A fluorescence stopped flow study of colchicine binding to tubulin

The kinetics of colchicine binding to tubulin has been studied, using a fluorescence stopped flow. The measurements of Garland (Garland, D. L. (1978) Biochemistry 17, 4266-4272) have been extended to high colchicine concentrations and different temperatures. The appearance of fluorescence is biphasic. Both phases depend in a nonlinear way on colchicine concentration. The presence of colchicine dimers at these concentrations has been taken into account. The fast phase is analyzed as a two-step mechanism. The thermodynamic parameters of the fast initial binding, and the activation energy of the slow conformational change, have been determined. The relative magnitude of the slow phase depends on temperature. It is interpreted as a slow preequilibrium between two tubulin conformers. The effect of the microtubule-associated proteins on the different processes is studied. The binding of colchicine to tubulin ring-like oligomers is discussed.