KINETIC-ANALYSIS OF LIGAND-RECEPTOR ASSOCIATION DATA THAT DISPLAY AN OVERSHOOT PHENOMENON

A binding overshoot was frequently observed in the time course of asso ciation of diazepam with rat brain membrane receptors shortly after th e start of the interaction. Such time profiles most likely reflect the ''receptor switch'' mechanism, assuming an equilibrium between two fo rms of a receptor (R and R) that possess different affinities to the ligand (L) in question. Similar effects could be caused by the presenc e of a slowly dissociating competitor. The kinetics of these mechanism s were verified by simulation of theoretical time courses. A computer program for simulation of the time course, and estimation of rate cons tants of the individual reaction steps, was developed and is described in this communication. It employs the Euler-Cauchy integration for si mulation of theoretical time courses. Optimised estimates of the rate constants were computed by simultaneous random variation of parameters within a pre-set interval. Stable solutions can be obtained for this system, thus enabling evaluation of equilibrium constants defined by t he model. The source code is available in Turbo-Pascal. It can be used , after re-writing the rate equations, for fitting of similar kinetic models to suitable experimental data.