Effect of temperature and pH on carbamoylation and phosphorylation of serum cholinesterases. Theoretical interpretation of activation energies in complex reactions.

Abstract

1. The effect of temperature and pH was studied on the kinetics of inhibition of horse serum and human serum cholinesterase by four organophosphorus compounds and five carbamates. 2. For all compounds, and at each pH and temperature, the inhibition followed the kinetics of a bimolecular reaction with the inhibitor in excess, and with a negligible concentration of the Michaelis complex. 3. The second-order rate constants (k(a)) for inhibition of human serum cholinesterase by one organophosphate and one carbamate increased from 5 degrees to 40 degrees C with an apparent activation energy of 46kJ/mol (11kcal/mol). 4. The k(a) constant for inhibition of horse serum cholinesterase increased with temperature from 5 degrees to 30 degrees C, and then decreased from 30 degrees to 40 degrees C. The theoretical interpretation of such an unusual effect of temperature is derived. 5. The increase of k(a) with pH (human serum cholinesterase) followed the dissociation curve for a single group on the enzyme (pK7.5). 6. Rate constants for decarbamoylation (k(+3)) were determined, and the time-course of inhibition was calculated from the k(a) and k(+3) constants.