Synthesis and binding to tubulin of an allocolchicine spin probe

A spin probe for the colchicine binding site on tubulin has been synthesized from allocolchicine. The probe is competitive to colchicine with an apparent inhibition constant of 11 microM while allocolchicine has an inhibition constant of 2 microM. Microtubule assembly is inhibited to 50% by 7 microM of the spin probe. As a mitotic poison the spin probe is less potent than colchicine. These data suggest that the probe binds to the same site on tubulin as colchicine and in spite of the somewhat lower efficiency, it seems to be a valuable tool for the study of the microtubule system.