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The interaction of colchicine and some related alkaloids with rat brain tubulin.

A study of the interaction of colchicine and purified rat brain microtubule protein is presented. Experimental data agree with a theory derived to describe the system. Protein stabilized with 0.1 mM vinblastine and 1 mM GTP lost the ability to bind colchicine in a first-order reaction, with a rate constant of 1.1 ± 0.1 x 10-5 sec -1. The colchicine-microtubule protein complex appeared to be stable. The rate constant for the second-order association of colchicine and microtubule protein is 128.7 ± 6.5 M-1 sec-1, while the rate constant for dissociation is 8.1 ± 0.8 x 10-6 sec-1. The dissociation constant calculated from these two numbers is 6.3 x 10-8 M. By using other ligands to perturb the interaction between colchicine and microtubule protein, the affinities of these ligands for microtubule protein could be measured. The methods developed are applicable in general for ligands of microtubule protein. Podophyllotoxin binds firmly to microtubule protein (Kd = 0.28 ± 0.06 µM), while picropodophyllin and lumicolchicine bind much more weakly than their respective isomeric partners. All three ligands appear to compete with colchicine for a binding site on microtubule protein. The diastereoisomeric pair of podophyllotoxin and picropodophyllin should be a useful tool with which to study the involvement of microtubules in physiological processes.