Two Further Serum Pseudocholinesterase Phenotypes as Causes of Suxamethonium Apnoea

Pseudocholinesterase polymorphism in an Irish population

BACKGROUND

Pseudocholinesterase polymorphism, as an example of pharmacogenetics with important clinical implications, has been widely studied and documented. However, data on a sample Irish population is lacking. We sought to provide this.

METHOD

In an assay involving Ellman's reaction, pseudocholinesterase activity, alone and with dibucaine or fluoride as an inhibitor, was quantified using propionylthiocholine iodide as substrate.

RESULTS

Pseudocholinesterase activities of 1.13-12.71 U/ml (mean +/- SD 6.74 +/- 2.04 U/ml) showed a normal distribution among our 116 healthy, non-medicated volunteers, aged 11-80 years (30.7 +/- 10.5 years) and weighing 46-114.6 kg (66.8 +/- 11.4 kg). However, dibucaine numbers from an inhibition study yielded a trimodal pattern consistent with the hypothesis of two allelic genes. Using an established nomenclature, 92 (79.3%) of our volunteers were homozygous for the usual form of the enzyme (E1uE1u). Of the 13 genotyped as E1uE1a, it is possible that 3 were misclassified and are probably E1kE1a. Only one volunteer was homozygous for the atypical form of the enzyme, with activity of 1.13 U/ml and dibucaine and fluoride number of 18.2 and 82.8, respectively.

CONCLUSION

The continuous variation in pseudocholinesterase activity and the trimodal pattern of dibucaine numbers are both in accord with observations in other population groups. Although dibucaine number yields a trimodal pattern, its use could lead to misclassification of some E1kE1a as E1uE1a.

RAPID ESTIMATION OF SERUM CHOLINESTERASE ACTIVITY USING THE ASTRUP MICRO EQUIPMENT

A rapid micro technique for the estimation of serum cholinesterase is described. Acetylcholine bromide is incubated with serum within the capillary of the Astrup electrode. The enzyme hydrolyses the substrate with the liberation of acetic acid. This causes a fall of pH which is seen on the galvanometer of the instrument and the rate of this fall is shown to be proportional to enzyme concentration. The method has been calibrated in international units and compared with a more conventional technique. The values found in homozygotes with normal dibucaine-resistant enzymes and in heterozygotes are reported, together with their dibucaine and fluoride numbers.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Determination of succinylcholine hydrolytic enzyme activity in human plasma

Succinylcholine is frequently employed in surgical procedures as a pre-anaesthetic. However, the lack of existing metabolic activity for this compound in some individuals entails a substantial risk. Normally, the risk is assessed indirectly through the measurement of pseudocholinesterase activity using other substrates, such as acetylcholine, rather than the agent itself. Thus, a method was devised to assess directly the hydrolysis of succinylcholine in human plasma samples. This method was applied to plasma samples derived from healthy men, healthy women, pregnant women and patients with silent pseudocholinesterasaemia.

Abnormal enzyme phenotype (E1a E1f): normal response to succinylcholine

The enzyme serum cholinesterase responsible for the hydrolysis of the muscle relaxant succinylcholine exists in the form of several variants. These may be identified in serum by using substances which inhibit their activity to different degrees. The heterozygote for the atypical and fluoride resistant enzymes (E1a E1f) is one of the phenotypes which has been reported to be sinsitive to succinylcholine. A case is described where succinylcholine given on two separate occasions did not induce apnoea in an individual phenotyped as E1a E1f by at least five methods of inhibition. This is the first reported example of such insensitivity to the drug in this phenotype. Temperature activities for the patient's serum over the range of 20 degrees C to 45 degrees C differed from that of an established E1a E1f phenotype used as a control. There was a progressive inactivation of the control serum at temperatures higher than 35 degrees C, as previously reported for this phenotype. Activity in the serum of the subject of this study did not exhibit the peak activity at 35 degrees C but continued to rise and probably reached a peak between 40 degrees C and 45 degrees C. The significance of these results in the context of current methods of phenotyping is discussed.

Prolonged apnoea after suxamethonium: an analysis of the first 225 cases reported to the Danish Cholinesterase Research Unit

During the last 4 years, 225 patients have been referred to the Danish Cholinesterase Research Unit following an episode of prolonged apnoea after suxamethonium. Fourteen patients (6.2%) were found to have a low serum cholinesterase activity due to an acquired deficiency (for instance, liver disease, chronic debilitating disease or carcinoma). One hundred and forty-eight patients (65.8%) had an inherited abnormal serum cholinesterase, and 105 of these patients (46.7%) were homozygous for the atypical enzyme (E1 Ea1). The mean period of apnoea in this latter group was 92 min (range: 25--240). Seventeen patients (7.6%) were heterozygous for the normal and the atypical enzyme (Eu1 Ea1), with a mean apnoea period of 25 min (range: 7--60 min). Twelve patients were found to be heterozygous for the atypical and the silent gene (E(a)1 E(s)1). The mean period of apnoea was 126 min (range: 45--210 min). Fourteen patients had other rare genotypes. The longest mean period of apnoea (170 min, range: 70--330) was found in patients homozygous for the silent gene (Es1 Es1). The silent gene and the fluoride-resistant gene were found in 8.9% and 2.7% of the patients, respectively. In 63 patients (28.1%) both the type and quantity of serum cholinesterase were normal. In 34 of these patients (15.2%), the prolonged apnoea was due to other causes; for example, suxamethonium overdose, hyperventilation and central as well as peripheral respiratory depression. However, in the other 29 patients (12.9%), the reason for the prolonged apnoea could not be established. The possibility therefore exists that these cases represent unknown genotypes.