Effect of the B ring and the C-7 substituent on the kinetics of colchicinoid-tubulin associations.

The kinetics of four B-ring derivatives of colchicine binding to tubulin has been examined quantitatively. The bindings of deacetamidocolchicine, deacetylcolchicine, demecolcine, and N-methyl-demecolcine to tubulin were biphasic processes. The association rate constants were determined as a function of temperature, and the thermodynamic parameters for the transition states of the fast phase were calculated. The kinetic parameters for the formation of the deacetylcolchicine-tubulin, demecolcine-tubulin, and N-methyldemecolcine-tubulin complexes were very similar to each other, but different from the parameters for the colchicine-tubulin association. In particular, the global activation enthalpies for the formation of the three aminocolchicinoid-tubulin complexes were 3-5 kcal/mol greater than the global activation enthalpy of colchicine binding to tubulin. These results indicate that electronic rather than steric properties of the B-ring substituent are of greater importance in the activation enthalpy of colchicinoids binding to tubulin. In contrast, the global activation enthalpy for deacetamidocolchicine, which lacks a substituent on the C-7 carbon, binding to tubulin was virtually identical to the global activation enthalpy previously found for the colchicine analog that lacks the B ring, 2-methoxy-5-(2,3,4-trimethoxyphenyl)tropone, binding to tubulin (Bane, S., Puett, D., Macdonald, T. L., & Williams, R. C., Jr. (1984) J. Biol. Chem. 259, 7391-7398). This result demonstrates that the carbons of the B ring are not involved in the transition state for the formation of colchicinoid-tubulin complexes. The first-order dissociation rate constants of the colchicinoid-tubulin complexes were determined at 37 degrees C. The dissociation profiles of the colchicinoid-tubulin complexes also consisted of two phases.(ABSTRACT TRUNCATED AT 250 WORDS)