Chemical mechanism of lysophosphatidylcholine: lysophosphatidylcholine acyltransferase from rabbit lung. pH-dependence of kinetic parameters

Lysophosphatidylcholine: lysophosphatidylcholine acyltransferase is an enzyme that catalyses two reactions: hydrolysis of lysophosphatidylcholine and transacylation between two molecules of lysophosphatidylcholine to give disaturated phosphatidylcholine. Following the kinetic model previously proposed for this enzyme [Martín, Pérez-Gil, Acebal & Arche (1990) Biochem. J. 266, 47-53], the values of essential pK values in free enzyme and substrate-enzyme complexes have now been determined. The chemical mechanism of catalysis was dependent on the deprotonation of a histidine residue with pK about 5.7. This result was supported by the perturbation of pK values by addition of organic solvent. Very high and exothermic enthalpy of ionization was measured, indicating that a conformational re-arrangement in the enzyme accompanies the ionization of the essential histidine residue. These results, as well as the results from previous studies, enabled the proposal of a chemical mechanism for the enzymic reactions catalysed by lysophosphatidylcholine: lysophosphatidylcholine acyltransferase from rabbit lung.

The standard enthalpy of ionization of water at 298 K from calorimetric measurements on iodine pentoxide

We have made calorimetric measurements of the enthalpies of reaction of I2O5(c) with water and with a small excess of OH−(aq) and have combined the results to obtain ΔH0 = 13.340 kcal mol−1 for the standard enthalpy of ionization of water. It is suggested that this approach can be applied usefully to investigation of the enthalpy of ionization of D2O for which there are still unresolved differences between reported values.