RAPID ESTIMATION OF SERUM CHOLINESTERASE ACTIVITY USING THE ASTRUP MICRO EQUIPMENT

Estimation of cholinesterase activity (EC 3.1.1.7; 3.1.1.8) in undiluted plasma and erythrocytes as a tool for measuring in vivo effects of reversible inhibitors

In vivo effects of reversible inhibitors of cholinesterase activity were determined radiometrically in undiluted samples of erythrocytes and plasma. [14C]acetylcholine at substrate saturation, 25 degrees C and pH 7.4 permitted rapid and precise determination of butyrylcholinesterase (EC 3.1.1.8) and acetylcholinesterase (EC 3.1.1.7) activities. Reference values for acetylcholinesterase and butyrylcholinesterase were estimated in the plasma and erythrocyte haemolysate of 102 healthy volunteers. The time course of in vitro inhibition was monitored, starting immediately after addition of 9-amino-1,2,3,4-tetrahydroacridine (tacrine), eserine or pyridostigmine to undiluted human plasma. Maximal inhibition (in vitro) was seen within 60 min with tacrine and eserine, in contrast to 180 min with pyridostigmine. The inhibition remained constant for more than 10 h except with eserine, from which enzyme activity showed an early recovery. Concentration response experiments were performed in undiluted human plasma and undiluted human erythrocyte haemolysate. Ki-values of tacrine, eserine and pyridostigmine were estimated. In contrast to pyridostigmine and eserine, tacrine was found to have a higher affinity for butyrylcholinesterase than for acetylcholinesterase. Tacrine at 2.5 mumol/l resulted in complete inhibition of butyrylcholinesterase and 70% inhibition of acetylcholinesterase activity. Dilution of these samples up to 100-fold was accompanied by almost complete recovery of acetylcholinesterase and by 50% recovery of butyrylcholinesterase.

Cholinesterase phenotyping: clinical aspects and laboratory applications

The measurement of cholinesterase activity in human serum is an important investigation, especially in patients suspected of poisoning with organophosphate insecticides, or those experiencing prolonged paralysis following treatment with the short acting muscle relaxant succinylcholine. Appropriate clinical management of affected individuals can only be instituted if enzyme activity is measured using a method capable of clear interpretation and phenotypic ascription of cholinesterase, ascertained by use of selected enzyme inhibitors, is reliable. This review considers factors which lead to significant cholinesterase abnormalities and advises on the techniques most appropriate to their investigation. It is concluded that no one method is suitable both for measurement of activity and determination of genotype. The use of propionyl thiocholine is recommended for the first procedure and benzoyl choline for the latter. Those laboratories in which a request for the assessment of cholinesterase status is an unusual event should make greater use of assistance from colleagues whose experience with the problem is greater.

Organophosphate insecticide poisoning and its management

The organophosphate insecticides have become widely used in agriculture and in home gardening. There has been an increase in accidental and intentional poisoning.

Almost all these compounds are potent cholinesterase inhibitors, and signs of poisoning are attributable to the accumulation of acetylcholine.

The local absorption of these compounds from the conjunctiva, upper respiratory tract and skin following exposure to the aerosol or dust used in agricultural work, produces mild symptoms and is rarely of consequence except in accidents due to careless handling. The severe type of poisoning is caused usually by ingestion of the compound(s), and accumulation of large amounts of acetylcholine is fatal if not antagonized rapidly by atropine in large and continuous doses, together with respiratory support.

Pyridine-2-Aldoxime Methiodide (P-2-AM) has been reported to be successful in treating Parathion poisoning. There is experimental evidence that it is not of similar value in some other organophosphorus insecticide poisoning.

Evidence for mutation being the source of the abnormal gene for plasma cholinesterase

Results of cholinesterase, dibucaine and fluoride numbers, and scoline hydrolysis rates are presented in a family found to have normal, silent and abnormal genes for plasma cholinesterase. Five sibs, confirmed by red cell grouping and tissue typing, have been shown to possess a cholinesterase pattern which cannot be explained on the basis of accepted theories of inheritance. In view of this it is suggested that, in this family, a mutation has occurred from a normal or silent gene to an abnormal gene.

An original approach to the diagnosis of scoline-induced apnoea

A new technique is described for measuring the rate of scoline hydrolysis by plasma cholinesterase, which may prove useful in the diagnosis of scoline sensitivity. The importance of extensive family screening is emphasized and three cases possessing a silent gene are described.