Vmax and Km 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 Vmax substrate to Vmax 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 Km 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 Km 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.
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