Benzoylcholine, as substrate

Nine of the remaining patients had dibucaine numbers (measured by the choline oxidase procedure) characteristic of the EyEy genotype and the sera of six had no activity with succinylcholine (Section 3.2). These 15 patients had cholinesterases whose properties appear to be quite different from those of any other variants that have been reported, with one possible exception. The six patients whose serum exhibited no activity with succinylcholine but did show the usual level of activity with benzoylcholine, represent a probable new phenotype. The nine patients who had DN values characteristic of the usual phenotype when using benzoylcholine as substrate but DN values characteristic of the atypical enzyme when using succinylcholine, must also be classified as representing... 

Irwin and Hein (13) reported a family in which DN and FN values indicated that the father was of the usual phenotype, the mother the atypical phenotype, and the two sons the intermediate phenotype. However, one of the sons had about 25% more cholinesterase activity with benzoylcholine as substrate than did his father. Furthermore, the son s cholinesterase activity was greater than that of his father when tested with several other substrates. The pattern of values for this son indicated that his cholinesterase was qualitatively different from that of anyone else in his family. An unidentified gene that affects the amount of enzyme activity, as well as the qualitative properties of cholinesterase, might exist in this family. However, the findings may have been complicated by a physiologically high level of cholinesterase activity which is found in some children, and which decreases to normal adult values at puberty. 

Todrick et al. (T7) studied the effect of n-butanol on the activity of horse serum cholinesterase. With benzoylcholine as substrate, the enzyme was activated to a maximum at a butanol concentration of 0.25 mol/liter. The activation decreased with increasing butanol concentration, and inhibition set in at 0.42 mol/liter. In contrast, with acetylcholine as substrate, the enzyme was inhibited and the inhibition increased progressively with increasing butanol concentration. 

Activation at high substrate concentrations not only explains the failure of butyrylcholinesterase to follow simple Michaelis-Menton kinetics, but also explains the enigma of substrate inhibition of the enzyme using either benzoylcholine (A21, T7) or acetyl- or butyryl-salicylcholine as substrates. The proposal made by Hastings is analogous to that of Myers (M24, M25) for the inhibition of acetylcholinesterase by excess substrate, in this case acetylcholine. 

Benzoylcholine has also been used as substrate in stud)dng the effects of alkanols on human serum cholinesterase variants (W8, W9). Increasing concentrations of alkanols were found to activate the usual enzyme up to a maximum, which increased in magnitude as the chain length of the alkyl group increased. At higher concentrations, irreversible inactivation, presumably due to denaturation, occurred. The atypical enzyme was much more readily inactivated with all the alkanols, and only slight ac-tivation was observed at low alkanol concentrations. 

TABLE 2. AChE and BuChE Activities in Male (M) and Female (F) Individuals Measured with Acetylthiocholine (ATCh), ButyrylthiochoTme (BTCh), Propionylthictchoiine (FTCh), and Benzoylcholine (BzCh) as Substrates ... 

The values for benzoylcholine were found to be 5 x 10" mol/liter for the usual enzyme, and 24 x 10" mol/liter for the homozygous atypical enzyme—a 4.8-fold difference. The value for the uncharged ester o-nitrophenylbutyrate was found to be 1.4 x 10" mol/liter for both enzymes. This is not in agreement with the report of McComb et al. (M2), who found that the for this substrate was about twice as great in the case of the atypical enzyme. 

Evans and Wroe (E22) conducted a study of the hydrolysis rates of four commonly used substrates as related to cholinesterase genotypes and suc-cinylcholine hypersensitivity. They concluded that the propionyl-thiocholine hydrolysis rate is capable of detecting over 90% of hypersensitive individuals, with no false positives. The cutoff value was the mean for the usual phenotype minus 2.5 SD. Acetylcholine and butyrylthiocholine were slightly inferior, and benzoylcholine was almost useless. If... 

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). 

Since the introduction of this assay procedure, a host of variations of the procedure have been reported. Many of these variations are purported to be improvements, but this is not always obvious. The most frequent variations involve using a different substrate in place of benzoylcholine. In general, with the assay conditions reported (Kll), the results obtained with other substrates tend to parallel those obtained with benzoylcholine, although it has been claimed that the efficiency of differentiation varies with the substrate and in certain cases, with the genotype (B27). Some of the other substrates used for DN determinations include acetylcholine (B2, B27, G3), butyrylcholine (B27, G4), pro-pionylthiocholine (D15-17, G3), and succinylcholine (A3). However, most laboratories have used the procedure of Kalow and Genest as briefly described above (Kll). A procedure using propionylthiocholine is described in Section 6.4. 

The most common mutant is the so-called dibucaine-resistant enzyme variant. It is characterized principally by its reduced afiSnity for a series of substrates and inhibitors as compared with the normal pseudocholinesterase. Although at high concentrations succinyldicholine can also be converted by the dibucaine-resistant enzyme variant, the absence of enzymatic hydrolysis under pharmacologic conditions is adequately explained by the large difference in the Michaelis constants of succinyldicholine with the normal enzyme with the dibucaine-resistant variant Usually the variant is identified by the reduced inhibition of its reaction with the substrate benzoylcholine produced by the local anaesthetic dibucaine (cinchocaine). The percentage of inhibition by dibucaine under standard conditions, defined as the dibucaine number (DN). is about 20 for the dibucaine-resistant variant and app oximately 80 for the ordinary enzyme. Serum activity from heterozygotes having both enzymes is inhibited about 40-70%. 

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