Determination of inhibition constants, I50 values and the type of inhibition for enzyme-catalyzed reactions

A procedure is proposed for determining whether an inhibitor of an enzyme-catalyzed reaction is competitive, noncompetitive, or uncompetitive with respect to the substrate. The method is based on fitting the equation for noncompetitive inhibition to data obtained by measuring the rate of the reaction over a range of substrate and inhibitor concentrations. The results of this fit may suggest that the inhibition may be either competitive or uncompetitive, whereupon the data are reanalyzed using the appropriate equation. Comparison of this second fit with the first using an F test permits a statistical decision to be made on the type of inhibition. The chosen fit yields values and standard errors for the Michaelis-Menten parameters (maximum velocity and Michaelis constant), as well as the inhibition constant(s). From these values it is then possible to predict the I50, and its standard error, at any chosen substrate concentration, thereby facilitating comparison with results obtained with similar inhibitors, for homologous enzymes, or in other laboratories.