A graphical method for determining inhibition constants

A new simple graphical method is described for the determination of inhibition type and inhibition constants of an enzyme reaction without any replot. The method consists of plotting experimental data as (V-v)/v versus the inhibitor concentration at two or more concentrations of substrate, where V and v represent the maximal velocity and the velocity in the absence and presence of inhibitor with given concentrations of the substrate, respectively. Competitive inhibition gives straight lines that converge on the abscissa at a point where [I] = −Ki. Uncompetitive inhibition gives parallel lines with the slope of 1/K'i. For mixed type inhibition, the intersection in the plot is given by [I] = −Ki and (V-v)/v = −Ki/K'i in the third quadrant, and in the special case where Ki = K'i (noncompetitive inhibition) the intersections occur at the point where [I] = −Ki and (V-v)/v = −1. The present method, the "quotient velocity plot," provides a simple way of determining the inhibition constants of all types of inhibitors.