A reexamination of the reaction between colchicine and tubulin

The rate of reversible dissociation of the colchicine-tubulin complex has been determined for purified porcine brain tubulin and for sea urchin tubulin from 100,000 times g supernatant. The rate constant, which is essentially the same for both species, is extremely slow; it corresponds to a half-life of approximately 36 hours for the reaction. This rate was the same whether or not sucrose was present, and was not influenced by low concentrations of vinblastine or by millimolar concentrations of colchicine. The association rate constant was determined for porcine brain tubulin with and without CaCl2. The sample without CaCl2 had been maximally polymerized prior to the addition of colchicine. A 4.7-fold difference was observed between the association rate constants under these two conditions. Equilibrium constants calculated from the measured rate constants are 5 to 20 times greater than directly measured values; this suggests that the interaction of microtubules and colchicine is more complex than was previously thought.