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One a- and 2 beta-tubulin isotypes (isotypes 1 and 2) from the parasitic nematode Haemonchus contortus were artificially expressed in E. coli and purified to obtain tubulin that was capable of polymerizing into microtubules. Binding of [14C] mebendazole (MBZ), a benzimidazole compound, to each individual unpolymerized isotype and to microtubules polymerized from recombinant alpha- and beta-tubulin was assessed and Kd and Bmax values determined. Mebendazole bound to the individual tubulin isotypes with a stoichiometry of 1:1. Binding occurred with highest affinity to alpha-tubulin followed by beta-tubulin isotype 2 and beta-tubulin isotype 1 indicating that alpha-tubulin may play a role in benzimidazole binding to microtubules. Upon polymerization of alpha- and beta-tubulin isotype 2 into microtubules the stoichiometry of binding increased to 2:1 (mebendazole : tubulin) while binding affinity remained the same. Mebendazole binding to alpha/beta-isotype 1 microtubules remained unchanged following polymerization. The increase in the number of benzimidazole receptors on alpha/beta-isotype 2 microtubules suggests the formation of a new benzimidazole receptor upon polymerization.

Trophozoites of the protozoan parasite Giardia duodenalis were exposed to various albendazole concentrations for 4 h, washed, fixed and incubated with antibodies raised against albendazole and its two major metabolites albendazole sulphoxide and albendazole sulphone. Tubulin antibodies were also used. A peroxidase- or FITC-conjugated secondary antibody was used to detect the primary antibody with transmission electron microscopy or confocal laser scanning microscopy, respectively. Albendazole, a benzimidazole compound, was detected in the mid-dorsal region of trophozoites, albendazole sulphoxide in the posterior-dorsal region and albendazole sulphone in clusters above the median bodies. Tubulin was recognised in the ventral disk. This is the first indication that G. duodenalis may be capable of metabolising albendazole and the potential path of the metabolised drug traced within the trophozoite. Fluorescence measurements revealed that albendazole sulphoxide binding decreased and albendazole sulphone binding increased with exposure of the trophozoites to increasing albendazole concentration. This indicates that if albendazole was being metabolised by trophozoites, it occurred to a greater extent following exposure to higher albendazole concentrations.