The Specificity of the Trimethylammonium Group in Acetylcholine

British Journal of Pharmacology and Chemotherapy

1965

It is commonly assumed by pharmacologists that the positively charged cationic group in such agonists as acetylcholine, noradrenaline and histamine is essential for their agonist action. The evidence in favour of this view is far from conclusive. Barlow & Hamilton (1962) examined the effect ofpH on the activity of nicotine in producing synaptic block of the rat phrenic nerve-diaphragm preparation. When compared with the quaternary nicotine N-methiodide the activity of nicotine was related to thepH in the way to be expected if the nicotinium ion were more active than the non-protonated form, but neither the range ofpH values studied nor the precision of the results were sufficient to establish the activity (if any) of the non-protonated nicotine relative to the nicotinium ion. Hamilton (1963) has shown a similar effect of pH in the action of nicotine on the frog rectus abdominus muscle, but in this case also the activity of the non-protonated form was not established. Ariens, Simonis & van Rossum (1964) were unable to discover any clear relationship between pH and the action of arecoline on the frog rectus, but did not extend their study over a wide enough pH range to obtain decisive results. Rocha e Silva (1961) examined the effect ofpH on the action of histamine. There was a clear pH-dependence below pH 7 but the action was independent of pH above 7. These results are compatible with the response to histamine being dependent on a group with a pKa 7.0. However, since the dissociation constants of the ionizable groups in histamine are pKaj, 5.9; pKa2, 9.7, the pH-dependence cannot be attributed to the change in histamine ionization and is probably due to a change in the charge on the receptors.An alternative approach to the problem is to study the activity of analogues of the agonist in which the basic nitrogen atom has been replaced by an atom with a different electron shell so that a cation is not formed. Banister & Whittaker (1951) examined the action of 3,3-dimethylbutyl acetate, the analogue of acetylcholine in which the basic nitrogen atom has been replaced by a carbon atom. On the frog rectus, this drug had about 1/12,000th the activity of acetylcholine and, on the guinea-pig ileum, the activity was small and maximal responses were not obtained. The analogue of histamine in which the primary amino-group is replaced by a hydroxyl group, H-(2-hydroxyethyl)-imidazole, is reported to have little or no histamine-like activity (Vartiainen, 1935;Schneedorf & Ivy, 1935;Grossman, Robertson & Rosiere, 1952). These latter pieces of evidence do suggest that uncharged analogues may have some activity and it seemed that the problem merited further examination in view of its importance for the theoretical interpretation of drugreceptor interactions.

Section: Resultssupporting

confidence: 63%

“…The reduction in activity found experimentally (600-fold) (Holton & Ing, 1949) is greater than this and presumably points to other factors in addition to reduced electrostatic interaction contributing to the lowered affinity when methyl groups are replaced by ethyl groups.…”

Section: Discussionmentioning

confidence: 74%

The Role of Ionic Interaction at the Muscarinic Receptor

British Journal of Pharmacology and Chemotherapy

1965

“…This receives some support from the binding profile of the effect of N-ethyl substitutions of acetylcholine. In the untreated high and low affinity agonist binding sites, the affinity falls progressively with each ethyl group, whereas after treatment with PCMB the affinity is increased by substitution of the first ethyl group as is commonly found with antagonists (Holton & Ing, 1949;Barlow, Scott & Stephenson, 1963). If this is the mechanism of the effect on agonist binding in Phase II it implies that there must be at the same time a loss of agonist activity at the receptor.…”

Section: Discussionmentioning

confidence: 97%

The effect of p‐chloromercuribenzoate on structure‐binding relationships of muscarinic receptors in the rat cerebral cortex

Birdsall

Hulme

et al. 1983

British J Pharmacology

Muscarinic receptors in the rat cerebral cortex, reacted with p‐chloromercuribenzoate (PCMB) under different conditions (Phase I and II), have modified binding sites. These exhibit remarkable changes in the structural dependence of the binding of drugs. In Phase I, the structure‐binding profile of agonists for both the high and low affinity agonist sites are altered. In Phase II, the structure‐binding profile of antagonists is also observed. In Phase II, the ability of potent agonists to discriminate between sub‐classes of agonist binding sites is eliminated. There is also a loss of heterogeneity in the binding of the selective antagonist pirenzepine. Of the 16 agonists examined, only pilocarpine has a heterogeneous binding profile in Phase II, the dispersity of binding being increased. The changes in binding properties of the receptors are discussed in terms of general theories of drug‐receptor interactions.

