A fluorescent probe and a photoaffinity labeling reagent to study the binding site of maytansine and rhizoxin on tubulin.

A fluorescent probe (20-demethoxy-20-[3-[[[5-(dimethylamino)naphthalen-1-yl]sulfonyl] amino]propyl]maytansinol 3-isobutyrate, Dan-PDM-3) and a photoaffinity labeling reagent (20-demethoxy-20-[(p-azidobenzoyl)oxy]maytansinol 3-isobutyrate, DABMI) were prepared by derivatization of ansamitocin P-3 (ASMP-3), a maytansinoid. Dan-PDM-3 consists of a tethered dansyl moiety and a maytansinoid moiety. DABMI contains a p-azidobenzoyl group instead of the tethered dansyl moiety of Dan-PDM-3. These compounds were synthesized by reacting 20-demethoxy-20-hydroxymaytansinol-3 isobutyrate (PDM-3) with the corresponding alkyl halide or benzoic acid. Both inhibit tubulin polymerization as potently as ASMP-3 and compete with ASMP-3 for binding to tubulin. The inhibition constants (Ki) of DABMI for the binding to tubulin of rhizoxin and ASMP-3 were 0.54 and 0.36 microM, respectively, which were nearly equal to the dissociation constant (Kd = 0.43 microM) of DABMI measured by the use of [14C]DABMI. The results suggest that Dan-PDM-3 and DABMI interacted with tubulin at the same site as rhizoxin and maytansine. DABMI is irreversibly bound to tubulin upon irradiation. Dan-PDM-3 and DABMI should be useful probes for studying the binding site.