Dibucaine cholinesterase inhibition

For many years, in about 800 cases with prolonged apnea, one of the present authors (W.K.) has faithfully determined both dibucaine and fluoride inhibition. Many of the fluoride inhibition data were inconsistent with the concept of there being a single, special allele conveying resistance to fluoride inhibition. Fluoride tests may be worth pursuing when the purpose is to conduct research into the genetics or biochemistry of cholinesterase. If the purpose is to explain prolonged apnea after suc-cinylcholine, the fluoride test scarcely repays the effort, because any abnormality of fluoride inhibition also shows up as an abnormality of cholinesterase inhibition by dibucaine (see Table 23). 

Pseudocholinesterase (EC 3.1.1.8). Enzyme present in serum but shows atypical kinetics Condition first discovered with introduction of suxamethonium (suc-cinyl dicholine) as muscle relaxant in electroconvulsion therapy. This drug is normally rapidly hydrolysed by pseudocholinesterase, and its effects last only a few minutes Afiected subjects (1 in 2,000 Europeans) develop prolonged muscular paralysis and apnea (up to 2 hours) after normal drug dose. Condition screened for by measuring inhibition of serum cholinesterase by dibucaine percent inhibition is called dibucaine number (80% for normal enzyme, 20% for atypical enzyme at lO M dibucaine). Dibucaine nmnbers of about 62 % also occur in 4 % of Europeans, who possess about equal amounts of normal and atypical forms... 

Neuromuscular blockade produced by succinylcholine and mivacurium can be prolonged in patients with a genetically abnormal variant of plasma cholinesterase. The dibucaine number is a measure of the ability of a patient to metabolize succinylcholine and can be used to identify at-risk patients. Under standardized test conditions, dibucaine inhibits the normal enzyme by 80% and the abnormal enzyme by only 20%. Many genetic variants of plasma cholinesterase have been identified, although the dibucaine-related variants are the most important. Given the rarity of these genetic variants, plasma cholinesterase testing is not a routine clinical procedure. 

Studies of cholinesterase structure and the biological mechanisms of inhibition are necessary for effective drug development. Medicinal compounds like Ortho-7, Dibucaine, and HI-6 are predicted as good targets for modeled AChE and BChE proteins based on docking studies [172],... 

Differentiation based on dibucaine inhibition. The most commonly used agent for differentiating serum cholinesterase variants is dibucaine, which acts as an inhibitor of the enzyme (Sections 2.2 and 6.2). Its use for the differentiation of cholinesterase variants was first introduced by Kalow and Genest (Kll) in 1957, and it has been and continues to be extensively used for this purpose. The assay is performed in two cuvettes, one with substrate alone and one with substrate plus dibucaine. Both cuvettes contain M/15 phosphate buffer (Sorensen) at pH 7.4 and 5 X 10 mol/liter benzoylcholine chloride. One of the cuvettes contains 1 X 10 mol/liter dibucaine. The initial velocity at 26°C is measured in both cuvettes by determining the rate of change of absorbance at 240 nm. The dibucaine number, DN, is the percentage of inhibition by dibucaine, and is calculated as DN = 100 (v - t> )/o, where V is the uninhibited velocity and v is the velocity measured in the presence of the inhibitor. DN values obtained by this method for the most widely studied cholinesterase variants are presented in Table 1 (see Section 2.1). 

Differentiation based on fluoride inhibition. None of the reported methods utilizing dibucaine as an inhibitor is capable of differentiating all the known cholinesterase variants. Therefore, fluoride ion is also commonly used to help distinguish the genetic variants. When... 

Cholinesterase inhibitors (eg, neostigmine) markedly prolong the neuromuscular blockade caused by succinylcholine. They increase ACh at the end plate, which intensifies depolarization, and they also inhibit succinylcholine metabolism by pseudocholinesterase. About one in 500 persons have a single abnormal gene for pseudochoUnesterase. Fewer than one in 3000 persons have two abnormal genes (homozygous atypical) that produce an enzyme with only 1% of the normal affinity for succinylcholine. The atypical enzyme is resistant to the inhibitory action of dibucaine. The answer is (E). 

Normal plasma cholinesterase (pseudocholinesterase) is 80% inhibited by the anaesthetic dibucaine, i.e. it has a dibucaine number of 80. In individuals with suxamethonium sensitivity, the cholinesterase differs from the normal form and is less susceptible to dibucaine inhibition, i.e. it has lower dibucaine numbers. This enzyme behaviour is used in phenotyping members of an affected family, since heterozygotes have dibucaine numbers intermediate between those of normals and homozygotes. 

The use of enzyme activity assays in the presence and absence of inhibitors has some merit and can be automated. Cholinesterase variants have been detected by Kalow and Genest (1957) and Harris and Whittaker (1961) using dibucaine and fluoride inhibition, respectively they have provided a means of obtaining both qualitative information (type of variant) and quantitative information (activity of cholinesterase in the sample) about the enzyme in serum. 

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