Plasma cholinesterase studies in some pathological conditions in man

Abstract

IN the last few years, there has been a growing interest in plasma cholinesterase. Until recently, study of tins subject has been mostly confined to p ,t~siological and biochemmal laboratorie,~. The disc~overy of organic phosphorous' ester-type war gases, the introduction c,f anti-cholinesterase substances:, such as insecticides in agrmulture, and the recen~ use of succmylcho'line as a relaxant in anaesthesiaall these events have made the knowledge of cholinesterase properties of very practical value (1, 2). Cholinesterases are enzymes widely distributed in animal tissues. They catalyse the hydrolyms of acetylcholiae. Two types of this enzyme have been described and can be identified by chemmal tests (3, 4): the specific cholinesterase of tissues and red cells, and the non-specific cholmesterase in the plasma also called pseudo-cholinesterase. There is now little doubt as to the physiological role of the specific acetylchohnesterase winch is bound to the cell membranes, its i:tmction seems to be ultra-rapid inactivation of flee acetylcholine. This reaclaon is essential for the development and propagatmn of the aclaon potential and for the metabolism of all nerve cells (5). The importance of this enzyme for normal ]fie has been demonstrated clea}ly by the Iact that selective inhibition of its activity by organic phosphorous ester-type war gases is very rapidly fatal to m~m and other animals. On the other hand, there is praclacally nothing known about the physiological role of "~e non-specific cholflmsterase which is present mainly in the plasma. No substrate l~as been found equivalent to acetylcholine which would explain the presence in plasma of tins highly achve and potent esterase. Selective poisoning of plasma cholinesterase by D.F.P. (di-isopropyl fluoro-phosphate) injections in experimental ammals and m man has not been fatal even when the enzyme activity was reduced to zero (18). Tins is in sharp contrast to the effect of complete reactivation of true cholmesterase, which is rapidly fatal. In spite of the apparent lack of importance and the surrounding mystery of its functaon, we have been interested in plasma cholinesterase because it has, been demonstrated to hydrolyse succmylcholine in vitro and in vivo (2). Studies of the behaviour of cholmesterase ha the ]plasma might be of some use to us ff we have to depend on its activaty in our dmly anaesthetic practice (6, 7). METHODS OF ESTI2VlATING CttOLINESTERASE All chemmal methods depend upon measurement of the acid produced by the action of cholinesterase on acetylc]holine. We have been uslhag the electrometric method of Michel (12). The drop in pH resulting from the liberation of acetic acid, when acetylcholine is hydrolysed, is measured with a glass electrode at