Butyrylthiocholine Assay for Identification of Cholinesterase Variants

Two unusual variants of cholinesterase (acylcholineacylhydrolase, 3.1.1.8) have been recognized by the use of differential inhibitors (Kalow and Genest, 1957; Harris and Whittaker, 1961). A third variant, the silent gene, determines an absent or greatly reduced serum cholinesterase level in the affected homozygote (Liddell, Lehmann, and Silk, 1962). These variants are alleles, and the affected homozygote is usually discovered because of an unexpected sensitivity to the muscle relaxant suxamethonium. Another variant, not an allele of the first group, is recognized by gel electrophoresis (Harris et al., 1963) and has not been investigated in this paper. Inhibitor studies on the first group have used an assay with benzoylcholine as substrate (Kalow and Lindsay, 1955). The results obtained with the different genotypes are summarized in Table I. Two nomenclatures have been proposed. Both are given in the Table, and that of Goedde and Baitsch (1964) has been used in this paper. Ellman et al. (1961) described a cholinesterase assay using acetylthiocholine. This has the disadvantage that acetylthiocholine is hydrolysed by