Comparison of Atypical and Usual Human Serum Cholinesterase

Atypical and usual human serum cholinesterases were purified and studied with the fluorescent probe, N-methyl(7-dimethylcarbamoxy)quinolinium iodide. Four active sites per tetramer were found in each enzyme. The turnover numbers of usual and atypical cholinesterases were the same: 15,000 pmol of benzoylcholine hydrol~zedlminl~mol of active site; 48,000 min-’ for o-nitrophenylbutyrate; and 0.0025 min-’ for N-methyl-(7-dimethylcarbamoxy)quinolinium iodide. They had identical rate constants for carbamylation, (5.0 min-‘) and for decarbamylation (0.15 h-l). The major difference between the two genetically determined forms of the enzyme was substrate affinity, Ku being 0.16 mM for usual and 5.4 mM for atypical cholinesterase, for the fluorescent probe substrate. K,,, for the uncharged ester, o-nitrophenylbutyrate, was 0.14 mM for both enzymes, whereas K,, for benzoylcholine was 0.005 mM for usual and 0.024 mM for atypical cholinesterase. We interpret these data to mean that the two enzymes differ only in the structure of their anionic site.