Calculation of inhibitor Ki and inhibitor type from the concentration of inhibitor for 50% inhibition for Michaelis-Menten enzymes

The use of I 50 (concentration of inhibitor required for 50% inhibition) for enzyme or drug studies has the disadvantage of not allowing easy comparison among data from different laboratories or under different substrate conditions. Modifications of the Michaelis-Menten equation for treatment of inhibitors can allow both the determination of the type of inhibition (competitive, noncompetitive, and uncompetitive) and the K i for the inhibitor. For competitive and uncompetitive inhibitors when the assay conditions are [ S] = K m , then K i = I 50 2 . For different conditions of [ S] there is a divergence between competitive and uncompetitive inhibitors that may be used to identify the type of inhibitor. The equation for K i also differs. For noncompetitive inhibitors the K i = I 50 and this relationship is valid with changing [ S]. The equations developed require a single substrate, reversible-type inhibitors, and kinetics of the Michaelis-Menten type. Examples of the use of the equations are illustrated with experimental data from scientific publications.