A fluorescence stopped flow study of the competition and displacement kinetics of podophyllotoxin and the colchicine analog 2-methoxy-5-(2',3',4'-trimethoxyphenyl) tropone on tubulin.

The colchicine analog 2-methoxy-5-(2',3',4'-trimethoxyphenol) tropone (AC) was used as a fluorescent probe to study the binding kinetics of podophyllotoxin at high concentrations. The observed pseudo-first order rate constant showed a linear concentration dependence up to 1 mM. The bimolecular rate constant (195 M-1 s-1 at 15 degrees C) and the activation energy (57 kJ/mol) correspond perfectly with those previously determined in the submicromolar range (Cortese, F., Bhattacharyya, B., and Wolf, J. (1977) J. Biol. Chem. 252, 1134-1140). Displacement kinetics of bound AC by podophyllotoxin, allow the determination of the dissociation rate constants for AC. By studying the temperature dependence, and combining with the binding rate constants previously determined (Engelborghs, Y., and Fitzgerald, T.J. (1986) Ann. N.Y. Acad. Sci. 466, 709-717) a full characterization of the kinetic pathway is possible. This is shown to differ considerably from the pathway of colchicine binding.