The effects of the anilinonaphthalenesulfonates on the alkylation of tubulin: correlation between the appearance of sulfhydryl groups and apolar binding sites

We have previously found that the sulfhydryl groups of tubulin are sensitive reporters of the effects of ligands on the tubulin molecule. In this study, we examined the effects of three anilinonaphthalenesulfonates on the interaction of tubulin with iodo[¹⁴C]acetamide and N,N′-ethylenebis(iodoacetamied). We found that 1,8-anilinonaphthalensulfonate (1,8-ANS) and 2,6-anilinonaphthalenesulfonate (2,6-ANS) had no effect on the ractiion with iodo[^14C]acetamide. In contrast, bis(1,8-anilinonaphthalenesulfonate) (BisANS), an inhibitor of microtubule assembly, had a complex effect. Low concentrations of BisANS, where presumably only the high-affinity binding site was saturated, had little or no effect on alkylation. Higher concentrations of BisANS cause a strong enhancement of alkylation. None of these compounds had any effect on the reaction with N,N′-ethylenebis(iodoacetamide). Our results suggest that the binding of BisANs, 2,6-ANS and 1,8-ANS to tubulin is complex and very different from that of the other anti-tubulin drugs. The correlatiion between the effects of drugs on alkylation of tubulin and the binding of BisANS is consistent with a model whereby the alkylatable sulfhydryls are located in apolar regions of the tubulin molecule.