Immunological comparison of the usual and atypical human serum cholinesterase phenotypes

Antiserum prepared against highly purified usual human serum cholinesterase (the most common phenotype) cross-reacted identically with the atypical serum cholinesterase. The level of circulating atypical enzyme protein, determined immunologically, was about 30% lower when the enzyme came from an atypical rather than a usual phenotype, and the level of enzyme activity measured enzymatically at Vmax with either o-nitrophenylbutyrate or benzoylcholine as substrate showed approximately the same degree of reduction. The average specific activity (activity at Vmax per microgram of enzyme protein) in sera from 28 usual and 20 atypical individuals did not differ significantly. These findings suggest that the atypical enzyme not only has altered catalytic properties (Km) but also might be synthesized more slowly, or cleared in vivo more rapidly, than the usual enzyme.