Thermal inactivation of the molecular forms of acetylcholinesterase and butyrylcholinesterase.

To compare acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) and butyrylcholinesterase (acylcholine acylhydrolase, EC 3.1.1.8), we utilized the physical parameter of thermolability. In serum or muscle extracts from mouse and rat, butyrylcholinesterase was inactivated as a unimodal function of temperature. Inactivation began at 51 degrees C and was complete at 54-57 degrees C (depending upon time of incubation). Acetylcholinesterase was inactivated in two stages. A 60% decrease in activity from 42 to 48 degrees C was followed by a plateau. The second stage of inactivation began at 51 degrees C and was complete at 57-60 degrees C (depending upon time of incubation). Sucrose density gradients revealed that the partial loss of acetylcholinesterase activity at 48 degrees C was due to inactivation of the monomeric 4 S enzyme, which was the most thermolabile molecular form in each tissue examined. When heated after isolation on density gradients, most of the forms of acetylcholinesterase and butyrylcholinesterase lost activity as a single exponential function of time. The monomers of both enzymes were inactivated fastest. Inactivation of the larger froms was slower and required higher temperatures. Tetrameric 10 S acetylcholinesterase was unique in following a time course that could only be fitted by a double exponential equation (i.e., when this form was heated to 55 degrees C, almost 60% of the activity showed a short half-life while the remainder showed a long half-life). This behavior did not reflect differences in the thermolability of soluble and membrane-derived tetramers.