociation constant of the complex. Loop II of fasciculin contains a cluster of hydrophobic residues that interact with the peripheral anionic site of the enzyme and sterically occlude substrate access to the catalytic site. Loop I fits in a crevice near the lip of the gorge to maximize the surface area of contact of loop II at the gorge entry. The fasciculin core surrounds a protruding loop on the enzyme surface and stabilizes the whole assembly. Upon binding of fasciculin, subtle structural rearrangements of AChE occur that could explain the observed residual catalytic activity of the fasciculin-enzyme complex.

Purification and sequence determination of a new muscarinic toxin (MT4) from the venom of the green mamba (Dendroaspis angusticeps)

A toxin which partially inhibited [3H]N-methylscopolamine binding to rat brain muscarinic receptors was purified from the venom of green mamba, Dendroaspis angusticeps. The N-terminal sequence (up to 45 amino acids) was determined by automated Edman degradation of the whole molecule. The complete sequence was elucidated after enzymatic cleavage with endoproteinase Arg-C or endoproteinase Lys-C and peptide fragments purification. The identity of the C-terminal amino acid was confirmed by hydrazinolysis. The new toxin (MT4) had eight half-cystines and 66 amino acids. It differed from muscarinic toxin MT1 by a single substitution in position 57 (arginine in MT1, histidine in MT4), proximal to the sixth half-cystine.

The functional role of the binding site aspartate in muscarinic acetylcholine receptors, probed by site-directed mutagenesis

Mutation of the Asp in transmembrane domain three of the muscarinic receptors to Asn (M1) or Glu (M1 and M2) strongly reduced the high-affinity component of agonist binding, and the M1 phosphoinositide response. Formation of the acetylcholine-receptor binary complex was also strongly inhibited. In contrast, binary complex formation by other agonists, as well as the antagonist (-)-N-methylscopolamine, was less affected. Ionic bonding between the carboxylate oxyanion and the positively-charged headgroup probably anchors acetylcholine when it is bound in its active conformation, but alternative, non-productive, binding modes, promoted by non-polar forces, may contribute to binary complex formation by other ligands.

Positive allosteric interactions on cardiac muscarinic receptors: effects of chemical modifications of disulphide and carboxyl groups

Changes in the allosteric effects of alcuronium on rat cardiac muscarinic receptors were investigated after chemical modifications of S-S bonds or free carboxyl groups. In membranes pretreated with dithiothreitol, alcuronium lost its positive action on the binding of [3H]methyl-N-scopolamine while its inhibitory effect on radioligand dissociation was preserved. In membranes pretreated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), known to modify free carboxyl groups in proteins, the ability to bind [3H]methyl-N-scopolamine was preserved if the pretreatment had been performed in the presence of alcuronium, methyl-N-scopolamine or carbachol, while the positive cooperative effect of alcuronium on [3H]methyl-N-scopolamine binding was only preserved in membranes that had been exposed to EDC in the presence of alcuronium. Methyl-N-scopolamine, carbachol and alcuronium differed in their ability to protect (against EDC) the action of alcuronium on the rate of [3H]methyl-N-scopolamine dissociation. The results suggest that the disulphide bridge connecting the first two extracellular loops of muscarinic receptors is important for the positive allosteric action of alcuronium and that three carboxyl groups (presumably aspartate residues) are involved in receptor interactions with alcuronium and methyl-N-scopolamine. The first group is important for the effect of alcuronium on the affinity for methyl-N-scopolamine, the second is critical for the effect of alcuronium on the rate of methyl-N-scopolamine dissociation, and the third is critical for methyl-N-scopolamine binding. Presumably, the two charged nitrogens of alcuronium associate with the first and the second of the three groups involved.

Selective inactivation of muscarinic receptor subtypes

Muscarinic receptors exist in multiple subtypes, denoted as M1, M2, M3 and M4, encoded by four distinct but related genes. A fifth gene product, m5, has also been predicted although this sequence awaits a pharmacological equivalent. Many tissues express more than one muscarinic receptor subtype, which may couple to different intracellular effectors and thus have different physiological roles. One way to characterize the role of each receptor is to selectively inactivate one receptor population, thus pharmacologically 'isolating' the muscarinic receptor subtype of interest. Selective receptor inactivation can be achieved using either a selective, irreversible antagonist, or protection using a selective, reversible antagonist against a non-selective irreversible antagonist. Therefore, combination of these two approaches may provide optimal selective inactivation. Several muscarinic alkylating agents have been identified, including phenoxybenzamine, EEDQ (N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) and propylbenzilylcholine mustard. These irreversible antagonists do not, in general, discriminate between muscarinic receptor subtypes and are frequently used to estimate the affinity and relative efficacy of muscarinic agonists. Consequently, use of these irreversible antagonists provides estimations of the 'receptor reserve' associated with a response mediated by muscarinic receptor activation. In contrast, 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl)piperidine) selectively inactivates M3 receptors, but will not discriminate between M1, M2 or M4 receptors. In the absence of highly selective alkylating agents, receptor protection by reversible antagonists may be used. Thus, reversible antagonists, such as pirenzepine, methoctramine or para-fluorohexahydrosiladifenidol, at appropriate fractional receptor occupancies, may protect M1, M2 or M3 receptors against alkylation by phenoxybenzamine. Selective alkylation of M3 receptors by 4-DAMP mustard is enhanced with concurrent M2 protection. This approach has been applied to defining the role of these muscarinic receptor subtypes in the control of ileal smooth muscle tone. These data suggest that, in ileum, M2 receptors may act to inhibit beta-adrenoceptor activation, thereby offsetting relaxation, while M3 receptors directly mediate contraction.