Hydantoin drugs inhibit polymerization of pure microtubular protein

5,5-Diphenylhydantoin or 5-(4-hydroxyphenyl)-5-phenylhydantoin, 3.6 X 10(-4) M, inhibited polymerization of purified pig brain tubulin 50% as measured by viscosometric assay and estimation of tubulin length by electron microscopy. 5,5-Diphenylhydantoin did not bind to the low molecular weight fraction of tubulin or to guanosinetriphosphate; it displaced colchicine weakly from its binding site. 5,5-Diphenylhydantoin did not depolymerize established microtubules, indicating that it differs from colchicine in site and mode of action.