The SH-SS exchange reaction between the Ellman's reagent and protein containing SH groups as a method for determining conformational states: tubulin.

Reactivity of tubulin SH groups, estimated by the slope to the curve in the SH-SS exchange reaction with 5-5' dithiobis (2-nitrobenzoic acid), was compared with that of bovine serum albumin (BSA) and studied in presence of various ligands. A small part of tubulin SH groups (12%) showed a higher reactivity, while the remaining portion had a smaller reactivity than that of BSA. The SH group reactivity of tubulin but not the total amount (12.8 mu/mole) diminished when assayed with colchicine and MgCl2 (0.1 and 0.2 mM, respectively); 0.2 mM CaCl2 increased the reactivity at the maximum level. On the basis of the biological role of tubulin SH groups and of the opposite effects exerted by Ca++ and Mg++ on the protein, the results presented here seem to indicate a correlation between conformational states of tubulin and its biological functions.