Measurement of individual rate constants of irreversible inhibition of a cysteine proteinase by an epoxysuccinyl inhibitor

The apparent binding constant (Ki) and the first order rate constant of inactivation (k3) have been obtained for EP-475 inhibition of papain at partially saturating concentrations of inhibitor and various substrate concentrations by global analysis of the data. The value of k3 (k3 = 0.71 ± 0.25 s−1) is larger whereas Ki (2.4 ± 0.91 μM) is comparable to or higher than those values for most nucleofuge methylketone inhibitors.