Irreversible inhibition of enzyme activity often results from covalent modification of the enzyme protein. Once the enzyme is covalently bound to an irreversible inhibitor it is permanently incapacitated. The inhibition is time-dependent and not freely reversible by procedures like dilution, dialysis, or gel filtration. Such inhibitors are often referred to as enzyme inactivators. Irreversible inhibition effectively decreases the concentration of the enzyme present—the net result being a reduced Vmax (because Vmax = kcat × [Et] and [Et] is actually reduced during irreversible inactivation)—while the KM of the remaining active enzyme is unaltered. This is reminiscent of a reversible noncompetitive inhibition pattern where both Vmax and Vmax/KM are affected (see Chap. 21 ). Therefore, for any new inhibitor, it is prudent to first establish whether that inhibitor is reversible or not! Without much hair-splitting on their nomenclature, we will consider three broad categories of enzyme inactivators in terms of their mechanism.