Binding of gossypol to purified tubulin and inhibition of its assembly into microtubules

1 Gossypol is a polyphenolic pigment, which is employed as a male antifertility drug. It inhibits, among other reported effects, the growth of cultured mammalian cells, spermiogenesis, flagellar motility in Trypanosoma and sperm, dynein ATPase and the lactate dehydrogenase X (LDH‐X) isozyme. 2 We have characterized the non‐covalent binding of gossypol to purified calf brain tubulin in 10 mM phosphate buffer, 0.1 mM GTP pH 7.0 at 25°C. Equilibrium measurements were performed by difference spectroscopy. A peak at 435 nm was produced by the perturbation of gossypol light absorption upon binding to tubulin. The experimental isotherm was fitted by 1.96 ± 0.06 gossypol binding sites per tubulin molecule, with identical apparent equilibrium binding constants of (7.5 ± 1.1) × 10 4 M −1 . The complex formed could be separated from free gossypol by gel chromatography. 3 Binding of gossypol was independent of the presence of 0.1 mM GTP in the buffer. Gossypol did not affect the binding of ligands to the colchicine site. Gossypol interacted with vinblastine but apparently did not bind to the vinblastine sites of tubulin. Gossypol did not displace anilinonaphthalene sulphonate (ANS) bound to tubulin, but caused a strong (fivefold) quenching of its fluorescence. This indicated that gossypol probably binds in the vicinity of the ANS site of tubulin. 4 Gossypol inhibited in vitro microtubule assembly at the same concentration range employed in the binding studies. An increase in the critical protein concentration required for polymerisation was observed, most simply interpreted by a stoichiometric mechanism. Gossypol did not induce any noticeable distortion of the microtubules observed under the electron microscope. This compound constitutes a new tubulin ligand and an inhibitor of microtubule assembly in vitro .