Association of thiocolchicine with tubulin

Abstract

Thiocolchicine, a colchicine analog in which the C-10 methoxy is replaced with a thiomethyl moiety, was shown to bind with high affinity to the colchicine site on tubulin (K a = 1.07 ± 0.14 × 10 6 M −1 at 23°C). Like colchicine, the association kinetics were biphasic, and the rate constants of both phases were temperature dependent. The rate constant of the fast phase of the association was 4 times greater than the rate constant for colchicine binding, and the activation energy was lower (19.1 ± 1.8 kcal/mol). X-ray crystallographic analysis shows that thiocolchicine displays greater puckering of the tropone C ring than colchicine ( Koerntogen, C. and Margulis, T. N. (1977) J. Pharm. Sci. 66, 1127-113). These results indicate that the conformation of the C ring may have little effect on the energetics of colchicinoids binding to tubulin.