Stability of butyrylcholinesterase: thermal inactivation in water and deuterium oxide

Abstract Irreversible thermal inactivation of the tetrameric form of human plasma butyrylcholinesterase (cholinesterase; EC 3.1.1.8) was studied in water and in deuterium oxide at pH 7 in the temperature range 53–65°C. The enzyme inactivation follows a complex kinetics that may be described by the sum of two apparent first-order processes. The Eyring plot for enzyme inactivation exhibits a wavelike discontinuity over a span of 2 C° around 58°C. This transition was interpreted in terms of equilibrium between two temperature-dependent conformational states. Though 2 H 2 O does not alter the overall multistep inactivation process, a slight solvent isotope effect was observed: a stabilizing effect and a shift in the transition temperature. A comparison between several enzyme preparations revealed differences in thermodynamic activation parameters of inactivation suggesting microheterogeneity in enzyme structures. Kinetics of inactivation of usual (E 1 u E 1 u ) and atypical (E 1 a E 1 a ) enzymes were compared. The atypical enzyme was found to be more stable than the usual phenotype.