Many active drugs have binding constants to their receptors or target enzymes in the low nanomolar or subnanomolar range. Conventional methods of kinetic analysis will not give correct parameter estimates for these inhibitors because one of their underlying assumptions, that the concentration of free drug is not appreciably depleted by binding to its target, is not valid.
The equation of Morrison (1969) correctly describes the kinetics of tight-binding inhibitors (often inaccurately described as “stoichiometric” inhibitors) and TIGHTFIT provides a convenient method of calculating Ki values for such inhibitors by performing nonlinear regression to the Morrison equation. It can be used with competitive and non-competitive inhibitors of enzymes that may be one-substrate enzymes or two-substrate enzymes with a range of kinetic mechanisms (random-order, rapid equilibrium random, ordered sequential, or ping-pong). Standard errors are calculated for the parameter estimates. Output is produced that can be used by graphics programs (including Microsoft Excel and R-graphics) to produce plots showing experimental points and the fitted inhibition curve.