Vincristine-induced self-association of calf brain tubulin

The vincristine-induced self-association of tubulin has been examined in a sedimentation velocity study as a function of free drug concentration in PG buffer (0.01 M NaPi and 10(-4) M GTP, pH 7.0) at 20 degrees C. Analysis of the weight-average sedimentation coefficient (S20,w) as a function of protein concentration showed a good fit with the model of an indefinite, isodesmic self-association mechanism. Analysis of the apparent association constants in terms of the Wyman linkage relations showed a good fit to mediation of the self-association by the binding of one ligand molecule. The intrinsic association constant for dimerization of the vincristine-liganded tubulin was found to be 3.8 X 10(5) M-1, and the intrinsic equilibrium constant for the binding of the self-association-linked vincristine molecule had a value of 3.5 X 10(4) M-1, consistent with that measured by fluorescence in our laboratory [Prakash, V., & Timasheff, S. N. (1983) J. Biol. Chem. 258, 1689-1697]. Both reactions are stronger in the presence of vincristine than of vinblastine, reflecting the oxidation of a -CH3 group to -CHO when going from the latter drug to the former one.