Irreversible inhibitors display time-dependent inhibition and their potency therefore cannot be characterised by an IC50 value. This is because the amount of active enzyme at a given concentration of irreversible inhibitor will be different depending on how long the inhibitor is pre-incubated with the enzyme. Instead, "k"obs/["I"] values are used, where "k"obs is the observed pseudo-first order rate of inactivation (obtained by plotting the log of % activity vs. time) and ["I"] is the concentration of inhibitor. The "k"obs/["I"] parameter is valid as long as the inhibitor does not saturate binding with the enzyme (in which case "k"obs = "k"inact).