An immunodisplacement assay based on a specific, solid-phase monoclonal antibody was designed to measure acetylcholinesterase in tissue extracts. Sample enzyme content was determined from the competitive reduction of binding of a purified acetylcholinesterase standard, with a detection limit of 5 ng or less. Washed erythrocyte membranes from six normal subjects averaged 1.8 units of acetylcholinesterase activity and 0.45 μg of acetylcholinesterase content per milligram of total protein. Enzyme activity and content in samples from three patients with paroxysmal nocturnal hemoglobinuria (PNH) were reduced approximately in parallel, by as much as 70%. The residual cholinesterase had almost the same homospecific activity as the normal enzyme and was bound with equivalent affinity by six different antibodies. Therefore, the cholinesterase defect was dominated by enzyme loss rather than by structural abnormalities affecting enzyme function. Fluorescence-activated sorting of antibody-labeled erythrocytes revealed a bimodal population distribution. Up to 66% of the PNH cells lacked cholinesterase, and the rest had a near-normal enzyme content. Inasmuch as enzyme-deficient cells represent the complement-sensitive population, cell sorting may help in assessing clinical status and, perhaps, in developing new therapeutic modalities for PNH.
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