Prediction of drug sensitivity in individuals with atypical serum cholinesterase based on in vitro biochemical studies

V_max and K_m values with twenty-five "atypical" and thirty-seven "usual" cholinesterase human sera were determined for the cholinesterase substrates procaine, tetracaine, benzoylcholine, o-nitro-phenylbutyrate, α-naphthylacetate and aspirin. Aspirin was demonstrated to be a substrate for serum cholinesterase. For each of these substrates the ratio of V_max substrate to V_max benzoylcholine was found to be similar with atypical and usual cholinesterase sera. Therefore, we concluded that the respective turnover numbers for atypical and usual cholinesterase were the same. Both atypical and usual cholinesterase sera had turnover numbers of 255 min^-1 for procaine, 74 min^-1 for tetracaine, 7200 min^-1 for aspirin in the presence of 50 mM CaCl₂ 36,000 min^-1 for α-naphthylacetate, and 48,000 min^-1 for o-nitrophenylbutyrate, at 25° in 0.1 M Tris-Cl buffer, pH 7.4. A comparison of K_m values for atypical and usual cholinesterase indicated that the positively charged substrates, as well as aspirin in the presence of CaCl₂, showed a lower affinity with atypical than with usual cholinesterase, while neutral esters had nearly the same K_m for atypical and usual cholinesterase. These results imply that individuals with atypical cholinesterase will hydrolyze therapeutic doses of positively charged substrates and aspirin at reduced rates, but neutral substrates should be hydrolyzed at normal rates.