Choline-esterase. An enzyme present in the blood-serum of the horse

THE question of the hydrolysis of acetylcholine by enzymes has been brought into prominence in connection with the investigations on the mechanism of nervous action which Loewi has carried out during the past decade. In his original paper on this subject Loewi [1921], working with isolated frogs' hearts, showed that stimulation of the vagus caused the formation or liberation in the heart of a substance which itself produces the effect previously con- sidered to be due to the action of the nerve. It was recognised by Loewi [1922] that this so-called vagus-substance was a choline derivative, while Loewi and Navratil [1926, 1] later advanced evidence which indicated that it was an ester of choline, possibly acetylcholine, although Witanowski [1925], working in Loewi's laboratory, had previously stated that it was more stable than the latter substance. Not only did Loewi and Navratil find that the physiological activities of the vagus-substance and acetylcholine were indistinguishable, but they discovered that both substances were destroyed by aqueous extracts of frog's heart. The destructive agent present in such extracts was thermolabile and possessed other properties characteristic of enzymes; it was therefore con- sidered to be an esterase. Following this work, Galehr and Plattner [1927, 1, 2] demonstrated that acetylcholine was similarly destroyed by defibrinated blood from various species. They were nevertheless of the opinion that no enzyme was involved but that the destruction was brought about by a process of adsorption catalysis, a view which was elaborated in subsequent papers by Plattner and his colleagues. It is unnecessary, however, to consider the latter work in detail since Engelhart and Loewi [1930] have now conclusively de- monstrated the enzymic nature of the destructive agent; results of a similar nature have, moreover, been obtained by Matthes [1930].