“…

…”

mentioning

confidence: 99%

“…These and other studies have shown that few modifications in the acetylcholine molecule are permissible if the same order of pharmacological activity is to be retained. For instance, replacement of one of the methyl groups by ethyl caused little change in activity, but the substitution of n-propyl or longer aliphatic groups, or the substitution of more than one methyl by ethyl, led to a pronounced decrease in activity (Holton and Ing, 1949). Substitution of a ,8-methyl group had little effect on muscarinic activity but almost abolished nicotinic activity (Simonart, 1932).…”

mentioning

confidence: 99%

The Specificity of Brain Choline Acetylase

British Journal of Pharmacology and Chemotherapy

Burke

Desbarats-Schonbaum

1956

The relationship between structure and pharmacological action in parasympathomimetic drugs has been extensively studied, notably by Hunt (1915), Hunt and Renshaw (1925, 1929, 1933, Bovet and Bovet-Nitti (1948), and Holton and Ing (1949). These and other studies have shown that few modifications in the acetylcholine molecule are permissible if the same order of pharmacological activity is to be retained. For instance, replacement of one of the methyl groups by ethyl caused little change in activity, but the substitution of n-propyl or longer aliphatic groups, or the substitution of more than one methyl by ethyl, led to a pronounced decrease in activity (Holton and Ing, 1949). Substitution of a ,8-methyl group had little effect on muscarinic activity but almost abolished nicotinic activity (Simonart, 1932). On the other hand a-methyl substitution reduced muscarinic activity without appreciable effect on nicotinic activity. Increase in the separation between the quaternary nitrogen and the ester group to 3 carbons in acetyl-y-homocholine had little effect on the activity (Hunt, 1915); and finally tertiary, secondary, and primary amino ethyl acetates were much less active than acetylcholine (Stehle, Melville, and Oldham, 1936).Similar studies on the specificity of the cholinesterases have shown that, although these enzymes are sensitive to the nature of the acyl group, they are extraordinarily unspecific for the alcoholic part of the ester (Adams, 1949), so that even non-basic esters are vigorously split and may sometimes be hydrolysed faster than acetylcholine (e.g., isopropenyl acetate-Burgen, unpublished).Until recently, in default of other evidence, it had been assumed that choline acetylase was specific. Doubt was thrown on this when Banister, Whittaker, and Wijesundera (1953) demonstrated the presence of propionylcholine in ox spleen. Gardiner and Whittaker (1954) and Berman, Wilson, and Nachmansohn (1953) were able to show that acetone powder extracts of pigeon brain and squid ganglia could synthesize propionylcholine or butyrylcholine if propionate or butyrate were added to the incubate in place of acetate. The only suggestion that choline acetylase might also be non-specific for choline, too, was the finding of Korey, Braganza, and Nachmansohn (1951) that choline acetylase from the squid head ganglion was able to acetylate dimethyl ethanolamine as fast as choline, but that ethanolamine, monomethylethanolamine, and diethylethanolamine were not acetylated at all.The present work was undertaken to study systematically the effects of modification of the choline molecule on the rate of acetylation by choline acetylase and also to attempt to find specific inhibitors of the acetylase.Since the acetyl esters of many of the bases studied have negligible biological activity it was not possible to use the usual bioassay methods for estimating the rate of acetylation. The most satisfactory solution to this problem was the use of a highly purified preparation of choline acetylase and the measurement of the rate at which...