The Effect of Ortho Substitution on the Hydrolysis of Benzoylcholine

Abstract

A series of mono-and di-ortho-substituted benzoylcholine compounds has been prepared and the rates of hydroxide ion and cholinesterase catalysed hydrolysis has been determined for each compound. The anti-acetylcholinesterase activities of the compounds have also been determined. It has been found that some of the compounds are stable to esterase catalysed hydrolysis and that the groups which confer stability do not prevent the formation of enzyme substrate complexes. The most suitable groups for ester stabilisation, when used as ortho substituents, are alkyl and iodo as mono substituents and most groups, except hydroxy, as di-ortho substituents.THE in vivo hydrolysis of benzoyl ester groups is a limiting factor in the use of some drugs and consequently it would be of value to stabilise such esters. The hydrolysis of benzoylcholine is catalysed both by hydroxide ions and by serum cholinesterase and therefore this molecule is suitable to examine the influence of ortho substitution on esterase catalysed hydrolysis of benzoyl esters. Ormerod (1953) prepared several meta and para substituted benzoylcholine compounds and determined their rates of cholinesterase catalysed hydrolysis. The small differences observed between experimental results and results calculated using the Hammett substituent constant were attributed to "differences in the enzyme substrate complex".Ortho substituents in benzoate esters have been shown (Evans, Gordon and Watson, 1937;Ingold, 1953a) to accelerate less or retard more the rate of ester hydrolysis than the same substituents in the meta and para positions. Consequently, ortho substitution of benzoylcholine may produce large changes in the rates of cholinesterase catalysed hydrolysis.The interactions between an ester group and an ortho substituent may be direct or transmitted or a combination of both these effects. The direct effect is exerted by way of electrostatic forces, van der Waal's forces and short range repulsive forces. The effects of the latter forces have been termed the primary steric effect (Ingold, 1953b). Transmitted interactions are due to induction and mesomerism. Where steric repulsion alters the geometry of a molecule, thereby modifying the transmitted interactions, the effect has been termed the secondary steric effect (Ingold, 1953~).Bender (1951) demonstrated the presence of a tetrahedral intermediate of the type I in the acid and alkali catalysed hydrolysis of esters. OH

The Effect of Ortho Substitution on the Pharmacology of Benzoylcholine

“…It appears that ortho substitution does not prevent the adsorption of these compounds to receptors of the isolated frog rectus and rat diaphragm preparations. Thomas and Stoker (1961) reached the same conclusions with respect to the in vitro interaction of the compounds with cholinesterase and acetylcholinesterase. * Via Thomas and Stoker (1961).…”

“…Thomas and Stoker (1961) reached the same conclusions with respect to the in vitro interaction of the compounds with cholinesterase and acetylcholinesterase. * Via Thomas and Stoker (1961). (a) = action of tubocurarine on the response to the compound;…”

“…The results from Thomas and Stoker (1961) of antiacetylcholinesterase activity as well as hydrolysis rates are presented together with the two sets of results obtained in the present investigation in Table 111. All the ortho substituted benzoylcholine derivatives tested inhibit the response of the rat diaphragm-phrenic nerve preparation to indirect supramaximal stimulation.…”

“…The nitro derivative was about as potent as benzoylcholine on the frog rectus but the assay was invalidated on the basis of the significant deviation from parallelism which existed between the standard and unknown dose response lines. Thomas and Stoker (1961) noted that o-nitrobenzoylcholine was anomalous in its behaviour with the cholinesterases.…”

“…(For a full discussion of the ortho effect see Stoker, 1959. ) To investigate the possibility of using ortho substitution as a general method of stabilising ester groups in drug molecules Thomas and Stoker (1961) synthesised a series of mono-and di-ortho substituted benzoylcholine derivatives. The hydrolysis of benzoylcholine is catalysed both by hydroxide ions and cholinesterase (Mendel, Mundell and Rudney, 1943) and was, therefore, a suitable molecule to use to compare the effect of ortho substitution on the relative rates of chemical and enzymic catalysed hydrolysis.…”