The Acetylcholinesterase Defect in Paroxysmal Nocturnal Hemoglobinuria: Evidence That the Enzyme Is Absent From the Cell Membrane

Paroxysmal nocturnal hemoglobinuria (PNH) is a myelodys-plastic disease characterized by erythrocytes that show abnormally increased sensitivity to complement-mediated lysis. Complement-sensitive PNH erythrocyte membranes have previously been shown to lack acetylcholinesterase (AchE) activity. but the molecular basis of this deficiency has been unclear. We have used monoclonal antibodies to four different epitopes on the AchE molecule to show that abnormal PNH erythrocytes failed to bind these antibodies. A CETYLCHOLIN ESTERASE ([AchE] EC 3. 1. 1.7) is found on the plasma membrane surface of all mamma-han erythrocytes. ’ However, the role of erythrocyte AchE remains unknown. Deficiency of erythrocyte AchE activity has been shown to occur as both inherited2 and acquired phenomena.37 Although many diseases have been described as associated with decreased AchE activity, paroxysmal nocturnal hemoglobinuria (PNH) is the only disease in which decreased AchE activity has been consistently found.’8 PNH is characterized by the existence of at least two populations of erythrocytes (E)-one of normal E (PNH I cells) and the other(s) of cells that exhibit varying degrees of abnormal sensitivity to lysis by complement (PNH II and III cells). Auditore and Hartmann6 first described decreased AchE activity in the blood and bone marrow of patients with PNH in 1959. The decrease in enzyme activity was soon shown to be proportional to the severity of the disease.7 Kunstling and Rosse showed that the abnormal complement-sensitive E of PNH patients had no detectable AchE activity, while AchE activity was present on the normal complement-insensitive (PNH I) E.8 However, the cause and possible importance of the loss of AchE activity on PNH cells have remained unclear. Investigators have postulated that the enzyme might be structurally abnormal, that its active sites might be blocked by an unknown factor, or that it is abnormally oriented in the lipid bilayer.9 In the present study, we have used four monocbonab antibodies to four different epitopes on human E AchE’#{176}to show that abnormal PNH E appear to lack the AchE protein rather than to have a protein with abnormal or no enzyme activity