INHIBITION OF BOVINE BRAIN MICROTUBULE ASSEMBLY INVITRO BY STYPOLDIONE

  • 1 January 1983
    • journal article
    • research article
    • Vol. 24  (3) , 493-499
Abstract
Stypoldione, an orthoquinone derived from the brown seaweed Stypopodium zonale, inhibited the polymerization of 3-cycle-purified bovine brain microtubule protein in vitro in a concentration-dependent manner. Fifty % inhibition of the extent of polymerization beginning under initiating conditions occurred at a stypoldione concentration of .apprx. 25 .mu.M, and 50% inhibiton of tubulin addition to the assembly ends of microtubules at steady state occurred at a concentration of .apprx. 8 .mu.M. Only slight structural abnormalities could be detected by negative stain EM in some of the microtubules that did assemble in the presence of the drug, and no aberrant structural forms of microtubule protein were detected. Stypoldione inhibited the binding of [3H]coclchicine to tubulin, with 50% inhibition of colchicine binding activity occurring at a stypoldione concentration of 12-15 .mu.M. Inhibition of colchicine binding activity appeared noncompetitive and was at least partially reversible, suggesting that stypoldione and colchicine bind at separate sites. By assuming that the inhibition constant for the ability of stypoldione to prevent the binding of colchicine to tubulin was equivalent to the Kd for the binding of stypoldione to tubulin, it was calculated that .apprx. 62% of the tubulin present free in solution under initiating conditions and 35-37% of the soluble tubuline under steady-state conditions was complexed with stypoldione when polymerization was inhibited by 50%. There may be a mechanism in which stypoldione interacts with soluble tubulin and inactivates the tubulin so that it is unable to add to microtubule ends has not been eliminated.