Thermal inactivation of butyrylcholinesterase and acetylcholinesterase.

Differences were observed in the extent of thermal inactivation of human butyrylcholinesterase (BuChE) and eel acetylcholinesterase (AChE). BuChE was more resistant to 57 degrees C inactivation than was AChE. Thermal inactivation of BuChE was reversible and followed first-order kinetics. AChE thermal inactivation was irreversible and did not follow first-order kinetics. AChE was marginally protected from thermal inactivation by the "nonspecific salts" ammonium sulfate and sodium chloride and to a greater extent by the "active site-specific salts" choline chloride, sodium acetate, and acetylcholine iodide. This protection was accompanied by a loss of absorbance at 280 nm. This data supports the hypothesis that thermal inactivation of AChE occurs by conformational scrambling and that aromatic amino acid residue(s) are involved in this process.