SUMMARY

and some rebated alkaloids with rat brain tubulin. Mol. Pharmacol., 13, 560-575. A study of the interaction of colchicine and purified rat brain microtubube protein is presented. Experimental data agree with a theory derived to describe the system. Protein stabilized with 0. 1 m vinbiastine 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 sec ‘. The colchicine-microtubule protein complex appeared to be stable. The rate constant for the second-order association of coichicine and microtubule protein is 128.7 ± 6.5 M1 sec’, while the rate constant for dissociation is 8. 1 ± 0.8 x 10 sec ’. The dissociation constant calculated from these two numbers is 6.3 x 10 M. By using other bigands to perturb the interaction between colchicine and micnotubule protein, the affinities of these ligands for microtubule protein could be measured. The methods developed are applicable in general for bigands ofmicnotubule protein. Podophylbotoxin binds firmly to microtubube protein (K(j- 0.28 ± 0.06 jiM), while picropodophyblin and lumicobchicine bind much more weakly than their respective isomenic partners. All three ligands appear to compete with colchicine for a binding site on micnotubule protein. The diastereoisomenic pair of podophylbotoxin and picropodophyllin should be a useful tool with which to study the involvement of microtubules in physiological processes