Clarification of Mechanism of Human Sputum Elastase Inhibition by a New Inhibitor, ONO-5046, Using Electrospray Ionization Mass Spectrometry

Liquid chromatography electrospray ionization mass spectrometry (LC/ESI-MS) to probe the nature of the covalent E–I complex was successfully applied to clarify the mechanism of human sputum elastase (HSE) inhibition by a new inhibitor, ONO-5046. The mass spectrum of the four HSE isozymes displayed their molecular ion peaks at m/z=26,018, 25,929, 25,200, and 25,054, respectively. Immediately after incubation, inactivation of HSE with ONO-5046 increased the four molecular ion peaks by approximately 84 amu, which was assigned to the mass unit of the pivaloyl moiety of ONO-5046. An additional minute of incubation of E–I complex restored the original molecular ion peaks. These observations strongly suggested that ONO-5046 inactivates HSE by a reversible ‘acylation–deacylation’ mechanism.