The duration and/or intensity of action of numerous drugs and other xenobiotics is determined by their rate of metabolism (biotransformation) by several cytochrome P450 enzymes that are localized primarily in liver endopiasmic reticulum (microsomes). If a compound is metabolized by a particular P450 enzyme, it will competitively inhibit the metabolism of other drugs metabolized by the same enzyme and, conversely, these same drugs will competitively inhibit the metabolism of that compound. The degree of inhibition will be determined by the concentrations of the given compound and of other drugs and their affinity (Km and Ki) for binding to the P450 enzyme that metabolizes them. Alternatively, compounds can be activated by P450 enzymes to metabolites that can reversibly or irreversibly inhibit P450 enzymes. This type of inhibitor most often exhibits kinetics characteristic of non-competitive inhibitors, although mixed inhibition (competitive and non-competitive) is also possible. The inhibitory potential of metabolism-dependent inhibitors will depend on the rate of inactivation of the P450 enzyme, the dose of inhibitor administered and the total content of the P450 enzyme in the liver.