Enzymic Aspects of Acetylcholine Metabolism

Abstract

Investigations have been performed involving choline acetyltransferase (ChAc), the enzyme responsible for the synthesis of acetylcholine (ACh) and cholinesterase (ChE), the enzyme responsible for the degradation of ACh. Attempts were made to purify ChAc in order to obtain a preparation which could be used to accurately characterise the enzyme .A comparison of preliminary purification procedures indicated that homogenisation in 0.1% Triton X-100/200 mM KCl was the best method for extracting ChAc from rat brain and human placenta. These studies were of value in obtaining enzyme preparations suitable for subsequent studies on the acetylation of various substrates by ChAc. Confirmation of earlier reports that inconsistent results are obtained on attempting to purify ChAc by ammonium sulphate fractionation suggested that this technique is of limited value in the purification of this enzyme. Purification of ChAc by the more specific technique of affinity chromatography was attempted but was unsuccessful. A study was made of the acetylation of various substrates by ChAc. Several methods of determining ChAc activity were compared and it was found that there were some differences in the suitability of the various methods for the study of substrates other than choline. Complications which arose in the determination of ChAc activity, such as the formation of choline within Chdc preparations and the hydrolysis of the acetyl coenzyme A used in the determinations, were either explained or resolved so that reliable estimates of the acetylation of substrates by ChAc could be made. Hemicholinium-3 (HC-3) and triethylcholine (TEC) were shown to be acetylated in vitro by ChAc preparations from rat brain homogenates. It was shown that a small percentage of the acetylation of TEC could probably be attributed to an enzyme other than ChAc present within the preparation. The possibility that carnitine acetyltransferase (CarAc) was involved in the acetylation of substrates by rat brain homogenates was investigated but the results suggested that, of the substrates tested, only carnitine was significantly acetylated by this enzyme. The acetylation by ChAc of edrophonium and of some analogues of hemicholinium was studied and this provided information on the possibility of a pre-synaptic component to the pharmacological actions of the compounds and also provided information on the structure/activity relationships between the various substrates with respect to their acetylation by ChAc. Two potential pesticides were tested for and found to possess cholinesterase inhibitory activity which correlated with thein vivo toxicity of the compounds observed previously in mice. It was thought possible that their hydrolysis in water to an inactive product would provide a useful method of rendering the compounds harmless after they had achieved their desired effect as pesticides. However, their potency in inhibiting cholinesterase did not decline in parallel with their hydrolysis and the value of the compounds as pesticides is probably therefore limited.