GRISEOFULVIN - A NOVEL INTERACTION WITH BOVINE BRAIN TUBULIN

Griseofulvin is an anti-fungal drug whose mechanism of action is direc ted against microtubules. Although it inhibits the assembly of mammali an brain tubulin, its binding to tubulin has not been directly measure d successfully. We have examined the interaction of griseofulvin with tubulin fluorometrically by measuring the quenching of tubulin tryptop han fluorescence by griseofulvin. From Scatchard analysis, we found th at griseofulvin bound to tubulin at one class of binding site with an affinity constant of 1.26 +/- 0.19 x 10(4) M(-1), and the binding was largely reversible. Griseofulvin caused a major change in the conforma tion of tubulin in that it increased the sulfhydryl titer of tubulin a pproximately 2-fold. The drug affected both the alpha and beta subunit s of tubulin equally. Interestingly, griseofulvin did not increase the sulfhydryl titer of the tubulin-colchicine complex although the bindi ng site of griseofulvin in was distinctly different from that of colch icine. The change of conformation of tubulin upon interaction with gri seofulvin did not affect the exposure of hydrophobic areas on tubulin as shown by binding of bis-5,5'-[8(N-phenyl)aminonapthalene-1-sulfonic acid] (BisANS). Even in combination with colchicine, griseofulvin had very little effect on BisANS binding to tubulin. Thus, griseofulvin a ppears to interact with tubulin in a manner that is very different fro m that of many other tubulin ligands.