Colchicine photosensitizes covalent tubulin dimerization

Pure rat brain tubulin can be cross-linked by ultraviolet irradiation of tubulin-colchicine complexes at the high-wavelength maximum of colchicine to form covalent dimers greater than trimers greater than tetramers. With colchicine concentrations approximately 3 x 10(-4) M (mole ratio to tubulin 3-12) and irradiation for 5-10 min at 95-109 mW/cm2, the yield of dimers is 11-17% and of trimers is 4-6% of the total tubulin. The oligomers show polydispersity and anomalously high apparent molecular masses that converge toward expected values in low-density gels. Maximal dimer yields are obtained with MTC and the decreasing photosensitizing potency is MTC greater than colchicine greater than colchicide greater than isocolchicine greater than thiocolchicine. Single-ring troponoids also promote dimerization. Evidence is presented suggesting that the initial, low-affinity, binding step of colchicine and its analogues is sufficient to photosensitize tubulin dimerization.