Molecular mechanisms of the interaction of anti-mitotic ligands with tubulin and microtubules: Special emphasis on taxol and vinblastine

Baccatin III's effect on in vitro assembly of tubulin has been re-examined under a variety of conditions. Baccatin III was found to be active in all circumstances in which Taxol is active. The effect of baccatin III on in vitro microtubule assembly was quantitatively assessed through determination of critical concentrations, which can be used to obtain the apparent equilibrium constants for the addition of tubulin subunits to growing microtubules. A fluorescent derivative of baccatin III (2-m-methoxybenzoyl-10-m-aminobenzoylbaccatin III) was used to study the interaction of baccatin III binding to microtubules. Our observations support the idea that the majority of the interactions between Taxol and tubulin that affect this equilibrium occur between the baccatin portion of the molecule and the binding site. The results also have significant bearing on the search of a common pharmacophore for Taxol like ligands.; A coumarin derivative of vinblastine, 17-deacetyl-O-(3-carbonylamino-7-diethylaminocoumarin)vinblastine (F-VLB), was designed to retain high affinity for tubulin. F-VLB was a potent inhibitor of in vitro microtubule assembly and F-VLB binding to tubulin was inhibited by vinblastine. Solvent studies and equilibrium binding studies of F-VLB with microtubules were performed. The results indicate that F-VLB can be a suitable probe to study the vinblastine-microtubule interaction in a molecular level.