Lipid-protein interactions in human erythrocyte-membrane acetylcholinesterase. Modulation of enzyme activity by lipids.

Purified human erythrocyte membrane acetylcholinesterase was incorporated into vesicles of various lipid compositions. The activities of the free and the lipid-associated enzyme were assayed at temperatures between 4 degrees C and 40 degrees C and the results were visualized as plots of log v versus 1/T (Arrhenius plots). For the purified, detergent-depleted enzyme a linear relation was obtained. If Triton X-100 was added to the assay medium a curved plot resulted. For acetylcholinesterase incorporated into dimyristoylphosphatidylcholine vesicles a clear break in the plot was observed at the phase transition temperature of the lipid. With lipids not experiencing a phase transition within the temperature range investigated, again a linear relation was obtained. These results show that the activity of human erythrocyte membrane acetylcholinesterase is strongly modulated by its hydrophobic environment.