N-SUBSTITUTED CHOLINE ANALOGUES AS SUBSTRATES FOR CHOLINE ACETYLTRANSFERASE

Abstract— Alkyl, phenyl, phenylalkyl and pyridiniumalkyl derivatives of choline were studied as substrates for choline acetyltransferase from bovine brain. When one methyl group of choline was replaced by an ethyl group, pK _m (negative logarithm of apparent K _m) decreased, whereas V _max was not significantly changed in comparison with choline. The n-propyl derivative showed an even lower pK _m with unchanged V _max Further elongation of the n-alkyl chain had little effect on the substrate parameters until the n-decyl derivative was reached, when a pronounced decrease of V _max occurred. The highest n-alkyl homologue studied, n-pentadecyl choline, was a very poor substrate. Phenylcholine was also a poor substrate, but introduction of an alkyl chain between the phenyl group and the quaternary nitrogen resulted in compounds with better substrate properties, although they were still inferior to choline. The lowest homologue of the pyridiniumalkyl cholines studied, pyridiniumpropylcholine, had a very low pK _m and a lower V _m in comparison with choline. Increasing the chain length of the alkyl residue resulted in an increase of pK _m, whereas V _max was little affected. The results demonstrate that replacement of one methyl group of choline with a more bulky substituent resulted in impaired substrate properties, presumably due to steric effects. No evidence was obtained for hydrophobic interactions between the enzyme and non-polar substituents in the choline analogues studied.