Abstract
This paper presents the results of electrospray ionization mass spectrometry (ESIMS) applied to an enzyme-inhibitor complex, using papain and cystatin with partly lagged N-terminus. It has been reported that the inhibitory activity of cystatin, a thiol protease inhibitor, toward papain decreases in several orders of magnitude when the N-terminal seven or eight residues are lost. In the absence of papain, multiply charged full-length cystatin was mainly observed accompanied by the signals of minor components, N-terminally truncated forms. When cystatin was mixed with equimolar quantity of papain, the relative intensity of the free full-length cystatin dramatically decreased. It might be caused by the higher binding affinity of the intact cystatin for papain than those of the truncated forms. The present study indicates the potential of ESIMS for the investigation of the structure-activity relationship without isolation of each inhibitor species carrying different level of inhibitory activity.
