IC50-to-Ki: a web-based tool for converting IC50 to Ki values for inhibitors of enzyme activity and ligand binding

Abstract

A new web-server tool estimates K(i) values from experimentally determined IC(50) values for inhibitors of enzymes and of binding reactions between macromolecules (e.g. proteins, polynucleic acids) and ligands. This converter was developed to enable end users to help gauge the quality of the underlying assumptions used in these calculations which depend on the type of mechanism of inhibitor action and the concentrations of the interacting molecular species. Additional calculations are performed for nonclassical, tightly bound inhibitors of enzyme-substrate or of macromolecule-ligand systems in which free, rather than total concentrations of the reacting species are required. Required user-defined input values include the total enzyme (or another target molecule) and substrate (or ligand) concentrations, the K(m) of the enzyme-substrate (or the K(d) of the target-ligand) reaction, and the IC(50) value. Assumptions and caveats for these calculations are discussed along with examples taken from the literature. The host database for this converter contains kinetic constants and other data for inhibitors of the proteolytic clostridial neurotoxins (http://botdb.abcc.ncifcrf.gov/toxin/kiConverter.jsp).

Figure 1.

Results page from the IC₅₀-to-K_i web tool for a tight-binding inhibitor of monoamine oxidase. The top table contains sample input data obtained from ref. 16. The middle table contains the results for a classic inhibitor that follows Michaelis–Menten kinetic Equations (1a–3a) for three kinetic reactions. The bottom table contains the results for nonclassic, tight-binding inhibitor uses Equations (1b–3b) for the same three reactions. The histograms summarize these results. Equations for each displayed mode of inhibition can be viewed by clicking on its label. A help list located on the upper right side is available for more detailed information about this tool.